Effect of nonunity Lewis number and finite-rate chemistry on the dynamics of a hydrogen-air jet diffusion flame

Abstract

In jet diffusion flames, buoyancy-influenced torroidal vortices roll outside the flame surface when the annulus air flow is low. The temporal and spatial evolution of these vortices changes the stretch along the flame surface, which results in a wrinkled laminar flame. A time-dependent, axisymmetric mathematical model having a detailed chemical-kinetics mechanism is used to simulate the wrinkled flame surface of a low-speed H 2-air diffusion flame. The effects of Lewis number and finite-rate chemistry on the steady-state and dynamic flame structures are examined. Results obtained with different models indicate that the size and shape of the outer structures are unaffected by the unity-Lewis-number and fast-chemistry assumptions. Experiments showed that the flame temperature tends to increase when the flame is bulging and decrease when it is squeezing. The lower Lewis number inside the flame—not the change in the Damkohler number—was found to be responsible for the observed fluctuations in the flame temperature. Preferential mass diffusion of different species causes an increase in water in the bulging regions of the flame and an increase in radicals in the squeezing regions.