A critical review and comparison of the initiation of combustion processes by conventional electric spark or thermal means with laser sources is presented. A description of the fundamentals of ignition processes is used as the basis for interpretation of experimental and theoretical studies of laser ignition. It is shown that many features of laser ignition can be understood on the basis of simple thermal concepts, particularly when the effects of thermal or radical losses are considered. It is proposed that the main advantages of laser sources are likely to be in the timing and placement of ignition rather than the inherent energy requirements. Potential applications to combustion systems of practical importance, e.g., high-speed propulsion systems, are discussed and instructive laboratory-scale experiments are suggested.

The role of chemical kinetics in the behavior of near-limit flames at rnicrogravity (μg) was investigated experimentally by comparison of the properties of these flames in mixtures having varying chemistry but similar diffusive transport properties. Conversely, the importance of unequal rates of diffusion of thermal energy and mass was evaluated by comparison of flames in mixtures having similar chemistry but varying Lewis numbers, It was found that the characteristics of near-limit flames at μg were affected much more by Lewis number effects than chemistry. It is concluded that there are at least three regimes of near-limit behavior at μg, depending upon the Lewis number (Le): (1) cellular flames, which are highly resistant to extinguishment, for mixtures with Le≪l, (2) self-extinguishing flames and well-defined flammability limits for mixtures with Le≃ 1, and (3) ignition energy limited flame propagation for Le ≪ 1, Gradual rather than abrupt transitions between regimes were observed.