Journal of Heat and Mass Transfer Research(JHMTR)Journal of Heat and Mass Transfer Research(JHMTR)
http://jhmtr.journals.semnan.ac.ir/
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Feed provided by Journal of Heat and Mass Transfer Research(JHMTR). Click to visit.Using Burnett Equations to Derive an Analytical Solution to Pressure-Driven Gas Flow and Heat ...
http://jhmtr.journals.semnan.ac.ir/article_3658_408.html
The aim of the present study is deriving an analytical solution to incompressible thermal flow in a micro-Couette geometry in the presence of a pressure gradient using Burnett equations with first- and second-order slip boundary conditions. The lower plate of the micro-Couette structure is stationary, whereas the upper plate moves at a constant velocity. Non-dimensional axial velocity and temperature profiles were obtained using the slip boundary conditions and were compared with the transition flow regime (0.1≤ Kn ≤10). The results showed that in this regime, rarefaction exerts a considerable effect on both the velocity and the temperature profiles. Because of the presence of the pressure gradient in the direction of the flow, the non-dimensional velocity and temperature profiles behave in a parabolic trend and flatten as the Knudsen number increases. The Poiseuille and Nusselt numbers, obtained using the derived analytical solution, decrease with increasing the Knudsen number. In the absence of an axial pressure gradient, the velocity profile behaves linearly and shows good agreement with the results of the previous works.Sun, 30 Sep 2018 20:30:00 +0100Energy and Exergy Analyses of a Diesel Engine Running on Biodiesel Fuel
http://jhmtr.journals.semnan.ac.ir/article_3659_408.html
Availability analysis is an effective approach to studying energy conversion in systems and identifying inefficiency. In the present study, a single-zone model was used to examine energy performance parameters and heat release rates. The governing equation of availability analysis was applied in the model, and the possibility of using biodiesel produced from sunflower oil as diesel engine was investigated via mathematical simulation. The different exergy components of pure diesel fuel and pure biodiesel were compared at different crank angles. Results indicated that combining the examined diesel engine with biodiesel fuel would decrease the energy and exergy efficiencies by about 2.72% and 2.61%, respectively. As a result, work exergy and heat transfer exergy decrease and exhaust gas exergy and irreversibility would increase. When biodiesel is replaced with diesel fuel, carbon monoxide decreases, carbon dioxide formation increases, and nitrogen oxide formation remains constant. However, considering the negligible decrease in the first- and second-law efficiencies of biodiesel fuel compared with the decrease in diesel fuel efficiencies, the former is regarded as a renewable fuel that produces less carbon monoxide. It can therefore serve as a substitute for diesel fuel.Sun, 30 Sep 2018 20:30:00 +0100Physico-Acoustic Study on Thermal Conductivity of Silver Nanofluid
http://jhmtr.journals.semnan.ac.ir/article_3660_408.html
Low transmission of heat is one of the major problems for heat exchanger fluids in many industrial and scientific applications. This includes cooling of the engines, high power transformers to heat exchangers in solar hot water panels or in refrigeration systems. In order to tackle these problems in thermal industries, nanofluids could play a significant role as excellent heat exchanger materials for thermal applications. Silver nanofluids can be used abundantly for thermal applications due to their low cost and high thermal conductivity. The present article describes the green synthesis of the silver nanoparticles from AgNO3 powder using some plant product like tannic acid. The silver nanoparticles are characterized by XRD, UV-visible spectrophotometer, TEM. The silver nanofluids of different concentrations are prepared by means of water as the base fluid. The ultrasonic velocity is calculated for different concentration at room temperature. Acoustical parameters like compressibility, intermolecular free length and acoustic impedance are calculated using ultrasonic velocity, density and viscosity and the results are discussed in terms of intermolecular interactions between the nanoparticles and the base fluid. The variation of ultrasonic velocity and other calculated acoustic parameters are used to analyze in amplification of heat conductivity of silver nanofluids.Sun, 30 Sep 2018 20:30:00 +0100Numerical Study of Spherical Vapor Layer Growth Due to Contact of a Hot Object and Water
http://jhmtr.journals.semnan.ac.ir/article_3661_408.html
Vapor film formation and growth due to contact of a hot body and other liquids arise in some industrial applications including nuclear fuel rods, foundry and production of paper. The possibility of a steam explosion remains in most of these cases which could result in injuries and financial damage. Due to the importance of such phenomenon, this study deals with vapor layer forming, growth, and its internal pressure. A mathematical model of a molten spherical droplet immersed in water has been developed, and the results of the numerical solution are discussed. The effects of changing various characteristics (e.g. hot body size, temperature, and hydrostatic effects, as well as the temperature of bulk fluid) were investigated. These parameters affect the vapor layer size, vapor internal pressure, and the saturated temperature at the interface between vapor and liquid phases. Finally, conclusions indicate that the internal vapor pressure jumps, being up to several times larger than that of the initial condition. These pressure pulses and related vapor layer thickness variations could cause thermal fragmentation of the droplet which in turn results in strong pressure shock build-up due to small pieces of the droplet in contact with the water, which could then escalate to become a propagating large-scale vapor explosion. The vapor explosions could be hazardous and threaten the system safety.Sun, 30 Sep 2018 20:30:00 +0100Constructal design of tree-shaped conductive pathways for cooling a heat generating volume
http://jhmtr.journals.semnan.ac.ir/article_3662_408.html
Constructal design is used to study heat removal from a square heated body with a tree shaped high thermal conductivity pathways embedded in the body. The objective is to minimize the defined maximum dimensionless temperature difference for the body. The thermal conductivity of the body is low, and there is a uniform heat generation on it. The volume of the body is fixed. The amount of high conductivity material for building the pathways are also fixed, but their length and diameter are variable. The effect of parameters such as the angle among the pathways, number of pathways, thermal conductivity coefficient, dimensionless area fraction and different length ratios are investigated. The results show that by optimizing the angle among the pathways, the operation of them improves up to %12. By increasing the number of blades, dimensionless temperature difference decreases, but the best heat removal would be achieved when the pathways place along the direction of the diagonal of the square body, since, as the simulations show, the maximum temperature in the body occurs at the corners of the square.Sun, 30 Sep 2018 20:30:00 +0100Mixed Convection Heat Transfer of Water-Alumina Nanofluid in an Inclined and Baffled C-Shaped ...
http://jhmtr.journals.semnan.ac.ir/article_3663_408.html
In this article, mixed convection heat transfer of alumina-water nanofluid in an inclined and baffled C-shape enclosure is studied. It is assumed that the flow is laminar and steady. There is no energy production, energy storage and viscous heat dissipation. Furthermore, the nanofluid is considered as a continuous, Newtonian and incompressible fluid. Governing equations are discretized by finite-difference method and solved by SIMPLE algorithm simultaneously. Reynolds number (10 < Re < 1000), rotation angle of enclosure ( < α < ), length of baffle (0.1 < Bf < 0.4), Richardson number (0.1 < Ri < 100) are changed. In addition, volume percent of nanoparticles are changed in the range of 0 < φ < 0.06. The results demonstrate that the Nusselt number increases with the Reynolds number. Adding nanoparticles always results in cooling enclosure. At high Reynolds number, increase of nanoparticles has less effect on the heat transfer rate than low ones. Finally, heat transfer increases with the Richardson number, the enclosure angle and the length of baffle.Sun, 30 Sep 2018 20:30:00 +0100Effect of opening diffuser and return vent location on air quality, thermal comfort and energy ...
http://jhmtr.journals.semnan.ac.ir/article_3664_408.html
Many investigations have recently been performed on return vent height and indicate that having 1.3 m distance from the floor is the optimized height for it. In this article, the effect of distance between opening diffuser and return vent on air quality, thermal comfort and energy saving was investigated. According to the results, by increasing the distance between opening and return vent up to 5 m, the return vent could be placed near the floor at height of 0.6 m without any unacceptable consequence in indices. Therefore in this case, energy saving of 15.8% could be achieved rather than 8%, 10.9% and 15.2% in other cases. However, the air quality was lower compared to the other cases. In the case having better air quality and more thermal comfort with acceptable energy saving of 15.2%, the opening and return vent were relocated at maximum distance between them (5 meters) and return vent was placed at the suggested height of 1.3 m from the floor, which was found to be the optimum scenario.Sun, 30 Sep 2018 20:30:00 +0100Spectral quasi-linearization approach for unsteady MHD boundary layer flow of casson fuid due ...
http://jhmtr.journals.semnan.ac.ir/article_422_0.html
The present paper seeks to examine a numerical method of solution called spectra quasi-linearization method (SQLM) to the problem of unsteady MHD boundary layer flow of Casson fluid due to an impulsively stretching surface under the influence of a transverse magnetic field, which is an important physical phenomena in engineering applications.The study extends the previous models to account for a classical non-newtonian fluid called Casson fluid under the influence of a transverse magnetic field. The flow model is described in terms of a highly nonlinear partial differential equations. The method of solution Spectral quasi-linearization methods(SQLM) seeks to linearised the original system of PDEs using the Newton-Raphson based quasilinearization method (QLM). The numerical resultsfor the surface shear stress are compared with those of the analytical approach results, and they are found to be in good agreement. The flow controlling parameters are found to have a profound effect on the resulting flow profiles.It is observed that there is a smooth transition from the small time solution to the large time solution. The magnetic field significantly affects the flow field and skin friction coefficient. Indeed, skin friction coefficient is found to decreaserapidly, initially, in small time interval before attaining a steady state for large time.Sun, 19 Jun 2016 19:30:00 +0100Analytical solution of pressure driven gas flow and heat transfer in micro-Couette using the ...
http://jhmtr.journals.semnan.ac.ir/article_2702_0.html
In the current work, an incompressible thermal flow in a micro-Couette in the presence of a pressure gradient is investigated utilizing the analytical solution of the Burnett equations with first-order and second-order slip boundary conditions, for the first time. The lower plate of this micro-Couette is stationary while the upper plate moves with a constant velocity. Both non-dimensional axial velocity and temperature profiles were obtained using two types of the slip boundary conditions and compared in transition flow regime (0.1≤ Kn ≤10). The results show that the effect of the rarefaction is considerable on both velocity and temperature profiles in this regime. Because of the presence of pressure gradient in direction of the flow, both the non-dimensional velocity and temperature profiles are obtained parabolically and become flatter as the Knudsen number increases. Besides, both Poiseuille number and Nusselt number were obtained using analytical solution. The obtained results show that the Poiseuille number and Nusselt number decrease with increasing the Knudsen number. It should be noted that at the absence of an axial pressure gradient, velocity profile is obtained linearly and show a good agreement with the other works in literature.Wed, 11 Oct 2017 20:30:00 +0100Energy and Exergy Analysis of a Diesel Engine Running with Biodiesel Fuel
http://jhmtr.journals.semnan.ac.ir/article_2705_0.html
Availability analysis is performed as an effective way to study of energy conversion for systems to identify the inefficiency. In this paper, a Single-zone model has been used in order to study the energy performance parameters and heat release rates. The governing equation of availability analysis is applied in this model; in addition, the possibility of using biodiesel that obtained from sunflower oil in diesel engine is investigated by mathematical simulation. Moreover, the different exergy parts of pure diesel fuel and pure biodiesel are compared in any crank angle. Results indicate that, by running considered diesel engine with biodiesel fuel, energy and exergy efficiencies would decrease about 2.72 and 2.61 percent respectively. As a result, work exergy and heat transfer exergy decreases and exhaust gases exergy and irreversibility increases. Consequently, when biodiesel is replaced by diesel fuel, CO amount is decreased and CO2 and NOx formation would be increased because of its molecular structure has more percentage of oxygen.Fri, 20 Oct 2017 20:30:00 +0100Physico-Acoustic Study on Thermal Conductivity of Silver Nanofluid
http://jhmtr.journals.semnan.ac.ir/article_2883_0.html
Low transmission of heat is one of the major problems for heat exchanger fluids in many industrial and scientific applications. This includes cooling of the engines, high power transformers to heat exchangers in solar hot water panels or in refrigeration systems. In order to meet these problems in thermal industries, nanofluids can have a significant role as excellent heat exchanger materials for thermal applications. Silver nanofluids can be used abundantly for thermal applications due to their low cost and high thermal conductivity. The present paper describes the green synthesis of silver nanoparticles from AgNO3 powder using some plant product like tannic acid. The silver nanoparticles are characterized by XRD, UV-visible spectrophotometer, TEM. The silver nanofluids of different concentrations are prepared using water as base fluid. The ultrasonic velocity is calculated for different concentration at room temperature. Acoustical parameters like compressibility, intermolecular free length and acoustic impedance are calculated using ultrasonic velocity, density and viscosity and the results are discussed in terms of intermolecular interactions between the nanoparticles and base fluid. The variation of ultrasonic velocity and other calculated acoustic parameters are used to analyse in amplification of heat conductivity of silver nanofluids.Thu, 08 Feb 2018 20:30:00 +0100Numerical Study of Spherical Vapor Layer Growth Due to Contact of a Hot Object and Water
http://jhmtr.journals.semnan.ac.ir/article_2884_0.html
Vapor film formation and growth due to contact of a hot body and other liquids arises in some industrial applications including nuclear fuel rods, foundry and production of paper.The possibility of a steam explosion remains in most of these cases which may result in injuries and financial damage. Since this phenomenon is important, this study deals vapor layer forming, growth and its internal pressure are investigated. A mathematical model of a molten spherical droplet immersed in water is developed, and results of the numerical solution are discussed. The effects of changing various characteristics (e.g. hot body size, temperature, and hydrostatic effects, as well as temperature of bulk fluid) are investigated. These parameters impact the vapor layer size, vapor internal pressure, and the saturated temperature at the interface between vapor & liquid phases. Finally, conclusions indicate that internal vapor pressure jump, being up to several times larger than that of the initial condition. These pressure pulses and related vapor layer thickness variations could cause thermal fragmentation of the droplet which in turn results in strong pressure shock build-up due to small pieces of the droplet in contact with the water, which could then escalate to become a propagating large scale vapor explosion. The vapor explosions could be hazardous and endanger the system safety.Thu, 15 Feb 2018 20:30:00 +0100Performance Investigation of Two Modified Two-Stage Trans-Critical Carbon Dioxide Refrigeration ...
http://jhmtr.journals.semnan.ac.ir/article_2894_0.html
In the present work, the performances of two new two-stage multi inter-cooler trans- critical carbon dioxide (CO2) refrigeration cycle with ejector and internal heat exchanger have been studied. In the new modified cycles, an internal heat exchanger is added to the cycles. Also second inter-cooler in modified cycles, cooled with the refrigeration of the cycle, so that in first cycle it is a branch of saturated vapour flow from separator and also in second cycle it is a branch of supersaturated steam from internal heat exchanger. Results are validated against those available in the literature. Comparisons of the results show that there is excellent agreement between them. The influences of important operational parameters in the cycle performance such as gas-cooler pressure, evaporator temperature and gas-cooler temperature on the cycle performance have been analysed. The obtained results show that if the cooling flow for second inter-cooler supply from saturated vapour from separator, maximum coefficient of performance can be improved 25% compared to the original cycle at the considered particular conditions for operation.Thu, 22 Feb 2018 20:30:00 +0100Constructul design of tree-shaped conductive pathways for cooling a heat generating volume
http://jhmtr.journals.semnan.ac.ir/article_2990_0.html
Constructal design is used to study heat removal from a square heated body with a tree shaped high thermal conductivity pathways which are embedded in the body. The objective is to minimize the defined maximum dimensionless temperature difference for the body. The thermal conductivity of the body is low, and there is a uniform heat generation on it. The volume of the body is fixed. The amount of high conductivity material for building the pathways are also fixed, but their length and diameter are variable. The effect of parameters such as the angle among the pathways, number of pathways, thermal conductivity coefficient, dimensionless area fraction and different length ratios are investigated. The results show that by optimizing the angle among the pathways, the operation of them improves up to %12. By increasing the number of blades, dimensionless temperature difference decreases, but the best heat removal achieves when the pathways place along the direction of the diagonal of the square body, because, as the simulations show, the maximum temperature in the body occurs at the corners of the square.Tue, 15 May 2018 19:30:00 +0100Mixed Convection Heat Transfer of Water-Alumina Nanofluid in an Inclined and Baffled C-Shaped ...
http://jhmtr.journals.semnan.ac.ir/article_2996_0.html
In this paper, mixed convection heat transfer of alumina-water nanofluid in an inclined and baffled c-shape enclosure is studied. It is assumed that the flow is laminar and steady. There is no energy production, energy storage and viscous heat dissipation. Also, the nanofluid is considered as a continuous, Newtonian and incompressible fluid. Governing equations are discretised by finite-difference method and solved by SIMPLE algorithm simultaneously. Reynolds number (10 < Re < 1000), rotation angle of enclosure ( < α < ), length of baffle (0.1 < Bf < 0.4), Richardson number (0.1 < Ri < 100) are changed. In addition, volume percent of nanoparticles are changed in the range of 0 < φ < 0.06. The results show that the Nusselt number increases with increase of Reynolds number. Adding nanoparticles always results in cooling enclosure. At high Reynolds number, increase of nanoparticles has less effect on the heat transfer rate. Furthermore, heat transfer increases with the Richardson number, the enclosure angle and the length of baffle.Thu, 24 May 2018 19:30:00 +0100Effect of opening diffuser and return vent location on air quality, thermal comfort and energy ...
http://jhmtr.journals.semnan.ac.ir/article_3020_0.html
Recently, many investigations have been performed on return vent height; showing that the height of 1.3 m from the floor is the best height for this vent. In this article, the effect of distance between opening diffuser and return vent was investigated on air quality, thermal comfort and energy saving. The results showed, by increasing the distance between opening and return vent up to 5 m, the return vent could be placed near the floor at height of 0.6 m without any unacceptable result in indices. Therefore in this case, energy saving of 15.8% could be achieved rather than 8%, 10.9% and 15.2% in other cases. But, the air quality was lower than other cases. The case which brought better air quality and more thermal comfort with acceptable energy saving of 15.2%, was when the opening and return vent were relocated at maximum distance between them (5 meters) and return vent was placed at the suggested height of 1.3 m from the floor, which was the optimum case in this research.Wed, 20 Jun 2018 19:30:00 +0100Influence of inclined Lorentz forces on entropy generation analysis for viscoelastic fluid over ...
http://jhmtr.journals.semnan.ac.ir/article_3084_0.html
In the present work, the entropy generation analysis for viscoelastic fluid flow including inclined magnetic field and non-linear thermal radiation aspects with the heat source and sink over a stretching sheet was investigated analytically. The boundary layer governing partial differential equations are transformed by appropriate similarity transformations to non-linear coupled ordinary differential equations. These equations are solved by applying Kummer's function and used to compute the entropy generation. Influences of various pertinent parameters on velocity, temperature and entropy generation were discussed in detail and illustrated graphically. Based on the results, it was revealed that the presence of radiation and heat source parameters reduced the entropy production and at the same time aligned magnetic field, Reynolds number, the dimensionless group parameter, Hartmann number, Prandtl number and viscoelastic parameters are produces more entropy. It is also found that the presence of inclined magnetic field and thermal radiation enhances the heat transfer rate. The wall temperature gradient was also calculated and compared with some previous publications, which show outstanding agreement.Wed, 01 Aug 2018 19:30:00 +0100Numerical study of flow and heat transfer characteristics of CuO/H2O nanofluid within a mini tube
http://jhmtr.journals.semnan.ac.ir/article_3091_0.html
Nanofluids are a new heat transfer fluids, which improve thermal performance and reduce the size of systems. In this study, the numerical domain as a three-dimensional copper mini tube was simulated to study the characteristics of flow and heat transfer of CuO/H2O nanofluid, flowed horizontally within it. The selected model for this study was two-phase mixture model. The results showed that nanofluids with the platelet nanoparticles have better thermal performance than other shapes of nanoparticles such as cylindrical, Blade, Brick and spherical nanoparticles, respectively. By studying the flow characteristics, it was found that the pressure drop and friction factor of the nanofluids are dependent on the shape of the nanoparticles so that the nanofluids containing spherical nanoparticles have the lowest reduction in the friction factor and nanofluids containing platelet-shaped nanoparticles have the highest reduction in friction factor. Also, as new formulas, we suggested two correlations to calculate the Nusselt number of nanofluids according to the effect of nanoparticle shape in the laminar and turbulent flow regimes.Thu, 09 Aug 2018 19:30:00 +0100Investigating Tubes Material Selection on Thermal Stress in Shell Side Inlet Zone of a Vertical ...
http://jhmtr.journals.semnan.ac.ir/article_3100_0.html
In this study, the effect of the tube material on the thermal stress generated in a vertical shell and tube heat exchanger is investigated. Shell and tube heat exchangers are the most commonly heat exchangers used in industries. One of the most common failures in these exchangers in the industry is tube failure at the junction of the tube to tubesheet. When the shell side and tube side fluid with temperature difference, flow in the heat exchangers, temperature gradient occurs in the tube. Temperature gradients cause thermal stress in the tube especially at the junction of the tube to tubesheet where there is no possibility of expansion and contraction. Therefore, in this study it is tried to make changes that reduce the effect of thermal stress in the failure. For this purpose, temperature distribution, thermal stress distribution and its effects on failure are investigated by changing the material. To perform the required analysis, three dimensional model of inlet zone of the shell side is created and steady state temperature distribution is obtained and the stress caused by temperature gradient is analyzed. Because of interference between fluid and structure in this study the indirect coupled field analysis is used. In this way the thermal analysis results are converted to indirect couple structural analysis as loading. Among the analyzed materials, the lowest rate of stress is for copper tubes. However, steel tubes have the best safety factor for thermal stress.Fri, 17 Aug 2018 19:30:00 +0100Investigation on Turbulent Nanofluid Flow in Helical Tube in Tube Heat Exchangers
http://jhmtr.journals.semnan.ac.ir/article_3101_0.html
The thermal characteristics of turbulent nanofluid flow in a helical tube in tube heat exchanger (HTTHE) are assessed numerically through computational fluid dynamics (CFD) simulation. The turbulent flow and heat transfer of two nanofluids (SiO2, Al2O3) and base fluid with respect to swirl dominated flow is simulated through the RNG model. It is concluded that at 1 % volume concentration of SiO2 nanoﬂuid and Al2O3 nanoﬂuid the heat transfer coefficient of both the nanofluids is 8.6 % and 8.2 % greater than the basefluid in annulus tube side, respectively, while this value is 4.3 % and 1.5 % in inner tube side. The findings of both the turbulent models: realizable k-epsion (k-ε) and re-normalisation group (RNG) k-epsilon are compared. The temperature distribution contours show that realizable and RNG k-ε models, together with the swirl dominated flow are of more uniform temperature distribution. The proper prediction of two-layer theory leads to having a uniform temperature distribution and proper dimensionless wall distance (Y+). Comparison between the CFD results with the predicted values for friction factor coefficient (f) and Nusselt number (Nu) calculated through experimental correlations indicate the maximum errors of 6.56% and 0.27%, respectively.Fri, 17 Aug 2018 19:30:00 +0100Numerical Investigation of Roofing Materials Effect on Solar Heat Gain in Different External ...
http://jhmtr.journals.semnan.ac.ir/article_3155_0.html
In this study, thermal performance of three kinds of roofs with different heat capacity and thermal conductivity under different external conditions is investigated by a numerical method. For this purpose, the combined solar radiation, conduction and convection heat transfer were calculated implicitly using a one-dimensional finite difference method. Different high and low solar radiation conditions in two common climates in the Middle East including hot-humid and hot-dry are considered. The effect of roofing materials is investigated in terms of their thermal storage and overall heat transfer coefficient. Also, the time lags and decrement factors are evaluated to compare roofs performance. The numerical model has been validated using EnergyPlus. The results show that the roof with high thermal storage and low thermal conductivity has better performance in comparison to others. However, the total heat gains are not linearly proportional to the overall heat transfer coefficients e.g. here, the ratios of the total load of roof 1 to roofs 2 and 3 are about 12 percent lower than the ratio of overall heat transfer coefficients. Also, the solar radiation intensity had considerable effects on time lags. Finally, the external conditions have no significant effect on the decrement factor.Fri, 07 Sep 2018 19:30:00 +0100Mixed convection study in a ventilated square cavity using nanofluids
http://jhmtr.journals.semnan.ac.ir/article_3485_0.html
This work shows a numerical study on the laminar heat transfer mixed convection in a square cavity with openings on the vertical walls through which nanofluid flows. The intention is to cool a square heat source placed at the center of the geometry. The nanofluid has Copper nanoparticles and water as its base-fluid. The velocity and temperature of the entrance flow are known. Some results are experimentally and numerically validated. A mesh independency study is carried out. Some parameters are ranged as follows: i) the Reynolds number from 50 to 500, the nanofluid volume fraction from 0 to 1%, the Grashof number from 1000 to 100000. Two main directions of the flow are studied: i) the ascending flow which goes from the bottom to the upper openings; ii) the descending flow which goes from the upper to the bottom openings. In some cases, the fluid seems to be stuck inside the cavity which weakens the heat transfer. The presence of nanoparticles also increases the heat transfer of 4% for the ascending primary flow inside the cavity.Sun, 02 Dec 2018 20:30:00 +0100Experimental Investigation of Thermal Conductivity of Liquid Paraffin/Alumina Nanofluids with a ...
http://jhmtr.journals.semnan.ac.ir/article_3489_0.html
Liquid paraffin as a coolant fluid can be used in electronic devices due to its suitable capabilities such as electrical insulating, high heat capacity, chemical and thermal stability, and high boiling point. However, the poor thermal conductivity of paraffin has been confined its thermal cooling application. Addition of high conductor nanoparticles to paraffin can fix this drawback properly. In this study, the influence of the nanoparticles on the thermal conductivity of base material was assessed. The effects of temperature (20 °C ≤ T ≤ 50 °C) and volume fraction (0 ≤ ϕ ≤ 3%) on the thermal conductivity of paraffin/alumina nanofluids have been presented. Nanofluid samples were prepared by the two-step method and thermal conductivity measurements were done by a KD2 pro instrument. The results showed that the thermal conductivity increased uniformly with the increase of solid volume fraction and temperature. Moreover, it can be observed that for more concentrated samples, the effect of temperature was more tangible. Thermal conductivity enhancement (TCE) and effective thermal conductivity (ETC) of the nanofluid was calculated and new correlations were reported to predict the values of them based on the volume fraction of nanoparticles and temperature of composite accurately.Sat, 08 Dec 2018 20:30:00 +0100Rapid mixing of Newtonian and non-Newtonian fluids in a three-dimensional micro-mixer using ...
http://jhmtr.journals.semnan.ac.ir/article_3504_0.html
The mixing of Newtonian and non-Newtonian fluids in a magnetic micro-mixer is studied numerically using 〖Fe〗_3 O_4 ferrofluid. The mixing process is performed in a three-dimensional steady-state micro-mixer. A magnetic source is mounted at the entrance of the micro-channel to oscillate the magnetic particles. The effects of electric current, inlet velocity, size of magnetic particles and non-Newtonian fluid are examined on the mixing efficiency. It is demonstrated that the mixing efficiency increases with applied current and the size of magnetic particles. The inlet velocity has an inverse effect on the enhancement of the mixing efficiency. It is found that electric currents of 0A and 50A lead to the mixing efficiency of 10% and 83%, respectively. Also, the results demonstrates that the effect of the size of magnetic particles is not considerable in comparison with the electric field In addition, the present results reveal that the mixing efficiency of a non-Newtonian fluid (blood) is smaller than that of Newtonian one.Tue, 11 Dec 2018 20:30:00 +0100Numerical investigation of heat transfer in a sintered porous fin in a channel flow with the ...
http://jhmtr.journals.semnan.ac.ir/article_3534_0.html
Extended surfaces are one of the most important approaches to increase the heat transfer rate. According to the Fourier law, the heat transfer increases by increasing the contact surface of body and fluid. In this study, the effect of heat transfer has been investigated on two sets of engineered porous fins, in which the balls with different materials are sintered together. The fluid flow through the channel is considered incompressible, steady and three-dimensional. In this study, fins made of copper, aluminum and steel balls with 0.6 and 1.7 mm diameters in single-row, two-row modes are studied, and the heat transfer and pressure drop through these fins are checked. Also, the surface and volume analysis of the rigid and porous fins is also provided. In addition, the effect of diameter and material of the balls on the temperature distribution and heat transfer coefficient is examined in two cases of constant flux and constant temperature at the base. The results show that the steel fin has a different heat transfer behavior compared to other fins; the suitable material for the constant pressure and constant flux are copper and aluminum, respectively. Also, it is found that utilization of this type of connection decrease the volume of the fin about 39% of and increase the surface are about 37%.Fri, 28 Dec 2018 20:30:00 +0100Numerical Study of turbulent free convection of liquid metal with constant and variable ...
http://jhmtr.journals.semnan.ac.ir/article_3624_0.html
In the present study, turbulent MHD convection of liquid metal with constant and variable properties is investigated numerically. The finite volume method is used to simulate the fluid flow and natural convection heat transfer in a square cavity. The fluid flow and heat transfer were calculated and compared for two cases constant and variable properties. It is observed that for the case variable properties in high Hartmann numbers (Ha) the temperature gradient near the hot wall is more than the cold wall. For both cases the temperature gradient near the hot and cold walls and also the slope of temperature profile is high. By applying magnetic field and increasing the Ha the temperature slope reduces so at Ha=800 the profile is linear. In the case constant properties the slope of temperature profile near the vertical walls is the same and the temperature profiles pass from one point at the center of the cavity. But in the case variable properties as it was expected the temperature profile doesn’t pass one point and the slope of lines at high Hartmann numbers near the hot and cold walls are partly different. Also it is observed that for the case constant properties the Nusselt number is less than the case variable properties.Thu, 31 Jan 2019 20:30:00 +01003D Simulation of the Effects of the Plasma Actuator on the Unsteady, Turbulent and Developing ...
http://jhmtr.journals.semnan.ac.ir/article_3628_0.html
The purpose of this paper is 3D simulation of unsteady, turbulent and developing flow within a circular duct in presence of the body force vector induced by Dielectric barrier discharge (DBD) plasma actuator. This study aims at investigating of using plasma actuator to control separation with special arrangement of electrodes. For this reason, the plasma actuator is modeled in OpenFOAM software and the results are validated. Then, a computational study is carried out on a 3D flow to examine the effect of the presence of the plasma actuator. Three phenomena of increasing the flow cross-sectional, developing flow and simultaneous flow in both tangential and radial directions determine the physics of the problem, especially the pressure gradients. Due to the geometry of the problem, the actuators are arranged differently and for the first time the electrodes are radially arranged. The results show that presence of a plasma actuator delay the separation point.Mon, 04 Feb 2019 20:30:00 +0100GDL construction Effects on distribution of reactants and electrical current density in PEMFC
http://jhmtr.journals.semnan.ac.ir/article_3770_0.html
A two dimensional pore scale model of polymeric fuel cell, which is promising of a clean and renewable energy production, is presented here. Let reactive gases behave as an ideal gas; inhomogeneous anisotropic structure of the gas diffusion layer, is considered as a random generated circular porous media. Lattice Boltzmann method is used to study the fluid flow and mass transfer within the cathode microstructure. All parts of the cathode have the same temperature and the electrochemical reaction on the surface of the catalyst layer enters the solution as a boundary condition. Effects of the gas diffusion layer structure (carbon fibers diameters changes) on the flow of reactive gases, molar fraction of various oxygen species, and water vapor within the various parts of the gas diffusion layer as well as the electrical current density are investigated. The results indicate increasing the diameter of the carbon fibers of the gas diffusion layer of a constant porosity facilitates both the flow of oxygen and vapor species inside the GDL while affecting the electrical current produced on the surface of the catalyst layer as well. Effects of increasing the carbon fiber diameter on the electrical current density are greater in a GDL with less porosity.Tue, 23 Apr 2019 19:30:00 +0100