Transported PDF modelling with detailed chemistry of pre- and auto-ignition in CH_4/air mixtures

摘要

The pre- and auto-ignition behavior of methane under varying levels of preheat in a turbulent flow field has been studied through the combination of detailed chemistry with a transported PDF approach closed at the joint-scalar level. The study considers the Cabra Burner configuration, which consists of a central methane/air jet issuing into a vitiated co-flow. The aim of the work is to explore the detailed thermochem-ical flow structure and to substantially reduce uncertainties associated with the chemical kinetics. The applied chemistry features 44 solved species and 256 reactions and includes low temperature oxygen adducts. The mechanism has, in related work, been shown to reproduce the spontaneous temperature limit for methane and ethane along with ignition delays times at higher temperatures. Radiation is accounted for through the RADCAL method and the inclusion of enthalpy into the joint-scalar PDF. Molecular mixing is closed using the modified Curl's model and a set of time-scale ratios (C_φ = 2.3, 2.5, 3.0 and 4.0) have been used to explore the model sensitivity. The impact on predictions of variations in the pilot stream composition have been explored by varying concentrations of OH and H_2 over a wide range. A detailed analysis of the flame structure, focusing on the chemical processes occurring before and during the ignition, suggests that the burner conditions lead to a classical auto-ignition pattern with the early formation of HO_2 and CH_2O prior to ignition. The work suggests that, under the current conditions, flame stabilization is dominated by turbulence-chemistry interactions rather than by specific modes of flame propagation. The work shows a significant sensitivity to the pilot stream composition and that residual H_2 acts as an ignition promoter. However, the sensitivity to the time-scale ratio C_φ is shown to be less than can be expected from studies of flame extinction using the same methodology.