The structure of planar hydrogen jet diffusion flames

Abstract

An experimental investigation was made of the structure of non-reacting planar jets and planar jet diffusion flames. The diagnostic techniques used include PLEF of OH and acetone vapor, and planar laser Mie scattering (PLMS) from seeded alumina particles. The non-reacting jet flow structure was dominated by large-scale antisymmetric vortices, which were largely absent in the reacting cases. For the non-reacting jets, the acetone PLIF conserved scalar fields were mapped to OH mass fraction fields using equilibrium chemistry. The structure of the simulated OH zones exhibited some similarities to those in the real flames, but were considerably more convoluted by the large-scale structures. The actual OH zones were highly laminar-like at low Reynolds number and became progressively more distributed by small-scale turbulence with increasing Reynolds number. Average Mie scattering images were obtained for reacting and non-reacting jets with equal jet densities. The particle concentration fields were then mapped to mixture fraction using an equilibrium chemistry assumption for the reacting case. The centerline value of the mixture fraction decays at a much higher rate for the non-reacting case showing the strong influence of heat release in reducing entrainment and thus increasing the flame length over that of cold flames. The observed changes in the nature of the large-scale structure suggests that this may be, at least in part, responsible for the reduced entrainment rate of the jet flames.