Stabilization of laminar inverted ultra-lean hydrogen-methane-air flames

Blowoff characteristics of inverted flames propagating in ultra-lean hydrogen-metane-air mixtures and stabilised at the end of a 2 mm diameter rod, have been studied experimentally for different hydrogen contents in the fuel gas blend. It was found that flames with hydrogen content in the fuel gas of 60% and more can be stabilsed at equivalence ratios well below the theoretical lean flammability limit for the corrsponding planar flames. Anomalous blowoff behavior was observed for flames in mixtures with a hydrogen content of 40% and more in the fuel gas. The blowoff of these flames occurred at a decreased flow velocity, after some critical value of the velocity was achieved. Below this value, the flame could not be stabilised. The stabilisation rod temperature, near its trailing edge, was measured with an infrared pyrometer. Low values of the measured rod temperature suggest that the flames studied were stabilised by flame stretch effects, rather than due to the heat losses to the stabilisation rod. The experimentally observed anomalous blowoff behaviour in mixtures with high hydrogen content in the fuel gas is attributed to strong combined flame stretch/preferential diffusion effects.