摘要:
可燃预混气点火过程研究是发动机燃烧领域最重要的课题之一。当前电火花强制点火广泛应用于各类发动机燃烧室中,其点火过程具有很强的瞬变性,从电火花产生到火焰完全形成的整个过程中,多种复杂因素联合影响点火火核的产生和发展。目前发动机高压、强湍流工况下的点火和火焰传播机理认识还不完善,亟需研究的科学问题是湍流和化学反应相互作用对点火和火焰传播的影响机制及其建模,包括湍流对点火的促进/抑制机制,湍流对火焰传播和火焰整体发展的影响规律,燃烧释热和火焰面不稳定性对湍流脉动速度(即火焰产生的湍流)的影响机制和对火焰传播速度的增强机制及由此导致的层流燃烧自加速转捩为湍流燃烧的理论,燃烧过程对标量通量输运(即反向或压力驱动输运)的影响机制。本文对强湍流下点火及火焰传播理论、实验和数值模拟方面的研究进展进行综述。
Abstract:
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.
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