Transition of heat transfer mechanism of concurrent flame spread over discrete fuels

Abstract

Scenarios of flame spread over discrete fuel elements have been widely simulated by fuel arrays. While investigation on flame spread characteristics of discrete fuel arrays has been performed, the role of convective heat transfer is not yet addressed well. By designing 90 arrangements of fuel arrays, the effects of fuel spacing (denoted by S), inclination angle (θ), and column number (n) on convective heat transfer are studied. The distribution factor φsinθ is introduced to characterize the influences of porosity (φ) and θ. It is found that the Nusselt number (Nu) increases along with increasing φsinθ. Moreover, when φsinθ>0.511, the inflection points of the experimental Nu occur, corresponding to the transition of the heat transfer mechanism. By introducing the Richardson number (Ri), the transition is identified at Ricrit≈1, where the convection pattern is categorized into natural convection (Ri≥1) and forced convection (Ri<1). In addition, Nu correlations under Ri≥1 and Ri<1 are determined by combining the ratio of length and width of the burning zone, porosity, and inclination angle. Finally, based on the newly developed Nu correlations, the prediction models of mass loss rate are proposed, which match the experimental results well.