Mechanisms of strong pressure wave generation in end-gas autoignition during knocking combustion

Abstract

A knocking combustion modeled using a one-dimensional constant volume reactor is simulated in a manner of direct numerical simulations, in which large detailed chemical kinetic mechanisms for two premixed gases, n-butane (113 species) and n-heptane (373 species), are directly and efficiently introduced. Detailed mechanisms of strong pressure wave generation during end-gas autoignition are clarified. Comparison of n-butane and n-heptane shows that the presence of the negative temperature coefficient (NTC) region significantly influences not only the timing of knocking occurrence but also the amplitude of pressure oscillations. In the case of n-heptane with the condition of an adiabatic wall, there is one large peak produced in the strength of knocking intensity for initial temperature between 450 and 1000K, whereas there is no peak produced in the case of n-butane. The peak generated around 650K is attributed to a pressure wave intensified through propagation in the end-gas, which is locally generated near the wall with the influence of the NTC region. It is also found that there is a transition of the autoignition position in the end-gas region from the wall for higher initial temperatures to the region ahead of the flame front for lower initial temperatures, leading to different mechanisms of knocking intensity generation. In the case of the isothermal wall condition, the peak around 650K is reduced due to the lack of a local temperature increase at the wall, demonstrating the influence of wall temperature conditions on the strength of knocking intensity.