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Environmental Modelling and Software (18736726) 193
Accurately modeling the turbulence characteristics of wind flow entering urban areas is essential for improving the reliability of air pollution simulations, particularly when utilizing the LES approach. In this study, the Consistent Discrete Random Flow Generation method was implemented within the OpenFOAM software and evaluated for the first time in the context of solving concentration equation. A comparison of four inlet boundary conditions was conducted using wind tunnel experimental data. It was found that CDRFG presents more accurate results than the other methods, with an average error of 18 %. Then, the performance of the method in a complex geometry was evaluated by comparison with both experimental data and field measurements. The simulation demonstrated a high degree of accuracy in predicting the average dimensionless concentration, showing a close match with the experimental results, with a mean error of 16 %, and with the field measurements, exhibiting a mean error of 37 %. © 2025 Elsevier Ltd
Atmospheric Pollution Research (13091042) 16(1)
Air pollution caused by traffic is a major contributor to unhealthy ambient air quality in the vicinity of urban highways and puts the health of residents and pedestrians at risk. Therefore, it is imperative to examine local pollution reduction methods. The present paper is concerned with Detached Eddy Simulation (DES) of an urban area with high concentration of air pollutants. As an example of a complex surrounding morphology, the studied domain encompasses a 2.5 km stretch of high-traffic highway, a bus terminal, and the nearby residential buildings. The numerical procedure is validated with some benchmark wind tunnel and numerical data. The locations of pollution accumulation have been identified and the effect of the ambient wind speed on the change of the maximum pollution concentration points has been investigated. Furthermore, potential pollution reduction strategies for such complex morphologies have been proposed based on the geometry change to prevent the formation of critical zones of pollution accumulation. The results show that critical zones are generally formed behind the walls of upstream buildings, adjacent to the upstream wall of the highway, and in front of the walls of downstream buildings. It was also found that changes in the ambient wind speed do not significantly alter the location of these critical zones. Furthermore, increasing the distance between adjacent buildings and the highway from 21 m to 30 m, can result in an average reduction of 76% in maximum carbon monoxide concentration values. An investigation into the impact of upstream buildings height indicates that reducing the height of certain buildings can effectively diminish pollution concentration to zero in residential areas and surrounding sidewalks. Additionally, increasing the depth of the highway and erecting 2 m solid barriers on either side of the highway are identified as two other effective techniques for reducing pollution concentration on both sides of the highway. The findings can be utilised to develop novel strategies aimed at enhancing air quality for residents and pedestrians. © 2024 Turkish National Committee for Air Pollution Research and Control
Applied Thermal Engineering (13594311) 279
Thermal management has a crucial role in proton exchange membrane fuel cells (PEMFC) to prevent the reduction of electrochemical reactions and membrane breakup. This paper presents a numerical modeling of the cooling plates and their integrated cooling channels and investigates heat removal performance in parallel (laminar) and serpentine (turbulent) flow fields by four distribution of position-dependent heat fluxes generated in PEMFCs. The generating heat, a current density function, is applied to the cooling plate. The results indicated that the distribution of the current density in the PEMFC, and consequently the heat flux distribution to the cooling plate impacts the PEMFC thermal performance. The transition from parallel to serpentine flow fields affects the thermal performance differently. Among the evaluated turbulence models the k-ɛ model demonstrated good predictive accuracy. The serpentine flow plate showed a 50 % lower maximum temperature difference at the studied surface in some cases. However, the pressure drop increases up to 930 kPa in the serpentine channel at the highest simulated water mass flow rate in comparison to the parallel flow field. All the temperature variables experienced lower values by applying serpentine flow filed over the parallel. The uniformity index considered as a final key parameter defining a more homogenous temperature distribution in PEMFC improved by 68 % maximum for serpentine flow field with turbulent flow. © 2025 Elsevier Ltd
Heat Transfer Engineering (15210537)
This paper deals with the three-dimensional modeling of heat and mass transfer in a miniature loop heat pipe operating with water-graphene nanofluid. Different diameters of the liquid and vapor lines are proposed to prevent the vapor from entering the liquid line. Comparison of the numerical outputs with available empirical data shows a good agreement with a reasonable discrepancy. The effects of heat load applied in the range of 20–380 W and nanofluid concentration in the range of 1–3%vf on the thermal performance of the system are investigated and discussed. The results indicate that, by increasing the heat load to 40 W, the average temperatures of the evaporator, vapor line, and condenser increase by 7.5 K, 4.9 K and 3 K, respectively. Also, the evaporation and condensation rates increase by 72.6% and 29.3%, respectively, indicating an improvement in the thermal performance. Using 1% water-graphene nanofluid instead of pure water leads to 1K reduction in the average temperature of evaporator and 4216.4 W/(m2 (Formula presented.) K) increase in its heat transfer coefficient. Moreover, the effective thermal conductivity increased 3.2% with nanoparticles, which indicates again an improvement in the system performance. However, the use of more concentrated nanofluid does not show a significant effect on performance. © 2025 Taylor & Francis Group, LLC.
International Journal of Hydrogen Energy (03603199) 48(99)pp. 39064-39083
In this study, energy, exergy and exergy-economic analyses of a novel system that simultaneously generates cooling effect, heat, electricity, hot water and desalinated water for a zero-energy building are presented. It is aimed to evaluate the feasibility of using a solar-geothermal system to meet the energy and water demands of a residential building using exergy-economic indexes. The multi-generation system operates based on solar and geothermal energies, and it consists of proton exchange membrane (PEM) electrolyser, PEM fuel cell, photovoltaic system, and a desalination system with a pressure exchanger. Results indicate that energy and exergy efficiencies in cooling mode are 13.27% and 32.44%, respectively, and in heating mode are 17.25% and 42.4%, respectively. The largest exergy destruction occurs in the photovoltaics and organic Rankine cycle. It is observed that the turbine and boiler have the highest portion in the exergy destruction of the organic Rankine cycle. The capital investment and operating and maintenance cost rate, and the cost of produced distilled water are 4.288 ($/h), 67.63 (c$/m3), respectively. Moreover, the unit exergy costs of power, heating and cooling effect are investigated. The exergy-economic factor and the cost of exergy destruction for the entire system are 57.38% and 4.288([Formula presented]), respectively. © 2023 Hydrogen Energy Publications LLC
Thermal Science and Engineering Progress (24519049) 40
Grooved heat pipes can work well in no gravity conditions such as aerospace applications. This study deals with the numerical simulation of nanofluid flow and heat transfer in a capillary-grooved heat pipe. We solve numerically the governing equations in three-dimensional form applying finite volume approach, and track the fluid–solid interface using volume of fluid method. Also, we perform some experimental measurements of temperature on a sample fabricated grooved heat pipe. The numerical results are compared with the measurements in case of acetone-fluid aluminum-wall heat pipe, and the difference is averagely 12%, which is acceptable for the problems deal with phase change processes. Moreover, we investigate the effect of working fluids, consisting of acetone, water, and water-CuO nanofluid with different volume fraction of nanoparticles. The results indicate that a grooved heat pipe has better thermal performance with water than with acetone, though the working temperature is higher in case of water working fluid. It was found that using nanofluid with 1% volume fraction significantly reduces the wall temperature, and the temperature difference between evaporator and condenser, which results in improvement in thermal performance of the heat pipe, with 18% reduction in its thermal resistance, and 21% enhancement in its effective thermal conductivity. We also observe that further enhancement in nanofluid concentration has no considerable effect on the performance of the heat pipe. © 2023 Elsevier Ltd
In this study, a novel system which generates cooling effect, heat, electricity, hot water and distilled water simultaneously for a green building is analyzed which operates based on solar and geothermal heat pump, proton exchange membrane (PEM) electrolysis, proton exchange membrane (PEM) fuel cell, photovoltaics, desalination system along with pressure exchanger. The results demonstrats that energy and exergy efficiencies in cooling mode are 13.27% and 32.44% respectively, and in heating mode are also 17.25% and 42.4%, respectively. The largest exergy destruction occurs in the photovoltaics and organic Rankine cycle. It is also worth noting that turbine and vaporizer have the highest portion in exergy losses of organic Rankine cycle. The unit exergy cost of cooling, and hot water, and the cost of produced distilled water and 84.5 $/GJ, 64 $/GJ and 67.63 c$/m3, respectively. © 2022 Proceedings of WHEC 2022 - 23rd World Hydrogen Energy Conference: Bridging Continents by H2. All rights reserved.
Energy Conversion and Management (01968904) 258
One of the proposed methods to improve the performance of a proton exchange membrane fuel cell is using metal foam within the channels. Here, we performed a 3D numerical simulation and studied the effect of structural properties of metal foam on the system performance. Also, we used artificial neural network to predict three criteria, i.e., maximum temperature, temperature uniformity index, and pressure drop, together with Genetic Algorithm for system optimization. Obtained results indicate that using metal foam can improve the temperature uniformity in the cell, so that maximum temperature and temperature uniformity index are decreased about 1.785 K and 0.7 K, respectively, while resultant increase in overall pressure drop of the system is only about 4.4%. The same amount of maximum temperature reduction is possible by increasing the flow rate; however, this scheme puts 60% more pressure drop upon the system. Moreover, we compared the performance of air- and water-cooling systems and found that for a given pressure drop, the maximum temperature as well as temperature uniform index of an air-cooling system is lower than that of a water-cooling system, while for the same system parasitic power, a system with water coolant has more uniform temperature distribution with lower maximum temperature. © 2022 Elsevier Ltd
Journal of Hydrology (00221694) 585
Horizontal wells have gained considerable interest among petroleum engineers and hydrologists in the last decades. Many attempts were made to reach a better understanding of fluid flow behaviour inside the reservoir/aquifer, especially in the near-wellbore regions. In most of the previous studies on horizontal wells, each well is treated as a volumetric source term in governing equations describing the fluid flow in the main reservoir/aquifer. Many important features affecting the fluid flow, such as the realistic wellbore geometry, have been ignored for mathematical convenience. In the present study, the three-dimensional single-phase fluid flow through a large size reservoir block is coupled to wellbore flow through imposing the pressure and flux continuity at the reservoir-well interface. As the momentum equations, the Darcy and turbulent Navier–Stokes equations are used in the reservoir block and wellbore, respectively. The governing equations are discretized on unstructured grids and solved by using the finite volume method. Using the computed pressure and velocity distributions, the horizontal well characteristics are numerically estimated and compared with available analytic data for various cases. It is shown that the calculated well index, a coefficient in reflecting the geometric features of the well-reservoir system, deviates from its analytic value, as a result of reservoir block boundary condition, the pattern of the wellbore open intervals, the horizontal well drilling path, and its vertical eccentricity. In addition, the proposed approach is used to improve Economides model, an analytical model for horizontal well index, by estimating its pseudo-skin factor in various cases of the well vertical eccentricity, horizontal orientation, and reservoir block conditions. © 2020 Elsevier B.V.
International Journal of Energy Research (1099114X) 44(7)pp. 5730-5748
Improvement in the cooling system performance by making the temperature distribution uniform is an essential part in design of polymer electrolyte membrane fuel cells. In this paper, we proposed to use water-CuO nanofluid as the coolant fluid and to fill the flow field in the cooling plates of the fuel cell stack by metal foam. We numerically investigated the effect of using nanofluid at different porosities, pore sizes, and thicknesses of metal foam, on the thermal performance of polymer electrolyte membrane fuel cell. The accuracy of present computations is increased by applying a three-dimensional modeling based on finite-volume method, a variable thermal heat flux as the thermal boundary condition, and a two-phase approach to obtain the distribution of nanoparticles volume fraction. The obtained results indicated that at low Reynolds numbers, the role of nanoparticles in improvement of temperature uniformity is more dominant. Moreover, metal foam can reduce the maximum temperature for about 16.5 K and make the temperature distribution uniform in the cooling channel, whereas increase in the pressure drop is not considerable. © 2020 John Wiley & Sons Ltd
International Journal of Thermal Sciences (12900729) 156
This study concerns with the effect of transverse vibration of an internal plate on mixed convection heat transfer inside a square enclosure of a glass recycling process dryer. The plate can oscillate in horizontal and vertical directions. The Reynolds-averaged Navier-Stokes equations are used to describe the unsteady turbulent flow field. The governing equations are solved numerically applying a finite-volume approach. OpenMP Parallel SIMPLE algorithm is employed with power-low scheme for the convective terms. Also, the turbulent flow properties are estimated by a two-layer zonal model based on k−εmodel and Wolfstein near wall treatment. Dimensionless effective parameters including Reynolds number (Re), Grashof number (Gr) and vibration angular frequency (ω) are varied to investigate inherent flow structures and the heat transfer rate. It was deduced from the results that at high Gr and low Re numbers, the effect of internal plate vibrations is negligible and the fluid flow is induced only by the buoyancy force. As Re and ω are increased, the effect of internal plate vibration becomes dominant. At high Re and ω values while Gr kept low, benchmark cavity heat transfer is increased up to 90 and 80 times due to the vibration of internal horizontal and vertical plates, respectively. The obtained results can be beneficial for the dryers of glass recycling industries in order to heat and smash the glass particles without a significant heat loss. Desired condition occurs at high Gr and ω when the flow Re number is comparatively low. © 2020
Physica Scripta (00318949) 94(6)
Operating temperature is one of the most important parameters affecting the performance of polymer electrolyte membrane (PEM) fuel cells. The cooling system of a PEM fuel cell maintains the temperature of the fuel cell stack at a specific value and removes the heat generated in the cell. This paper presents 3D numerical simulation of a water-cooled PEM fuel cell cooling system, with a metal foam insert instead of traditional cooling channels. We explore the possibility of using metal foams for thermal management of fuel cells. We consider the turbulent flow of the coolant through the porous medium and investigate the effect of metal foam properties, including porosity and pore size, on the performance of the cooling system. The Brinkman-Darcy-Forchheimer equation is employed to analyze the flow field in the porous medium and the k - ϵ model is utilized for turbulence studies. The numerical results indicate that, at a specific Reynolds number, by increasing the porosity and the pore size of the metal foam, the pressure drop decreases, while the maximum temperature difference occurs in the cooling plate. The temperature uniformity index also increases. The latter result indicates that the temperature distribution in the cooling plate becomes more uniform. The obtained results of present study are in good agreement with available experimental and analytical data, confirming that the presented computational fluid dynamics modeling using the selected turbulence model can accurately predict the flow field parameters in the cooling channels of PEM fuel cells. © 2019 IOP Publishing Ltd.
Journal Of Applied Fluid Mechanics (17353645) 11(3)pp. 637-645
In this paper, we presented a similarity solution for turbulent film condensation of stationary vapor on an isothermal vertical flat plate. In this method, some similarity transformations are employed and the set of governing partial differential equations (PDE) of conservation together with transport equations of turbulent kinetic energy and dissipation rate are transformed into a set of ordinary differential equations (ODE). Calculated data for the flow field, velocity profile, wall shear stress, condensate film thickness, turbulent kinetic energy, rate of dissipation, and heat transfer properties are discussed. The effect of Prandtl (Pr) number was also investigated in a wide range of variations. The obtained results showed that at high Prandtl numbers, the velocity profile becomes more uniform across the condensation film and therefore, the kinetic energy of turbulence is reduced. Furthermore, the effect of change in Pr is negligible at high Pr numbers and consequently, the flow parameters have no significant change in this range. The friction coefficient changes linearly through the condensation film and the slope of friction lines diminishes slightly by the Pr number. The rate of turbulent kinetic energy increases linearly from the wall up to about 20% of condensate film, then rises asymptotically and converges to a constant value near the liquid-vapor interface. Also, the rate of turbulent dissipation grows linearly up to 40% of condensate film thickness and then increases slightly while it oscillates. © 2018, Isfahan University of Technology.
Applied Thermal Engineering (13594311) 144pp. 769-778
This paper concerns with the problem of natural convection heat transfer inside an industrial oven with an internal plate. The model is simulated numerically as a square cavity with an isolated plate laid horizontally or vertically inside it. Different ranges of Rayleigh number from laminar to turbulent flow regimes were investigated numerically and inherent flow structures and the amount of transferred heat were obtained. The governing Reynolds-averaged Navier-Stokes and energy equations are discretized and solved applying finite-volume method. Moreover, a two-layer zonal model is applied for near-wall turbulent properties. Comparing the present results with similar previous studies and experimental data shows that the employed zonal model is more accurate than other existing turbulent models. Our results also demonstrate that the total rate of heat transferred by a cavity with internal isolated plate is always lower than that of a bare cavity. As the distance of internal plate from the cavity wall decreases, its effect on the reduction of overall heat transfer increases accordingly. We also determined a distance between the plate and the cavity wall with less than 2% change in relative Nusselt number and deduced that the maximum distance of vertical plate is always greater than that of the horizontal plate at each Rayleigh number. Furthermore, a dead zone is created between the hot wall and the plate with low velocities, the temperatures close to the hot wall temperature, and very weak circulation, which can significantly reduce the overall heat transfer in the cavity. The results of this work can be used in glass industries and reduction of heat loss in industrial ovens by placing the glass sheets inside them in appropriate position. © 2018 Elsevier Ltd
Chinese Journal of Chemical Engineering (10049541) 25(10)pp. 1352-1359
The aim of this study is to use a new configuration of porous media in a heat exchanger in continuous hydrothermal flow synthesis (CHFS) system to enhance the heat transfer and minimize the required length of the heat exchanger. For this purpose, numerous numerical simulations are performed to investigate performance of the system with porous media. First, the numerical simulation for the heat exchanger in CHFS system is validated by experimental data. Then, porous media is added to the system and six different thicknesses for the porous media are examined to obtain the optimum thickness, based on the minimum required length of the heat exchanger. Finally, by changing the flow rate and inlet temperature of the product as well as the cooling water flow rate, the minimum required length of the heat exchanger with porous media for various inlet conditions is assessed. The investigations indicate that using porous media with the proper thickness in the heat exchanger increases the cooling rate of the product by almost 40% and reduces the required length of the heat exchanger by approximately 35%. The results also illustrate that the most proper thickness of the porous media is approximately equal to 90% of the product tube's thickness. Results of this study lead to design a porous heat exchanger in CHFS system for various inlet conditions. © 2017 Elsevier B.V.
Journal of Thermal Science and Engineering Applications (19485093) 9(2)
This paper concerns with calculation of heat transfer and pressure drop in a mixedconvection nanofluid flow on a permeable inclined flat plate. Solution of governing boundary layer equations is presented for some values of injection/suction parameter (f0), surface angle (γ), Galileo number (Ga), mixed-convection parameter (λ), volume fraction (φ), and type of nanoparticles. The numerical outcomes are presented in terms of average skin friction coefficient (Cf) and Nusselt number (Nu). The results indicate that adding nanoparticles to the base fluid enhances both average friction factor and Nusselt number for a wide range of other effective parameters. We found that for a nanofluid with φ=0.6, injection from the wall (f0=0.2) offers an enhancement of 30% in Cf than the base fluid, while this growth is about 35% for the same case with wall suction (f0=0.2). However, increasing the wall suction will linearly raise the heat transfer rate from the surface, similar for all range of nanoparticles volume fraction. The computations also showed that by changing the surface angle from horizontal state to 60 deg, the friction factor becomes 2.4 times by average for all φ's, while 25% increase yields in Nusselt number for the same case. For assisting flow, there is a favorable pressure gradient due to the buoyancy forces, which results in larger Cf and Nu than in opposing flows. We can also see that for all u values, enhancing Ga/Re2 parameter from 0 to 0.005 makes the friction factor 4.5 times, while causes 50% increase in heat transfer coefficient. Finally, we realized that among the studied nanoparticles, the maximum influence on the friction and heat transfer belongs to copper nanoparticles. © 2017 by ASME.
Journal of the Energy Institute (17460220) 90(5)pp. 752-763
This paper concerns with numerical modeling of fluid flow through a zigzag-shaped channel to be used as the cooling plate for polymer electrolyte membrane fuel cells. In general, large scale PEM fuel cells are cooled by liquid water flows through coolant flow channels, and the shape of these channels has a key role in the cooling performance. We perform a three-dimensional numerical simulation to obtain the flow field and heat transfer rate in square area cooling plates. The performance of zigzag flow channels is evaluated in terms of maximum surface temperature, temperature uniformity and pressure drop. The results indicate that in the zigzag channels model, maximum surface temperature, surface temperature difference and temperature uniformity index, respectively, reduce about 5%, 23%, and 8% with respect to straight channels model. Hence, the cooling performance of fuel cells can be improved by implementing the zigzag channels model as the coolant fluid distributors, although the coolant pressure drop is higher than straight channels in this model. © 2016 Energy Institute
Applied Thermal Engineering (13594311) 110pp. 399-411
In this paper, the performance of a novel multi-generation solar system is evaluated at different working conditions. The proposed system is able to produce electric power, distilled water and hot water, concurrently. The performance of the main components including thermoelectric generator, thermal energy storage, desalination unit and hot water tank is modeled mathematically, and then evaluated at different working conditions using the second law of thermodynamics. The effects of working temperature and solar irradiation are also investigated as the key operational parameters. Moreover, the Genetic algorithm is employed to determine the optimum working conditions for the minimum thermodynamic irreversibility. The results indicate that using 960 MJ/d of incident solar energy, the system produces 896 kJ/d electrical energy, 177 kg/d distilled water and 29,000 kg/d hot water. The maximum energy and exergy efficiency of the integrated system is about 46 and 1.5%, respectively. The outcomes of this research confirm the suitability of such a system for residential application. © 2016 Elsevier Ltd
International Journal of Hydrogen Energy (03603199) 41(3)pp. 1902-1912
This paper deals with the usage of metal foams in coolant flow field of bipolar or cooling plates in PEMFC stack rather than conventional machined channel designs. A three-dimensional model is employed to simulate the fluid flow and heat transfer in cooling plates and the capabilities of four different coolant flow field designs, include one parallel, two serpentine and one metal foam porous media field, are investigated and compared based on the maximum surface temperature, uniformity of temperature and the pressure drop. The numerical results indicated that a model with the porous flow field made by metal foam is the best choice for reducing the surface temperature difference, maximum surface temperature and average surface temperature, among the studied models. Furthermore, due to its high permeable coefficient, the coolant pressure drop is very low in this model. Consequently, this model can be well-used as a coolant fluid distributor to improve the PEM fuel cell performance. © 2015 Hydrogen Energy Publications, LLC.
Modern Physics Letters B (02179849) 30(16)
In PEM fuel cells, during electrochemical generation of electricity more than half of the chemical energy of hydrogen is converted to heat. This heat of reactions, if not exhausted properly, would impair the performance and durability of the cell. In general, large scale PEM fuel cells are cooled by liquid water that circulates through coolant flow channels formed in bipolar plates or in dedicated cooling plates. In this paper, a numerical method has been presented to study cooling and temperature distribution of a polymer membrane fuel cell stack. The heat flux on the cooling plate is variable. A three-dimensional model of fluid flow and heat transfer in cooling plates with 15 cm × 15 cm square area is considered and the performances of four different coolant flow field designs, parallel field and serpentine fields are compared in terms of maximum surface temperature, temperature uniformity and pressure drop characteristics. By comparing the results in two cases, the constant and variable heat flux, it is observed that applying constant heat flux instead of variable heat flux which is actually occurring in the fuel cells is not an accurate assumption. The numerical results indicated that the straight flow field model has temperature uniformity index and almost the same temperature difference with the serpentine models, while its pressure drop is less than all of the serpentine models. Another important advantage of this model is the much easier design and building than the spiral models. © 2016 World Scientific Publishing Company.
Physica Scripta (00318949) 91(3)
In this paper, we perform a numerical study on the heat transfer and pressure drop in hydraulically and thermally developing turbulent flow of nanofluid through an internally ribbed pipe. The effects of volume fraction of nanoparticles and the distance between the ribs are investigated on the heat transfer and skin friction coefficients at the entrance region of the pipe. The set of governing equations followed by a two-layer zonal turbulence model are solved numerically by a velocity-pressure coupling algorithm based on finite-volume method. Moreover, available empirical relations are used to calculate the nanofluid properties in terms of the nanoparticles and the base fluid. The obtained results illustrate that increasing the volume fraction of nanoparticles makes the thermal entrance length decrease and consequently, the heat transfer increases. It reveals that 10% increase in the volume fraction of nanoparticles causes about 15% rise in average Nusselt number. In addition, it is found that the friction factor rises by increasing the volume fraction of nanoparticles compared with turbulent flow of the base-fluid. Also, the average Nusselt number in nanofluid flow increases with the interval between the ribs compared with pure-fluid flow. © 2016 The Royal Swedish Academy of Sciences.
Applied Thermal Engineering (13594311) 99pp. 373-382
This paper concerns with modeling nanofluid boundary layer flow in the presence of a magneto hydrodynamic field over a horizontal permeable stretching flat plate using artificial neural network. The flow is generated due to the linear stretch of the sheet. The governing PDEs are transformed into ODEs and numerically solved using a double precision Euler's procedure. We studied numerically the effects of injection or suction from the surface, the volume fraction of nanoparticles, the viscous dissipation, and the magnetic parameter on the skin friction factor, Nusselt number and hydraulic and thermal boundary layer thicknesses. The results show that both the friction factor and Nusselt number increase as the volume fraction of nanoparticles increases. Moreover, the magnetic field increases the friction factor while reduces the Nusselt number. By a multilayer neural network model, we also calculated the skin friction factor and Nusselt number with respect to the aforementioned effective parameters. The model is able to compute the test data set with mean relative errors of 0.19% and 0.36% for the friction factor and Nusselt number, respectively. This means the applied neural network model can accurately predict the output results. © 2016 Elsevier Ltd. All rights reserved.
Physica Scripta (00318949) 88(T155)
This paper concerns the study of laminar and turbulent force convection heat transfer and pressure drop between horizontal parallel plates with a nanofluid composed of Al2O3 and water. A set of governing equations are solved using a non-staggered SIMPLE procedure for the velocity-pressure coupling. For the convection-diffusion terms a power-law scheme is employed. A modified k-ε model with a two-layer technique for the near-wall region has been used to predict the turbulent viscosity. The effects of nanoparticle volume fraction in the base fluid on laminar and turbulent flow variables are presented and discussed. The velocity and temperature profiles, friction factor, pressure coefficient and Nusselt number at different Reynolds numbers in the entrance region for both the laminar and turbulent flow regimes are reported under different thermal boundary conditions. The results show that the effect of the presence of nanoparticles in the base fluid on hydraulic and thermal parameters for the turbulent flow is not very significant, while the rate of heat transfer for the laminar flow with nanoparticles is greater than that of the base liquid. Furthermore, the thermal boundary layer and consequently the Nusselt number more quickly reach their fully developed values by increasing the percentage of nanoparticles in the base fluid for the laminar flow regime, while no changes in the trend are observed for the turbulent flow. © 2013 The Royal Swedish Academy of Sciences.
Physica Scripta T (02811847) 142
In this paper, a two-dimensional numerical scheme is presented for the simulation of turbulent, viscous, transient compressible flows in the simultaneously developing hydraulic and thermal boundary layer region. The numerical procedure is a finite-volume-based finite-element method applied to unstructured grids. This combination together with a new method applied for the boundary conditions allows for accurate computation of the variables in the entrance region and for a wide range of flow fields from subsonic to transonic. The Roe-Riemann solver is used for the convective terms, whereas the standard Galerkin technique is applied for the viscous terms. A modified κ-ε model with a two-layer equation for the near-wall region combined with a compressibility correction is used to predict the turbulent viscosity. Parallel processing is also employed to divide the computational domain among the different processors to reduce the computational time. The method is applied to some test cases in order to verify the numerical accuracy. The results show significant differences between incompressible and compressible flows in the friction coefficient, Nusselt number, shear stress and the ratio of the compressible turbulent viscosity to the molecular viscosity along the developing region. A transient flow generated after an accidental rupture in a pipeline was also studied as a test case. The results show that the present numerical scheme is stable, accurate and efficient enough to solve the problem of transient wall-bounded flow. © 2010 The Royal Swedish Academy of Sciences.
Scientia Iranica (23453605) 17(2 B)pp. 108-120
The transient flow of a compressible gas generated in a pipeline after an accidental rupture is studied numerically. The numerical simulation is performed by solving the conservation equations of an axisymmetric, transient, viscous, subsonic flow in a circular pipe including the breakpoint. The numerical technique is a combined finite element-finite volume method applied on the unstructured grid. A modified K - ε model with a two-layer equation for the near wall region and compressibility correction is used to predict the turbulent viscosity. The results show that, for example, after a time period of 0.16 seconds, the pressure at a distance of 61.5 m upstream of the breakpoint reduces about 8%, while this value for the downstream pressure located at the same distance from the rupture is about 14% at the same time. Also, the mass flow rate released from the rupture point will reach 2.4 times its initial value and become constant when the sonic condition occurs at this point after 0.16 seconds. Also the average pressure of the rupture reduced to 60% of its initial value and remained constant at the same time and under the same condition. The results are compared with available experimental and numerical studies for steady compressible pipe flow. © Sharif University of Technology, April 2010.
Journal of Thermophysics and Heat Transfer (15336808) 23(4)pp. 801-809
This study investigates the effects of wall heating and skin friction on the characteristics of a compressible turbulent flow in developing and developed regions of a pipe. The numerical solution is performed by finite-element-based finite volume method applied on unstructured grids. A modified κ-ε model with a two-layer equation for the near-wall region and a compressibility correction are used to predict turbulent viscosity. The results show that shear stress in fully developed flow is nearly constant from the centerline up to 75% of the pipe radius, then increases sharply next to the wall, and the ratio of the turbulent viscosity to the molecular one is less than 0.2. Under a uniform wall heat flux condition, the friction factor decreases in the entrance region and will be fully developed after Z/D > 50, but the Nusselt number increases first and then will be fully developed after Z/D > 10. In addition, the heat flux accelerates the developing compressible flow and causes the entrance length to decrease, unlike the incompressible flow.
International Journal of Heat and Mass Transfer (00179310) 52(25-26)pp. 5751-5758
The aim of this work is to analyze the gas flow in high pressure buried pipelines subjected to wall friction and heat transfer. The governing equations for one-dimensional compressible pipe flow are derived and solved numerically. The effects of friction, heat transfer from the wall and inlet temperature on various parameters such as pressure, temperature, Mach number and mass flow rate of the gas are investigated. The numerical scheme and numerical solution was confirmed by some previous numerical studies and available experimental data. The results show that the rate of heat transfer has not a considerable effect on inflow Mach number, but it can reduce the choking length in larger f DL/D values. The temperature loss will also increase in this case, if smaller pressure drop is desired along the pipe. The results also indicate that for fDL/D = 150, decreasing the rate of heat transfer from the pipe wall, indicated here by Biot number from 100 to 0.001, will cause an increase of about 7% in the rate of mass flow carried by the pipeline, while for f DL/D = 50, the change in the rate of mass flow has not a considerable effect. Furthermore, the mass flow rate of choked flow could be increased if the gas flow is cooled before entrance to the pipe. © 2009 Elsevier Ltd. All rights reserved.
Physica Scripta T (02811847) 132
In this paper, the compressible gas flow through a pipe subjected to wall heat flux in unsteady condition in the entrance region is investigated numerically. The coupled conservation equations governing turbulent compressible viscous flow in the developing region of a pipe are solved numerically under different thermal boundary conditions. The numerical procedure is a finite-volume-based finite-element method applied to unstructured grids. The convection terms are discretized by the well-defined Roe method, whereas the diffusion terms are discretized by a Galerkin finite-element formulation. The temporal terms are evaluated based on an explicit fourth-order Runge-Kutta scheme. The effect of different thermal conditions on the pressure loss of unsteady flow is investigated. The results show that increase in the inflow temperature or pipe-wall heat flux increases the pressure drop or decreases the mass flow rate in the pipe. © 2008 The Royal Swedish Academy of Sciences.
Journal of Fluids and Structures (10958622) 22(4)pp. 529-540
An efficient algorithm for the design optimization of the compressible fluid flow problem through a flexible structure is presented. The methodology has three essential parts: first the behavior of compressible flow in a supersonic diffuser was studied numerically in quasi-one-dimensional form using a flux splitting method. Second, a fully coupled sequential iterative procedure was used to solve the steady state aeroelastic problem of a flexible wall diffuser. Finally, a robust Genetic Algorithm was implemented and used to calculate the optimum shape of the flexible wall diffuser for a prescribed pressure distribution. © 2006 Elsevier Ltd. All rights reserved.
This paper deals with design and analysis of intermittent supersonic wind tunnels. System can be constructed by allowing air at atmospheric pressure to pass through a converging-diverging nozzle, a test section and a diffuser into a vacuum tank. The governing equations of compressible fluid flow have been solved numerically using flux vector splitting method to obtain running time under which it works at the design Mach number. The formulation has been tested on the theory of quasi one-dimensional compressible flow. The numerical results are in good agreement with the results of the theory.
