Convection of Radiating Gas in a Vertical Channel through Porous Media

The free convection flow of radiating gas between two vertical thermally conducting walls through porous medium in the presence of a uniform gravitational field has been studied. Closed form solutions for the velocity and temperature have been obtained in the optically thin limit case when the wall temperatures are varying linearly with the vertical distance. It is observed that the fluid velocity increases and the temperature difference between the walls and the fluid decreases with an increase in the radiation parameter. It is also observed that both the fluid velocity and temperature in the flow field increase with an increase in the porosity parameter. It is found that the fluid velocity decreases while the temperature increases with an increase in the thermal conductance of the walls. Further, it is found that radiation causes to decrease the rate of heat transfer to the fluid, thereby reducing the effect of natural convection.


Introduction
In recent years, free convection flow of viscous fluids through porous medium has attracted the attention of a number of researchers in view of its wide application to geophysics, astrophysics, meteorology, aerodynamics, boundary layer control and so on.In addition, convective flow through a porous medium has the application in the field of chemical engineering for filtration and purification processes.In petroleum technology, to study the movement of natural gas oil and water through oil channels/reservoirs and in the field of agriculture engineering to study the underground resources, the channel flows through porous medium have numerous engineering and geophysical applications.However, these studies are confined to normal temperatures of the surrounding medium.If the temperature of the surrounding fluid is rather high, radiation effects play an important role and this situation does exist in space technology.In such cases, one has to take into account the effects of radiation and free convection.Channels are frequently used in various applications in designing ventilating and heating of buildings, cooling electronic components, drying several types of agriculture products grain and food, and packed bed thermal storage.Convective flows in channels driven by temperature differences at bounding walls have been studied and reported extensively in literature.Radiative convective flows are frequently encountered in many scientific and environmental processes such as astrophysical flows, water evaporation from open reservoirs, heating and cooling of chambers and solar power technology.Several researchers have investigated convective flow in porous medium such as Nield and Bejan [1], Sparrow and Cess [2], Burmeister [3], Bejan [4], Kaviany [5] and Vafai [6].Raptis [7,8] has studied the effect of radiation on free convection flow through a porous medium.The natural convection cooling of vertical rectangular channels in air considering radiation and wall conduction has been studied by Hall et al. [9].Al-Nimr and Haddad [10] have described the fully developed free convection in open-ended vertical channels partially filled with porous material.Thermal dispersion-radiation effects on non-Darcy natural convection in a fluid saturated porous medium have been investigated by Mohammadein and El-Amin [11].The effect of wall conductances on free convection between asymmetrically heated vertical plates has been studied by Kim et al. [12].Greif et al. [13] have made an analysis on the laminar convection of a radiating gas in a vertical channel.Effect of wall conductances on convective magnetohydrodynamic channel flow has been investigated by Yu and Yang [14].Gupta and Gupta [16] have studied the radiation effect on hydromagnetic convection in a vertical channel.Datta and Jana [15] have studied the effect of wall conductances on hydromagnetic convection of a radiation gas in a vertical channel.Makinde and Mhone [17] investigated the effect of thermal radiation on MHD oscillatory flow in a channel filled with saturated porous medium and non-uniform wall temperatures.Narahari [18] has investiagted the effects of thermal radiation and free convection currents on the unsteady Couette flow between two vertical parallel plates with constant heat flux at one boundary.
In this paper, we have studied the fully developed free convection flow of radiating gas between two vertical thermally conducting walls embedded in porous medium.The governing equations are solved analytically.The effects of the permeability of the porous medium and the influences of radiation parameter and thermal wall conductances on velocity filed and temperature distribution are investigated and analyzed with the help of their graphical representations.It is observed that the fluid velocity decreases with an increase in the radiation parameter F. It is also observed that both the fluid velocity  and temperature     in the flow field increase with an increase in the porosity parameter  .It is found that the fluid velocity decreases while the temperature increases with an increase in the thermal conductance  .Further, it is found that radiation causes to decrease the rate of heat transport to the fluid thereby reducing the effect of natural convection.The rate of flow increases with an increase in either radiation parameter F or Rayleigh number

Formulation of the Problem and Its Solutions
Consider a fully developed flow of a viscous incompressible fluid flow in a vertical channel embedded in porous medium.The distance between the channel walls is 2L.Employ a Cartesian coordinates system with zaxis vertically upwards along the direction of flow and y-axis perpendicular to it.The origin of the axes is such that the channel walls are at positions = y L  and = y L (see Figure 1).
For the fully developed laminar flow in porous medium, the velocity and the temperature field have only a vertical component and all of the physical variables except temperature and pressure are functions of y.The temperature inside the fluid can be written as where N is the vertical temperature gradient.On the use of (1), the momentum and energy equations are simplified to the following form   where 0 , v is the kinematic coefficient of fluid viscosity, g the acceleration due to gravity, k permeability of the porous medium and = w T T     the thermal conductivity.
In the optically thin limit, the fluid does not absorb its own emitted radiation.This means that there is no selfabsorption but the fluid does absorb radiation emitted by the boundaries.Cogley et al. [19] showed that in the optically thin limit for a non-grey gas near equilibrium, the following relation holds where K  , is the absorption coefficient, h e  is the Planck function and the subscript w refers to values at the wall.Further simplifications can be made concerning the spectral properties of radiating gases ( [20]) but are not necessary for our investigation.
On the use of (5), Equation (4) becomes where Subscript "0" indicates that all quantities have been evaluated at the entrance temperature which is the temperature of the wall at .
On use of (8), Equation (2) becomes where Introducing the non-dimensional variables and on using (11), Equations ( 9) and ( 6) become The dimensionless velocity and the temperature boundary conditions are where  is the thermal conductance ratio.
Eliminating from ( 12) and ( 13), we obtain The solution of     satisfying the boundary condi- tions ( 14) is easily obtained.Achieving     , one can determine   1 u  from (12) using the boundary condi- tions (14).
The solutions for     and   1 u  subject to the boundary conditions (14) are where It is observed from the Equations ( 16) and ( 17) that the velocity and temperature depend on the parameters where and are given by ( 19).
where and are given by (19).

Results and Discussion
To study the effects of radiation and porosity of the porous medium on the velocity field 1 and temperature distribution          Ra. Figure 11 reveals that the magnitude of  in- creases with an increase in porosity parameter  .On the other hand, it is seen from Figure 12 that for fixed values of F, the magnitude of  decreases with an de- crease in  .
The rate of heat transfer across the channel's wall is given as Copyright © 2011 SciRes.WJM  The non-dimensional flow rate is given by On the other hand, it is seen from Figure 18 that for fixed values of F, W decreases with an decrease in  .

Conclusions
The fully developed free convection flow of a radiating gas between two vertical thermally conducting walls embedded in porous medium has been studied.The effects of the permeability of the porous medium and the influences of radiation parameter and thermal wall conductances on velocity and temperature fields are investigated and analyzed with the help of their graphical representa-    Copyright © 2011 SciRes.WJM

Figure 1 .
Figure 1.Geometry of the problem.

u
 for constant wall temperature are given by

u
 for thermally insulated walls are given by

uFigure 2
Figure 2 that the velocity

Figure 3
shows that the velocity at any point in the flow region decreases with an increase in Rayleigh number .The Rayleigh number is viewed as the ratio of buoyancy forces and the product of thermal and momentum diffusivities.Increasing value of Rayleigh number opposes the natural convection and hence the fluid velocity decreases.It is observed from Figure 4 that the velocity 1 increases with an increase in porosity parameter Ra u  .Porosity is a measure of the void (or empty) spaces in a porous medium and is a fraction of the volume of voids over the total volume.Porosity influences the convection flow and so the fluid velocity increases.It is seen from Figure 5 that the velocity decreases with an

Figure 9 . 1 gures 10 - 12 .
Figure 9. Variations of θ(η) for F = 1, σ = 0.1 and Ra = 1.increase in thermal conductance parameter  .Figures 6 and 7 reveal that the temperature  decreases with an increase in either radiation parameter F or Rayleigh number .Radiation tends to increase the rate of heat transport to the fluid.Thus the effect of radiation reduces the influence of natural convection by causing a reduction in the temperature difference between the fluid and the channel walls.The increase in radiation parameter F means the release of heat energy from the flow region and so the fluid temperature significantly decreases.It is found from Figures 8 and 9 that the temperature Ra