Premixed flame ignition: Theoretical development

Premixed flame ignition is a fundamental issue in combustion. A basic understanding of this phenomenon is crucial for fire safety control and for the development of advanced combustion engines. Significant efforts have been devoted to understanding the mechanisms of ignition and determining critical ignition conditions, such as critical flame radius, minimum ignition energy, and minimum ignition power, which have remained challenging research topics for centuries. This review provides an in-depth investigation of the forced-ignition of laminar premixed flames in a quiescent flammable mixture, with emphasis on theoretical developments, particularly those based on activation energy analysis. First, the fundamental concepts are overviewed, including spark ignition, characteristic time scales, and critical ignition conditions. Then, the chronological development of premixed flame ignition theories is discussed, including homogeneous explosion, thermal ignition theory, flame ball theory, quasi-steady ignition theory, and, more importantly, transient ignition theory. Premixed flame ignition consists of three stages: flame kernel formation, flame kernel expansion, and transition to a selfsustaining flame. These stages are profoundly affected by the coupling of positive stretch with preferential diffusion, characterized by the Lewis number. Specifically, positive stretch makes the expanding ignition kernel weaker at larger Lewis numbers, consequently increasing the critical ignition radius and MIE. The premixed flame ignition process is dominated by flame propagation dynamics. Both quasi-steady and transient ignition theories demonstrate that the critical flame radius for premixed ignition differs from either flame thickness (by thermal ignition theory) or flame ball radius (by flame ball theory). Particularly, the transient ignition theory appropriately acknowledges the "memory effect" of external heating, offering the most accurate description of the evolution of the ignition kernel and the most sensible evaluation of minimum ignition energy. In addition, the effects of transport and chain-branching reactions of radicals, finite droplet vaporization, and repetitive heating pulses on premixed flame ignition are discussed. Finally, a summary of major advances is provided, along with comments on the applications of premixed flame ignition theory in ignition enhancement. Suggested directions for future research are presented.