The influences of laminar transport and chemical kinetics on the time-mean reaction rate in a turbulent flame

Abstract

The ESCIMO theory of turbulent combustion is applied to a one-step exothermic chemical reaction between pre-mixed fuel and air in a steady-flow well-stirred reactor. The performance is computed as a function of the three independent dimensionless ratios which can be formed from the four characteristic times, namely those of: residence, molecular diffusion, chemical kinetics and eddy stretching. Major attention is given to the burning process in a single “fold”, composed initially of a layer of fresh gas and a layer of burned gas. Two main regimes are observed. that of internal flame propagation, and that of self-ignition. The first regime, which is often found in practice, occurs when the chemical-kinetic time is much shorter than the time of diffusion, and the eddy-stretching time is not too large. Then the burn-out time of the fold is almost proportional to the stretching time; and the diffusion, and chemical times play minor roles. The second regime occurs when the reaction time is of the same order as the diffusion or stretching time. Then the fold becomes uniform in composition well before significant reaction has proceeded; the burn-out time is now dependent only on the chemical-kinetic time. Attention is also given, in the paper, to: • the characteristics of the population of “folds”, existing in the stirred reactor; • the corresponding implications for reactor stability, and for the interpretation of experimental datal • certain features of the numerical-computation process which affect its economy; and • the possibility of extending the calculations to mixtures exhibiting complex chemistry.