Makina Müh.

The fluid flow and heat transfer characteristics and entropy generation of zirconia, ZrO2-water, nanofluid flow through a rectangular micro-channel are numerically investigated. The flow is considered under single-phase 3-D steady-state incompressible laminar flow conditions. The constant heat flux is applied to the bottom surface of micro-channel. The finite volume method is used to discretize the governing equations. As a result, the average Nusselt number decreases with increasing nanoparticle volume fraction, while the average Darcy friction factor is not affected. Moreover, the total entropy generation decreases with increase in nanoparticle volume fraction, while the Bejan number is almost not affected.

In this paper, convective heat transfer and entropy generation of ZnO-EG nanofluid flow through a square microchannel are numerically investigated. Flow is modelled by using Eulerian-Eulerian two phase flow model. Nanoparticle volume fraction of ZnO-EG nanofluid ranged between %1.0 and %4.0. As a result, it is found that the convective heat transfer coefficient of flow increased from 9718.15 W/m 2 K to 23010.79 W/m 2 K when 4.0% ZnO nanoparticle addition to pure EG at Re=100. Total entropy generation of ZnO-EG nanofluid decreases with increase in nanoparticle volume fraction of ZnO-EG nanofluid. It is also observed that the Bejan number decreases with increase in nanoparticle volume fraction of ZnO-EG nanofluid.

The fact that the properties of thermoelectric materials are to be estimated with Artificial Neural Networks without production and measurement will help researchers in terms of time and cost. For this purpose, figure of merit, which is the performance value of thermoelectric materials, is estimated by Artificial Neural Networks without an experimental study. P-and n-type thermoelectric bulk samples were obtained in 19 different compositions by doping different elements into Ca 2.7 Ag 0.3 Co 4 O 9 -and Zn 0.98 Al 0.02 O-based oxide thermoelectric materials. The Seebeck coefficient, electrical resistivity and thermal diffusivity values of the bulk samples were measured from 200 °C to 800 °C with an increase rate of 100 °C, and figure of merit values were calculated. 7 different Artificial Neural Network models were created using 123 measured results of experimental data and the molar masses of the doping elements. In this system aiming to predict the electrical resistivity, thermal diffusivity and figure of merit values of thermoelectric materials, the average R value and accuracy rate of these values were estimated to be 94% and 80%, respectively.

The objective of this study is to investigate experimentally and theoretically whether sodium carbonate (Na 2 CO 3) salt is suitable for establishing a salinity gradient in a salt-gradient solar-pond (SGSP). For this purpose, a small-scale prismatic solar-pond was constructed. Experiments were conducted in the laboratory under the incident radiation from two halogenlamps acting as a solar simulator. Furthermore, a one-dimensional transient mathematical model that describes the heat and mass transfer behaviour of the SGSP was developed. The differential equations obtained were solved numerically using a finite-difference method. It was found from the experiments that the density gradient, achieved using sodium carbonate salt, can suppress convection from the bottom to the surface of the pond.

The objective of this study is to develop an artificial neural network (ANN) model to predict the thermal conductivity of ethylene glycol-water solutions based on experimentally measured variables. The thermal conductivity of solutions at different concentrations and various temperatures was measured using the cylindrical cell method that physical properties of the solution are being determined fills the annular space between two concentric cylinders. During the experiment, heat flows in the radial direction outwards through the test liquid filled in the annual gap to cooling water. In the steady state, conduction inside the cell was described by the Fourier equation in cylindrical coordinates, with boundary conditions corresponding to heat transfer between the solution and cooling water. The performance of ANN was evaluated by a regression analysis between the predicted and the experimental values. The ANN predictions yield R 2 in the range of 0.9999 and MAPE in the range of 0.7984% for the test data set. The regression analysis indicated that the ANN model can successfully be used for the prediction of the thermal conductivity of ethylene glycol-water solutions with a high degree of accuracy.

This paper presents a two-dimensional Computational Fluid Dynamics (CFDs) simulation for the steady, laminar flow of an incompressible magnetorheological (MR) fluid between two fixed parallel plates in the presence of a uniform magnetic field. The purpose of this study is to develop a numerical tool that is able to simulate MR fluids flow in valve mode and determine B 0 , applied magnetic field effect on flow velocities and pressure distributions. A uniform transverse external magnetic field is applied perpendicular to the flow direction. The equations governing the steady flow of an incompressible MR fluid are implemented in the commercial code Ansys 14.0 Fluent which is a flexible CFD code based on finite volume approach. The governing differential equations describing the flow between parallel plates under magnetic field are solved numerically by using Fluent MHD module based on solving the magnetic induction equation method. The external applied magnetic field, B 0 , takes values between 0 and 1.5 T with 0.5 T step size, was applied to determine B 0 effects on fluid flow. The numerical solutions for velocity and pressure distributions were obtained for different magnetic fields. It was observed that increase in B 0 leads to decrease flow velocity. Results, obtained from numerical study was plotted graphically and disgusted in the present paper.

In this study, the effects of air gap on the optimum insulation thickness, insulation, and total costs, energy saving, payback period, fuel consumption, and emissions of CO 2 and SO 2 in the composite wall construction have been investigated for a prototype building in a sample city, Karabuk. For this reason two wall constructions, one with and the other without air gap, have been considered and the air gap thickness has been taken from 0 to 6 cm. Based on LCC analysis, optimum insulation thickness is determined for different air gap thicknesses. The results show that when 2, 4, and 6 cm of the air gap thicknesses were used; the optimum insulation thickness, insulation cost, and payback period were decreased by 10.02, 20.03, and 30.07% while the total cost, fuel consumption, and CO 2 and SO 2 emissions were decreased by 27.5, 44.98, and 54.46%, respectively. The insulation material installed at its optimum insulation thickness with air gaps in the composite wall will not only reduce the heat loss from the building walls but also has economical and environmental advantages.

In this study, thermal performance of the salt gradient solar pond (SGSP), which of density gradient is artificially with sodium carbonate solution, was tested under Karabuk prevailing weather conditions in Turkey. A small-scale prismatic glass tank was constructed with an area of 0.45 Â 0.20 m 2 and a depth of 0.25 m as solar pond. A series of experiments with four different density levels were conducted in July-August 2004. The variations of the temperature and density profiles were observed for each of experiment for a week. It was found that the maximum temperature difference between the bottom and surface of the pond is 218C and maximum temperature in the lower convective zone (LCZ) has been measured as 498C at the first experiment. The efficiency of the pond was evaluated 13.33% weekly mean radiation intensity of 524 W m À2 for the first experiment.

The effect of box geometry, such as cylindrical and rectangular, on the performance of solar cookers has not been investigated. In this study, two different model box type solar cookers, which are in rectangular and cylindrical geometries, were constructed using the same material and were tested to investigate the effects of box geometries on the cooker performance. The experimental studies were conducted under the same operating conditions. The obtained results from the experiments were used to calculate the thermal efficiency and specific and characteristic boiling times. These parameters of the cookers were determined for 0.5, 1, and 1.5 kg of fresh water. It is observed that the cylindrical model has higher temperatures than the rectangular model under the same operating conditions. The thermal efficiency increased from 12.7 to 36.98% for cylindrical and 9.85 to 28.25% for rectangular model, when the quantity of water was increased from 0.5 to 1.5 kg; similarly, the characteristic boiling time was decreased from 39.56 to 14.35 for the cylindrical and 43.74 to 15.77 (min m -2 kg -1 ) for the rectangular model, respectively. The cylindrical model provided high thermal performance, which is indicated by high thermal efficiency and low characteristic boiling time, in comparison with the rectangular model.

This study examines the effect of the electrical and magnetic field which is applied perpendicular to the flow and each other on the twophase steady flow in a glass pipe. Micron-sized iron powder, which are highly conductive, magnetizable, are used for the first phase of the fluid and then pure water which is not magnetizable and has very low electrical conductivity is used for the second phase. The mathematical model is derived by adding a term representing the impact of electromagnetic force to the momentum equation of the multiphase fluids in the interactive magnetizable phase. The derived model is analytically solved by using the methods of Laplace and D'Alambert. According to obtained results, when only magnetic and electrical fields are applied perpendicular to the flow of the mixture, local flow velocity of the first phase is decreased due to the direct effect of the magnetic field. The second phase local flow velocity is decreased due to the indirect effect of the magnetic field which is caused by the interaction of the phases. As a result, it is seen that the electromagnetic force is effecting the nonconductor phase of the mixture through the conductor phase which it can directly affect.

The unsteady viscous incompressible and electrically conducting of two-phase fluid flow in circular pipes with external magnetic and electrical field is considered in this present study. Effects of both uniform transverse external magnetic and electrical fields applied perpendicular to the fluid and each other on the two-phase (solid/liquid) unsteady flow is investigated numerically. While iron powders are being used as the first phase of two-phase fluid, pure water was used as the second phase. The system of the derived governing equations, which are based on the NaviereStokes equations including Maxwell equations, are solved numerically by using Pdex4 function on the Matlab for both phases. The originality of this study is that, in addition to magnetic field, the effect of electrical field on two-phase unsteady fluids is being examined. The magnetic field which is applied on flow decreases the velocity of both phases, whereas the electrical field applied along with magnetic field acted to increase and decrease the velocity values depending on the direction of electrical field. Electrical field alone did not display any impact on two-phase flow. On the other hand, analytical and numerical results are compared and favorable agreements have been obtained.

Work to date has shown that Artificial Neural Network (ANN) has not been used for predicting thermal performance parameters of a solar cooker. The objective of this study is to predict thermal performance parameters such as absorber plate, enclosure air and pot water temperatures of the experimentally investigated box type solar cooker by using the ANN. Data set is obtained from the box type solar cooker which was tested under various experimental conditions. A feed-forward neural network based on back propagation algorithm was developed to predict the thermal performance of solar cooker with and without reflector. Mathematical formulations derived from the ANN model are presented for each predicting temperatures. The experimental data set consists of 126 values. These were divided into two groups, of which the 96 values were used for training/learning of the network and the rest of the data (30 values) for testing/validation of the network performance. The performance of the ANN predictions was evaluated by comparing the prediction results with the experimental results. The results showed a good regression analysis with the correlation coefficients in the range of 0.9950-0.9987 and mean relative errors (MREs) in the range of 3.92516-7.040% for the test data set. The regression coefficients indicated that the ANN model can successfully be used for the prediction of the thermal performance parameters of a box type solar cooker with a high degree of accuracy.

Bu calismada, tek basamakli sasirtma levhasi kullanilan govde borulu isi degistiricisinin govde tarafi isil performansi literaturde onerilen Kern, Bell-Delaware ve basitlestirilmis Bell-Delaware yontemleri ile belirlenmis, elde edilen sonuclar literaturde bulunan deneysel veriler ile karsilastirilarak teorik sonuclarin dogrulamasi yapilmistir. Bu yontemlere gore belirlenen govde tarafi toplam isi transfer katsayisi, isi degistiricinin toplam isi transfer katsayisinin belirlenmesinde kullanilmistir. Isi degistiricisi icin belirlenen toplam isi transfer katsayi, deneysel verilerden LMTD ve ?-NTU yontemleri yardimiyla elde edilen toplam isi transfer katsayi ile karsilastirilmistir. Hesaplamalar MATLAP programinda yazilan bir grafik ara yuze sahip hesaplayici ile gerceklestirilmistir. Toplam dokuz deney verisine gore yapilan hesaplamalar sonucunda, Bell-Delaware yonteminin isi degistiricinin toplam isi transfer katsayisini deneysel sonuclara gore en yuksek dogrulukla sagladigi gorulmustur.

The fluid flow and heat transfer characteristics of ZnO-Ethylene glycol (EG) nanofluid through rectangular microchannels having different aspect ratios (α is 1 to 2) are numerically investigated. The different nanoparticle volume percentages (φ is 1 % to 4 %) of ZnO-EG nanofluid are used. The flow is considered under single-phase, three-dimensional, steady-state, incompressible, thermally developing, laminar flow conditions. As a result, the microchannel with an aspect ratio value of 1 has the highest convection heat transfer coefficient and the lowest pressure drop. It is also observed that the convection heat transfer coefficient and pressure drop increases with an increase in nanoparticle volume fraction value of nanofluid. However, the Nusselt number decreases with increasing nanoparticle volume fraction, while the Darcy friction factor is not affected.

An Organic Rankine Cycle is theoretically modelled and thermoeconomically investigated for different evaporator effectiveness. In thermoeconomic analysis, the Specific Exergy Costing (SPECO) method is used and the effect of evaporator effectiveness on unit exergy cost of electricity generated by the system is investigated. Exergy value of electricity generated by the system increases with increase in evaporator effectiveness. On the other hand, increase in evaporator effectiveness causes to an increase in hourly capital cost of system components and overall system. The results showed that the unit exergy cost of electricity generated by the system is 36.72 $/kJ for evaporator effectiveness of  =0.5, while it is 41.64 $/kJ for  =0.95.

In this paper, Ca2.8Pr0.2Co4O9 powders were synthesized by sol-gel method and thermal and structural characterization of the powders were systematically examined for high temperature thermoelectric generator applications. Solution characteristics of precursors were examined by Turbidity and pH measurements using a turbidimeter and a pH meter. Differential Thermal Analysis-Thermogravimetry (DTA-TG) was used to specify appropriate thermal regime of the powders for calcination process. Chemical structure and reaction type of intermediate temperature products were defined by Fourier Transform Infrared (FTIR) Spectroscopy. Structural properties of the powders were implemented by X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) was used to specify chemical composition and empirical formula of the elements existed within the powders. Drying, oxidation and phase transformation temperatures of the powders were optimized according to the DTA-TG and FTIR results. Considering ...

In this study, a spark ignition (SI) engine performance is exergetically investigated for different crankshaft speeds. The test engine is VSG413 SI type engine fueled with 95 octane gasoline. The test engine has 1.3 l volume, four stroke, four cylinder, 45 kW maximum power capacity and 98 Nm maximum torque capacity. The experiments are realized under fully loaded test conditions. The crankshaft speed is changed between 1200 rpm and 5000 rpm. In the calculations, actual operating data obtained by experiments are used. It is found that maximum energy and exergy efficiencies are obtained at crankshaft speed of 2500 rpm, which are 29.78% and 27.77%, respectively. Entropy generation rate increases with increase in crankshaft speed and ranged from 0.059 kW/K to 0.253 kW/K for all study. Bu çalışmada, buji ateşlemeli bir motorun performansı farklı krank mili hızları için ekserjetik olarak incelenmektedir. Test motoru 95 oktan benzin yakıtlı VSG413 SI türü motordur. Test motoru 1.3 l hacim, dört strok, dört silindir, 45 kW'lık maksimum güç kapasitesi ve 98 Nm'lik maksimum tork kapasitesine sahiptir. Deneyler tam yük test koşulları altında gerçekleştirilmiştir. Krank mili hızı 1200 d/d ve 5000 d/d aralığında değiştirilmiştir. Hesaplamalarda, deneylerden elde edilen gerçek çalışma verileri kullanılmıştır. %29.78'lik maksimum enerji verimi ve %27.77'lik maksimum ekserji veriminin 2500 rpm'lik krank mili hızında elde edildiği tespit edilmiştir. Entropi üretimi krank mili hızının artmasıyla artmaktadır ve bütün çalışma boyunca 0.059 kW/K ile 0.253 kW/K aralığında değişmektedir.

Relations between effectiveness (e) and expense coefficients (f) were derived, and an economic simulation model was developed to simulate heat exchangers (HE) and HE networks (HEN) in all flow types for the first time. e values of parallel flow, counter flow, cross flow and all HEs under the condition of C r = 0 were derived in terms of f, NTU (Number of Transfer Unit) and minimum heat capacities (C min). e values obtained from economic calculations were used for developing economic simulation model of HEs. Vectors including outlet temperatures and inlet temperatures of flows were obtained from static simulation to utilize in economic simulation model. Then, case studies were performed with counter flow HE and e values randomly determined in a sample HEN. Use (N), expense (P) and savings (E) of all HEs in a HEN were calculated easily by the way of linear equation systems without any complex processes, iterations, software and special hardware, in terms of both cold and hot flows properties by using economic simulation model.

Bir ısı değiştiricisi, iki veya daha fazla sıvı arasındaki, katı veya sıvı arasındaki veya katı parçacıklar ve akışkan arasındaki termal iç enerjisini dışarıdan ısı ve iş girişi olmaksızın gerçekleştiren cihazlardır. Isı değiştiricileri kullanım amaçlarına göre değişik konstrüksiyonlarda, kapasitelerde, boyutlarda ve tiplerde olabilmektedirler. Isı geri kazanım amaçlı kullanılan ısı değiştiricileri sanayi kuruluşlarında yatırım maliyetinde önemli bir yer tutmaktadır. Bu nedenle ısı değiştiricisi seçilirken veya ısı değiştiricisi devreleri tasarlanırken oldukça dikkat edilmeli ve ısı geri kazanımının maksimum olması için doğru ısı değiştiricisi veya efektif bir ısı değiştiricisi devresi tasarlanması gerekmektedir. Bu çalışmada, bir kümen üretim tesisinde bulunan ısı değiştiricisi devresinin statik simülasyonu yapılmıştır. Devreye giriş yapan ve devreden çıkış yapan sıcak ve soğuk akışların koordinatları matris denklemleriyle tanımlanarak devre içerisindeki sıcaklık dağılımları ve devre çıkışındaki sıcaklıklar iterasyona başvurulmadan, özel yazılım ve donanımlara başvurmaksızın yüksek bir doğrulukta elde edilmiştir. Simulasyon sonucunda ısı değiştiricisi devresindeki her bir hücreden geçen sıcak akış ve soğuk akışa ait sıcaklıklar işletme şartları ile örtüşecek şekilde tam doğrulukta elde edilmiştir. Ayrıca devre içerisindeki ısı değiştiricilerindeki sıcaklık dağılımları elde edilerek devrenin statik durumunun izlenmesi sağlanmıştır.