Turbulent Diffusion Flame Properties Behind a Backward-Facing Step

Experiments and analysis are reported for a subsonic combustion wind-tunnel experiment, in which a turbulent diffusion flame is stabilized behind a backward-facing step. A mixture of hydrogen—plus a diluent—is introduced through the porous floor behind the step. Velocity and temperature distributions were measured using laser Doppler velocimetry and Raman spectroscopy. The flowfleld was modeled analytically using a two-equation turbulence model with Favre-averaged equations using the conserved scalar approach. Dominant features of the flow is a flame anchored primarily in the shear layer, a relatively cool recirculation zone, and a long reattachment region, compared with that for cold flow. The intensity of the Stokes line of nitrogen is shown to be an indication of the temperature of the flow. The analysis, which gave excellent agreement with experimental results in the absence of combustion, underpredicts the extent of the recirculation zone and places the region of highest temperature closer to the floor than was observed experimentally, once a flame is present. Nomenclature c = concentration H = step height T = temperature U = axial velocity V = vertical velocity X = axial distance Y = distance above the tunnel floor Subscripts