Constrained-temperature solutions of coflow laminar diffusion flames

Abstract

We investigate the use of an experimental 2-D temperature profile to constrain detailed numerical solutions of a sooting coflow laminar diffusion flame. Experimentally, four optical diagnostic techniques are used to measure the two-dimensional temperature field in an ethylene-air coflow flame. This experimental temperature field is then used to impose the temperature in the solution process, thus obviating the need to solve the energy equation and, in particular, to incorporate costly models of radiative losses in the flame. Results are presented for a 40% ethylene-air flame on the Yale Coflow Burner. In the unconstrained solution of the complete set of governing equations, the location of maximum temperature is found along the flame wings, whereas the experimental temperature field has its maximum along the centerline. Similarly, the location of peak soot volume fraction migrates from along the flame wings in the unconstrained calculation, where soot surface growth processes dominate, to the centerline in the constrained case, where soot inception is the dominant condensed-phase formation mechanism. The distribution of soot in the constrained solution is much more consistent with experimental observations, and this fact illustrates how the validation of a soot sub-model may be complicated by the necessity of modeling distributed heat losses in the flame.