Analysis of blowoff dynamics from flames with stratified fueling

In some compact combustors with bluff body flame stabilization, fuel injection may be too closely coupled to permit uniform mixing. This stratification in the fuel profile can be asymmetric about the bluff body, and the flame equivalence ratio may differ across the recirculation zone. An experimental study of asymmetric fueling about a bluff body is reported in this paper with a focus on the impact of stratification on flame blowoff. In order to understand the blowoff dynamics of turbulent flames with stratified fueling, high speed chemiluminescence imaging was performed for five levels of fuel stratification through blowoff. Physical probe measurements of the local equivalence ratios were used to characterize the stratification. It was found that for overall fuel lean flames (averaged across the bluff body), fuel stratification increases the flame stability such that the overall equivalence ratio at blowoff is decreased for increased stratification. It was also found that the stronger (richer) shear layer determines the flame dynamics near blowoff. The leaner branch of the flame extinguishes earlier and the richer branch is shown to pilot the overall flame and therefore be responsible for the increase in flame stability at larger gradients. Proper orthogonal decomposition (POD) was applied to the high speed chemiluminescence images of the flames and used to quantitatively track the flame front dynamics through blowoff. All flames were found to exhibit Benard-von Karman vortex shedding just prior to blowoff. The POD time constants demonstrate that a strong fuel gradient decreases the time it takes for the flame to shift from its mean fully burning shape to blowoff and decreases the dwell time of the flame in the recirculation zone following local extinction. These time scales indicate a faster blowoff process with fuel stratification. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.