THREE-DIMENSIONAL NUMERICAL SIMULATION OF A THREE-LAYERED RADIANT POROUS HEAT EXCHANGER WITH VARIABLE GAS PROPERTIES

This paper deals with the thermal analysis of a new type of porous heat exchanger (PHE). This system operates based on energy conversion between gas enthalpy and thermal radiation. The proposed PHE has one high temperature (HT) and two heat recovery (H RI. and H R-2) sections. In the HT section, the enthalpy of flowing high temperature gas that is converted to thermal radiation emitted towards the two heat recovery sections where the reverse energy conversion from thermal radiation to gas enthalpy occurs. In each section, a 3D cubic porous layer 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 nonlocal thermal equilibrium and separate energy equations are used for these two phases. For thermal analysis of the proposed PHE, the coupled energy equations for the gas and porous layer at each section are numerically solved using the finite difference method. The radiative transfer equation is solved by the discrete ordinates method (DOM) to compute the distribution of radiative heat flux in the porous layer. The numerical results consist of the gas and porous temperature distributions. The variations of radiative heat flux and gas property are also presented. Furthermore, the effects of scattering albedo, optical thickness and inlet gas temperature on the efficiency of the proposed PHE are investigated. It is shown that this type of porous heat exchanger has a very high efficiency especially when the porous layers have high optical thickness. The present results are compared with those reported theoretically by other researchers and good agreement is found.