An experimental study on flame geometry and radiation flux of line-source fire over inclined surface

Abstract

An experimental study was performed on line-source fire over an inclined surface (ground) to simulate downhill fire spread behavior. The flame geometry and the thermal radiation to both far-field surroundings and near-field inclined surface were investigated. As a basic configuration for wildland fire over a slope, the buoyancy induced natural convection flow along the inclined surface and the constraint of air entrainment by the inclined surface change the flame geometry as well as its radiation emission. Various surface (ground) inclination angles (from 0°-80°), fire source heat release rates and fuels were considered comprehensively with a total of 126 test conditions. Results showed that the flame perpendicular height decreased, while both the flame parallel length and base drag length along the inclined surface increased, with the increased inclination angle. A dimensional analysis was then performed based on the controlling mechanisms, with the dimensionless heat release rate, the density ratio of fuel vapor to air, along with sinα and cosα involved to represent the components in the parallel and perpendicular directions. The flame geometry parameters were well represented by the proposed dimensional analysis. Both the radiation fluxes to far-field surroundings and to near-field inclined surface decreased with the increased inclination angle. The far-field radiation was found to be well characterized by a model based on the soot volume fraction analysis according to single point source model. Concerning the near-field radiation to inclined surface, an inclined cuboid radiative modeling was developed. The predicting results by the proposed model and the experimental values showed good agreement. The present study has explained the controlling physics and proposed non-dimensional functions for flame geometry and modeling the downslope radiation of the line-source fire over the inclined surface, which facilitates the understanding of the wildland fire spread behavior over a slopping ground in the downhill direction.