Effects of ambient pressure on flame structure of CO/H2/N2 counterflow diffusion flame

Abstract

A numerical study has been conducted for the understanding of flame structure with the fuel composition of 40 per cent CO, 30 per cent H2, 30 per cent N2 and an oxidizer composition of 79 per cent N2 and 21 per cent O2 in a counterflow diffusion flame. Numerical results are obtained for flames over wide-ranged conditions of axial velocity gradients and ambient pressures. Main concern is specially given to effects of axial velocity gradient and ambient pressure on flame structure. It is seen that the role of axial velocity gradient on combustion processes is globally opposite to that of ambient pressure, on the basis of the Damköhler number considering the balance of convection, diffusion, and chemical reaction. That is, chemical non-equilibrium effects become dominant with increasing axial velocity gradient, but are suppressed with increasing ambient pressure. Flame strength is globally weakened by the increase of axial velocity gradient but is augmented by the increase of ambient pressure. However, flame extinction is described better on the basis of only chemical reaction and in this case axial velocity gradient and ambient pressure play a similar role conceptually such that the increase of axial velocity gradient and ambient pressure cause the flame not to be extinguished and extend the extinction limit, respectively. It is suggested that a combustion process like flame extinction is mainly influenced by the competition between the radical formation reaction and the third-body recombination reaction. Copyright © 2001 John Wiley & Sons, Ltd.