Flame propagation through an air-fuel spray mixture with transient droplet vaporization

Abstract

Flame initiation and propagation through an air/fuel vapor/fuel drop system is numerically modeled in a cylindrical one-dimensional closed combustor. An unsteady formulation of the flow problem eliminates the cold-boundary difficulty and gas-phase ignition problem. A velocity lag between the gas and the liquid phase is allowed and unsteady heat transfer to the droplets is taken into account. The surface temperature of the droplet is evaluated by using an unsteady spherically symmetric formulation of the droplet heat conduction problem with no internal motion and with a time-varying heat flux specified at the surface as a boundary condition. Results have been obtained for two commercially important fuels, namely, n-octane and n-decane. The activation energy and the preexponential factor in the Arrhenius-type expression for chemical rate, along with initial temperature, initial droplet size, stoichiometric ratio, and diffusivity are parametrically varied and flame speed and flame temperatures are observed. Flame speed is seen to increase with increasing preexponential factor, decreasing activation energy, increasing ambient temperature, decreasing initial droplet radii, and increasing diffusivities. It is also observed that unlike premixed combustion, heterogeneous combustion gives rise to local variations of equivalence in the axial direction. This phenomenon could give rise to a secondary diffusion flame in the wake of a propagating flame and produce local variations in the flame temperature.