Concentration-, Pressure-, and Temperature-Dependence of the Flame Velocity in Hydrogen-Oxygen-Nitrogen Mixtures

Abstract

A mechanism consisting of 18 elementary chemical reactions is demonstrated to give a complete quantitative description of the concentration-, temperature-, and pressure-dependence of the flame velocity in H₂-O₂-N₂ mixtures. Whereas concentration- and temperature-dependence show the expected behaviour, the pressure dependence is determined by the two different chain mechanisms occurring. At low and at high pressures the pressure exponent of the flame velocity is positive in agreement with simple flame theory. In between there is a transition region with negative pressure exponents due to the change of the chain mechanism, which does not contradict simple flame theory as sometimes asserted in the literature. Furthermore, the partial equilibrium assumption is shown to be valid only at temperatures T>1700 K, whereas at lower temperatures this hypothesis fails by orders of magnitude. Experiments on NO formation in H₂-air flames can be reproduced, if the larger of the two rate constants recommended in the literature is used for the calculations.