Effect of backward facing step on combustion stability in a constant contact area cylindrical meso-scale combustor

Abstract

This experiment investigates the effect of backward facing step size variation on combustion stability in the cylindrical meso-scale combustor with the constant contact area. The backward facing step was varied by changing the combustor inlet diameter while the combustor diameter at the combustion zone was kept constant, i.e. has a constant contact area. Butane gas (C4H10) was used as fuel with air as an oxidizing agent. The result shows that the backward facing step has an important role in the combustion stabilization mechanism. Stable flame could be stabilized inside the meso-scale combustor with the backward facing step. Without the backward facing step, the flame blows out, then stable at the combustor rim. Recirculation flow occurs in the area behind the backward facing step. The increasing backward facing step size leads to an increase in reactant inlet velocity, recirculation flow size and shear stress in the area near the backward facing step. At large backward facing step size, the high reactant inlet velocity together with the large shear stress quenches the flame while the heat recovered by recirculation flow is less sufficient to stabilize flame so that the flame drifts to the downstream position. Hence bigger backward facing step size causes narrower flame stability limit area. The smaller the backward facing step size the wider the flame stability limit which shifts more toward lower equivalence ratio and high reactant velocity regions. Decreasing the backward facing step size decreases the reactant velocity into the combustion reaction zone as well as decreases the recirculation flow and the shear stress, so that the quenching effect decreases. The smaller recirculation flow has a better function for the flame holder to increase flame stability inside the meso-scale combustor. Therefore, small backward facing step size has a very important role in recovering heat energy stabilizing the flame in the meso-scale combustor.