Blowoff characteristics of bluff-body stabilized conical premixed flames under upstream velocity modulation

This article presents experimental findings on the blowoff characteristics of conical premixed flames anchored at their apex by three different flame holders (rod, disk, and cone) in the presence of upstream velocity oscillations. Experiments were performed with propane-air mixtures at mixture velocities approaching the flame holder of 5, 10, and 15 m/s. The flow speed was modulated sinusoidally at frequencies up to 400 Hz with a constant-velocity modulation amplitude of u(rms)/U-m = 0.08 upstream of the flame holder. It was found that the blowoff equivalence ratio exhibits a dependence on the flow modulation frequency. Specifically, at low approach velocities (5 m/s), the effect of upstream flow modulation is to improve flame stability as evidenced by lower flame blowoff equivalence ratios for all three types of flame holders considered. At higher approach velocities (10 and 15 m/s), the disk- and cone-shaped flame holders exhibit less stability with increasing excitation frequency. The rod-shaped flame holder behavior is different at these higher velocities in that the flow modulation still provides enhanced flame stability. The flame stability results are supplemented with a detailed analysis of the flow field in the flame stabilization zone obtained by particle image velocimetry. (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.