Heat release imaging in turbulent premixed methane–air flames close to blow-off

Abstract

Estimates of the heat release (HR) of unconfined lean premixed methane–air flames stabilized on an axisymmetric bluff body have been measured for conditions increasingly closer to blow-off. Simultaneous imaging of OH- and CH2O-PLIF was performed, and HR measurements obtained using the pixel-by-pixel multiplication of the OH- and CH2O-PLIF images. Blow-off was approached by slowly reducing the fuel flow rate. At conditions far from blow-off, HR occurred along the shear layer, whereas at conditions near blow-off, HR was also observed inside the recirculation zone (RZ). Localised extinctions along the flame front were seen at conditions away from blow-off, and increased in frequency and size as blow-off was approached. At conditions near blow-off, HR was detected on the boundary of flame pockets inside the RZ which had detached from the fragmented flame at the attachment point. Regions void of OH in the RZ near blow-off were often seen to be filled with CH2O. Regions void of both OH and CH2O were also observed, but less often, indicating the presence of both preheated gases and fresh reactants inside the RZ. Such images do not show a connection with the annular air jet, implying the cold reactants entered the RZ from the top. HR was observed to increase as a function of the absolute value of flame front curvature for the near unity Lewis number flames investigated. The measurements reported here are useful for model validation and for exploring the changes in turbulent premixed flame structure as extinction is approached.