Numerical experiments on reaction front propagation in n-heptane/air mixture with temperature gradient

Usually different autoignition modes can be generated by a hot spot in which ignition occurs earlier than that in the surrounding mixture. However, for large hydrocarbon fuels with negative temperature coefficient (NTC) behavior, ignition happens earlier at lower temperature than that at higher temperature when the temperature is within the NTC regime. Consequently, a cool spot may also result in different autoignition modes. In this study, the modes of reaction front propagation caused by temperature gradient in a one dimensional planar configuration are investigated numerically for n-heptane/air mixture at initial temperature within and below the NTC regime. For the first time, different supersonic autoignition modes caused by a cool spot with positive temperature gradient are identified. It is found that the initial temperature gradient has strong impact on autoignition modes. With the increase of the positive temperature gradient of the cool spot, supersonic autoignitive deflagration, detonation, shock-detonation, and shock-deflagration are sequentially observed. It is found that shock compression of the mixture between the deflagration wave and leading shock wave produces an additional ignition kernel, which determines the autoignition modes. Furthermore, the cool spot is compared with the hot spot with temperature below the NTC regime. Similar autoignition modes are observed for the hot and cool spots. Different autoignition modes in the considered simplified configuration are summarized in terms of the normalized temperature gradient and acoustic-to-excitation time scale ratio. It is shown that the transition between different autoignition modes is not greatly affected by the NTC behavior. Therefore, our 1-D simulation indicates that like hot spot, the cool spot may also generate knock in engines when fuels with NTC behavior is used and the temperature is within the NTC regime.