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 blowoff. Simultaneous imaging of OH-and CH 2 O-PLIF was performed, and HR measurements obtained using the pixel-by-pixel multiplication of the OH-and CH 2 O-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 CH 2 O. Regions void of both OH and CH 2 O 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.

Joint PLIF measurements of CH2O and OH were performed in a swirl-stabilized nheptane spray flame at conditions close to extinction. Simulations of laminar counterflow heptane flames at different strain rates showed that the heat release could be approximately represented by the product CH2OxOH, but also that this product cannot visualise the heat release at the lean side of stoichiometry. The simulations suggest that the outline of CH2O regions in PLIF images could be an approximate indicator of the stoichiometric mixture fraction iso-line. Due to the intense turbulence and local extinction, individual PLIF images show a very variable behaviour. They indicate rich zones, reaction sheet breaks, lift-off, and they suggest that this flame is mostly of non-premixed character. The mean heat release rate as represented here is consistent with inverse Abel-transformed OH* chemiluminescence imaging. The usefulness of this technique for spray flames is discussed.

The focus of this work is to visualise the regions of CH 2 O and heat release (HR) of an unconfined turbulent premixed bluff body stabilised ethylene-air flame at conditions approaching lean blow-off using simultaneous imaging of OH-and CH 2 O-PLIF. The HR regions are estimated from the product of the OH and CH 2 O profiles. At conditions near blow-off, wide regions of CH 2 O are observed inside the recirculation zone (RZ). The presence of CH 2 O and HR inside the RZ is observed to follow fragmentation of the downstream flame parts near the top of the RZ. The presence of wide regions void of both OH and CH 2 O inside the RZ at conditions very close to blow-off indicates the possible entrainment of un-reacted gases into the RZ. The behaviour of the lean ethylene-air flame with Lewis number (Le) greater than 1 is compared to that of a lean methane-air flame with Le of approximately 1. For both fuels, qualitatively similar observations of flame fragmentation downstream followed by build-up of CH 2 O and HR inside the RZ are observed at conditions near lean blow-off. Also, a similar trend of flame front curvature conditioned on HR was observed for both the ethylene-air and methane-air flames, where the magnitude of HR was observed to increase with the absolute value of curvature.

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