Direct Modeling of Auto-Ignition and Flame Propagation of N-heptane-Air Mixtures at HCCI Conditions by Using Dynamic Multi-Timescale Method

Abstract

The ignition, flame propagation, and transition to detonation of n-heptane-air mixtures in a one-dimensional, cylindrical chamber are numerically modeled at homogeneously charged compression ignition (HCCI) conditions by using a multi-time scale (MTS) method with a comprehensively reduced kinetic mechanism. It is found that depending on the initial temperature and temperature gradient, there exist many new combustion regimes. At low temperatures, it is shown that there is a coupled low temperature flame (LTF) and high temperature flame (HTF) propagation regime. At intermediate temperatures, the results demonstrated that there are six different combustion regimes, an initial single flame front propagation regime, a coupled LTF and HTF double flame regime, a decoupled LTF and HTF double flame regime, a low temperature ignition regime, a single HTF regime, and a hot ignition regime. At high temperatures, only HTF and hot ignition are observed. Furthermore, it is found that the existence negative temperature coefficient (NTC) region dramatically changes the critical temperature for flame acoustic coupling. The rapid increase of the magnitude of critical temperature in the NTC region enhances the occurrence of supersonic ignition regime and suppresses detonation transition. The results show that the low temperature flame chemistry affects dramatically the flame regimes, flame transitions to ignition and detonation, and the temporal histories of pressure and heat releases.