Direct numerical simulation of low temperature reactions affecting n-dodecane spray autoignition

Spray autoignition are key issues for diesel engine. In this study, different levels of direct numerical simulations (DNS) including zero dimensional (0D) and two dimensional (2D) configurations have been conducted to identify the role of cool flames in the process of spray autoignition. Two quite different types of ignition processes are identified: (1) When initial air temperature is relatively low, the entire vaporization and mixing region could be influenced by the low temperature reactions. The first-stage ignition first occurs near stoichiometry and then cool flames propagate to richer regions. With elevated temperature and chemical reactivity in the domain with cool flame occurrence, the second-stage ignition is triggered and eventually leads to hot flames. (2) When initial air temperature is relatively high, two-stage ignition only occurs in the rich region with cool flames surrounding individual droplet, while a hot flame is triggered directly in the lean region and further propagates into the rich core region. Analysis of cross-correlation coefficient between chi and HR and representative species are conducted, which indicate that the first-stage ignition is negatively correlated with chi. Length scale of second stage ignition kernels is found much larger than that of first-stage ignition. This is because more uniform field is obtained when second-stage ignition occurs, as large concentration and temperature stratification are greatly reduced under the influences of turbulent mixing and cool flame propagation. Budget analysis is further conducted to analyze the local flame structure and identify all the combustion modes of reaction fronts during spray autoignition.