A Study Energy Conversion by a Porous Combustor-Heat Exchanger with Cyclic Flow Reversal Combustion
Abstract
Present observations of energy conversion from low-grade fuels aimed at producing thermal energy, strongly suggest that cyclic flow reversal combustion in a porous medium (CFRC) is a promising approach for future applications. The CFRC is very advantageous from the aspects of fuel conservation, efficient, combustion intensity and emission pollutants because of its prominent ability in creating an efficient internal heat recirculation from the hot exhaust gases to the unburned mixture when compared with that of conventional burners. Against this background, a novel porous combustor-heat exchanger (PCHE) equipped with the CFRC was developed for abstracting heat from a typical low-grade gaseous fuel. The PCHE incorporates features of a porous medium, which can be used as an efficient compact combustor or a heat exchanger. With this combination, the PCHE in the form of a concentric cylinder is arranged in such a way that the inner cylinder is the porous combustor, which serves as a radiant burner equipped with CFRC. The outer cylinder is the porous heat exchanger, which acts as an integral function of a radiative heat absorber, a compact heat exchanger and a thermal insulator for obtaining maximum thermal shielding. The radiative heat flux converted from the heat of combustion is, therefore, effectively converted into a substantial increase in the enthalpy of the process air flowing through the porous heat exchanger. Performance of the PCHE is verified by performing parametric studies of some dominating parameters, i.e., half-period, equivalence ratio and thermal input, which affect thermal efficiencies and emission characteristics. The PCHE equipped with the CFRC effectively abstracts heat from the typical fuel with a minimum apparent heat content of 0.62 MJ/m3 [normal] at which the maximum preheated process air temperature of 150°C is obtained with relatively low emission pollutants. Optimum operating condition for the PCHE should be at relatively short half-period, low equivalence ratio and high thermal input.