Direct numerical simulations of methane-air turbulent premixed flames propagating in two-dimensional homogeneous isotropic turbulence are conducted to investigate the effects of turbulence length scale on the local flame structure and NO formation mechanism in turbulence. Detailed kinetic mechanism including 49 reactive species and 279 elementary reactions is used to simulate CH4-O2-N2 reaction in turbulence. DNS are conducted for the case of turbulence integral length scale of about 1, 2.5 and 5 times of the laminar flame thickness, and the results of DNS are compared with those of the hydrogen-air turbulent premixed flames with same turbulence condition. In the methane-air turbulent premixed flame, turbulent burning velocities (ST/SL) are less than those of the hydrogen-air turbulent premixed flames. In the case of methane-air turbulent premixed flame with l/δL=1 and u'rms/SL=20, some of species shows complex and thickened distribution, while the others show smooth and thinner distribution like heat release rate. These trends can be categorized by the ratio of lifetime of chemical species to turbulent characteristic time scale. This flame structure may correspond to the flame structure in well-stirred reactor regime. In the case of hydrogen-air premixed flame, most probable local heat release rate does not depend on l/δL, while, in the case of methane-air flame, most probable local heat release rate increases with the decrease of l/δL. In this study, Prompt NO formation mechanism in turbulent premixed flame is also investigated. Stretched thin flame elements show smaller NO production rate, because NO production reactions: R214: HNO+H ⇔ H2+NO, R190: NH+O ⇔ NO+H are suppressed and NO decomposition reactions: R246: CH+NO ⇔ HCN+O, R249: CH2+NO ⇔ H+HNCO are enhanced in the thinner flame elements.
A strut is expected as a key technology of stable combustion for SCRAM jet engine. We took up a two-staged combustion strut having an air-intake at the front end, a preburner inside and an injection nozzle at the rear end of the strut. Intake air from the main air stream with high total temperature and high total pressure is effective for preburning hydrogen. The preburned hydrogen is injected into the recirculation zone generated at the rear end of the strut, showing a good flame-holding performance. We could obtain the relationship between the air-intake flow rate, the equivalence ratio and the combustion performance with a unique numerical simulation.