A Numerical Study of Flame Spread and Blowoff over a Thermally-Thin Solid Fuel in an Opposed Air Flow

Abstract

Flame spread and blowoff in an opposed air stream over a thermally-thin solid fuel is studied theoretically. The model includes the quasi-steady, two dimensional Navier-Slokes/ momentum, energy and species equations with one-step overall chemical reaction and second-order, finite-rate Arrhenius kinetics in gas phase. In a reference frame attached to the flame front, the flame spread rate v¯ _f) becomes an eigenvalue for this problem. The solid phase equations become steady, consisting of an energy balance coupled with the heat flux from the gas phase and a mass balance including Arrhenius pyrolysis kinetics. The parametric study is based on a variable Damkohler number (Da) which is a function of opposed flow velocity (u∞ ). The spread rate v¯ _fand the flame size are reduced and the flame becomes weaker as Da is decreased or u∞ is increased. A blowoff limit is reached when Da is lowered to a critical value. Heat conduction in the solid fuel contributes to higher VF and is the dominant process near the blowoff limit. The flame structures for both far and near-limit flames are presented graphically. Ahead of the flame front, a flow recirculation zone is found for every case of compulalion. The structural analysis shows that the flame has both premixed- and diffusion-flame characteristics. Finally, the comparison between the two cases shows the effect of flame stretch by increasing the opposed flow velocity.