Waste Heat Recovery By Enthalpy-Radiation Energy Conversion in Porous Enclosures.

Abstract

Radiation-enthalpy energy conversion in a porous medium has important applications in high temperature facilities such as heat exchangers, and industrial furnaces. The present study concerns the recovery of waste heat by placing a porous segment in the path of the hot exhaust from a furnace. In the theoretical analysis, the Discrete Ordinates Method is employed to formulate a one dimensional problem with different angular distribution of intensities. The effects of the governing parameters on the performance of the system are determined by the operation temperature, gas flow velocity, optical thickness of the porous segment, scattering albedo, shape factor of the phase function, porosity of the porous medium, and incident radiation. The results show that a porous segment with a large optical thickness has a better efficiency of waste heat recovery than a porous segment with a smaller optical thickness. The efficiency rises rapidly with increases in the optical thickness from 0 to 5. Beyond that range, however, the efficiency increases very slowly because the effective portion of the porous medium to emit radiation energy back to upstream direction is only the small portion, which faces to the upstream direction, of the porous medium. For small incident radiation, porous material with a lower scattering albedo is found to be better than that with a high scattering albedo. In case of a strong incident radiation, however, porous material with a high scattering albedo is better than that with a lower scattering albedo. The shape factor of the phase function is important only in high scattering albedo cases. For the strong incident radiation situation, the solid temperature is higher than the gas flow temperature at the front part of porous segment, i.e., radiation energy is converted to gas enthalpy. Higher operation temperatures or lower gas flow velocities or both increase the efficiency of waste heat recovery. Extremely high operation temperatures, however, result in radiative equilibrium and break down the radiation-enthalpy conversion mechanism, especially in the low scattering albedo case. Experiments are carried out using various porous media. Aluminum screens were found to cause largest conversion of enthalpy to radiation energy. The experimental mean temperature profiles show qualitative agreement with the theoretical predictions.