Laminar flame speeds of methane/air mixtures at engine conditions: Performance of different kinetic models and power-law correlations

Abstract

The laminar flame speed is an important input in turbulent premixed combustion modelling of spark ignition engines. At engine-relevant temperatures and pressures, its measurement is challenging or not possible and thereby it is usually obtained from simulations based on chemical models or power-law correlations. This work aims to investigate the performance of different models and power-law correlations in terms of predicting laminar flame speeds of methane/air at engine conditions. The propagation of spherically expanding laminar flames in a closed chamber was simulated and laminar flame speeds were computed over a broad range of pressures (1-120 atm) and temperatures (300-1100 K) for methane/air mixtures based on seven kinetic models. It was found that at engine conditions, there are notable discrepancies among the predictions. GRI Mech. 3.0 and USC Mech. II respectively predict the largest and smallest values at high pressure conditions. This was explained by the difference in CH3 oxidation and recombination according to reaction pathway analysis. Additionally, laminar flame speeds of methane flames were experimentally determined under engine-relevant conditions. It was shown that the recently developed Foundational Fuel Chemistry Model Version 1.0 model predicts closely the data at high pressures and temperatures. Therefore, it was chosen as the reference model for the comparisons. Thirteen published power-law correlations for laminar flame speeds of CH4/air were implemented, and their performance in predicting the laminar flame speeds at engine conditions was investigated. Most of these correlations have been derived for a narrow range of temperatures and pressures, which are lower than those encountered in engines. A new power-law correlation was derived based on predictions by the Foundational Fuel Chemistry Model Version 1.0. This new correlation is expected to provide reliable predictions at engine conditions for a stoichiometric methane/air mixture and thereby it is recommended to be used in modeling turbulent premixed combustion in spark-ignition engine simulations.