Experimental analysis of shear strength and strain rate on H2/CH4 pinch-off flames

Abstract

An experiment was conducted under acoustic excitation conditions for the physical understanding of the pinch-off phenomenon in non-premixed flames. For fuel conditions, H2/CH4 has a volume ratio of 75/25 %, and for acoustic excitation conditions, experiments were performed at a forcing frequency of 80 Hz and velocity perturbation intensity of 50 %. In addition, experiments were performed in the range of 1.0–4.5 m/s of air and fuel velocity. OH* chemiluminescence analysis using a high-speed camera was performed for flame structure analysis, while particle image velocimetry was performed for flow-field analysis. Simultaneous measurement of OH* chemiluminescence and particle image velocimetry allowed observing the flame separation. Strain rate and shear strength were introduced for pinch-off and non-pinch-off analysis, and the correlation between the two factors was analyzed. Pinch-off mapping analysis at various velocities confirms the presence of a physical boundary. The direction of vortex roll-up reverses depending on the fuel and air velocities, indicating the formation of different flame structures. When acoustic excitation dominates, the strain rate increases and flame pinch-off occurs. In contrast, when the shear effect dominates, a non-pinch-off flame is obtained and the shear strength decreases. Accordingly, the strain rate and shear strength are correlated, and the approximate pinch-off threshold can be determined at various experimental points. These findings suggest that the shear strength and the strain rate can be used as parameters to distinguish pinch-off.