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In this paper, the statistical analyses of the ammonia-water absorption refrigerant system (ARS) are presented. For this purpose, different surface response methods, including the Central composite design (CCD) and Box- Behnken design (BBD), have been studied and compared first to obtain the best response. An attempt is made to study the effect of some factors (mass flow rate, isentropic efficiency of pump, pressure ratio, maximum pressure of cycle, concentration difference of strong and weak solution, concentration of strong solution and effectiveness factor of heat exchanger and pure ammonia concentration) on the system’s response. Based on the design of experiment analysis, regression models are presented to quantify the effects of these parameters on coefficient of performance (COP) and cooling load of the ammonia-water absorption refrigerant cycle. The proposed correlations obtained by the analysis have remarkably satisfactory performance for all simulation data. The error analysis is applied to find the maximum error of 0.482% in numerical computations for ARS by CCD method in midpoint values of parameters.

The annealing time is an important affecting factor in the performance of many furnaces. The present work deals with the transient simulation of annealing process in a cubic furnace in which a solid element is placed in its center. As the working gas can have some radiating features, a set of governing equations including the energy balance with the radiative transfer equation (RTE) for the gray radiating medium and the conduction equation inside the solid product are numerically solved with progressing in time. Numerical results which are validated against both analytical and theoretical findings in the literature demonstrate that during the starting period, a high rate of radiant energy transfers into the solid body even at small optical thickness. This behavior which hastens the rate of heat transfer at low values of the radiation conduction parameter, causes a fast annealing process in which the solid body warms up to its maximum temperature. Moreover, it is revealed that the rate of heat transfer is an increasing function of radiation-conduction parameter.

In this paper, the statistical analyses are presented to study the thermal efficiency and power output of gas turbine (GT) power plants. For analyzing gas turbine operation and performance, a novel approach is developed utilizing the response surface methodology (RSM) which is based on the central composite design (CCD) method. An attempt is made to study the effect of some operational factors (inlet temperatures of turbine and compressor, inlet pressure of compressor, lower heating value of fuel (LHV) and air mass flow rate) and design parameters (pressure drop of combustion chamber and isentropic efficiency of equipment) on the system’s response. Based on the DoE analysis, regression models are presented to quantify the effects of these parameters on thermal efficiency and net work of the Brayton based gas turbine cycle. The proposed correlations obtained by the analysis have a remarkably satisfactory performance for all simulation data. The error analysis shows a maximum error of 5.5% in numerical computations of response functions for GT power plants. The optimal point of the thermal efficiency and net power output based on the optimized conditions were found to be 45.71% and 4.182 MW, respectively

In this paper, the thermal characteristics of single and double flow plane solar heaters with radiating working gas were analyzed and compared by numerical analysis for the first time. The laminar mixed convection gas flow in the heaters was numerically simulated by the CFD method using the finite volume technique. The set of governing equations included the conservation of mass, momentum and energy for the convective gas flows and the conduction equation for solid parts. Besides, the radiative transfer equation was solved by the discrete ordinate method for radiant intensity computation. From numerical results, the thermal efficiency of single flow heater found very sensitive to gas optical thickness, such that gas radiation always has positive influence on the performance of system. The efficiency increase about 50% was computed for optical thicknesses more than 2 in the test cases. However, for double flow gas heater with less sensitivity to gas radiation and reciprocating trend for thermal efficiency with optical thickness, 15% increase in thermal efficiency was seen at the optimum optical thickness. Comparison between the present numerical results and those reported in literature, showed good consistency.

Due to high energy demand and adverse effect of global warming, improving the performance of solar thermal systems is of great importance. In this respect, the transient flow and thermal behavior of turbulent naturally ventilated airflow in a flat plate solar air heater (SAH) are investigated numerically. To achieve this aim, conservation of mass, momentum, and energy equations are solved for the turbulent airflow concurrently with the conduction equation inside the solid element using finite element method (FEM). The low Reynolds turbulence model is applied in the calculation of turbulent stress and heat flux, and the Boussinesq approximation is applied for computing buoyancy forces associated with the density gradient. The surface radiation is also considered explicitly in the calculations to obtain more accurate and reliable results. It is found that although based on the considered location the maximum incident radiation occurs at the optimum angle of 30°, the maximum flow rate reach to its peak when the inclination angle is 60°, in which the total increase of incident radiation is nearly 17%. It is also found that the rate of heat transfer is a decreasing function of the inclination angle ( 30 ⩽ θ ⩽ 90 ), such that the overall rate of heat transfer reduction is around 50%. Further, investigation of the impacts of solar radiation and air duct width on thermal performance showed that by increasing both parameter the rate of heat transfer and air flow increase by about 35% and 100% respectively. The exclusive analysis of the effects of sudden climate change on the SAH’s behavior revealed that a short time climate change leads to zero incident radiation, and considerable fluctuations in outlet temperature and flow rate.

Immersed boundary method (IBM) is introduced for solving the radiative transfer equation (RTE) to simulate the radiation heat transfer with natural convection in irregular geometries. IBM is organized based on the direct forcing method and sharp interface scheme. The Lattice Boltzmann method (LBM) with double multiple-relaxation-time (MRT)-D2Q9 is adopted in computing the momentum and energy equations, and the finite volume method (FVM) is applied in the numerical solution of RTE with a pseudo-time-stepping approach. The FVM-LBM code is tested in benchmark geometries in two cases. The ability and performance of IBM are examined in an irregular geometry for Rayleigh number, Planck number, and optical thickness on thermal and flow fields. Static and Hopf bifurcation as an attractive effect of radiation heat transfer is noted. The significant benefit of IBM is using a unique mesh in simulating these problems.

In the current study, a numerical investigation of two-dimensional combined convection-radiation heat transfer of turbulent gas flow over a backward-facing step (BFS) in a horizontal rectangular duct is presented. The computational domain contains two different parts including gas flow and solid element that makes the problem as a conjugate one. The gas phase is considered to be a radiating media that can absorb, emit and scatter thermal radiation, where in solid phase, heat transfer takes place by conduction. The set of governing equations for gas flow is solved numerically using the CFD technique and the k − ε $$k - \varepsilon $$ model is employed for computation of turbulence fluctuations. To evaluate the radiative term in the gas energy equation, the radiative transfer equation (RTE) is solved by the discrete ordinates method (DOM). Inside the solid phase, the conduction equation is solved to obtain the temperature distribution. The effects of conduction ratio, optical thickness, radiation-conduction parameter and albedo coefficient on heat transfer behavior of the system are carried out.

This paper presents a numerical investigation for laminar mixed convection flow of a radiating gas in a lid-driven cavity with a rectangular-shaped obstacle attached on the bottom wall. The vertical walls of the square cavity are assumed to be adiabatic, while other walls of cavity and obstacle are kept at constant temperature. The fluid is treated as a gray, absorbing, emitting and scattering medium. The governing differential equations consisting the continuity, momentum and energy are solved numerically by the computational fluid dynamics techniques to obtain the velocity and temperature fields. Discretized forms of these equations are obtained by the finite volume method and solved using the SIMPLE algorithm. Since the gas is considered as a radiating medium, besides convection and conduction, radiative heat transfer also takes place in the gas flow. For computation of the radiative term in the gas energy equation, the radiative transfer equation is solved numerically by the discrete ordinate method. The streamline and isotherm plots and the distributions of convective, radiative and total Nusselt numbers along the bottom wall of cavity are presented. The effects of Richardson number, obstacle location, radiation–conduction parameter, optical thickness and albedo coefficient on the flow and temperature distributions are carried out. Comparison between the present numerical results with those obtained by other investigators in the cases of conduction–radiation and pure convection systems shows good consistencies.

This paper deals with studying fluid flow and heat transfer of nanofluid through a forward facing step channel which is affected by a uniform magnetic field transverse to fluid flow. All the channel walls are assumed to be in constant temperature and the fluid temperature at the channel inlet is less than that of the walls. Also, the nanofluid is considered as a single-phase Newtonian fluid and the proper correlations were utilized to determine the thermophysical properties of nanofluid. Therefore, a code has been developed and two-dimensional continuity, momentum and energy equations were solved, using CFD technique. The computations were conducted for different values of the Reynolds and Hartmann numbers, and contraction ratio and an extensive range of nanoparticles volume fraction. The results indicated that flow separation and reattachment phenomena, in vicinity of the step edge, could be influenced strongly by magnetic field and the average Nusselt number is increased significantly by increasing nanoparticles volume fraction and Hartmann number.

Radiation is a major component of heat transfer in the modeling of furnaces. In the present work, transient coupled radiative and conductive heat transfer problems are analyzed in two-dimensional irregular geometries with a simple Cartesian practical method named the blocked-off. By this technique, Cartesian coordinate-based ray-tracing algorithm can be applied to the geometries with inclined or curved boundaries. The S–N discrete ordinates method used to solve the radiative transfer equation and finite difference method by applying the Taylor series for solving the energy equation to obtain the transient temperature distributions inside the participating medium. The walls of the enclosures are opaque, diffuse, and gray with specified temperatures. The medium was considered to be absorbing-emitting and linear anisotropic scattering with variable thermal conductivity, variable density, and variable specific heat, as a function of temperature. In the case of constant thermophysical gas properties, results have been compared with those reported in the literature, and good agreement was found.

In the current study, a numerical investigation of three-dimensional combined convection-radiation heat transfer over an inclined forward facing step (FFS) in a horizontal rectangular duct is presented. The fluid is treated as a gray, absorbing, emitting and scattering medium. To simulate the incline surface of FFS, the blocked-off method is employed in this study. The set of governing equations for gas flow are solved numerically using the CFD technique to obtain the temperature and velocity fields. Since the gas is considered as a radiating medium, all of the convection, conduction and radiation heat transfer mechanisms are presented in the energy equation. For computation of radiative term in energy equation, the radiative transfer equation (RTE) is solved numerically by the discrete ordinates method (DOM) to find the divergence of radiative heat flux distribution inside the radiating medium. The effects of optical thickness, radiation-conduction parameter and albedo coefficient on heat transfer behavior of the system are carried out.

The effect of radiation on turbulent mixed convection flow, generated by two plane wall jets with different temperatures inside a cavity was studied numerically. The medium is treated as a gray, absorbing, emitting and scattering. The two-dimensional Reynolds-average Navier-Stokes equations, coupled with the energy equation are solved by using the computational fluid dynamic (CFD) techniques, while the AKN low-Reynolds-number model is employed for computation of turbulence fluctuations. The Boussinesq approximation is used to calculate the buoyancy term, and the radiation part of the problem is solved by numerical solution of the radiative transfer equation (RTE) with the well known discrete ordinate method (DOM). The governing equations are discretized by the finite volume technique into algebraic equations and solved with the SIMPLE algorithm. The effects of radiation conduction parameter, scattering albedo, optical thickness and Richardson number on the thermal behavior of the system are carried out. Results show that the gas radiation has a significant effect on the temperature distribution inside the turbulent mixed convection flow.

The journal bearings which are designed for heavy-duty operations could experience different lubricant density due to high bearing loads. In the present work, hydrodynamic behaviour of finite length journal bearings under laminar and isoviscous flow with variable density are investigated. For this purpose, three-dimensional continuity and momentum equations along with a proper density-pressure relation are solved numerically, using CFD technique. Also, an appropriate cavitation model based on mass conservation is involved in the computation. Because of complex geometry of journal bearing, a conformal mapping is employed to generate an orthogonal grid and the governing equations are transformed in the computational domain. Since the degree of oil compressibility can be depended to the type of lubricant, typical mineral and synthetic oils treatments are modelled, separately. Results indicate that the oil compressibility effect leads to increasing load carrying capacity such that this increase is slightly more for the synthetic oil.

In the present work, the interaction between non-gray radiation and forced convection in a laminar radiating gas flow over a recess including two backward and forward facing steps in a duct is investigated numerically. Distributions of absorption coefficients across the spectrum (50 cm−1 < η < 20,000 cm−1) are obtained from the HITRAN2008 database. The full-spectrum k-distribution method is used to account for non-gray radiation properties, while the gray radiation calculations are carried out using the Planck mean absorption coefficient. To find the divergence of radiative heat flux distribution, the radiative transfer equation is solved by the discrete ordinates method. The effects of radiation–conduction parameter, wall emissivity, scattering coefficient and recess length on heat transfer behaviors of the convection–radiation system are investigated for both gray and non-gray mediums. In addition, the results of gray medium are compared with non-gray results in order to judge if the differences between these two approaches are significant enough to justify the usage of non-gray models. Results show that for air mixture with 10 % CO2 and 20 % H2O, use of gray model for the radiative properties may cause significant errors and should be avoided.

Based on an effective energy conversion method between flowing gas enthalpy and thermal radiation, a three-layered type of porous heat exchanger (PHE) has been proposed. The PHE has one high temperature (HT) and two heat recovery (HR1 and HR2) sections. In HT section, the enthalpy of gas flow converts to thermal radiation and the opposite process happens in HR1 and HR2. In each section, a 2-D rectangular porous medium which is assumed to be absorbing, emitting and scattering is presented. For theoretical analysis of the PHE, the gas and solid phases are considered in non-local thermal equilibrium and separate energy equations are used for these two phases. Besides, in the gas flow simulation, the Fluent code is used to obtain the velocity distribution in the PHE from inlet to outlet section. For thermal analysis of the PHE, the coupled energy equations for gas and porous layer at each section are numerically solved using the finite difference method. In the computation of radiative heat flux distribution, the radiative transfer equation (RTE) is solved by the discrete ordinates method (DOM). The effects of scattering albedo, optical thickness, particle size of porous medium and inlet gas temperature on the efficiency of PHE are explored. Numerical results show that this type of PHE has high efficiency especially when the porous layers have high optical thickness. The present results are compared with those reported theoretically by other investigators and reasonable agreement is found.

PurposeThe purpose of this paper is to focus on thermal characteristics behavior of forced convection flow in a duct over forward facing step (FFS), in which all of the heat transfer mechanisms, including convection, conduction and radiation, take place simultaneously in the fluid flow.Design/methodology/approachThe fluid is treated as a gray, absorbing, emitting and scattering medium. The Navier‐Stokes and energy equations are solved numerically by computational fluid dynamics (CFD) techniques to obtain the velocity and temperature fields. Discretized forms of these equations are obtained by the finite volume method and solved using the SIMPLE algorithm. Since the gas is considered as a radiating medium, all of the convection, conduction and radiation heat transfer take place simultaneously in the gas flow. For computation of the radiative term in the gas energy equation, the radiative transfer equation (RTE) is solved numerically by the discrete ordinate method (DOM) to find the radiative heat flux distribution inside the radiating medium. By this numerical approach, the velocity, pressure and temperature fields are calculated.FindingsThe effect of wall emissivity, optical thickness, albedo coefficient and the radiation‐conduction parameter on heat transfer behavior of the system are also investigated. The numerical results for two cases of convection‐conduction and conduction‐radiation problems are compared with the available data published in open literature and good agreement was obtained.Originality/valueThis is the first time in which flow over FFS in a duct, considering all heat transfer mechanisms including conduction, convection and radiation, is solved numerically.

Purpose – The purpose of this paper is to assess the turbulent thermohydrodynamic (THD) performance characteristics of an axially grooved finite journal bearing for a variety of simulated operating conditions.Design/methodology/approach – The set of governing equations consisting the Navier‐Stokes, turbulent kinetic energy and its dissipation rate equations coupled with the energy equation in the lubricant flow and the heat conduction equation for the bush are solved to obtain the three dimensional steady state THD characteristics of journal bearings. The lubricant flow in turbulent regime is modelled using the AKN low‐Re k−ϵ turbulence model. The problem is formulated mathematically and solved numerically using the computational fluid dynamics (CFD) approach with appropriate boundary conditions.Findings – It was found that shaft rotational speed has dramatic effects on the maximum temperature of the bearing and lubricant, also on the maximum hydrodynamic pressure and the oil flow rate. Besides, it was re...

The present work details a numerical simulation of forced convective laminar flow in a channel with a heated obstacle attached to one wall. The second law analysis is employed to investigate the distribution of entropy generation in the flow domain to demonstrate the rate of irreversibilities in thermal system. The conjugate problem including the convection heat transfer in the fluid flow and conduction one inside the obstacle is solved numerically to obtain the velocity and temperature fields in both gas and solid phases. To reach this goal, the set of governing equations including momentum and energy equations for the gas phase and conduction equation for the obstacle are solved by CFD technique to determine the hydrodynamic and thermal behaviors of the fluid flow around the obstacle and the temperature distribution in the solid element. An attempt is made to detail the local Nusselt number distribution and mean Nusselt number and also the local entropy generation distribution for the individual exposed obstacle faces. A good consistency is found between the present numerical results with experiment.

The main goal of this paper is to analyze the thermal and hydrodynamic behaviors of laminar mixed convection flow of a nongray radiating gas over an inclined step in an inclined duct. The fluid is considered an air mixture with 10% CO2 and 20% H2O mole fractions, which is treated as homogeneous, absorbing, emitting, and nonscattering medium. The full-spectrum k-distribution (FSK) method is used to handle the nongray part of the problem, while the radiative transfer equation (RTE) is solved using the discrete ordinate method (DOM). In addition, the results are obtained for different medium assumptions such as pure mixed convection and gray medium to compare with the nongray calculations as a real case. The results show that in many cases, neglecting the radiation part in computations and also use of gray simulations are not acceptable and lead to considerable errors, especially at high values of the Grashof number in mixed convection flow.

This study presents a numerical analysis of three-dimensional laminar forced convection flow of a radiating gas over an inclined backward-facing step in a rectangular duct under bleeding condition. The fluid is treated as a gray, absorbing, emitting, and scattering medium. The three-dimensional Cartesian coordinate system is used to solve the governing equations which are the conservations of mass, momentum, and energy. These equations are solved numerically using the computational fluid dynamic techniques to obtain the temperature and velocity fields, while the blocked-off method is employed to simulate the incline surface. Discretized forms of these equations are obtained by the finite volume method and solved using the SIMPLE algorithm. Since the gas is considered as a radiating medium, besides the convective and conductive terms in the energy equation, the radiative term also presented. For computation of this term, the radiative transfer equation is solved numerically by the discrete ordinates method to find the divergence of radiative heat flux distribution inside the radiating medium. By this numerical procedure, the role of radiation heat transfer on convection flow of a radiating gas which has many engineering applications (for example in heat exchangers and combustion chambers) is studied in detail. Beside, the effects of bleeding coefficient, albedo coefficient, optical thickness, and the radiation–conduction parameter on heat transfer behavior of the system are investigated. Comparison of numerical results with the available data published in the open literature shows a good agreement.

The interaction of non-gray radiation and natural convection in a 2-D cavity is studied numerically. The aim of this work is to analyze the thermal and flow behaviors of real cases with a small temperature difference using the full-spectrum k-distribution method. In addition, the results are obtained for different medium assumptions like pure convection, transparent medium, and gray medium to investigate which assumption is appropriate to be used instead of non-gray calculations. The results show that the gray medium assumption for radiative heat transfer is suitable to be used in cavities with normal air mixture at normal room conditions.

This article deals with analyzing the effect of radiative heat transfer on natural convection heat transfer in a square cavity under normal room conditions. The governing equations of natural convection and radiative transfer are solved simultaneously to obtain the temperature, velocity and heat flux distributions inside the participating medium. The finite volume method has been adopted to solve the governing equations and the discrete ordinates method (DOM) is used to model the radiative transfer in absorbing-emitting media. The radiative–convective model is validated by comparison with test cases solutions from the literature. Then, the effects of Rayleigh number from 102 to 106 and optical thickness in a broad range from 0 to 100 on temperature and velocity distributions and Nusselt numbers are investigated. The results show that even under normal room conditions with a low temperature difference, the radiation plays a significant role on temperature distribution and flow pattern in the cavity. Also, several interesting effects of radiation are observed such as a sweep behavior on the isotherms, streamlines and velocity distributions of the cavity along the optical thickness and a reverse behavior on maximum stream function and convective Nusselt number at different Rayleigh numbers.

The present study deals with the theoretical modeling aspects of coupled conductive and radiative heat transfer in the presence of absorbing, emitting and scattering gray medium within two- dimensional square furnace including two heat sources. The gray radiative medium is bounded by isothermal walls which are considered to be opaque, diffuse and gray. The well known discrete ordinate method (DOM) is employed to solve the radiative transfer equation (RTE) to obtain the radiative intensity distribution in the participating medium. In order to compute the temperature field inside the medium, the gas energy equation including the conductive and radiative term is solved by finite volume method. By this technique, the effect of radiation conduction parameter, which has a main role on thermal behavior of the conduction-radiation systems, is explored.

This paper presents thermohydrodynamic solutions for a finite-width journal bearing with single axial groove on the crown by considering turbulent effects in lubricant film. Appropriate governing equations and boundary conditions are used for the fluid flow and heat transfer process occurring in finite full journal bearings. A general computer program is developed to numerically solve the set of governing equations. Results are provided for THD characteristics of journal bearing in terms of Reynolds number, clearance ratio and eccentricity ratio. Good agreement with published experimental results was achieved. The results indicate that the bearing temperature field and pressure field are considerably influenced by the journal bearing parameters.

In the present work, convection heat transfer of water at supercritical pressure in a narrow annulus at low Reynolds numbers (less than 1500) has been investigated numerically. The continuity, momentum and energy equations have been solved simultaneously using computational fluid dynamics techniques with the inlet Reynolds number ranging from 250 to 1000, Grashof number from 2.5 × 10 5 to 1 × 10 6 and the inlet fluid temperature from 360 °C to 380 °C. In all of the case studies, a sub-cooled water flow at supercritical pressure (25 MPa) and a temperature close to the pseudo-critical point enters the annular channel with constant heat flux at inner wall surface and insulated at outer wall. To calculate the velocity and temperature distributions of the flow, discretized form of the governing equations in the cylindrical coordinate system are obtained by the finite volume method and solved by the SIMPLE algorithm. It has been shown that the effect of buoyancy is strong and causes extensive increase in velocity near the inner wall, and consequently an increase in the convective heat transfer, which is desirable. Besides, the effects of inlet Reynolds number, Grashof number and inlet temperature on the velocity distribution and also on the heat transfer have been investigated.

The present work deals with the fluid flow simulation and thermal analysis of a two-dimensional heat recovery system using porous media. A basic high-temperature flow system is considered in which a high-temperature non-radiating gas flows through a random porous matrix. The porous medium, in addition to its convective heat exchange with the gas, may absorb, emit and scatter thermal radiation. It is desirable to have large amount of radiative heat flux from the porous segment in the upstream direction (towards the thermal system). The lattice Boltzmann method (LBM) is used to simulate fluid flow in the porous medium. The gas and solid phases are considered in non-local thermal equilibrium, and separate energy equations are applied to these phases. Convection, conduction and radiation heat transfers take place simultaneously in solid phase, but in the gas flow, heat transfer occurs by conduction and convection. In order to analyze the thermal characteristics of the heat recovery system, volume-averaged velocities through the porous matrix obtained by LBM are used in the gas energy equation and then the coupled energy equations for gas and porous medium are numerically solved using finite difference method. For computing of radiative heat flux in the porous medium, discrete ordinates method is used to solve the radiative transfer equation. Finally the effect of various parameters on the performance of porous heat recovery system is studied.

In this study, hydrodynamic characteristics of finite length journal bearings running with incompressible laminar flow under magnetic field are investigated. Ferrofluid as a lubricant contains nanoparticles of magnetite (Fe3O4). The analysis is based on the numerical solution of the full Navier-Stokes equations using computational fluid dynamics techniques. Considering the complexity of the physical geometry, conformal mapping is used to generate an orthogonal grid, and the governing equations are transformed in the computational domain. Discretized forms of the transformed equations are obtained by the control volume method and solved by the SIMPLE algorithm. Results indicated that when compared with the conventional lubricant, the ferrofluid as a lubricant enhances the hydrodynamic characteristics of journal bearings and provides a higher load capacity. Besides, it was found that the other bearing characteristics are considerably affected by the applied magnetic field.

This paper describes a theoretical study to investigate the heat transfer characteristics of porous radiant burners (PRBs). In the present work, a 2-D rectangular model is used to solve the governing equations for porous medium and gas flow before the premixed flame to the exhaust gas. The gas and the solid phases are considered in non-local thermal equilibrium and combustion in the porous medium is modeled by considering a non-uniform heat generation zone. The homogeneous porous media, in addition to its convective heat exchange with the gas, may absorb, emit and scatter thermal radiation. The radiation effect in the gas flow is neglected but the conductive heat transfer is taken into account. In order to analyze the thermal characteristics of porous burners, the coupled energy equations for the gas and porous medium in steady condition are solved numerically and the discrete ordinates method (DOM) is used to obtain the distribution of radiative heat flux in the porous media. Finally, the effects of various parameters on the performance of porous radiant burners are examined. The present results are compared with some reported theoretical and experimental results by other investigators and good agreement is found.

Thermal characteristics of a new type of multi-layered porous heat exchanger (PHE) are identified in the present study. This system works based on the energy conversion between gas enthalpy and thermal radiation. It has a five-layered structure consisting of two high temperature, two heat recovery and one low temperature sections. These sections are separated from each other by four quartz glass windows. In the two high temperature sections, the enthalpy of high temperature gas flow is converted to thermal radiation that is emitted toward the three adjacent layers where the low temperature air flows are effectively heated by the reverse conversion from thermal radiation into gas enthalpy. The gaseous radiation is also considered, such that in each section, a transient theoretical analysis is conducted for a one-dimensional system where convection, conduction and radiation take place simultaneously in both gas and solid phases. The coupled energy equations for the gas flows and porous layers based on the two-flux radiation model are solved numerically to identify the transient heat transfer characteristics of the system. It is shown that this type of porous heat exchanger has a very high efficiency especially when the porous layers have high optical thicknesses.

This work presents the heat transfer characteristics of a new type of porous radiant air heater (PRAH) that operates on the basis of an effective energy conversion method between flowing gas enthalpy and thermal radiation. In this system, four porous layers consisting of porous radiant burner (PRB), high-temperature (HT) section, first heat recovery section, and second heat recovery section are considered. These layers are separated from each other by three quartz glass walls. The PRB generates a large amount of thermal radiative energy, which is converted into gas enthalpy in heat recovery layers. In the HT section, the gas enthalpy of the exhaust flow from PRB converts into thermal radiation. At each layer, the gas and solid phases are considered in non-local thermal equilibrium and combustion in the PRB is modelled by considering a non-uniform heat generation zone. The homogeneous porous media, in addition to its convective heat exchange with the gas, may absorb, emit, and scatter thermal radiation. In order to determine the thermal characteristics of the proposed PRAH, a two-dimensional model is used to solve the governing equations for porous medium and gas flow and the discrete ordinates method is used to obtain the distribution of radiative heat flux in the porous media. Computational results show that this type of air heater has much higher efficiency than the conventional ones. In addition, it is clarified that the high optical thickness and low scattering coefficient for porous layers improve the heater efficiency. In order to validate the applied numerical method at the present analysis, the results are compared with some reported theoretical and experimental results by other investigators and good agreement is found.

To calculate complex turbulent flows with heat transfer, we have developed a numerical procedure to solve the governing equations based on the orthogonal grids generation by the Schwarz–Christoffel transformation. In the present work, the turbulent fluid flow over a single forward facing step is studied to examine the effects of step length and its inclination angle on flow and heat transfer distributions. The continuity, Navier–Stokes and energy equations are solved numerically and the k – e model is employed for computation of turbulence fluctuations. Because of the complex flow geometry, the governing equations are transformed in the computational domain and the discretized forms of these equations are obtained with finite volume method and solved by the SIMPLE Algorithm. The present results reveal that the coefficient of heat transfer and also the hydrodynamic behavior of the flow are strongly dependent to step length and inclination angle.

In this study, the thermohydrodynamic (THD) characteristics of journal bearings with turbulent lubricant flow are determined using computational fluid dynamic techniques. The bearing has infinite length and operates under incompressible and steady conditions. The numerical solution of two-dimensional Navier—Stokes equations, with the equations governing the kinetic energy of turbulence and the dissipation rate, coupled with the energy equation in the lubricant flow and the heat conduction equation in the bearing is carried out. The AKN low- Re k–ε turbulence model is employed for computation of turbulence fluctuations. Because of the complex physical geometry, the governing equations are transformed for use in the computational domain such that the relation between computational and physical domains is obtained by conformal mapping. Discretized forms of the transformed equations are obtained by the control volume method and solved by the SIMPLE algorithm. Cavitation effects are also considered by using an appropriate cavitation model. The liquid fraction in the cavitated region is computed based on the continuity requirements, and rather than the two-phase flow of lubricant in this region, a homogeneous mixture with equivalent property is assumed and the governing equations still apply in the cavitated domain. In this study, the effects of Reynolds number, clearance ratio, and eccentricity ratio on THD behaviour of journal bearings are examined. Comparison between the present numerical results and experiment shows good agreement.

In this paper, two-dimensional laminar convective heat transfer of water at a supercritical pressure of 25 MPa in a horizontal rectangular duct has been investigated. The objectives were to understand the thermal and hydrodynamic behavior of fluid flow under different conditions. A set of governing equations containing the continuity, momentum and energy are solved simultaneously using CFD techniques. The finite volume method is employed to obtain the discretized forms of the governing equations. In the numerical solution of these equations, the SIMPLE algorithm is used for pressure-velocity computations. In all the cases studied in this paper, sub-cooled water at a supercritical pressure enters a duct with constant surface temperature which is near to the fluid pseudo- critical temperature. For this type of fluid flow with forced convective heat transfer, the pressure, velocity and temperature fields are calculated. Numerical results show that as the fluid temperature becomes closer to the pseudo...

In this paper, the turbulent fluid flow over a single forward facing step with inclined angle θ is studied numerically. The continuity and Navier-Stokes equations are solved numerically using CFD techniques. The standard k−e turbulence model is used to calculate the turbulence fluctuations. Because of the flow geometry, the governing equations are transformed in the computational domain and the discretized forms of these equations are obtained by finite volume approach. Conformal mapping is used to generate an orthogonal grid by the Schwarz-Christoffel transformation. In this study, the effects of geometrical parameters including step length and inclined angle θ on the fluid flow are examined. The present results reveal that the hydrodynamic behavior of the flow is strongly dependent on step length and inclined angle θ and especially the extension of recirculation zones varies with these parameters.

In this work, an advance type of air heater is developed and analysed using porous radiant burners (PRBs). This system operates on the basis of the energy conversion process between gas enthalpy and thermal radiation via porous media. In the five-layered structure, which is analysed in this work, there are five porous layers that are separated by four special quartz glass windows. The main layer operates as a PRB that generates a large amount of thermal radiative energy and in the other layers, the energy conversion process takes place between gas enthalpy and thermal radiation. In order to determine the thermal characteristics of a newly proposed porous air heater (PAH), the coupled energy equations for the gas flows and porous layers and the radiative transfer equation are solved numerically by an iterative method. The spherical harmonics approximation is used to obtain the thermal radiative heat flux distribution inside the solidmatrices. Computational results show a very high efficiency for this type of PAH, in comparison with conventional ones.

On the basis of an effective energy conversion phenomenon between gas enthalpy and thermal radiation, the thermal characteristics of a new type of multi-layered porous heat exchanger (PHE) are obtained in this study. This system has a five-layered structure consisting of two high temperature section, two heat recovery sections, and one low-temperature section. In each section, a transient theoretical analysis is conducted for a one-dimensional system in which convection, conduction, and radiation take place simultaneously in porous layers. The coupled energy equations for the gas flows and porous layers based on the two-flux radiation model are solved numerically to investigate the transient heat transfer characteristics of the system. It is clarified that this type of PHE operates efficiently, especially when the porous layers have high optical thickness.