Turbulent Flame Speed Dependencies in Lean Methane-Air Mixtures under Engine Relevant Conditions

Direct numerical simulations are carried out to study the behavior of turbulent flames propagating in lean methane-air mixtures of equivalence ratio 0.5 and for non-dimensional turbulence intensities (urms/SL) of 2-25. The turbulent Reynolds number (ReT) varies from 44-549, the Damköhler (Da) number from 0.26-3.2, and the Karlovitz number (Ka) from 1.1-49.4. The mixture pressure is 20 bar and temperature is 810 K to simulate approximate conditions in lean-burn natural gas engines. A 13-species reduced mechanism and a global mechanism are employed in the study. It is shown that the normalized turbulent flame speeds (ST/SL) can be related to the flame area enhancement (AT/AL) resulting from turbulence interactions with the laminar flame and the efficiency factor (Io) which is close to unity (∼ 1.07 ± 0.04) when evaluated at the temperature of peak heat release rate. This area enhancement increases with increasing ReT and decreasing Ka. The effect of integral length scale on flame area enhancement is related to the effect induced by the spectrum of eddies of the turbulent cascade. A correlation is proposed for the turbulent flame speed. While the global mechanism is adequate for predicting flame speed and area enhancement, some differences in the structure of the flame are observed between predictions with the global and reduced mechanisms.