Flame-Shock Wave Interaction And Its Effects On End-Gas Autoignition And Detonation Development In A Confined Space

Turbocharged and downsized spark ignition engines with gasoline direct injection play a significant role in energy conservation and emission reduction.However,owing to the increasing heat load in downsized engines,knocking combustion（including conventional knock and super-knock）is likely to occur,which causes high pressure oscillation,deteriorates the engine performance,and damages the engine components.Knocking combustion restricts the further improvement of thermal efficiency in downsized engines.It is generally accepted that conventional knock is caused by the end-gas autoignition,while super-knock is the result of pre-ignition and accompanied by detonation.The formation of autoignition and detonation is strongly related to the flame-pressure/shock wave interaction.Thus,the researches on flame-pressure/shock wave interaction and its effect on end-gas autoignition and detonation development are the key to revealing the formation mechanism of knocking combustion.Because it is a small-scale transient phenomenon,end-gas autoignition during knocking combustion is difficult to measure even with various experimental techniques.Based on the“flame acceleration through obstructed channel”theory,an optical constant-volume combustion bomb（CVCB-TJU）is developed to investigate the flame-shock wave interaction and its effect on autoignition and detonation.The premixed flame propagation dynamic and flame-shock wave interaction in a confined space are first explored.Then,formation of end-gas autoignition and detonation development induced by flame-shock wave interaction,and the stochastic characteristics of detonation are discussed.Finally the influence of flame propagation velocity,inert gases and fuel characteristics on end-gas autoignition and detonation is investigated.This study provides new insights into the knocking combustion and deflagration to detonation transition（DDT）phenomenon in engines and industries,and fundamentals of turbulent combustion.Firstly,the flame propagation,shock wave formation and development,as well as flame-shock wave interaction in a confined combustion chamber are investigated based on CVCB-TJU.A perforated plate is mounted in the combustion chamber to promote the flame acceleration and shock wave formation.According to the flame morphology and flame tip velocity,the evolution of flame propagation is divided into three stages:the laminar flame stage before the perforated plate,jet flame stage in the immediate proximity of the perforated plate downstream,and turbulent flame stage in the remaining region.In the laminar flame stage,caused by Darrieus–Landau instability and thermal-diffusion instability,a cellular structure appears on the flame surface,consequently causing slight flame acceleration.After passing through the perforated plate,the flame is split into several jet flames with a sharp acceleration by an order of magnitude.The sharp flame acceleration is caused by various factors.On the one hand,the delayed burning before the flame passes through the perforated plate produce a powerful jet flow and subsequently lead to an extremely high flame propagation velocity.On the other hand,the Rayleigh-Taylor and Kelvin-Helmoltz instabilities with shear effect are triggered when the flame suddenly passes through an obstacle or a vent,which promotes the flame surface to increase and wrinkle with faster flame burning.Finally,the jet flames coalesce with each other,forming a wrinkled turbulent flame.Compressed by the fast turbulent flame,a pressure wave or shock wave is formed ahead of the flame front,which is reflected at the wall and oscillates in the combustion chamber.Impacted by the flame-shock wave interactions,periodic oscillation of the flame tip velocity is observed.It is found that the intensity of the shock wave is positively related to the flame tip velocity.Then,the formation of end-gas autoignition and detonation development are explored,which are induced by the flame-shock wave interaction.The shock wave oscillates in the combustion chamber and compresses the unburned mixture repeatedly.Once the shock waves are sufficiently strong to induce locally a strong fresh gas temperature increase,the end-gas autoignition occurs.Then,the coupling between the autoignition and shock wave results in the formation of detonation,leading to extremely high pressure.As time passes,the detonation gradually decays and becomes a shock wave in the burnt region.According to the beginning time of the autoignition,two kinds of shock wave trigging autoignition modes are identified.Under mode 1,the autoignition occurs after two shock wave reflections occur at the end wall,while under mode 2,the end-gas experiences one compression by the shock wave before autoignition.Increasing the oxygen concentration promotes the transition from mode 1to mode 2.Moreover,under mode 2,the in-cylinder pressure and pressure oscillation are bigger than those under mode 1,because there exists more unburnt mixture when autoignition occurs under mode 2.And it is found that the autoignition occurrence probability and location are stochastic even though great efforts are devoted to ensuring equal experimental conditions.With increasing oxygen concentration,the occurrence probability of autoignition increases.The reasons are as follows.On the one hand,the flame propagation velocity increases with increasing oxygen concentration,subsequently inducing a shock wave of higher intensity.On the other hand,increasing the oxygen concentration improves the mixture reactivity.After the oxygen concentration attains a critical value,autoignition with detonation is inevitable.Thus,by regulating the oxygen concentration,controllable autoignition with detonation is achieved,which lays the foundation for further studies.Finally,the influences of flame velocity,inert gases and fuel characteristics on the end-gas autoignition and detonation is investigated.By decreasing the porosity or increasing the number of perforated plate,the flame propagation velocity increases.With an increase in the flame propagation velocity,the combustion modes transmit from normal combustion to end-gas autoignition without detonation,then to end-wall detonation,and finally,to side-wall detonation.Under the current experimental scenario,increasing the flame propagation velocity promotes the formation of autoignition and detonation because it induces shock waves of higher intensity.Then,the effect of inert gases is explored using Ar,N₂ and CO₂,which may provide guidance for the inhibition of knocking combustion.When compared with Ar and N₂,CO₂ requires the highest flame propagation velocity to initiate the autoignition,and the resulting pressure oscillation is the lowest.To be closer to practical engines,gasoline is employed as test fuel.It is found that the combustion modes and transition mechanism of gasoline are consistent with hydrogen.However,unlike hydrogen,gasoline needs relatively lower velocity without shock wave to trigger the autoignition and detonation.