International Journal of Heat and Mass Transfer, 2020
This study investigated the heat transfer performance of three nanostructured surfaces and two pl... more This study investigated the heat transfer performance of three nanostructured surfaces and two plain surfaces: one roughened and one polished during the saturated pool boiling of refrigerants R-134a at 5 and 25 °C and R-245fa at 20 °C. Nanocoatings were applied to polished copper tubes through a layer-by-layer (LbL) process that deposited silica nanoparticles, a chemical oxidation process where an intertwined mat of sharp copper oxide (CuO) structures were generated and a commercial nanocoating process (nanoFLUX). A polished copper tube and a roughened copper tube were tested as comparison cases. All tubes were tested in the horizontal position in pool boiling over heat fluxes of 20 to 100 kW/m2, followed by a further increase in heat flux in an attempt to reach critical heat flux. The tubes were internally water heated and Wilson plots were conducted to characterise the internal heat transfer characteristics. The nanoFLUX surface had the highest heat transfer coefficients, the LbL and polished surfaces had the lowest heat transfer coefficients, and the CuO and roughened surfaces had intermediate heat transfer coefficients. The nanoFLUX surface had between 40 and 200% higher heat transfer coefficients than those of the polished tube. Both roughened tubes and nanocoated tubes showed typical exponentially increased heat transfer coefficients as heat flux was increased. However, the nanoFLUX and CuO surfaces displayed more heat flux sensitivity compared with the other surfaces. The nanoFLUX surfaces outperformed the other nanostructured surfaces due to a higher nucleation site density and outperformed the roughened tube due to a unique heat transfer mechanism. The nanoFLUX and CuO surfaces also experienced reduced critical heat flux compared with plain surfaces, thought to be caused by the trapping of vapour in the fibrous nanostructures, resulting in reduced wetting in the Cassie-Baxter state.
The existing constant heat flux boundary condition flow regime maps are accurate for high Prandtl... more The existing constant heat flux boundary condition flow regime maps are accurate for high Prandtl number fluids, however, they are inaccurate at lower Prandtl numbers and tube diameters and specifically for water as the working fluid. The purpose of this paper was to develop a flow regime map that can be used for both high and low Prandtl number fluids for a wide range of tube diameters. Furthermore, not only to develop a map for fully developed flow, but also for developing flow. An experimental setup was designed, built and validated against results from literature. Two smooth circular test sections with inner diameters of 4 mm and 11.5 mm, respectively, and lengths of 5.5 m and 9.5 m, respectively, were used. Heat transfer measurements were taken at a range of Reynolds numbers from laminar to turbulent. Water was used as the test fluid and the Prandtl number was limited between 3 and 7. Experimental results from literature complemented the test matrix used for this study, especially with high Prandtl number data. Six flow regime maps were developed for a constant heat flux boundary condition and a square-edged inlet, and these flow regime maps are original for four reasons. Firstly, it contains contour lines that show the Nusselt number enhancements due to free convection effects. Secondly, it is valid for a wide range of tube diameters and Prandtl numbers. Thirdly, the flow regime maps were developed as a function of temperature difference (Grashof number) and heat flux (modified Grashof number). Finally, four of the six flow regime maps are not only valid for fully developed flow, but also developing flow.
Limited work has been done specifically focussing on the relationship between pressure drop and h... more Limited work has been done specifically focussing on the relationship between pressure drop and heat transfer in the transitional flow regime. The purpose of this study was therefore to experimentally investigate and compare the pressure drop and heat transfer characteristics of developing and fully developed flow in smooth horizontal circular tubes in the laminar, transitional, quasi-turbulent and turbulent flow regimes. An experimental setup was designed, built and validated against data from literature. A smooth circular test section with an inner diameter of 11.5 mm, and maximum length-to-diameter ratio of 872, was used. Pressure drop and heat transfer measurements were taken at Reynolds numbers between 500 and 10,000 at different heat fluxes. Water was used as the test fluid and the Prandtl number ranged between 3 and 7. A total of 317 mass flow rate measurements, 34,553 temperature measurements and 2536 pressure drop measurements were taken. Pressure drop and heat transfer measurements were taken simultaneously and the relationship between pressure drop and heat transfer was investigated. It was found that the Reynolds numbers at which transition started and ended was the same for the pressure drop and heat transfer results. Correlations were developed to determine the relationship between heat transfer and pressure drop, as well as the average Nusselt numbers, in the laminar, transitional, quasi-turbulent and turbulent flow regimes, for both developing and fully developed flow in mixed con-vection conditions. It was found that the relationship between heat transfer and pressure drop can be used as an additional criterion to distinguish the different flow regimes.
Limited experimental work has been done specifically focussing on how the heat transfer character... more Limited experimental work has been done specifically focussing on how the heat transfer characteristics of developing flow in the transitional flow regime changes along the tube length, and how it compares with fully developed flow. The purpose of this study was therefore to experimentally investigate the heat transfer characteristics of developing and fully developed flow in smooth horizontal tubes in the transitional flow regime at a constant heat flux. An experimental setup was designed, built and validated against literature. Two smooth circular test sections with inner diameters of 4 mm and 11.5 mm, respectively , were used and the maximum length-to-diameter ratios were 1373 and 872, respectively. Heat transfer measurements were taken at Reynolds numbers between 700 and 10 000 at different heat fluxes. The boundaries of the different flow regimes were defined mathematically, and terminology to define the transition characteristics were presented. It was found that the Reynolds number at which transition started was independent of axial position, and transition occurred at the same moment in time along the whole tube length. However, the end of transition was dependent on axial position and occurred earlier as the flow approached fully developed flow. Free convection effects affected both the start and end of the transitional flow regime, and caused the Reynolds number range of the transitional flow regime to decrease. Correlations were developed to determine the start and end of the transitional flow regime for developing and fully developed flow for mixed convection conditions. The transitional flow regime of developing flow was divided into three regions. In the first region, the width of the transitional flow regime decreased significantly with axial position and free convection effects were negligible. In region 2, the width of the transitional flow regime decreased with both axial position and free convection effects. In the fully developed region, the width of the transitional flow regime was independent of axial position, but free convection effects caused it to decrease and even to become negligible. It was concluded that the heat transfer characteristics of developing flow in the transitional flow regime changes as the flow develops and differ significantly from fully developed flow.
Correlations to calculate the local and average heat transfer coefficients for single-phase lamin... more Correlations to calculate the local and average heat transfer coefficients for single-phase laminar flow in horizontal circular tubes with a constant heat flux boundary condition are usually restricted to fully developed flow, high Prandtl numbers or constant fluid properties. What further complicates the heat transfer characteristics of developing flow, is the local transition from laminar to turbulent flow along the tube length, above the critical Reynolds number. The purpose of this study was to investigate the effects of free convection on the development of the local heat transfer characteristics in smooth horizontal circular tubes heated with a constant heat flux. An experimental setup was designed, built, and results were validated against literature. Two smooth circular test sections with inner diameters of 4 mm and 11.5 mm were used, and the maximum length-to-diameter ratios were 1373 and 872, respectively. Heat transfer measurements were taken at Reynolds numbers between 500 and 10,000 at different heat fluxes. A total of 1046 mass flow rate measurements and 89,459 temperature measurements were taken. Water was used as the test fluid and the Prandtl number ranged between 3 and 7. Three different regions were identified for developing laminar flow and were qualitatively and quantitatively defined. Correlations were developed to determine the thermal entrance lengths, as well as local and average Nusselt numbers for developing and fully developed laminar flow in mixed convection conditions with a constant heat flux boundary condition. In the transitional flow regime, the laminar-turbulent transition along the tube length was divided into four regions, and it was found that the flow transitioned faster with increasing free convection effects and Reynolds number.
The purpose of this study was to experimentally investigate the heat transfer and pressure drop c... more The purpose of this study was to experimentally investigate the heat transfer and pressure drop characteristics in the transitional flow regime of twisted tape inserts in a circular tube. Experiments were conducted in a circular tube with an internal diameter of 19.0 mm and a length of 5.27 m, and twisted tape inserts with twist ratios of 3, 4 and 5. A square-edged geometry was used at the tube inlet and it was experimentally operated with water flowing through it while the tube was heated with a constant heat flux. The experiments were conducted at three different heat fluxes of 2, 3 and 4 kW/m 2. The experimental setup was operated between Reynolds numbers of 400 and 11 400, and the Prandtl numbers varied between 2.9 and 6.7. Two methods were used to identify the transition points of the different heat fluxes and twist ratios. The first method used the standard deviation of the temperature measurements, and the second method used three linear curve fits on a log–log scale. The curve fits made it possible for correlations to be developed for the non-dimensionalised heat transfer coefficients and friction factors, which took twist ratio, heat flux and Reynolds number into consideration. For the same heat flux, it was found that the Colburn j-factors increased as the twist ratios decreased, and transition started earlier. When the twist ratio was kept constant and the heat flux was varied, higher heat fluxes delayed the transition from laminar to transitional flow. The friction factors were found to increase as the twist ratio decreased. When both the twist ratio and the Reynolds number were kept constant, an increase in heat flux was found to decrease the friction factor.
A B S T R A C T One of the methods to improve the efficiency of a wide range of energy systems is... more A B S T R A C T One of the methods to improve the efficiency of a wide range of energy systems is to enhance the performance of the heat transfer fluids. Nanofluids consisting of multi-walled carbon nanotubes in water base were studied experimentally in a square cavity with differentially heated side walls. The investigation was carried out for particle volume concentrations of 0 to 1% at Ra number 10 8. The thermal conductivities and viscosities for the nanofluids were experimentally determined. However, it was observed that the available correlations from literature did not agree well with the experimental data. The nondimensional Nusselt number characterised the heat transfer performance. Thermal conductivity was measured for the range of volume concentration 0–1% and maximum enhancement of 6% was found to be at 1%. Viscosity was measured and observed to increase by 58% over the 0–1% particle volume concentration range tested. The experimental results on natural convection yielded a maximum enhancement in heat transfer performance of 45% at volume concentration of 0.1%. This research supports the idea that " for nanofluids with effective thermal conductivity greater than the thermal conductivity of the base fluid, there may exist an optimum concentration which maximizes the heat transfer. "
A B S T R A C T Cavity design is an important aspect in thermal systems, and proper cavity design... more A B S T R A C T Cavity design is an important aspect in thermal systems, and proper cavity design saves plenty of energy as losses are minimised through better design. In this work, the influence that the Aspect Ratio (AR) of a rectangular cavity filled with nanofluids has on the natural convection process is studied experimentally. Three different cavities with the AR of 1, 2 and 4 are fabricated, and the heat transfer performance is studied using two different fluids namely de-ionised water and Al 2 O 3 /Water nanofluids. It is found that the AR of the cavity has a significant effect on the heat transfer coefficient and Nusselt number. More importantly, the optimum nanofluid concentration for maximum heat transfer varies with the AR of the cavity. It also found that the Rayleigh number has a strong effect on the Nusselt number as well as nanofluid buoyancy.
The architecture of heat exchangers is a classical subject that has been studied extensively in t... more The architecture of heat exchangers is a classical subject that has been studied extensively in the past. In this paper, we address the fundamental question of what the size of the heat exchanger should be, in addition to what architectural features it should have. The answer to the size question follows from the tradeoff between (1), the useful power lost because of heat transfer and fluid flow and (2), the power destroyed during transportation, manufacturing, and maintenance. Changes in heat exchanger size induce changes in the opposite sign in the power requirements (1), and (2). This fundamental tradeoff regarding size is illustrated by considering one side of a heat exchanger (one flow passage) in laminar flow and in fully rough turbulent flow, with several duct cross sectional shapes and arrays of channels in parallel. The size tradeoff is present in heat exchanger applications across the board, from vehicles to stationary power plants.
Experimental work has shown that the flowfield around a wing-body configuration can be successful... more Experimental work has shown that the flowfield around a wing-body configuration can be successfully modified with a short Kutta edge tail, so named because, by controlling the rear stagnation point, the circulation about the body can be effectively modified. The precise nature of the Kutta edge and body interaction were not considered, rather only the global flowfield effects. Therefore, the purpose of this study was to investigate the lift potential of low-drag bodies with Kutta edges, by numerically solving the flowfield around two low-drag bodies selected from literature. The drag and lift of the bodies were compared with experimental and numerical results in literature with good agreement. The geometries, computational grids, and boundary conditions of the two benchmark cases were then modified by adding short Kutta edges, for aftbody deflection angles of 2, 4, 6, and 8 deg at Reynolds numbers of 1.2 × 10 6 and 10 7. Both of the low-drag bodies showed similar average increases in lift and in pressure drag with the addition of the Kutta edge at increasing deflection angles. Though the configuration study is not yet complete, the results indicate a design space where there is potential for improvement in flight efficiency. Nomenclature A m = body cross-sectional area at maximum diameter, m 2 AR = aspect ratio (Kutta edge width/Kutta edge length) C DV = volume-based drag coefficient C f = skin friction coefficient C L = lift coefficient C P = pressure coefficient D = drag force, N d = body diameter, m L = lift force, N l = body length, m P = pressure, Pa r = body radius, m Re = ρUl∕μ, Reynolds number U = freestream velocity, m∕s U x = streamwise velocity component, m∕s U y = cross-stream velocity component, m∕s U z = vertical velocity component, m∕s V b = body volume, m 3 x = streamwise direction, m y = vertical (wall-normal) direction, m y ‡ = nondimensional wall distance z = horizontal direction, m δ = aftbody deflection angle, deg μ = dynamic viscosity, kg∕…m · s† ρ = density, kg∕m 3 τ w = wall shear stress, Pa Subscripts F = F-57 low-drag body f = friction component L, α = lift slope at angle of attack L, δ = lift slope at angle of deflection M = Myring low-drag body p = pressure component
Standard pin-fins in the heat transfer channels are shaped to reduce the pressure penalty and inc... more Standard pin-fins in the heat transfer channels are shaped to reduce the pressure penalty and increase the thermal performance. The paper presents experimental results of the wall-static pressure distributions in an array of modified cylindrical short pin-fins in a channel. Standard cylindrical pin-fins with a smooth surface and a similar array configuration are also evaluated as a baseline for comparisons. The pin-fins with a height to diameter ratio of 1.28 are arranged in a staggered array consisting of 13 rows in a rectangular channel of aspect ratio 1:7.8. The cylindrical pins are modified by the machined slots at the tips. The slots in the pins are aligned in the streamwise direction. The static pressure distributions are measured on the endwall between the pin-rows and on the pin surface. The Reynolds number based on the channel hydraulic diameter ranges from 10,000 to 50,000. The slots in the pins reduce the friction factor and wall-static pressure drop between the pin-rows by up to 50%. The objectives of the investigation are to reduce the pressure penalty in the cylindrical pin-fin channel to provide increased thermal performance.
The natural convection heat transfer of a magnetic nanofluid in a differentially heated cavity is... more The natural convection heat transfer of a magnetic nanofluid in a differentially heated cavity is investigated with and without an applied external magnetic field. The effects of volume fraction, magnetic field configuration, and magnetic field strength are investigated. Spherical Fe 2 O 3 nanoparticles with a diameter of 15–20 nm are used in the nanofluids. Volume fractions ranging between 0.05% and 0.3% are tested for the case with no magnetic field, while only a volume fraction of 0.1% was tested in an externally applied magnetic field. The experiments were conducted for a range of Rayleigh numbers in 1.7 Â 10 8 < Ra < 4.2 Â 10 8. The viscosity of the nanofluid was determined experimentally. An empirical correlation for the viscosity was determined, and the stability of various nanofluids was investigated. Using heat transfer data obtained from the cavity, the average heat transfer coefficient and average Nusselt number for the nanofluids are determined. It was found that a volume fraction of 0.1% showed a maximum increase of 5.63% to the Nu at the maximum Ra. For the magnetic field study, it was found that the best-performing magnetic field enhanced the heat transfer behaviour by an additional 2.81% in Nu at Ra = 3.8 Â 10 8 .
Increasing the efficiency of concentrating solar power (CSP) technologies by means of optimisatio... more Increasing the efficiency of concentrating solar power (CSP) technologies by means of optimisation tools is one of the current topics of solar thermal researchers. Of these technologies, Linear Fresnel collectors (LFCs) are the least developed. Therefore, there is plenty of room for the optimisation of this technology. One of the goals of this paper, in addition to the optimisation of an LFC plant, is introducing an applicable optimisation procedure that can be applied for any type of CSP plant. This paper focuses on harvesting maximum solar energy (maximising plant optical efficiency), as well as minimising plant thermal heat loss (maximising plant thermal efficiency), and plant cost (the economic optimisation of the plant), which leads to the generation of cheaper solar electricity from an LFC plant with a fixed power plant cycle (The performance optimisation of this study is based on the plant performance throughout an imaginary summer day). A multi-tube cavity receiver is considered in this study since there is plenty of room for its optimization. For the receiver, optimal cavity shape, tube bundle arrangement, tube numbers, cavity mounting height and insulation thickness are considered, while for the mirror field, the number of mirrors , mirror width, mirror gaps and mirror focal length are considered to achieve the optimisation goals. A multi-stage optimisation process is followed. Firstly, optical (using SolTrace), thermal (using a view area approach) and economic performance are combined in a multi-objective genetic algorithm as incorporated in ANSYS DesignXplorer (DX). This leads to an optimal LFC with a variable focal length for each mirror. After determining a fixed optimal focal length for all the mirrors, a Computational Fluid Dynamics (CFD) approach is used to optimise the thermal insulation of the cavity receiver for minimal heat loss and minimal insulation material. The process is automated through the use of ANSYS Workbench and Excel (coding with Visual Basic for Application (VBA) and LK Scripting in SolTrace). The view area approach provides an inexpensive way of calculating radiation heat loss from the receiver that is shown in the subsequent CFD analysis to be dominating the heat transfer loss mechanisms. The optimised receiver is evaluated at different LFC plant tube temperatures to assess its performance.
CFD is presented as a ray-tracing tool for linear focus CSP. Test cases from literature with Mont... more CFD is presented as a ray-tracing tool for linear focus CSP. Test cases from literature with Monte Carlo solutions are used as reference. Ray effect and false scattering errors are reduced with refinement. Refinement includes mesh refinement as well as discrete ordinates count. Linear Fresnel and parabolic trough test cases show good comparison. a b s t r a c t The modelling of solar irradiation in concentrated solar power (CSP) applications is traditionally done with ray-tracing methods, e.g. the Monte Carlo method. For the evaluation of CSP receivers, the results from ray-tracing codes are typically used to provide boundary conditions to Computational Fluid Dynamics (CFD) codes for the solution of conjugate heat transfer in the receivers. There are both advantages and disadvantages to using separate software for the irradiation and heat transfer modelling. For traditional ray-tracing methods, advantages are the cost-effectiveness of the Monte Carlo method in modelling reflections from specular surfaces; the ability to statistically assign a sun shape to the rays; the statistical treatment of reflectivity and optical errors (e.g. surface slope errors), to name a few. When considering a complex mirror field and a complex receiver with secondary reflective surfaces, especially with selective coatings to enhance absorption and limit re-radiation losses, standard ray trac-ers may be limited in specifying emissivity and absorptivity, which are both specular and temperature dependent, and are hence not suitable as radiation analysis tool. This type of scenario can be modelled accurately using CFD, through the finite volume (FV) treatment of the radiative transfer equation (RTE) and a banded spectrum approach at an increased computational cost. This paper evaluates the use of CFD in the form of the commercial CFD code ANSYS Fluent v15 and v16 to model the reflection, transmission and absorption of solar irradiation from diffuse and specular surfaces found in linear CSP applications. 2-D CFD solutions were considered, i.e. line concentration. To illustrate and validate the method, two sources were used. The first source was test cases from literature with published solutions and the second a combined modelling approach where solutions were obtained using both FV and ray tracing (with SolTrace). For all the test cases, good agreement was found when suitable modelling settings were used to limit both ray-effect and false scattering errors.
In this paper the two dimensional numerical topology optimization of a high conductive conduit ma... more In this paper the two dimensional numerical topology optimization of a high conductive conduit material , distributed within a heat-generating material, is investigated with regards to the effect of orthotropic materials. Specifically, materials with orthotropic thermal conductivities (different primary and secondary principal thermal conductivities). Two cases are considered in this study, namely the optimal distribution of an isotropic conduit material within an orthotropic heat generating material; and the optimal distribution of an orthotropic conduit material within an isotropic heat-generating material. A finite volume method (FVM) code, coupled with the method of moving asymptotes (MMA); the solid isotropic with material penalization (SIMP) scheme; and the discrete adjoint method, was used to find the optimal distribution of the high conductive conduit material within the heat generating material. For the optimal distribution of an isotropic conduit material within an orthotropic heat-generating material is was found that a heat-generating material angle 10 6 h 0 6 60 is preferred, for a higher thermal performance, and a heat-generating material angle h 0 < 10 and h 0 > 60 should be avoided. For the optimal distribution of an orthotropic conduit material within an isotropic heat-generating material is was found that an optimal conduit material angle exists giving the best thermal performance (lowest s max). It was found that the optimal conduit material angle remains the same for different conductivity ratios and different heat-generating material angles. It was also found that the optimal conduit material angle directly corresponds to the domain aspect ratio, h 1;opt ¼ tan À1 ð2H=LÞ, with a minimum improvement of 3% and a maximum improvement of 50% of the thermal performance when using an orthotropic conduit material over that of an isotropic conduit material. A 50% improvement of the thermal performance effectively translates to either double the allowable heat generation or half the peak operating temperature of the isotropic heat-generating material.
The numerical study of nanofluids as a two-phase flow (both as solid nanoparticles and in a liqui... more The numerical study of nanofluids as a two-phase flow (both as solid nanoparticles and in a liquid phase) has brought about a new approach to simulation in this area. Due to the lack of hybrid models to fully predict the flow characteristics of nanofluids under different conditions, a case can be made for developing homogenous models from numerical simulations. In this study, the convective heat transfer and hydrodynamic characteristics of nanofluids are investigated by simulation with ANSYS-FLUENT. Accordingly, four common types of nanofluids in horizontal turbulent pipe flows have been chosen from experimental data available in literature for modelling purposes. These nanofluids are Al 2 O 3 , ZrO 2 , TiO 2 and SiO 2. The simulations are done using the built-in models of ANSYS-FLUENT, namely the Mixture model and Discrete Phase Modelling (DPM). Comparing various appropriate turbulence models, the Realisable and Standard k À 3 models have provided the same results in most of the simulations. The Reynolds stress model (RSM) overestimates pressure drops compared with the other k À 3 models, while the re-normalisation group (RNG) model overestimates heat transfer coefficient. The anisotropy of instantaneous velocity in the RSM gives higher turbulent kinetic energy, dissipation rate and slip velocity between the particles and the main flow, which makes it an essential part of simulations. All the DPM results have shown the same trend, but with different percentages from measured data, which means that the number of particles plays a key role in the simulations. Any small weaknesses in DPM have a significant influence on the results due to the higher number of nanoparticles.
Non-uniform heat flux profiles on circular tubes are found in a number of heat transfer applicati... more Non-uniform heat flux profiles on circular tubes are found in a number of heat transfer applications, including solar heating. In this numerical study the influence of the circumferential angle spans of non-uniform heat flux distributions are considered on the secondary buoyancy-driven flow, internal fluid heat transfer coefficients, and friction factors in horizontal absorber tubes in parabolic trough solar collector applications for water heating in the laminar flow regime. Inlet Reynolds numbers ranging from 130 to 2200 for 10 m long tubes with different inner diameters were considered. Sinusoidal type incident heat flux distributions, tube-wall heat conduction and heat losses were taken into account. It was found that due to buoyancy-driven secondary flow, overall and local internal heat transfer coefficients were increased significantly due to the non-uniformity of the incident heat flux. Average internal heat transfer coefficient increased with the heat flux intensity, the incident heat flux angle span and the inlet fluid temperature. The effective friction factor decreased with an increase in the absorber tube inlet fluid temperature. It was found that improved thermal efficiencies can be achieved for low mass flow rate water heating applications by employing parabolic trough collector systems compared to flat plate systems.
An experimental study was conducted to determine the lower and upper Reynolds number limits of th... more An experimental study was conducted to determine the lower and upper Reynolds number limits of the transitional flow regime, and the characteristics of the heat transfer coefficients and friction factors for annular passages with different hydraulic diameters and diameter ratios in the transitional flow regime. Water was used in this investigation during isothermal, heating and cooling cases. Four horizontal concentric counter-flow tube-in-tube heat exchangers with conventional inlet geometries were considered to obtain the required data. The flow was both hydrodynamic and thermally developing, and the transitional flow was composed of mixed and forced convection types. The wall temperature on the inner surface of the annular passages was approximately uniform, while the outer surface was isothermal. Average Nusselt numbers were obtained for both the heating and cooling cases, while friction factors were obtained for heating, cooling and isothermal adiabatic conditions. Isothermal adiabatic condition was considered for reference purposes. The geometric size of the annular passage and direction of the heat flux (heating and cooling cases of annular fluid) had a significant influence on the heat transfer coefficients , friction factors and Reynolds number span of the transitional flow regime. The annular geometric parameters that represent the geometric size of the annular passage were proposed and found to describe the heat transfer coefficient and friction factors well. Subsequently, correlations for predicting the Nusselt numbers and friction factors in the transitional flow regime were developed.
Increasing the efficiency of concentrating solar power (CSP) technologies by means of optimisatio... more Increasing the efficiency of concentrating solar power (CSP) technologies by means of optimisation tools is one of the current topics of solar thermal researchers. Of these technologies, Linear Fresnel collectors (LFCs) are the least developed. Therefore, there is plenty of room for the optimisation of this technology. One of the goals of this paper, in addition to the optimisation of an LFC plant, is introducing an applicable optimisation procedure that can be applied for any type of CSP plant. This paper focuses on harvesting maximum solar energy (maximising plant optical efficiency), as well as minimising plant thermal heat loss (maximising plant thermal efficiency), and plant cost (the economic optimisation of the plant), which leads to the generation of cheaper solar electricity from an LFC plant with a fixed power plant cycle (The performance optimisation of this study is based on the plant performance throughout an imaginary summer day). A multi-tube cavity receiver is considered in this study since there is plenty of room for its optimization. For the receiver, optimal cavity shape, tube bundle arrangement, tube numbers, cavity mounting height and insulation thickness are considered, while for the mirror field, the number of mirrors , mirror width, mirror gaps and mirror focal length are considered to achieve the optimisation goals. A multi-stage optimisation process is followed. Firstly, optical (using SolTrace), thermal (using a view area approach) and economic performance are combined in a multi-objective genetic algorithm as incorporated in ANSYS DesignXplorer (DX). This leads to an optimal LFC with a variable focal length for each mirror. After determining a fixed optimal focal length for all the mirrors, a Computational Fluid Dynamics (CFD) approach is used to optimise the thermal insulation of the cavity receiver for minimal heat loss and minimal insulation material. The process is automated through the use of ANSYS Workbench and Excel (coding with Visual Basic for Application (VBA) and LK Scripting in SolTrace). The view area approach provides an inexpensive way of calculating radiation heat loss from the receiver that is shown in the subsequent CFD analysis to be dominating the heat transfer loss mechanisms. The optimised receiver is evaluated at different LFC plant tube temperatures to assess its performance.
The natural convection heat transfer of a magnetic nanofluid in a differentially heated cavity is... more The natural convection heat transfer of a magnetic nanofluid in a differentially heated cavity is investigated with and without an applied external magnetic field. The effects of volume fraction, magnetic field configuration, and magnetic field strength are investigated. Spherical Fe 2 O 3 nanoparticles with a diameter of 15–20 nm are used in the nanofluids. Volume fractions ranging between 0.05% and 0.3% are tested for the case with no magnetic field, while only a volume fraction of 0.1% was tested in an externally applied magnetic field. The experiments were conducted for a range of Rayleigh numbers in 1.7 Â 10 8 < Ra < 4.2 Â 10 8. The viscosity of the nanofluid was determined experimentally. An empirical correlation for the viscosity was determined, and the stability of various nanofluids was investigated. Using heat transfer data obtained from the cavity, the average heat transfer coefficient and average Nusselt number for the nanofluids are determined. It was found that a volume fraction of 0.1% showed a maximum increase of 5.63% to the Nu at the maximum Ra. For the magnetic field study, it was found that the best-performing magnetic field enhanced the heat transfer behaviour by an additional 2.81% in Nu at Ra = 3.8 Â 10 8 .
International Journal of Heat and Mass Transfer, 2020
This study investigated the heat transfer performance of three nanostructured surfaces and two pl... more This study investigated the heat transfer performance of three nanostructured surfaces and two plain surfaces: one roughened and one polished during the saturated pool boiling of refrigerants R-134a at 5 and 25 °C and R-245fa at 20 °C. Nanocoatings were applied to polished copper tubes through a layer-by-layer (LbL) process that deposited silica nanoparticles, a chemical oxidation process where an intertwined mat of sharp copper oxide (CuO) structures were generated and a commercial nanocoating process (nanoFLUX). A polished copper tube and a roughened copper tube were tested as comparison cases. All tubes were tested in the horizontal position in pool boiling over heat fluxes of 20 to 100 kW/m2, followed by a further increase in heat flux in an attempt to reach critical heat flux. The tubes were internally water heated and Wilson plots were conducted to characterise the internal heat transfer characteristics. The nanoFLUX surface had the highest heat transfer coefficients, the LbL and polished surfaces had the lowest heat transfer coefficients, and the CuO and roughened surfaces had intermediate heat transfer coefficients. The nanoFLUX surface had between 40 and 200% higher heat transfer coefficients than those of the polished tube. Both roughened tubes and nanocoated tubes showed typical exponentially increased heat transfer coefficients as heat flux was increased. However, the nanoFLUX and CuO surfaces displayed more heat flux sensitivity compared with the other surfaces. The nanoFLUX surfaces outperformed the other nanostructured surfaces due to a higher nucleation site density and outperformed the roughened tube due to a unique heat transfer mechanism. The nanoFLUX and CuO surfaces also experienced reduced critical heat flux compared with plain surfaces, thought to be caused by the trapping of vapour in the fibrous nanostructures, resulting in reduced wetting in the Cassie-Baxter state.
The existing constant heat flux boundary condition flow regime maps are accurate for high Prandtl... more The existing constant heat flux boundary condition flow regime maps are accurate for high Prandtl number fluids, however, they are inaccurate at lower Prandtl numbers and tube diameters and specifically for water as the working fluid. The purpose of this paper was to develop a flow regime map that can be used for both high and low Prandtl number fluids for a wide range of tube diameters. Furthermore, not only to develop a map for fully developed flow, but also for developing flow. An experimental setup was designed, built and validated against results from literature. Two smooth circular test sections with inner diameters of 4 mm and 11.5 mm, respectively, and lengths of 5.5 m and 9.5 m, respectively, were used. Heat transfer measurements were taken at a range of Reynolds numbers from laminar to turbulent. Water was used as the test fluid and the Prandtl number was limited between 3 and 7. Experimental results from literature complemented the test matrix used for this study, especially with high Prandtl number data. Six flow regime maps were developed for a constant heat flux boundary condition and a square-edged inlet, and these flow regime maps are original for four reasons. Firstly, it contains contour lines that show the Nusselt number enhancements due to free convection effects. Secondly, it is valid for a wide range of tube diameters and Prandtl numbers. Thirdly, the flow regime maps were developed as a function of temperature difference (Grashof number) and heat flux (modified Grashof number). Finally, four of the six flow regime maps are not only valid for fully developed flow, but also developing flow.
Limited work has been done specifically focussing on the relationship between pressure drop and h... more Limited work has been done specifically focussing on the relationship between pressure drop and heat transfer in the transitional flow regime. The purpose of this study was therefore to experimentally investigate and compare the pressure drop and heat transfer characteristics of developing and fully developed flow in smooth horizontal circular tubes in the laminar, transitional, quasi-turbulent and turbulent flow regimes. An experimental setup was designed, built and validated against data from literature. A smooth circular test section with an inner diameter of 11.5 mm, and maximum length-to-diameter ratio of 872, was used. Pressure drop and heat transfer measurements were taken at Reynolds numbers between 500 and 10,000 at different heat fluxes. Water was used as the test fluid and the Prandtl number ranged between 3 and 7. A total of 317 mass flow rate measurements, 34,553 temperature measurements and 2536 pressure drop measurements were taken. Pressure drop and heat transfer measurements were taken simultaneously and the relationship between pressure drop and heat transfer was investigated. It was found that the Reynolds numbers at which transition started and ended was the same for the pressure drop and heat transfer results. Correlations were developed to determine the relationship between heat transfer and pressure drop, as well as the average Nusselt numbers, in the laminar, transitional, quasi-turbulent and turbulent flow regimes, for both developing and fully developed flow in mixed con-vection conditions. It was found that the relationship between heat transfer and pressure drop can be used as an additional criterion to distinguish the different flow regimes.
Limited experimental work has been done specifically focussing on how the heat transfer character... more Limited experimental work has been done specifically focussing on how the heat transfer characteristics of developing flow in the transitional flow regime changes along the tube length, and how it compares with fully developed flow. The purpose of this study was therefore to experimentally investigate the heat transfer characteristics of developing and fully developed flow in smooth horizontal tubes in the transitional flow regime at a constant heat flux. An experimental setup was designed, built and validated against literature. Two smooth circular test sections with inner diameters of 4 mm and 11.5 mm, respectively , were used and the maximum length-to-diameter ratios were 1373 and 872, respectively. Heat transfer measurements were taken at Reynolds numbers between 700 and 10 000 at different heat fluxes. The boundaries of the different flow regimes were defined mathematically, and terminology to define the transition characteristics were presented. It was found that the Reynolds number at which transition started was independent of axial position, and transition occurred at the same moment in time along the whole tube length. However, the end of transition was dependent on axial position and occurred earlier as the flow approached fully developed flow. Free convection effects affected both the start and end of the transitional flow regime, and caused the Reynolds number range of the transitional flow regime to decrease. Correlations were developed to determine the start and end of the transitional flow regime for developing and fully developed flow for mixed convection conditions. The transitional flow regime of developing flow was divided into three regions. In the first region, the width of the transitional flow regime decreased significantly with axial position and free convection effects were negligible. In region 2, the width of the transitional flow regime decreased with both axial position and free convection effects. In the fully developed region, the width of the transitional flow regime was independent of axial position, but free convection effects caused it to decrease and even to become negligible. It was concluded that the heat transfer characteristics of developing flow in the transitional flow regime changes as the flow develops and differ significantly from fully developed flow.
Correlations to calculate the local and average heat transfer coefficients for single-phase lamin... more Correlations to calculate the local and average heat transfer coefficients for single-phase laminar flow in horizontal circular tubes with a constant heat flux boundary condition are usually restricted to fully developed flow, high Prandtl numbers or constant fluid properties. What further complicates the heat transfer characteristics of developing flow, is the local transition from laminar to turbulent flow along the tube length, above the critical Reynolds number. The purpose of this study was to investigate the effects of free convection on the development of the local heat transfer characteristics in smooth horizontal circular tubes heated with a constant heat flux. An experimental setup was designed, built, and results were validated against literature. Two smooth circular test sections with inner diameters of 4 mm and 11.5 mm were used, and the maximum length-to-diameter ratios were 1373 and 872, respectively. Heat transfer measurements were taken at Reynolds numbers between 500 and 10,000 at different heat fluxes. A total of 1046 mass flow rate measurements and 89,459 temperature measurements were taken. Water was used as the test fluid and the Prandtl number ranged between 3 and 7. Three different regions were identified for developing laminar flow and were qualitatively and quantitatively defined. Correlations were developed to determine the thermal entrance lengths, as well as local and average Nusselt numbers for developing and fully developed laminar flow in mixed convection conditions with a constant heat flux boundary condition. In the transitional flow regime, the laminar-turbulent transition along the tube length was divided into four regions, and it was found that the flow transitioned faster with increasing free convection effects and Reynolds number.
The purpose of this study was to experimentally investigate the heat transfer and pressure drop c... more The purpose of this study was to experimentally investigate the heat transfer and pressure drop characteristics in the transitional flow regime of twisted tape inserts in a circular tube. Experiments were conducted in a circular tube with an internal diameter of 19.0 mm and a length of 5.27 m, and twisted tape inserts with twist ratios of 3, 4 and 5. A square-edged geometry was used at the tube inlet and it was experimentally operated with water flowing through it while the tube was heated with a constant heat flux. The experiments were conducted at three different heat fluxes of 2, 3 and 4 kW/m 2. The experimental setup was operated between Reynolds numbers of 400 and 11 400, and the Prandtl numbers varied between 2.9 and 6.7. Two methods were used to identify the transition points of the different heat fluxes and twist ratios. The first method used the standard deviation of the temperature measurements, and the second method used three linear curve fits on a log–log scale. The curve fits made it possible for correlations to be developed for the non-dimensionalised heat transfer coefficients and friction factors, which took twist ratio, heat flux and Reynolds number into consideration. For the same heat flux, it was found that the Colburn j-factors increased as the twist ratios decreased, and transition started earlier. When the twist ratio was kept constant and the heat flux was varied, higher heat fluxes delayed the transition from laminar to transitional flow. The friction factors were found to increase as the twist ratio decreased. When both the twist ratio and the Reynolds number were kept constant, an increase in heat flux was found to decrease the friction factor.
A B S T R A C T One of the methods to improve the efficiency of a wide range of energy systems is... more A B S T R A C T One of the methods to improve the efficiency of a wide range of energy systems is to enhance the performance of the heat transfer fluids. Nanofluids consisting of multi-walled carbon nanotubes in water base were studied experimentally in a square cavity with differentially heated side walls. The investigation was carried out for particle volume concentrations of 0 to 1% at Ra number 10 8. The thermal conductivities and viscosities for the nanofluids were experimentally determined. However, it was observed that the available correlations from literature did not agree well with the experimental data. The nondimensional Nusselt number characterised the heat transfer performance. Thermal conductivity was measured for the range of volume concentration 0–1% and maximum enhancement of 6% was found to be at 1%. Viscosity was measured and observed to increase by 58% over the 0–1% particle volume concentration range tested. The experimental results on natural convection yielded a maximum enhancement in heat transfer performance of 45% at volume concentration of 0.1%. This research supports the idea that " for nanofluids with effective thermal conductivity greater than the thermal conductivity of the base fluid, there may exist an optimum concentration which maximizes the heat transfer. "
A B S T R A C T Cavity design is an important aspect in thermal systems, and proper cavity design... more A B S T R A C T Cavity design is an important aspect in thermal systems, and proper cavity design saves plenty of energy as losses are minimised through better design. In this work, the influence that the Aspect Ratio (AR) of a rectangular cavity filled with nanofluids has on the natural convection process is studied experimentally. Three different cavities with the AR of 1, 2 and 4 are fabricated, and the heat transfer performance is studied using two different fluids namely de-ionised water and Al 2 O 3 /Water nanofluids. It is found that the AR of the cavity has a significant effect on the heat transfer coefficient and Nusselt number. More importantly, the optimum nanofluid concentration for maximum heat transfer varies with the AR of the cavity. It also found that the Rayleigh number has a strong effect on the Nusselt number as well as nanofluid buoyancy.
The architecture of heat exchangers is a classical subject that has been studied extensively in t... more The architecture of heat exchangers is a classical subject that has been studied extensively in the past. In this paper, we address the fundamental question of what the size of the heat exchanger should be, in addition to what architectural features it should have. The answer to the size question follows from the tradeoff between (1), the useful power lost because of heat transfer and fluid flow and (2), the power destroyed during transportation, manufacturing, and maintenance. Changes in heat exchanger size induce changes in the opposite sign in the power requirements (1), and (2). This fundamental tradeoff regarding size is illustrated by considering one side of a heat exchanger (one flow passage) in laminar flow and in fully rough turbulent flow, with several duct cross sectional shapes and arrays of channels in parallel. The size tradeoff is present in heat exchanger applications across the board, from vehicles to stationary power plants.
Experimental work has shown that the flowfield around a wing-body configuration can be successful... more Experimental work has shown that the flowfield around a wing-body configuration can be successfully modified with a short Kutta edge tail, so named because, by controlling the rear stagnation point, the circulation about the body can be effectively modified. The precise nature of the Kutta edge and body interaction were not considered, rather only the global flowfield effects. Therefore, the purpose of this study was to investigate the lift potential of low-drag bodies with Kutta edges, by numerically solving the flowfield around two low-drag bodies selected from literature. The drag and lift of the bodies were compared with experimental and numerical results in literature with good agreement. The geometries, computational grids, and boundary conditions of the two benchmark cases were then modified by adding short Kutta edges, for aftbody deflection angles of 2, 4, 6, and 8 deg at Reynolds numbers of 1.2 × 10 6 and 10 7. Both of the low-drag bodies showed similar average increases in lift and in pressure drag with the addition of the Kutta edge at increasing deflection angles. Though the configuration study is not yet complete, the results indicate a design space where there is potential for improvement in flight efficiency. Nomenclature A m = body cross-sectional area at maximum diameter, m 2 AR = aspect ratio (Kutta edge width/Kutta edge length) C DV = volume-based drag coefficient C f = skin friction coefficient C L = lift coefficient C P = pressure coefficient D = drag force, N d = body diameter, m L = lift force, N l = body length, m P = pressure, Pa r = body radius, m Re = ρUl∕μ, Reynolds number U = freestream velocity, m∕s U x = streamwise velocity component, m∕s U y = cross-stream velocity component, m∕s U z = vertical velocity component, m∕s V b = body volume, m 3 x = streamwise direction, m y = vertical (wall-normal) direction, m y ‡ = nondimensional wall distance z = horizontal direction, m δ = aftbody deflection angle, deg μ = dynamic viscosity, kg∕…m · s† ρ = density, kg∕m 3 τ w = wall shear stress, Pa Subscripts F = F-57 low-drag body f = friction component L, α = lift slope at angle of attack L, δ = lift slope at angle of deflection M = Myring low-drag body p = pressure component
Standard pin-fins in the heat transfer channels are shaped to reduce the pressure penalty and inc... more Standard pin-fins in the heat transfer channels are shaped to reduce the pressure penalty and increase the thermal performance. The paper presents experimental results of the wall-static pressure distributions in an array of modified cylindrical short pin-fins in a channel. Standard cylindrical pin-fins with a smooth surface and a similar array configuration are also evaluated as a baseline for comparisons. The pin-fins with a height to diameter ratio of 1.28 are arranged in a staggered array consisting of 13 rows in a rectangular channel of aspect ratio 1:7.8. The cylindrical pins are modified by the machined slots at the tips. The slots in the pins are aligned in the streamwise direction. The static pressure distributions are measured on the endwall between the pin-rows and on the pin surface. The Reynolds number based on the channel hydraulic diameter ranges from 10,000 to 50,000. The slots in the pins reduce the friction factor and wall-static pressure drop between the pin-rows by up to 50%. The objectives of the investigation are to reduce the pressure penalty in the cylindrical pin-fin channel to provide increased thermal performance.
The natural convection heat transfer of a magnetic nanofluid in a differentially heated cavity is... more The natural convection heat transfer of a magnetic nanofluid in a differentially heated cavity is investigated with and without an applied external magnetic field. The effects of volume fraction, magnetic field configuration, and magnetic field strength are investigated. Spherical Fe 2 O 3 nanoparticles with a diameter of 15–20 nm are used in the nanofluids. Volume fractions ranging between 0.05% and 0.3% are tested for the case with no magnetic field, while only a volume fraction of 0.1% was tested in an externally applied magnetic field. The experiments were conducted for a range of Rayleigh numbers in 1.7 Â 10 8 < Ra < 4.2 Â 10 8. The viscosity of the nanofluid was determined experimentally. An empirical correlation for the viscosity was determined, and the stability of various nanofluids was investigated. Using heat transfer data obtained from the cavity, the average heat transfer coefficient and average Nusselt number for the nanofluids are determined. It was found that a volume fraction of 0.1% showed a maximum increase of 5.63% to the Nu at the maximum Ra. For the magnetic field study, it was found that the best-performing magnetic field enhanced the heat transfer behaviour by an additional 2.81% in Nu at Ra = 3.8 Â 10 8 .
Increasing the efficiency of concentrating solar power (CSP) technologies by means of optimisatio... more Increasing the efficiency of concentrating solar power (CSP) technologies by means of optimisation tools is one of the current topics of solar thermal researchers. Of these technologies, Linear Fresnel collectors (LFCs) are the least developed. Therefore, there is plenty of room for the optimisation of this technology. One of the goals of this paper, in addition to the optimisation of an LFC plant, is introducing an applicable optimisation procedure that can be applied for any type of CSP plant. This paper focuses on harvesting maximum solar energy (maximising plant optical efficiency), as well as minimising plant thermal heat loss (maximising plant thermal efficiency), and plant cost (the economic optimisation of the plant), which leads to the generation of cheaper solar electricity from an LFC plant with a fixed power plant cycle (The performance optimisation of this study is based on the plant performance throughout an imaginary summer day). A multi-tube cavity receiver is considered in this study since there is plenty of room for its optimization. For the receiver, optimal cavity shape, tube bundle arrangement, tube numbers, cavity mounting height and insulation thickness are considered, while for the mirror field, the number of mirrors , mirror width, mirror gaps and mirror focal length are considered to achieve the optimisation goals. A multi-stage optimisation process is followed. Firstly, optical (using SolTrace), thermal (using a view area approach) and economic performance are combined in a multi-objective genetic algorithm as incorporated in ANSYS DesignXplorer (DX). This leads to an optimal LFC with a variable focal length for each mirror. After determining a fixed optimal focal length for all the mirrors, a Computational Fluid Dynamics (CFD) approach is used to optimise the thermal insulation of the cavity receiver for minimal heat loss and minimal insulation material. The process is automated through the use of ANSYS Workbench and Excel (coding with Visual Basic for Application (VBA) and LK Scripting in SolTrace). The view area approach provides an inexpensive way of calculating radiation heat loss from the receiver that is shown in the subsequent CFD analysis to be dominating the heat transfer loss mechanisms. The optimised receiver is evaluated at different LFC plant tube temperatures to assess its performance.
CFD is presented as a ray-tracing tool for linear focus CSP. Test cases from literature with Mont... more CFD is presented as a ray-tracing tool for linear focus CSP. Test cases from literature with Monte Carlo solutions are used as reference. Ray effect and false scattering errors are reduced with refinement. Refinement includes mesh refinement as well as discrete ordinates count. Linear Fresnel and parabolic trough test cases show good comparison. a b s t r a c t The modelling of solar irradiation in concentrated solar power (CSP) applications is traditionally done with ray-tracing methods, e.g. the Monte Carlo method. For the evaluation of CSP receivers, the results from ray-tracing codes are typically used to provide boundary conditions to Computational Fluid Dynamics (CFD) codes for the solution of conjugate heat transfer in the receivers. There are both advantages and disadvantages to using separate software for the irradiation and heat transfer modelling. For traditional ray-tracing methods, advantages are the cost-effectiveness of the Monte Carlo method in modelling reflections from specular surfaces; the ability to statistically assign a sun shape to the rays; the statistical treatment of reflectivity and optical errors (e.g. surface slope errors), to name a few. When considering a complex mirror field and a complex receiver with secondary reflective surfaces, especially with selective coatings to enhance absorption and limit re-radiation losses, standard ray trac-ers may be limited in specifying emissivity and absorptivity, which are both specular and temperature dependent, and are hence not suitable as radiation analysis tool. This type of scenario can be modelled accurately using CFD, through the finite volume (FV) treatment of the radiative transfer equation (RTE) and a banded spectrum approach at an increased computational cost. This paper evaluates the use of CFD in the form of the commercial CFD code ANSYS Fluent v15 and v16 to model the reflection, transmission and absorption of solar irradiation from diffuse and specular surfaces found in linear CSP applications. 2-D CFD solutions were considered, i.e. line concentration. To illustrate and validate the method, two sources were used. The first source was test cases from literature with published solutions and the second a combined modelling approach where solutions were obtained using both FV and ray tracing (with SolTrace). For all the test cases, good agreement was found when suitable modelling settings were used to limit both ray-effect and false scattering errors.
In this paper the two dimensional numerical topology optimization of a high conductive conduit ma... more In this paper the two dimensional numerical topology optimization of a high conductive conduit material , distributed within a heat-generating material, is investigated with regards to the effect of orthotropic materials. Specifically, materials with orthotropic thermal conductivities (different primary and secondary principal thermal conductivities). Two cases are considered in this study, namely the optimal distribution of an isotropic conduit material within an orthotropic heat generating material; and the optimal distribution of an orthotropic conduit material within an isotropic heat-generating material. A finite volume method (FVM) code, coupled with the method of moving asymptotes (MMA); the solid isotropic with material penalization (SIMP) scheme; and the discrete adjoint method, was used to find the optimal distribution of the high conductive conduit material within the heat generating material. For the optimal distribution of an isotropic conduit material within an orthotropic heat-generating material is was found that a heat-generating material angle 10 6 h 0 6 60 is preferred, for a higher thermal performance, and a heat-generating material angle h 0 < 10 and h 0 > 60 should be avoided. For the optimal distribution of an orthotropic conduit material within an isotropic heat-generating material is was found that an optimal conduit material angle exists giving the best thermal performance (lowest s max). It was found that the optimal conduit material angle remains the same for different conductivity ratios and different heat-generating material angles. It was also found that the optimal conduit material angle directly corresponds to the domain aspect ratio, h 1;opt ¼ tan À1 ð2H=LÞ, with a minimum improvement of 3% and a maximum improvement of 50% of the thermal performance when using an orthotropic conduit material over that of an isotropic conduit material. A 50% improvement of the thermal performance effectively translates to either double the allowable heat generation or half the peak operating temperature of the isotropic heat-generating material.
The numerical study of nanofluids as a two-phase flow (both as solid nanoparticles and in a liqui... more The numerical study of nanofluids as a two-phase flow (both as solid nanoparticles and in a liquid phase) has brought about a new approach to simulation in this area. Due to the lack of hybrid models to fully predict the flow characteristics of nanofluids under different conditions, a case can be made for developing homogenous models from numerical simulations. In this study, the convective heat transfer and hydrodynamic characteristics of nanofluids are investigated by simulation with ANSYS-FLUENT. Accordingly, four common types of nanofluids in horizontal turbulent pipe flows have been chosen from experimental data available in literature for modelling purposes. These nanofluids are Al 2 O 3 , ZrO 2 , TiO 2 and SiO 2. The simulations are done using the built-in models of ANSYS-FLUENT, namely the Mixture model and Discrete Phase Modelling (DPM). Comparing various appropriate turbulence models, the Realisable and Standard k À 3 models have provided the same results in most of the simulations. The Reynolds stress model (RSM) overestimates pressure drops compared with the other k À 3 models, while the re-normalisation group (RNG) model overestimates heat transfer coefficient. The anisotropy of instantaneous velocity in the RSM gives higher turbulent kinetic energy, dissipation rate and slip velocity between the particles and the main flow, which makes it an essential part of simulations. All the DPM results have shown the same trend, but with different percentages from measured data, which means that the number of particles plays a key role in the simulations. Any small weaknesses in DPM have a significant influence on the results due to the higher number of nanoparticles.
Non-uniform heat flux profiles on circular tubes are found in a number of heat transfer applicati... more Non-uniform heat flux profiles on circular tubes are found in a number of heat transfer applications, including solar heating. In this numerical study the influence of the circumferential angle spans of non-uniform heat flux distributions are considered on the secondary buoyancy-driven flow, internal fluid heat transfer coefficients, and friction factors in horizontal absorber tubes in parabolic trough solar collector applications for water heating in the laminar flow regime. Inlet Reynolds numbers ranging from 130 to 2200 for 10 m long tubes with different inner diameters were considered. Sinusoidal type incident heat flux distributions, tube-wall heat conduction and heat losses were taken into account. It was found that due to buoyancy-driven secondary flow, overall and local internal heat transfer coefficients were increased significantly due to the non-uniformity of the incident heat flux. Average internal heat transfer coefficient increased with the heat flux intensity, the incident heat flux angle span and the inlet fluid temperature. The effective friction factor decreased with an increase in the absorber tube inlet fluid temperature. It was found that improved thermal efficiencies can be achieved for low mass flow rate water heating applications by employing parabolic trough collector systems compared to flat plate systems.
An experimental study was conducted to determine the lower and upper Reynolds number limits of th... more An experimental study was conducted to determine the lower and upper Reynolds number limits of the transitional flow regime, and the characteristics of the heat transfer coefficients and friction factors for annular passages with different hydraulic diameters and diameter ratios in the transitional flow regime. Water was used in this investigation during isothermal, heating and cooling cases. Four horizontal concentric counter-flow tube-in-tube heat exchangers with conventional inlet geometries were considered to obtain the required data. The flow was both hydrodynamic and thermally developing, and the transitional flow was composed of mixed and forced convection types. The wall temperature on the inner surface of the annular passages was approximately uniform, while the outer surface was isothermal. Average Nusselt numbers were obtained for both the heating and cooling cases, while friction factors were obtained for heating, cooling and isothermal adiabatic conditions. Isothermal adiabatic condition was considered for reference purposes. The geometric size of the annular passage and direction of the heat flux (heating and cooling cases of annular fluid) had a significant influence on the heat transfer coefficients , friction factors and Reynolds number span of the transitional flow regime. The annular geometric parameters that represent the geometric size of the annular passage were proposed and found to describe the heat transfer coefficient and friction factors well. Subsequently, correlations for predicting the Nusselt numbers and friction factors in the transitional flow regime were developed.
Increasing the efficiency of concentrating solar power (CSP) technologies by means of optimisatio... more Increasing the efficiency of concentrating solar power (CSP) technologies by means of optimisation tools is one of the current topics of solar thermal researchers. Of these technologies, Linear Fresnel collectors (LFCs) are the least developed. Therefore, there is plenty of room for the optimisation of this technology. One of the goals of this paper, in addition to the optimisation of an LFC plant, is introducing an applicable optimisation procedure that can be applied for any type of CSP plant. This paper focuses on harvesting maximum solar energy (maximising plant optical efficiency), as well as minimising plant thermal heat loss (maximising plant thermal efficiency), and plant cost (the economic optimisation of the plant), which leads to the generation of cheaper solar electricity from an LFC plant with a fixed power plant cycle (The performance optimisation of this study is based on the plant performance throughout an imaginary summer day). A multi-tube cavity receiver is considered in this study since there is plenty of room for its optimization. For the receiver, optimal cavity shape, tube bundle arrangement, tube numbers, cavity mounting height and insulation thickness are considered, while for the mirror field, the number of mirrors , mirror width, mirror gaps and mirror focal length are considered to achieve the optimisation goals. A multi-stage optimisation process is followed. Firstly, optical (using SolTrace), thermal (using a view area approach) and economic performance are combined in a multi-objective genetic algorithm as incorporated in ANSYS DesignXplorer (DX). This leads to an optimal LFC with a variable focal length for each mirror. After determining a fixed optimal focal length for all the mirrors, a Computational Fluid Dynamics (CFD) approach is used to optimise the thermal insulation of the cavity receiver for minimal heat loss and minimal insulation material. The process is automated through the use of ANSYS Workbench and Excel (coding with Visual Basic for Application (VBA) and LK Scripting in SolTrace). The view area approach provides an inexpensive way of calculating radiation heat loss from the receiver that is shown in the subsequent CFD analysis to be dominating the heat transfer loss mechanisms. The optimised receiver is evaluated at different LFC plant tube temperatures to assess its performance.
The natural convection heat transfer of a magnetic nanofluid in a differentially heated cavity is... more The natural convection heat transfer of a magnetic nanofluid in a differentially heated cavity is investigated with and without an applied external magnetic field. The effects of volume fraction, magnetic field configuration, and magnetic field strength are investigated. Spherical Fe 2 O 3 nanoparticles with a diameter of 15–20 nm are used in the nanofluids. Volume fractions ranging between 0.05% and 0.3% are tested for the case with no magnetic field, while only a volume fraction of 0.1% was tested in an externally applied magnetic field. The experiments were conducted for a range of Rayleigh numbers in 1.7 Â 10 8 < Ra < 4.2 Â 10 8. The viscosity of the nanofluid was determined experimentally. An empirical correlation for the viscosity was determined, and the stability of various nanofluids was investigated. Using heat transfer data obtained from the cavity, the average heat transfer coefficient and average Nusselt number for the nanofluids are determined. It was found that a volume fraction of 0.1% showed a maximum increase of 5.63% to the Nu at the maximum Ra. For the magnetic field study, it was found that the best-performing magnetic field enhanced the heat transfer behaviour by an additional 2.81% in Nu at Ra = 3.8 Â 10 8 .
16th International Heat Transfer Conference (IHTC-16), 2018
The open Brayton cycle with open cavity receiver utilises a parabolic dish to concentrate solar i... more The open Brayton cycle with open cavity receiver utilises a parabolic dish to concentrate solar irradiance so that it may be captured by the working fluid (air). The cycle has been analysed and optimised to work with a simple receiver so that complexity and cost may be reduced. To maintain sufficient cycle effectiveness, a large efficient recuperator has to be implemented to allow for the high temperatures in order of 1000 K needed by the Brayton cycle. The proposed micro-turbine, an automotive turbocharger, cannot operate at high pressure ratios. The recuperator allows for lower pressure ratios to be considered. The purpose of this research is to test a design for a low-pressure and high-temperature recuperator that can be implemented within the solar Brayton cycle, and can be locally manufactured for a relatively low cost, as no current solution for such a cycle exists. Current solutions involve complex designs, expensive manufacturing processes and permanent joining methods that would eliminate the possibility for inspection and maintenance. The key element in the proposed design would see a high temperature sealant being used along with a clamped plate heat exchanger layout, in place of welding, allowing for ease of assembly as well as the ability to dismantle the unit for inspection. To ascertain the possibility of implementing the design, the recuperator was first modelled and shown to adhere to the necessary criteria. Results from the theoretical model show that at the proposed cycle conditions the recuperator plate bank would consist of 350 channels with an effectiveness of 90% and a total pressure drop of 3.49 kPa. A small scale model of the recuperator was constructed and tested, using both in-stream and surface thermocouples. Due to combustion issues with the LPG, the data was slightly skewed, however enough results were attained to show that the design could prove effective with a few modifications and further testing, and that the high temperature sealant works well with the clamped plate design.
Heat transfer in the transitional flow regime has many applications in industry. One of the most ... more Heat transfer in the transitional flow regime has many applications in industry. One of the most important is in flow channels or conduits which is used as one of the two sides of heat exchangers. However, recent works show that during the design process, the best compromise between high heat transfer coefficients and relatively low pressure drops is usually found in the transitional flow regime. Unfortunately, very limited experimental work has been conducted in this regime. The purpose of this paper is threefold. Firstly, an historical overview is given. Secondly, to present the state of the art in transitional flow. Thirdly, to give an overview of completed work preliminary results of work in progress in the transitional flow regime. It was concluded from a historical overview and the state of the art that very little work has been conducted on heat transfer in the transitional flow regime. Completed work and preliminary results from six projects which are work in progress are discussed.
In this paper the effect of inclination angle on the condensation heat transfer coefficient, pres... more In this paper the effect of inclination angle on the condensation heat transfer coefficient, pressure drop and flow regime inside a smooth tube was investigated numerically. The working fluid was R134a at a saturation temperature of 40°C. The Volume of Fluid (VOF) multiphase flow formulation was utilized to solve the governing equations. Simulations were conducted at a heat flux of 5 kW/m 2 , at mass fluxes of 100 – 600 kg/m 2 .s, and the inclination angles were varied from vertical downward to vertical upward. The simulation results were successfully validated with the experimental data. The results showed that an optimum downward inclination angle of between-30° and-15° exists, for the heat transfer coefficients. It was also found that the effect of inclination angle on the pressure drop and void fraction became negligible at high mass fluxes and vapour qualities.
In this paper, experimental investigation on heat transfer and friction characteristics of a duct... more In this paper, experimental investigation on heat transfer and friction characteristics of a duct inserted with a short length angular cut twisted tape has been carried out. The effects of different lengths of angular cut twisted tapes on heat transfer augmentation were calculated through a circular duct using air as working fluid. The experimental investigations were conducted using the twisted tapes with different lengths over a Reynolds number range of 10,000 – 50,000. The experimental results were in good agreement with correlations. The obtained results indicate that angular cut twisted tapes show promising results when compared to plain twisted tapes in terms of heat transfer. The full length angular cut twisted tape inserts performs better in terms of heat transfer in comparison with other full length tape. A large data set has been generated for heat transfer and thermal-hydraulic performance which is useful for the design of solar thermal heaters and heat exchangers. 45
The flow regime at which a fluid flows is very important in heat transfer engineering and also an... more The flow regime at which a fluid flows is very important in heat transfer engineering and also analogous to the rate of energy consumption in the domestic or industrial applications of heat exchangers. In this article, the influence of twisted tape insert on the heat transfer coefficients in the transitional flow regime was experimentally investigated and reported. A thin typical twisted tape of twist ratio of 5 was inserted into a plain circular copper tube with water as working fluid, square-edge entry and at a constant heat flux boundary condition of 2 kW/m 2 over a Reynolds number range of 500 to 10 394 and Prandtl number range of 4.32 to 6.72. The transitional flow regime was easily identified and differentiated from laminar and turbulent flow regime by plotting the local heat transfer coefficients in terms of local Colburn j-factors against local Reynolds numbers in the fully developed flow region. The local station considered in this study had a length-to-diameter ratio of 141. The results showed that the transitional flow regime commenced at a local Reynolds number of 981 and ended at 1 447 for the tube with twisted tape insert. In the plain tube, which was used for validation, the critical Reynolds number was attained at a local Reynolds number of 3 005 and the transitional flow regime ended at a local Reynolds number of 3 318.
Numerous studies reveal that the heat transfer capability of thermal systems has been significant... more Numerous studies reveal that the heat transfer capability of thermal systems has been significantly enhanced with the use of nanofluids. On the other hand, the hazardous nature of the nanoparticles is evident. Recent studies clearly indicate that the nanoparticles affect the human health as well as the environment. Therefore environmentally safe bio-nanofluids are currently under investigation. In this study, a novel heat transfer fluid with bio-nanomaterial is prepared and its natural convection heat transfer characteristics are studied. The bio-nanomaterial considered in this study is powdered mango bark. A two-step process is employed to prepare stable nanofluids. The effect of particles concentration, the temperature difference between the hot and cold side, and Rayleigh number on the natural convection heat transfer process is studied. The experimental results show that the natural convection process is deteriorated with the addition of mango nanoparticles in de-ionized water.
Due to design limitations or system upgrade, heat exchangers are required to operate in the lamin... more Due to design limitations or system upgrade, heat exchangers are required to operate in the laminar-to-turbulent transition region in order to achieve a high heat transfer with low pressure drop. The present research investigates the effect of vertical upflow with heating on the single-phase heat transfer in the transitional flow regime of a vertical tube. The experimental setup consists of a swinging test bench which allows for horizontal flow and vertical upflow direction under constant heat flux boundary condition. A smooth circular tube with inner diameter of 5.1 mm and a heated length of 4.52 m was used as the test section with water at Prandtl number ranging between 5 to 7 as working fluid. The experiment covers Reynolds number range of 1 000 to 10 000 at horizontal and vertical orientations of the test section using a squared-edged inlet geometry. For fully developed vertical upflow direction, transition is delayed when compared to horizontal flow direction where the effect of secondary flow increases the laminar flow heat transfer and causes transition to occur much earlier. The width of transition region for vertical tubes is significantly smaller than that of the horizontal tubes.
The transitional flow regime has been mostly avoided by designers due to uncertainty and perceive... more The transitional flow regime has been mostly avoided by designers due to uncertainty and perceived chaotic behavior. Previous work done in the transitional flow regime did not focus specifically on the effects of free convection on the local heat transfer characteristics along the tube length. Therefore, the purpose of this study was to investigate the effects of free convection on the heat transfer characteristics of developing and fully developed flow in the transitional flow regime; and is work in progress. An experimental setup was designed, built and validated and heat transfer measurements were taken at a heat fluxes of 1, 3 and 8 kW/m 2 between Reynolds
There are still some unknown aspects of nanofluids regarding simulations. Pool boiling flows are ... more There are still some unknown aspects of nanofluids regarding simulations. Pool boiling flows are complicated in terms of numerical modelling and presence of particles can noticeably expand this complexity. Two-dimensional pool boiling flow with nanoparticles is numerically solved around a horizontal cylinder for validation purposes. For further study, a tube bundle with four tubes is considered with nanofluid pool boiling. Unsteady Eulerian two-fluid model in ANSYS-Fluent is used to simulate the liquid and vapour flows in the computational domain. On the other hand, the particles are injected and tracked in the domain in the Lagrangian frame via the discrete model. Coupling between two frames is carried out through some user defined functions and the effects of final outcomes appear as thermos-physical properties. It means that nanofluid mixture properties are a function of local volume fraction. Superheat temperature and heat transfer coefficient from modelling are compared with experimental measurements. The good agreement is found and further discussion regarding particles migration and deposition are presented. Since the real fluctuation of the surface roughness cannot be introduced, an estimation of deposition predicted by the discrete model is presented. It is found that the percentage of deposition depend on heat flux and a weak function of particle concentration.
In this experimental study, the Reynolds number limits of the transitional flow regime and fricti... more In this experimental study, the Reynolds number limits of the transitional flow regime and friction factor characteristics were determined in the transitional flow regime of annuli. Isothermal and diabatic tests were conducted using four horizontal concentric annular passages with hydraulic diameters of 26.2, 23, 20.2 and 17 mm and respective diameter ratios of 0.327, 0.409, 0.386 and 0.483. The water in the annular passage was either heated or cooled at non-uniform temperature on the inner wall while the outer wall was insulated. The flow was both hydrodynamic and thermally developing, and the transitional flow was associated with mixed convection. The geometric size of the annular passage had a significant influence on the friction factors and Reynolds number span of the transitional flow regime. The annular geometric parameter that represent the geometric size of the annular passage was proposed and found to describe the friction factors well. Subsequently, correlations for predicting the friction factors in the transitional flow regime were developed.
A novel heat transfer fluid with bio-nanomaterial, which is environmentally safe, is prepared and... more A novel heat transfer fluid with bio-nanomaterial, which is environmentally safe, is prepared and its thermo-physical properties such as thermal conductivity and viscosity are measured. The bio-nanomaterial considered in this study is mango bark. A two-step process is employed to prepare a stable nanofluid. The average particle size was measured using scanning electron microscope and is found to be 100nm. The stability of the nanofluid is checked by measuring the absorbance and viscosity at a constant temperature. The concentration of nanofluid and temperature are varied between 0.1 to 1 vol% and 10 to 60 o C, respectively for the measurement of viscosity and thermal conductivity. The measurement shows that the measured thermal conductivity of the water is comparable with the standard data presented by
In this paper, the potential for improved thermal performance of a high concentration ratio parab... more In this paper, the potential for improved thermal performance of a high concentration ratio parabolic trough solar energy system working with high thermal conductivity single-walled carbon nanotubes (SWCNTs) and Therminol®VP-1 nanofluid is numerically investigated. In the numerical analysis, the practical heat flux profiles expected for parabolic trough receivers were obtained using Monte-Carlo ray tracing and coupled with a computational fluid dynamics tool using user defined functions to investigate the thermal performance of the parabolic trough solar energy system. A parabolic trough system with a concentration ratio of 113 was considered in this study and heat transfer fluid inlet temperatures between 400 K and 650 K were used. The volume fraction of SWCNTs in the base fluid was in the range 0% to 2.5% and the flow rates used were in the range 0.82 to 69.41 m 3 /h. Results show improvements in the convective heat transfer performance and receiver thermal efficiency as well as a considerable reduction of the receiver thermal losses with increasing volume fractions. The heat transfer performance increases up to 64% while the thermal efficiency increases by about 4.4%. Higher increments are observed at low flow rates and inlet temperatures. The receiver thermodynamic performance also increases significantly with the use of nanofluids. Entropy generation rates reduce by about 30% for the range of parameters considered.
Vacuum infusion is one of the usual processes to build composite materials. To find the place for... more Vacuum infusion is one of the usual processes to build composite materials. To find the place for resin injection or vacuum suction, the experts usually employ their experiences. In this study a hemisphere is simulated by ANSYS-FLUENT 15 commercial software for prediction of vacuum infusion process to find the best place for resin injection as well as the number of required resin inlets. For the simulations, the hemisphere with radius of 300 mm considered and the fibers thickness of 5.53 mm as a porous medium with 0.494 porosity. Different conditions have investigated for the simulations of the geometry. The results showed that Computational Fluids Dynamics (CFD) simulation is a strong tool to predict the best condition for the vacuum infusion process.
Publication on cavity flow natural convection by using nanofluids has increased in recent years. ... more Publication on cavity flow natural convection by using nanofluids has increased in recent years. On the other hand, contrary results offered by different researchers, both in experimental and numerical works. In this research it is tried to indicate that the accuracy of the viscosity and the thermal conductivity of the nanofluids is the most important reason of the contrary results. Therefore, cavity flow natural convection of CuO-water nanofluids has considered experimentally for volume fractions 0.5% and 1% in this research. The results show that the natural convection of nanofluids are more sensitive on accuracy of the viscosity than the thermal conductivity as well as it recommends to measure the viscosity and thermal conductivity in experimental natural convection works. However, for the range of the volume fractions tested in this research, CuO-water nanofluids (30-50nm) have not shown heat transfer advantage.
In this experimental investigation the influence of buoyancy driven secondary flow on the effecti... more In this experimental investigation the influence of buoyancy driven secondary flow on the effective lengthwise heat transfer coefficient in a circular horizontal tube is considered for liquid water at different applied external circumferential heat flux distributions. The test section consisted of a 2 m long stainless steel tube with an inner diameter of 27.8 mm and a wall thickness of 2.77 mm. Hydrodynamic fully developed inlet flow at a uniform temperature of approximately 20°C was considered for a Reynolds number range from 650 to 2600.
This study numerically investigated influence of non-uniform circumferential heat flux distributi... more This study numerically investigated influence of non-uniform circumferential heat flux distributions boundaries on secondary flow, internal heat transfer and friction factor characteristics of a horizontal circular tube in turbulent mixed convection regime. A three dimensional steady-state numerical simulation for inlet Reynolds number of 3 030 to 202 400 was implemented on ANSYS Fluent version 14. The circumferential non-uniform heat flux distribution was simulated as a sinusoidal function of heat flux incident on the tube model. The k-ε model was used to simulate the turbulent flow of the heat transfer fluid through the tube model. A steel tube with wall thickness of 5.2 mm, length to inner-diameter ratio of 160 and thermal conductivity of 16.27 W/mK was used. The tube-wall heat conduction and the external heat flux losses via convection and radiation were also considered. It was found that circumferential spans of non-uniform heat flux distributions boundaries have significant effects on the buoyancy-driven secondary flow for Reynolds number range of 3 030 to 9 100. The Richardson number increased with the circumferential span of the heat flux boundary due to buoyancy-effects and the internal heat transfer coefficient was higher than where buoyancy-effect was neglected. Internal heat transfer coefficients and friction factors for non-uniform heat flux cases were found to be higher than the uniform heat flux cases. These revealed that at Re less than 9 100, secondary flow effects, heat flux intensities and heat flux distributions boundary type must be considered in determining internal heat transfer and friction factors characteristics of the tube. Internal heat transfer coefficients increased with fluid inlet temperatures, while friction factor decreased with an increase in fluid inlet temperatures. For Re above 9 100, internal heat transfer coefficients and friction factors are independent of secondary flow effects, heat flux intensities, circumferential spans of heat flux distributions and heat flux boundary type. This indicates that classical correlations are suitable for higher Reynolds turbulent flow, but in laminar and low Reynolds turbulent flow regimes, classical equations were not suitable for non-uniform heating.
In this experimental study, hydrodynamic and thermally developing flow associated with mixed conv... more In this experimental study, hydrodynamic and thermally developing flow associated with mixed convection was examined in the transitional flow regime for a horizontal concentric annular passage. The test facility, which consisted of a counter-flow heat exchanger having an annular diameter ratio was 0.483 with an inner passage wall diameter of 15.90 mm, was operated at different degrees of longitudinal Wall Temperature Uniformity (WTU) for heated flow applications using a conventional annular inlet geometry type. The degree of WTU is described in this paper as a ratio of the wall temperature (value in Kelvin) at the outlet to the wall temperature (value in Kelvin) at the inlet of the inner surface of annular passage. Three different degrees of WTUs of 0.99, 0.975 and 0.965 were examined. Critical Reynolds numbers based on heat transfer were analyzed. It was found that the degree of WTU have an influence on critical Reynolds number and the heat transfer coefficient. A model which takes into account WTU is proposed to predict heat transfer in the transition regime.
The use of high concentration ratio parabolic trough systems is emerging as one of the ways to fu... more The use of high concentration ratio parabolic trough systems is emerging as one of the ways to further reduce the cost of energy from these systems. This paper reports on the thermal efficiency of a parabolic trough system and entropy generation in the parabolic trough receiver at different concentration ratios. In this study, the geometric concentration ratios considered were in the range 88-113, while the collector rim angle was taken as 80 o. A combined Monte Carlo ray tracing and computational fluid dynamics procedure was implemented to obtain the numerical solution of the thermal and thermodynamic performance of the system. Results show that the thermal efficiency reduces by about 4.5% as the concentration ratio increases from 88 to 113 at a given flow rate, while the entropy generation rate is shown to increase with increasing concentration ratio. Results further show that there is potential for improved performance with heat transfer enhancement using Cu-Therminol ® VP-1 nanofluid as the heat transfer fluid in the receiver's absorber tube. For a system with a concentration ratio of 113 using Cu-Therminol ® VP-1 nanofluid as the heat transfer fluid, the thermal efficiency increases by about 12% as the nanoparticle volume fraction increases from 0 to 6%.
In this research Lagrangian method is employed to track the nanoparticles inside a differentially... more In this research Lagrangian method is employed to track the nanoparticles inside a differentially heated walls nanofluid-filled cavity in natural convective flow. The thermo-physical properties of the solid-liquid mixture is also included in Eulerian frame. There are multiple interactions between fluid and particles which turns the simulations for solid-liquid highly complicated. ANSYS FLUENT 15.0 was used in this research to model submicron particles. Moreover, important interaction forces are implemented in the software as a User Defined Function (UDF) to modify the Lagrangian model. Brownian movement of the particles is assumed as a diffusion force acting on each particle during small user defined time step. Pressure gradient, gravity, virtual mass and Thermophoresis forces have also added to the force balance equation. Electrostatic forces are the contributing factors for stability of the nanofluid which they are needed to be present in dynamic equation of the particles. The importance of the forces on distribution of the particles is compared. The results show that the presence of these forces has considerable impact on nanoparticles concentration profiles.
An experimental study of heat transfer and flow pattern visualisation during the condensation of ... more An experimental study of heat transfer and flow pattern visualisation during the condensation of R134a was conducted in a smooth horizontal tube at low mass fluxes. Most previous experimental and analytical studies on in-tube condensation were conducted at high mass fluxes. In these studies, it was found that the heat transfer coefficient was not a function of the temperature difference between the tube wall temperature and condensation temperature. In addition, most heat transfer models developed were for high mass fluxes and failed to predict heat transfer coefficients at low mass fluxes properly. However, the most recent predictive heat transfer models have been based on studying and analysing the flow patterns. In all of these, only very few experimental studies have been coupled with flow pattern identification at different controlled temperature differences and mean vapour qualities at low mass fluxes. Therefore, the purpose of this study was the investigation of R134a condensing at low mass fluxes (20 –100 kg/m 2 s) and the identification and analysis of the flow patterns observed. The experiments were conducted in a smooth horizontal tube 8.38 mm in internal diameter with a length of 1.5 m at different mean vapour qualities and controlled temperature differences. The average saturation temperature was maintained at 40 ° C. The flow patterns were recorded simultaneously with a high speed video camera at the inlet and outlet of the test section through transparent sight glasses. The results showed that stratified flow and stratified-wavy were the dominant flow patterns.
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Papers by Josua P Meyer
past. In this paper, we address the fundamental question of what the size of the heat exchanger
should be, in addition to what architectural features it should have. The answer to the size question
follows from the tradeoff between (1), the useful power lost because of heat transfer and fluid flow
and (2), the power destroyed during transportation, manufacturing, and maintenance. Changes in
heat exchanger size induce changes in the opposite sign in the power requirements (1), and (2).
This fundamental tradeoff regarding size is illustrated by considering one side of a heat exchanger
(one flow passage) in laminar flow and in fully rough turbulent flow, with several duct cross
sectional shapes and arrays of channels in parallel. The size tradeoff is present in heat exchanger
applications across the board, from vehicles to stationary power plants.
past. In this paper, we address the fundamental question of what the size of the heat exchanger
should be, in addition to what architectural features it should have. The answer to the size question
follows from the tradeoff between (1), the useful power lost because of heat transfer and fluid flow
and (2), the power destroyed during transportation, manufacturing, and maintenance. Changes in
heat exchanger size induce changes in the opposite sign in the power requirements (1), and (2).
This fundamental tradeoff regarding size is illustrated by considering one side of a heat exchanger
(one flow passage) in laminar flow and in fully rough turbulent flow, with several duct cross
sectional shapes and arrays of channels in parallel. The size tradeoff is present in heat exchanger
applications across the board, from vehicles to stationary power plants.