A NUMERICAL STUDY OF FLAME SPREAD AND BLOWOFF OVER A THERMALLY-THIN SOLID FUEL IN AN OPPOSED AIR-FLOW

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-Stokes’ 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 (VF) 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 (Du) which is a function of opposed flow velocity (uœ). The spread rate Vf and 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 compulation. 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. © 1990, Taylor & Francis Group, LLC. All rights reserved.