Entropy and nitrogen oxides production in steady detonation wave propagating in hydrogen-air mixtures

Abstract

Detonation engine, relying on the Fickett-Jacob cycle, may offer higher thermodynamic efficiency than engine relying on the Brayton (constant pressure) or Humphrey (constant volume) cycles. The present work quantitatively investigates the entropy production in steady detonation wave propagating in hydrogen-air mixtures. In particular, the contribution of each elementary chemical reaction to the total entropy production was investigated. The change of entropy during chemical processes was divided into the sensible and the reaction contributions. The study of the influence of several parameters including equivalence ratio, reactor model, initial pressure and temperature was embedded in a parametric study. It is shown that detonation conditions favor higher entropy production than constant volume and pressure combustion processes when considering the von Neumann state as the reference thermodynamic state. In general, the largest modifications in the entropy production pathways are induced by a change of equivalence ratio. In most cases, the largest contributions to total entropy production comes from the reactions H2 + OHH2O + H and H + OH + M = H2O + M, which are also the largest contributors to the energy release. The contributions of the sensible and reaction parts to the total entropy production demonstrate complex variations whose details depend on which specific reaction is considered. Besides, we studied the evolution of the main radicals and nitrogen oxides within the Taylor-Zeldovich expansion wave which brings the flow to rest. Significant change of the nitrogen oxides mole fraction occurs in this region, which indicates that this process should be accounted for when estimating pollutant formation in detonation.