Prediction of ignition modes of NTC-fuel/air mixtures with temperature and concentration fluctuations

Abstract

The ignition regime criteria proposed by Im et al. (2015) are extended to account for temperature and concentration fluctuations. The newly-developed criteria are applied to negative temperature coefficient (NTC) and non-NTC fuels. The statistical volume-averaged Sankaran number, S¯a, and the volumetric fraction of Sa < 1, FSa,S, are proposed as new metrics to determine a priori the combustion modes, combustion intensity, and ignition delay times, with FSa,S being used to quantify the heat release fraction from spontaneous ignition. It is demonstrated that FSa,S agrees acceptably with the fraction of heat release from spontaneous ignition obtained by the Damköhler-based analysis from direct numerical simulation (DNS) data. The ignition modes can also be predicted by S¯a regardless of the NTC and non-NTC characteristics of fuels over a wide range of initial mean temperatures and different fluctuation levels. Besides, the magnitude of S¯a can be used to estimate FSa,S due to its strong correlation with FSa,S. Additionally, the predicted Sa by the theory, Sap, is compared with the statistical mean S¯a showing a consistent agreement and they are found to correlate with the combustion intensity that is characterized by the maximum heat release rate. Finally, the ignition delay time can be correlated with Sap for single-stage fuels, and for NTC fuels if the initial mean temperature lies outside the NTC regime.