A Numerical Study of the Characteristics of Autoignited Laminar Lifted Methane/Hydrogen Jet Flames in a Diluted Coflow Air

Abstract

The numerical study on the autoignited laminar lifted methane/hydrogen jet flame in a diluted coflow air is conducted. For the numerical simulations, laminarSMOKE is adopted as a solver with SanDiego mechanism, which is a detailed mechanism of 57-species for methane oxidation. In the previous experimental and numerical studies, the unusual decreasing liftoff height, HL, was observed as the fuel jet velocity, U0, increases. It results from the differential diffusion effect between binary fuel species. Extending the previous studies, a series of numerical simulations are performed by varying the mole fraction of oxygen in the coflow air, XO2 , to investigate the characteristics and stabilization mechanism of the flame in practical MILD combustion mode. It is found that the decreasing tendency of HL with increasing U0 becomes much significant with the decreasing XO2 , and eventually, the difference of HL for various XO2 becomes marginal at the relatively-high U0. The hydrogen species can be easily diffused to the flamebase regardless of flow residence time due to its high diffusive nature. On the other hand, the amount of methane species transported by diffusion to the flamebase depends on U0. In the high U0 regime, a limited amount of methane species can be reached to the flamebase due to the short diffusion time scale. Consequently, in the high U0 regime, the local hydrogen ratio, RH, at flamebase becomes higher, such that the decreasing ignition delay, τig, by RH overcomes the increasing τig by flame intensity. The species transport budget analysis verifies that the lifted flame is stabilized by autoignition-assisted flame propagation for undiluted air, and it transits to autoignition as XO2 decreases. Also, the flame propagation has slightly greater effect on flame stabilization in the high U0 regime due to relatively-high RH. Chemical explosive mode analysis (CEMA) identifies that the RH has greater effect on the flame stabilization for relatively-high U0.