Flame stabilization mechanisms in hydrogen enriched methane-air low-swirl flames

A newly designed low-swirl combustor by the Korea Institute of Machinery and Materials was tested for fully premixed H$_{\mathrm{2}}$-CH$_{\mathrm{4}}$/air reactant mixtures with equivalence ratios 0.6 to 0.9 and H$_{\mathrm{2}}$ fractions 0{\%} to 80{\%}. Using simultaneous OH/CH$_{\mathrm{2}}$O planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (S-PIV), flame stabilization mechanisms and flame shape transitions were observed for three distinct flame shapes, namely lifted bowl-, lifted W-, and attached crown-shapes. The three flame shapes were stabilized through the dynamic balance between flame speed and flow velocity in the non-swirling, central flow region. Bowl-flames were generally shrouded by the inner shear layer, while the W- and crown-shaped flames involved flame segments reaching the outer shear layer of the flow, and were stabilized by shed eddies in the former and were hardware-stabilized in the latter. In the effective operating window of the burner, the flame could transition from one shape to another based on the equivalence ratio, H$_{\mathrm{2}}$ fraction, and bulk flow velocity. The transition from W- to bowl-flames also led to an unusual internal blow-out zone within the global flame stability limits, where the existence of this zone was related to the laminar-flame extinction strain rate.