Effects of heat release and gravity on an unsteady diffusion flame

Abstract

In this paper, we present time-dependent axisymmetric numerical simulations of an unsteady diffusion flame formed between a H2−N2 jet and a coflowing air stream. The computations show the evolution of two types of instabilities commonly observed in experiments. Higher-frequency instabilities form in the mixing region between the high and low velocity fluids. Low-frequency instabilities form outside the flame zone in the low-velocity region. Our computations include the effects of convection, molecular diffusion, thermal conduction, viscosity, gravitational forces, and chemical reactions with energy release. We discuss the results from a series of computations to show the effects of reaction and gravitational forces. Results for several different Reynolds and Grashoff numbers are discussed. Our computations show that Kelvin-Helmholtz instabilities develop in the region between the high-velocity and low-velocity fluid when there are no chemical reactions. When reaction is included, these instabilities are dempened resulting in a mixing region which is almost steady. The effect of a temperature-dependent viscosity is insignificant so we conclude that the flow field is stabilized by the volumetric expansion from the heat release. Gravitational effects are insignificant in the nonreacting jet because the flow field is isothermal. When the flow of the reacting jet is upward, low-frequency instabilities form in the co-flow region. When the gravitational field is reduced by a half, however, the instabilities are very weak and are not clearly evident. When the flow of the reacting jet is downward, there are no low-frequency instabilities in the flow field.