Parametric examination of the destruction of availability due to combustion for a range of conditions and fuels

Abstract

A comprehensive second law analysis of combustion for a range of conditions and fuels was completed. Constant pressure, constant volume and constant temperature combustion processes were examined. The parameters studied were reactant temperature, reactant pressure, equivalence ratio and the fuels themselves. In addition, the contribution and relative significance of the various components (thermo-mechanical, reactive and diffusion) to the mixture availability was examined. Also, the effect of reactant mixture dissociation was incorporated into the combustion analysis. It was found that for similar initial conditions, constant pressure combustion and constant volume combustion exhibited similar trends. For constant temperature combustion, the trend is significantly different from the constant pressure and constant volume combustion, with almost the entire reactant availability being destroyed due to combustion at lower temperatures. Amongst the parameters examined, reactant mixture temperature had the most significant effect on the fraction of availability destroyed during combustion. The percentage availability destroyed reduced from 25 to 30% at 300 K to about 5% at 6000 K for constant pressure and constant volume combustion processes. The effect of the reactant mixture pressure on the fraction of availability destroyed was more modest. The values for the percentage availability destroyed for pressures ranging from 50 kPa to 5000 kPa were found to lie within a range of 5%. The effect of equivalence ratio on the fraction of reactant mixture availability destroyed was also documented. In general, it was found that the destruction of availability decreased with increasing equivalence ratios. This value, however, accounts for the availability due to fuel like species in the product mixture. Therefore, for practical applications, combustion of the stoichiometric mixture would be preferred over the rich equivalence ratios. It was found that the fraction of reactant availability destroyed increased with increasing complexity of the fuel??s molecular structure. In addition, it was shown that the diffusion availability terms is small and may be neglected, while the reactive availability and thermo-mechanical availability are more significant.