Numerical evaluation of boundary-layer assumptions used for the prediction of the standoff distance of a laminar diffusion flame

Abstract

Numerical simulations of a diffusion flame established over a flat plate with a flow of oxidizer parallelto its surface are presented. All simulations are intended to describe experiments conducted in microgravity and using gaseous fuel injection. The numerical tool uses direct numerical simulation and a combustion model based on infinite chemistry and a mixture fraction approach. The purpose of this study is to better understand the effect of the flame on the flow and to validate the use of boundary-layer approximations in the analytical modeling of the stand off distance. This study does not attempt description of the leading edge of the flame, but concentrates on the description of the flame geometry downstream of this region. Special attention is given to velocity overshoots previously reported in experimental studies. The study of the different contributions to the local acceleration provides insight on the origins and mechanisms leading to these overshoots. It was observed that the acceleration is mainly due to an injection-induced pressure gradient at the leading edge of the porous burner that is significantly amplified by the flame due to the local decrease in density. The fuel injection velocity defines two different flame regimes. For low injection velocity, the flow conforms to the boundary-layer assumptions. The streamlines show that for this regime, soot will be formed far from the flame, and therefore soot oxidation is small. Experiments have shown that blue flames, negligible radiative feedback, and unstable flames characterize this regime. Separation of the flow follows an increase in injection velocity leading to flow conditions that cannot be described by a boundary-layer approach. The streamlines demonstrate that soot will be convected toward the flame leading to high oxidation rates. Experiments have shown that yellow flames, significant radiative feedback, and a stable flame characterize this regime.