Comparison of differential-diffusion effects in flamelet models for combustion with normal and oxygen-enriched air

Increased attention to fuel costs and CO2 emissions in industrial combustion processes has led to renewed interest in oxygen-enriched combustion as a means of reducing energy costs and greenhouse gas emissions. In many cases, optimal benefits are achieved by increasing the oxygen concentration in the combustion air from 21% to about 30%. While many industrial studies have documented process benefits associated with oxygen-enriched combustion, there have been few technical studies addressing fundamental combustion science issues resulting from oxygen enrichment. In particular, there have been no studies to date examining the effect of oxygen enrichment on differential-diffusion effects, which introduce errors in flamelet models used for mixing-chemistry coupling in simulations of industrial combustion processes. Flamelet libraries are most commonly computed via equilibrium calculators like CEA, in which there are no spatial gradients and thus no differential diffusion. An alternative is to incorporate differential diffusion via the equilibrium limit of a one-dimensional opposed-flow diffusion flame calculated using OPPDIF. The resulting one-dimensional spatial gradients allow differential diffusion among individual chemical species and temperature, and thus the flamelet library differs from that obtained via CEA. An open question is whether differences in CEA and OPPDIF flamelet libraries are sufficiently large that the choice of flamelet library is a substantial contributor to the overall accuracy of the combustion process simulation. The present results clarify this by comparing chemical species fields Y_i(x,t) and reaction rate fields w_i(x,t) obtained from flamelet libraries generated with CEA and OPPDIF. It does so for both normal (21% O2) combustion air and for oxygen-enriched (30% O2) combustion air, to examine the effects of oxygen enrichment on differential diffusion effects in the flamelet approach. This allows conclusions to be drawn regarding the importance of differential diffusion effects in flamelet libraries. By comparing results for both normal (21% O2) and oxygen-enriched (30% O2) combustion air, the present results allow conclusions to be drawn regarding the effects of oxygen enrichment on differential diffusion in the flamelet simulations of combustion processes.