Effects of differential diffusion on nonpremixed-flame temperature

Abstract

This numerical and analytical study investigates effects of differential diffusion on nonpremixed-flame temperatures. To focus more directly on transport effects the work considers a single irreversible reaction with an infinitely fast rate, with Schab-Zel'dovich coupling functions introduced to write the conservation equations of energy and reactants in a chemistry-free form accounting for non-unity values of the fuel Lewis number L-F. Different flow configurations of increasing complexity are analyzed, beginning with canonical flamelet models that are reducible to ordinary differential equations, for which the variation of the flame temperature with fuel-feed dilution and L-F is quantified, revealing larger departures from adiabatic values in dilute configurations with oxidizer-to-fuel stoichiometric ratios S of order unity. Marble&#39;s problem of an unsteady flame wrapped by a line vortex is considered next, with specific attention given to large-Peclet-number solutions. Unexpected effects of differential diffusion are encountered for S < 1 near the vortex core, including superadiabatic/subadibatic flame temperatures occurring for values of L-F larger/smaller than unity as well as temperature profiles peaking on the oxidizer side of the flame. Direct numerical simulations of diffusion flames in a temporal turbulent mixing layer are used to further investigate these unexpected differential- diffusion effects. The results, confirming and extending previous findings, underscore the nontrivial role of differential diffusion in nonpremixed-combustion systems.