The fuel inclination and wind velocity play a significant role on the forward flame spread behavior. It deserves further study since the heat transfer mechanism and flame spread characteristics coupled these two factors are not clear yet. In this paper, a thermally thin slab of PMMA that could be inclined from a horizontal (0°) to a vertical (90°) angle is used to investigate flame spread behavior under the condition of the concurrent ambient airflow. A wind tunnel is utilized to provide a uniform concurrent airflow, ranging from 0 (quiescent) to 3 m/s with an interval of 0.5 m/s. Essential flame characteristic parameters are collected to quantify the flame spread process, including flame spread rate (FSR), burning rate, heat release rate as well as heat flux feedback both in the pyrolysis and preheating zones. A mechanism, including the competition between the acceleration of buoyancy brought by inclination and the cooling effect of ambient airflow for relatively high wind velocity, is developed. The relationship between flame length and pyrolysis length is investigated. Moreover, the evolution of both heat release rate per unit width and standoff distance as a function of pyrolysis length are analyzed. A dimensionless heat release rate for upward flame at different wind velocities is used to scale the dimensionless flame length with a power-law exponent of 0.77 and −1.92. In addition, the dimensionless heat flux in preheated zone decay with distance as a function of power law. A predictive formulation of FSR coupled with inclination angle and wind velocity is proposed. This study facilitates the understanding of the interaction of fuel inclination angle and horizontal ambient airflow from aspect of heat and mass transfer.
