Understanding the effects of inclination angle and fuel bed width on concurrent flame spread over discrete fuel arrays

Abstract

Concurrent flame spread over discrete fuel arrays is a dangerous flame configuration where flame spread has the same direction with induced airflow. However, inclined upward flame spread over discrete fuels is rarely concerned, especially for the varied fuel bed width. In this study, birch rods were mounted in the regular array spaced 9 mm apart. 36 groups of arrays by varying six inclination angles (θ, 15°–90° with an interval of 15°) and six kinds of column numbers (n, 1–11 with an interval of 2) were designed to study the combined effect of inclination angle and array fuel bed width on flame spread over discrete fuel arrays. Average flame spread rate was found to be independent on the fuel bed width for n>1, and mass loss rate per total mass is restricted by air entrainment when θ⩾75° or θ=60° and n>5. As the inclination angle increases, the heat transfer mechanism changed from radiant dominance to convective dominance. Moreover, in the cases of radiant dominance, average flame spread rate was proportional to the squared incident heat flux. By simplifying burning zone, a prediction model of mass loss rate was developed. Based on the aspect ratio, burning zone was further divided into line (λ⩾3) and rectangular (λ<3) fire sources, and then dimensionless correlations between the flame length and the heat release rate were proposed both for λ⩾3 and λ<3. In addition, during the upward flame spread process, flame length was found to be controlled by the competition between the accelerated upward buoyancy flow and the restriction of lateral air entrainment. This study provides a comprehensive insight for flame spread behavior of discrete fuels.