Effects of low-temperature chemistry on direct detonation initiation by a hot spot under engine conditions

This study investigates the effect of low-temperature chemistry (LTC) on the minimum runup distance, lm, of direct detonation initiation by an autoignitive hot spot under engine conditions. A newly predictive model is proposed to determine an a priori lm . The temperature and pressure increase and the radical build-up prior to the main ignition resulting from LTC are incorporated in the model that allows a better prediction of ignition modes. A series of 1D simulations are performed by varying the initial temperature, the hot spot size and its temperature difference at constant-volume and engine conditions. DME (ethanol) is used to represent a two (single) stage ignition fuel. It was found that taking the transient evolution of the mixture state as an initial condition to the ZND model results in a better representative exothermic characteristic length scale, which exhibits a strong linear correlation with lm regardless of fuel types over a wide range of conditions. The LTC oxidation is found to reduce lm by approximately a factor of two for the low-temperature cases, leading to a comparable value of lm to that of the high-temperature cases. It was also found that the ignition modes are better predicted by identifying an alternative characteristic length scale based on the variation of ignition delay time Tig within its range of monotonic distribution. For a hot spot that has temperature variation spanning across the NTC regime, the runup distance was found to be shortened by approximately a factor of two, such that a longer hot-spot size is required to form detonation. In addition, a better prediction of ignition modes was achieved by defining the normalized front speed as a statistical mean for each runup-distance element than that determined at the midpoint of a hot spot.