Study of Mass Fraction Oscillation Effects on the Laminar Diffusion Flame Structure

In this work، the unsteady response of a laminar diffusion flame to harmonic mass fraction oscillations is investigated. Flame-sheet assumption is utilized to model the laminar unsteady two-dimensional co-flow diffusion flame. The flow is assumed to be subsonic، inviscid، and uniform. The convection-diffusion equation for conserved scalar with appropriate boundary conditions is solved. Considering the stoichiometric mass fraction surface to be the flame surface، it is possible to obtain the flame zone. Assuming that unburnt species cannot pass across the flame surface and that the diffusion coefficient is constant، heat release rate can be related to the flame area. To best of our knowledge، this is the first time this approach has been applied to a diffusion flame to calculate heat release rate. Flame response function is acquired as oscillations of heat release rate to fluctuations of fuel mass fraction. At each Peclet number، frequency domain is divided into three regions، namely diffusion-dominated region، convection-diffusion region، and convection-dominated region. Our results indicate that the magnitude of response function decreases as excitation frequency increases، while phase difference approaches a constant value. Also، as Peclet number increases، the amplitude of oscillations in heat release rate increases in diffusion-dominated and convection-diffusion regions، but it does not change significantly in convection-dominated region.