Forced convection in thermally developing region of a channel partially filled with a porous material and optimal porous fraction

Local Nusselt numbers have been obtained on the porous side and the fluid side of the parallel plate channel. Plots to obtain wall heat transfer directly have been presented. Change in wall heat transfer has been examined to establish that the maximum enhancement in heat transfer occurs at a porous fraction of 0.8 at a Darcy number of 0.001. Correspondingly, the maximum enhancement per unit pressure drop occurs at a porous fraction of 0.7. As Darcy number increases, the porous fraction at which the maximum enhancement in heat transfer occurs decreases.     

Natural convection along a vertical porous surface

This paper describes an experimental study on natural convection along a vertical porous surface consisting of a bank of parallel plates with constant gaps. Compared with a smooth surface, the heat transfer for a porous surface with streamwise gaps is somewhat enhanced owing to enthalpy transport due to flow within the gaps and that with spanwise gaps is enhanced due to the leading- and trailing-edge effects of solid-phase micro surfaces. Observations show that no clear transition to turbulent flow occurs at a critical Rayleigh number for a smooth surface and that the boundary layer oscillates with a dominant frequency at higher Rayleigh numbers. The dominant frequency for a porous surface is nearly the same as that for a smooth surface. This fact obviously indicates that the oscillations of natural convection are almost independent of the porous structure of the heating surface and are mainly dependent on the behavior of the outer boundary layer. © 1998 Scripta Technica. Heat Trans Jpn Res, 26(6): 385–397, 1997     

Thermal behavior of a porous electric heater

The performance of a proposed porous electric heater is investigated. The porous heater exchanges heat with the working fluid through its large volumetric surface area. As a result, it produces lower surface temperature as compared with the conventional heater for the same imposed heating power. Two mathematical models are presented to describe the thermal behavior of both heaters. Axial diffusion is included in the governing equation of the solid conventional heater. The predictions of both models are compared at different operating conditions where it is found that porous heaters have much better thermal performance than the conventional heaters.     

Augmentation of heat transfer from a solid cylinder wrapped with a porous layer

In the present study, the heat transfer from a porous wrapped solid cylinder is considered. The heated cylinder is placed horizontally and is subjected to a uniform cross-flow. The aim is to investigate the heat transfer augmentation through the inclusion of a porous wrapper. The porous layer is of foam material with high porosity and thermal conductivity. The mixed convection is studied for different values of flow parameters such as Reynolds number (based on radius of solid cylinder and stream velocity), Grashof number, permeability and thermal conductivity of the porous material. The optimal value of porous layer thickness for heat transfer augmentation and its dependence on other properties of the porous foam is obtained. The flow field is analyzed through a single domain approach in which the porous layer is considered as a pseudo-fluid and the composite region as a continuum. A pressure correction based iterative algorithm is used for computation. Our results show that a thin porous wrapper of high thermal conductivity can enhance the rate of heat transfer substantially. Periodic vortex shedding is observed from the porous shrouded solid cylinder for high values of Reynolds number. The frequency of oscillation due to vortex shedding is dampened due to the presence of the porous coating. Beyond a critical value of the porous layer thickness, the average rate of heat transfer approaches asymptotically the value corresponding to the case where the heated cylinder is embedded in an unbounded porous medium.     

Heat transfer from a porous composite sphere immersed in a moving stream

Convection heat transfer in the low Reynolds number regions from an isothermal sphere surrounded by a porous shell is numerically evaluated. In the limit of large porous medium Peclet numbers, the average Nusselt number based on the porous medium thermal conductivity becomes independent of the fluid Peclet number and becomes proportional to the porous medium Peclet number raised to the power one-third. In the limit of small fluid and porous medium Peclet numbers, the average Nusselt number approaches that given by the limiting case of pure conduction.     

Natural convection in a square porous annulus

The current study is focused to investigate the natural convective heat transfer characteristics in a porous square annulus. Finite element method is used as a tool to simplify the partial differential equations that govern the heat and fluid flow characteristics inside the porous medium. A simple three noded triangular element is used to divide the porous domain into smaller segments known as elements. The algebraic set of equations resulting from the finite element equation are assembled into a global matrix and then solved iteratively to get the solution variables. Thermal equilibrium as well as non equilibrium in porous domain is considered. The effect of various geometric and physical parameters are investigated. The boundary conditions are such that the inner walls of the annulus are heated isothermally to temperature T_h, and the outer surfaces are exposed to cool temperature T_c. The width ratio defined as the ratio of hollow portion to the length of the cavity is varied from 0.125 to 0.875. Results are discussed with respect to width ratio, Rayleigh number, radiation parameter and viscous dissipation parameter.     

Natural convection in a concentric annulus with a porous sleeve

Analytical solutions obtained through perturbation method and Fourier transform are presented for natural convection in concentric cylinders with a porous sleeve. The porous sleeve is press-fitted to the inner surface of the outer cylinder. Both the inner and outer cylinders are kept at constant temperatures with the inner surface at a slightly higher temperature than that of the outer. The main objective of the present study is to investigate the buoyancy-induced flow as affected by the presence of the porous layer. A parametric study has been performed to investigate the effects of Rayleigh number, Darcy number, porous sleeve thickness, and relative thermal conductivity on the heat transfer results.     

Effects of a porous medium on forced convection of a reciprocating curved channel

Enhancement of heat transfer rates of a reciprocating curved channel partially installed by a porous medium is investigated numerically. The distribution of heat transfer rates on the heat surface of the reciprocating curved channel is rather non-uniform that easily causes a thermal damage to destroy the channel. A method of using the porous medium to enhance heat transfer rates of the channel is then developed to solve the thermal damage. The arbitrary Lagrangian–Eulerian method is firstly modified for treating a moving boundary problem of the porous medium. Main parameters of Reynolds numbers, porosities, frequencies and amplitudes are examined. The results show that the enhancements of heat transfer rates of most porous medium situations are achieved. However, heat transfer rates of a few porous medium situations are unexpectedly inferior to those of without porous medium situations.     

往复式多孔介质燃烧器流动特性的试验研究

研究了往复式多孔介质燃烧器在冷态条件下气流在其中的流动特性规律。在多孔介质的类型和运行温度不变的情况下，往复式多孔介质燃烧器的压降与空截面流速的平方成正比，与多孔介质的厚度成正比；燃烧器的稳定时间基本不受多孔介质厚度和空截面流速的影响。建立了计算压降的简单数学模型，理论模型的计算结果与实验数据符合较好。     

Disintegration process and performance of a coaxial porous injector

In order to understand the breakup performance of coaxial porous injectors, the sprays of coaxial porous injectors with two different porous material cylinder lengths were compared with those of conventional shear coaxial injectors. To allow comparison, the wall injection lengths were designed to be equivalent to the value of the recess depth. Cold flow sprays were visualized using back-lit photography methods and analyzed quantitatively with a laser diffraction apparatus, in order to study the effects of the momentum flux ratio and Weber number on the breakup for each type of injector. In case of the shear coaxial injector, the large liquid core was observed in low air mass flow rate condition. However, the destabilization of the liquid jet from the coaxial porous injector is almost complete within the inner region, near the injector face plate. Additionally, better breakup performance in low gas flow rate condition was obtained when the porous cylinder length decreased, while the shear coaxial injectors showed better breakup efficiency when the recess length increased. In conclusion, the different breakup process caused by the radial momentum in the inner region of the porous injector disintegrated the liquid core.     

Enhanced thermal stratification in a liquid storage tank with a porous manifold

Experiments were conducted to investigate the effectiveness of a porous manifold in the formation and maintenance of thermal stratification in a liquid storage tank. A thermal storage tank with a capacity of 315 L and a height-to-radius ratio of 4 was used for the experiment. The porous manifold used was made from rolling up a nylon screen into the shape of a tube. Stratification was observed at a Richardson number as low as 0.615. Flow visualization was also performed to confirm the effectiveness of the porous manifold in the promotion and maintenance of stable thermal stratification. From the results of flow visualization, one can conclude that a porous manifold is able to reduce the shear-induced mixing between fluids of different temperature, and thus is able to promote and maintain a stable stratification.     

Heat transfer in a conical cylinder with porous medium

In this paper, numerical study of heat transfer in a conical annular cylinder fixed with saturated porous medium is presented. The heat transfer is assumed to take place by natural convection and radiation. The inner surface of conical cylinder is maintained at uniform wall temperature. The governing partial differential equations are non-dimensionalised using suitable non-dimensional parameters and then solved by using finite element method. The porous medium is divided using triangular elements with uneven element size. A computer software is used to solve the coupled momentum and energy equations in an iterative manner. The results are discussed for various values of geometric and physical parameters of porous medium with emphasis on cone angle of the cylinder. It is seen that the cone angle plays a vital role in heat transfer from the hot surface to porous medium.     

Radiation and free convection flow through a porous medium

Steady two dimensional free convection flow through a very porous medium bounded by a vertical infinite porous plate by the presence of radiation is condsidered. Expressions for the velocity and temperature are obtained. Effects of the radiation on the velocity field are discussed.     

Combined free and forced convection flow through a porous medium

Local similarity solutions of the forced and free convection flow past a porous medium bounded by a semi-infinite vertical porous plate are obtained numerically.     

Thermal analysis on the cooling performance of a porous evaporative plate for building

In this paper, a wet porous cooling plate has been used for a building wall. Cooling can be achieved due to the evaporation in the porous layer. A mathematical model on the heat and mass transfer in the unsaturated porous media is developed to analyze the influences of ambient conditions and the porous layer thickness on the cooling performance of the porous evaporative plate. With a decrease in ambient relative humidity and an increase in ambient temperature, more cooling of the porous evaporative plate can be supplied for the inside of the room. The heat exchange between the inside surface of the porous plate and the air in the room should be intensified to achieve a higher cooling efficiency of the porous plate. The ambient wind speed and the thickness of the porous plate also have significant influence on the average temperature of the porous plate. All these results should be taken into account for the utilization of the porous evaporative cooling plate. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20284     

Thermosolutal convection in a fluid‐porous composite medium

In the present study analysis has been performed for thermosolutal convection in a fluid‐porous composite medium, consisting of a fluid‐saturated porous medium followed by an overlaying clean medium. The fluid‐porous composite medium is subjected to both a horizontal solutal and a thermal gradient. Top and bottom walls of the fluid‐porous composite medium are assumed to be impermeable and adiabatic. The Darcy‐Brinkman‐Forchheimer model is used to study the flow through the fluid‐porous composite medium. A single domain approach is taken into consideration for numerical simulation. The solution is done by control volume integration. A comprehensive analysis has been performed for various pertinent parameters to delineate their behavior. Results of the transport phenomenon have been provided in graphical and tabular form, for the complete understanding of the complex phenomenon. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21048     

Effect of thermal radiation on unsteady mixed convection flow and heat transfer over a porous stretching surface in porous medium

An analysis is performed to investigate the effects of thermal radiation on unsteady boundary layer mixed convection heat transfer problem from a vertical porous stretching surface embedded in porous medium. The fluid is assumed to be viscous and incompressible. Numerical computations are carried out for different values of the parameters involved in this study and the analysis of the results obtained shows that the flow field is influenced appreciably by the unsteadiness parameter, mixed convection parameter, parameter of the porous medium and thermal radiation and suction at wall surface. With increasing values of the unsteadiness parameter, fluid velocity and temperature are found to decrease in both cases of porous and non-porous media. Fluid velocity decreases due to increasing values of the parameter of the porous medium resulting an increase in the temperature field in steady as well as unsteady case.     

Comparison of the waves of temperature difference between the solid and fluid phases in a porous slab and in a semi-infinite porous body

In this paper an analytical investigation of the temperature difference between the solid and fluid phases in a porous slab initially at a low temperature whose inlet boundary is suddenly subjected to a flow of a higher temperature incompressible fluid is carried out. A two energy equation model for the porous bed is utilized. It is shown that the temperature difference forms a wave localized in space and propagating from the fluid inlet boundary. Using the perturbation technique, an analytical expression describing the wave of temperature difference is obtained. Results of the present investigation are compared against the results obtained in ref. [1] for a semi-infinite porous body.     

On double diffusion in a Brinkman heat generating porous layer

This paper relies on linear stability analysis to define the parametric domain in which convective motions exist in a horizontal unstably stratified porous layer. The paper focuses on the combined effect of three distinct factors on the onset of convection: the presence of a second diffusing component (in addition to heat), the presence of internal heat generation in the porous medium and the departure from the Darcy model of flow in porous medium by using Brinkman's modification of Darcy's law.