Rotating counterflow twin premixed flame (RCTF) of hydrogen air was numerically simulated with detailed chemistry to explore the possibility of ultra-lean combustion. As a result, it was found that ultra-lean RCTF of equivalence ratio U = 0.052, which is far leaner than the generally-recognized flammability limit U = 0.10, is realized. It was also found that under ultra-lean conditions the flame temperature of RCTF largely exceeds the adiabatic flame temperature; e.g., at O = 0.06 the former is 1171 K, while the latter is 503 K. This increase of burned gas temperature is attributed to the so-called low Lewis number effect within the flammability limit, but under an ultra-lean condition some other mechanism to increase temperature is dominant. The 'pseudo local equivalence ratio' of burned gas of RCTF differs largely from that of the unburned gas due to the extraordinarily high concentration of H 2O. This suggests the possibility that the local condition at the reaction zone is much richer than the unburned gas, which brings about the large temperature increase.
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