CONDITIONAL MOMENT CLOSURE MODELLING FOR HCCI FEATURING COMPRESSION HEATING AND EXPANSION COOLING

Abstract

This paper presents a conditional moment closure (CMC) model for ignition of a lean ethanol/air mixture under homogeneous charge compression ignition (HCCI) conditions. A set of direct numerical simulations (DNSs) presented by Bhagatwala et al. (2014) is used to evaluate the performance of the CMC model. The DNS data includes five cases with a mean temperature of 924 K and three different levels of thermal stratification. The effect of compression heating and expansion cooling is considered in the first three cases with T ′ = 15, 25 and 40 K. For this purpose, an inert mass source term is added to the governing equations. However, the other two cases with T ′ = 15 and 40 K do not consider compression heating and expansion cooling. The results show a better agreement between the CMC and DNS for cases in which compression heating and expansion cooling is considered. Further investigation of the DNS data shows that the contribution of the diffusion term in the CMC equations, representing the importance of deflagration mode, is only significant for the case which has the largest level of thermal stratification and does not involve compression heating and expansion cooling.