Bluff-body flame stabilization: Blockage effects

Abstract

Experiments have been performed to define the influence of blockage on flame stabilization by bluff bodies in ducted flow. Flameholders of a particularly simple geometry were studied over a wide range of blockage ratios. The studies were made while combustion was taking place and showed that flow speeds and flame geometry depend strongly on blockage. However, the experiments demonstrated convincingly that, at flame blowoff, the particular combination of these variables known as the characteristic mechanical time is independent of blockage as well as of other gross fluid dynamic parameters. Further experiments explored the influence of Mach number on the flows and showed quantitatively the changes in flow speeds and flame geometry to be expected at high Mach numbers. The experiments showed that the value of the mechanical time at blowoff remains unchanged at high Mach numbers despite large changes in the flow speeds and lengths that constitute this mechanical time. As a guide for the experiments, a free-streamlike theory was developed. This purely fluid dynamic theory, supplemented by a few simple experimental results, suffices to predict most of the features of bluff-body flameholding. A result of practical importance, predicted by the theory and confirmed by experiment, is that maximum blowoff speed occurs at a relatively low blockage ratio.