Effect of pressure wave disturbance on auto-ignition mode transition and knocking intensity under enclosed conditions

Pressure wave propagation behavior is an essential feature for the combustion under enclosed conditions, e.g. internal combustion engines. Previous work by Pan et al. (2016) and Yu et al. (2015) showed that pressure wave disturbance not only affects hot-spot formation and knocking origin, but also induces detonation wave through a coupling mechanism between pressure wave and flame front. On this basis, this study further investigates the role of pressure wave disturbance in auto-ignition mode and knocking intensity by means of detailed numerical simulations with stoichiometric H2/air mixture. Firstly, the pressure waves with different levels in strength have been obtained by adjusting ignition temperature of hot ignition kernel. It shows that as ignition temperature is raised at each initial temperature, pressure wave strength is decreased monotonously, with declining compression ratio and temperature rise caused by pressure wave disturbance. Secondly, three auto-ignition modes have been observed with the variations of pressure wave strength, i.e. detonation, mixed mode and supersonic deflagration. As the weakness of pressure wave strength, there is an auto-ignition mode transition from detonation to supersonic deflagration, accompanied by rapid decreases in pressure peak, obvious pre-flame partial reaction and significant increases in auto-ignition reaction front speed. These observations are still maintained at elevated initial pressure conditions. Finally, such auto-ignition modes and knocking intensity for the detailed computations are summarized in the non-dimensional Bradley's diagram. The results show that both auto-ignition mode and initial thermodynamic state can affect knocking intensity, and the modifications in knocking intensity by pressure wave disturbance are mainly through auto-ignition mode transition. This is qualitatively consistent with the distribution of combustion regimes in Bradley's diagram, even though some deviations do exist because the diagram is constructed on basis of initially non-reactive flows.