Investigations On Shock-wave Induced Detonation

As the energy problems and environmental issues become increasingly prominent,the stricter limits on fuel consumption and emissions are carried out over the world.Turbocharge is commonly adopted in gasoline engine for the aim of energy conservation and emission reduction.But a phenomenon called detonation prone to happen in supercharged engine at conditions of low speed and high load.Detonation can damage the engine instantly because there is detonation wave which propagates in the combustion chamber.This paper presents a two-dimensional numerical study of the circular combustion chamber.The formation process of shock from hot spot with temperature gradient and the propagation process of the wall reflection shock wave in combustion chamber are simulated.According to the picture photographed by high speed camera in a rapid compression machine,two modes of shock wave induced detonation are investigated by simulation: shock wave reflection induced detonation and flame acceleration induced detonation.The process that shock wave reflection induced detonation can be divided into four parts: 1)the ignition process of hot spot located at the end of combustion chamber;2)the formation process of shock wave;3)the formation process of high-temperature region near the wall produced by the reflection of shock wave;4)the initiation process of detonation in the high-temperature region.The numerical simulation demonstrates that Mach reflection occurs near the wall during the propagation of the shock wave in the combustion chamber.The temperature of the region between the slip plane and the wall reaches 1200 K,which provides conditions for detonation.Comparative study is performed with or without chemical exothermic reactions in the combustion chamber.It is found that shock reflection play a leading role,although both shock reflection near wall and chemical exothermic reactions can increase the temperature and pressure in the reflection region.Different phenomena could be simulated by changing the temperature of unburned mixture in the combustion chamber.The results showed that the decrease of unburned mixture temperature in the combustion chamber will retard the detonation.In the real engines,as unburned mixture only maintains high temperature and pressure in a short time,low temperature of unburned mixture will greatly increase the timerequired for initiation of detonation.Thus,reducing the temperature of unburned mixture in the combustion chamber is an effective way to inhibit detonation.The process of shock wave intersection induced detonation is simulated.And the results show that the shock wave spreads faster in the burned region than that in the unburned region.As the two shock waves with different speed intersect in the region of flame,the flame front accelerated by the increasing pressure eventually catch up with the shock wave,then detonation happens.