Research progress on ignition and flame propagation in highly turbulent flows

The ignition and flame propagation of premixed mixture are commonly recognized as the key processes for reliable engine operations.Spark ignition,the most common ignition mode in engine combustion,effectively converts the electrical energy into a small heat deposit with the formation of radicals.The ignition process is constituted of two main phases,that are the ignition kernel formation and the kernel propagation.The forced ignition phenomena ob-served are highly transient in nature with various factors influencing the development of spark kernels from the moment of spark deposit until complete flame establishment.The most favora-ble ignition spots for sparking in engine combustors are influenced not just by the local equiva-lence ratio,but also by the time history of the velocity fluctuations,which would transport the flame kernel away from the ignition spot.To date,some fundamental physico-chemical processes of the ignition process are still unclear in highly turbulent flows which are commonly found under engine operating conditions.The problems urgent to be studied include the turbulence promo-tion/suppression mechanism for the ignition process,the effect of turbulence on flame propaga-tion and the scaling law for turbulent flame speed over a wide range of pressures and turbulent in-tensity,the effect of heat release and flame instability on turbulent fluctuating velocity (e.g., the flame generated turbulence)and flame propagation,the transition from a laminar flame to a turbulent flame resulting from propagation acceleration due to flame instability,and the effect of combustion process on the transport of reactive scalars.In this paper,recent theoretical,experi-mental and numerical studies of the ignition and flame propagation in turbulent flows are re-viewed together with discussions on the spark ignition in turbulent nonpremixed flames and the challenges for high-fidelity simulations of the ignition process.The needs of experimental techniques and turbulent combustion modelling for the study of the ignition process in highly tur-bulent flows especially under engine operating condition are also discussed.