Effective gas-to-gas heat exchanger by means of energy conversion between gas enthalpy and thermal radiation

Abstract

This paper presents the heat transfer characteristics of a new type of gas-to-gas heat exchanger that operates on the basis of energy conversion between gas enthalpy and thermal radiation. A theoretical analysis is conducted for the one-dimensional system where convection and radiation take place simultaneously in three porous layers. In the high-temperature section, the enthalpy of gas flow converts to thermal radiation and the reverse direction of this process occurs in two recovery sections. The porous medium is assumed to be a homogeneous continuum that absorbs, emits and scatters thermal radiation. In order to investigate the thermal behaviour of gas flows and porous layers, the coupled energy equations for the gas and porous media, based on a two-flux radiation model, are solved numerically using an iterative method. Computational results show that this type of heat exchanger has a very high efficiency, especially when the porous layers have a high optical thickness and a low scattering coefficient. To confirm the validity of the present analysis, the numerical results for a simple energy recovery system are compared with theoretical results of other investigators and reasonable agreement is found.A surface area (m 2 ) b backscattered fraction factor B incoming radiation (W/m 2 ) B 0 non-dimensional incoming radiation ¼ B=sT 4 g0 c specific heat (J/kg K) h heat transfer coefficient (W/m 2 K) k thermal conductivity (W/m K) N S number density of solid particles (m 23 ) P dimensionless group ¼ h w =r g c g u g q þ,2 forward and backward radiative heat flux3 g0 R 0 radius of the porous layer (m) T temperature (K) u velocity (m/s) x coordinate along the flow direction (m) d thickness of the porous layer (m) u p,g non-dimensional temperature ¼ T p,g =T g0 r density (kg/m 3 ) s Stefan -Boltzman constant (W/m 2 K 4 ) s a absorbing coefficient (m 21 ) s e summation of the absorbing and scattering coefficients (m 21 ) s s scattering coefficient (m 21 ) t optical depth ¼ s e (x À x 1 ) t 0 optical thickness ¼ s e d v scattering albedo ¼ s s =s e

Heat transfer characteristics of a cylindrical porous radiant air heater under the influence of a two-dimensional axisymmetric radiative field

“…It is clarified from these experiments that the optical thickness of about eight is enough to obtain sufficient total HR. For the purpose of improving the heat exchanger efficiency, Gandjalikhan Nassab [9] proposed a self-insulated type of gas-to-gas heat exchanger with a three-layered structure. He found that the optical thickness and scattering albedo are the main design parameters for this type of heat exchanger.…”