Effects of Globally Stratified Temperature Distributions and NTC Characteristics on End-gas Combustion Modes

Abstract

ABSTRACT This study numerically investigates the effects of stratified temperature distributions on the end-gas combustion mode in a constant volume reactor. The initial temperature in the reactor was globally stratified with linear gradients. The effects of negative temperature coefficient (NTC) characteristics are addressed through a comparison between the results of a non-NTC fuel and an NTC fuel. The compressible Navier–Stokes equations are solved with detailed chemistry. The results show that the addition of large temperature gradients such as −10 or −5 K/cm can prevent the pressure wave development associated with end-gas autoignition, and the knocking intensity is significantly reduced. In contrast, small temperature gradients such as −1 K/cm lead to the generation of the developing detonation mode and thus to large knocking intensities. In the NTC fuel case, the knocking intensity becomes relatively large because the spatial gradient of the ignition delay time decreases even with the addition of large temperature gradients, especially in the NTC regime. Based on all the results, a correlation is presented between the initial temperature and the reaction front speed (the spatial gradient of the ignition delay time) for the prediction of end-gas combustion mode. The correlation suggests that the possibility of preventing large knocking intensities by temperature gradients is reduced under high-pressure conditions because of the increase in the reaction front speed. Consequently, this study suggests that very large temperature gradients would be required under high-pressure conditions to prevent large knocking intensities under the concept of thermal stratification.