Prediction of blowoff in a fully controllable low-swirl burner burning alternative fuels: Effects of burner geometry, swirl, and fuel composition

Abstract

Although the lean-premixed low-swirl burner (LSB) has been successfully demonstrated in a variety of applications, the mechanisms of flame stabilization are not fully understood and practical approaches for predicting stability limits are lacking. Using a unique fully controllable LSB coupled with a stereoscopic particle image velocimetry system, flow field characteristics and flame stability were investigated over a wide range of inlet conditions (reactant flow rates, 400<Qtot<1200 SLPM; heat release rates, 15<HRRHHV<75kW), scales (38.1, 50.8 and 76.2mm LSB nozzles), swirl configurations (swirl angles, α of 0°, 35°, 37.5°, 40.4°, 44.4° and 47°), and fuel compositions (reference fuel of methane and 10 different alternative fuel mixtures). Two distinct stability modes were documented within the regime classically identified as low-swirl. The dominant ‘W-type’ flames were observed to be stabilized in the shear layer between the central core flow and annular flow of the LSB. The less common ‘R-type’ flames were characterized by a noticeable axial recirculation, even at low swirl numbers. Damköhler number calculations revealed that these two flame modes were separated by a transition region of roughly 0.3<Da<0.75, with blowoff in a W- and R-type mode generally occurring above and below this range. For hydrogen containing fuel mixtures, a third, attached-flame stabilization-mode was also observed at low burner exit velocities and/or high equivalence ratios. For the W-type regime, a semi-empirical correlation was developed based on the strength of the annular shear layer (Vdiff), which linearly related data for all available fuel mixtures and LSB sizes to the average exit velocity, Uave, such that (Vdiff/Uave)(SL/SL,CH4s)0.17=1.05±15% for all available data at blowoff. This result provides new insight into LSB flame stabilization and is a useful tool for LSB design applicable to a wide range of fuels, equivalence ratios, heat release rates, and scales.