A Study of Flame Spread Over Convectively Ignited Solid Fuel in a Sudden-Expansion Combustor

Abstract

The flame spread over a convectively ignited solid fuel (PMMA) slab in a suddenexpansion combustor is studied experimentally. The PMMA slab, which is installed downstream of the backward-facing step in the sudden-expansion combustor, is ignited convectively under the oxidizing stream of u0 = 22 m/s and T0 = 810C supplied by combustion of LPG in a vitiator. The process of flame spread during the ignition transient is recorded by a high-speed CMOS camera at 1000 fps. Corresponding flow visualization and PIV (Particle Image Velocimetry) techniques are also established to analyze the vortical structures. The mechanisms of this specific flame spread process are attempted to be clarified from the perspective of flame-vortex interactions. Instead of being ignited near the backward-facing step, ignition was observed to occur at the downstream end. The higher temperature at the downstream significantly decreases the characteristic reaction time and small eddies increases the mixing. Three phases of flame spread during ignition are identified during the ignition transient. In Phase 1, the flame spread is dominated by the characters of boundary layer and partial premixed flame. It also behaves as a buffer zone to provide adequate energy for the upstream flame spread. In Phase 2, the impinging of vortices as well as the character of premixed flame dominated the reattachment zone. The upstream propagating flames experience periodical extinction-reignition cycles due to the high stress from the impinging of vortex. At last in Phase 3, the incoming large vortices roll up portions of heat back to their upstream coherent vortices and ignite the shear layer. The shear layer ignition possesses the characteristic of a diffusion flame.