Experimental study of compact swirl flames with lean premixed CH₄/H₂/air mixtures at stable and near blow-off conditions

An experimental study on the compact lean premixed flames stabilized on a bluff-body and swirl burner is performed. The flames are featured with a strengthened Inner Recirculation Zone (IRZ). The objective is to identify the critical causes of blow-off near the lean limit. Meanwhile, the effects of hydrogen addition on the flame stabilization are also examined. Premixed CH₄/H₂/air mixtures are adopted with varied hydrogen fractions up to 80%. Flames far from and close to the blow-off conditions are both obtained for comparison. Simultaneous PIV/OH-PLIF measurements are performed, and flow-flame dynamics are analyzed. Results show that the flow straining near the flame root is dominant to induce the root extinction and initiate the blow-off. The strain is observed to mainly result from the shear deformation. The weakened root burning intensity is demonstrated with the CH* chemiluminescence. With hydrogen addition, the flame root is strengthened due to the enhanced resistance to the strain. The increased burning velocity and temperature with hydrogen are also beneficial for the flame stabilization. The inner shear layer vortices are anticipated to promote the blow-off by convecting cold unburned mixture into the IRZ.