Effect of sample thickness on concurrent steady spread behavior of floor-and ceiling flames

摘要

This paper presents an experimental investigation of the sample thickness effect on the concurrent flamesteady spread behavior. Two configurations, ceiling flame and floor flame, as the fundamental diffusioncombustion problems and representative scenarios of horizontal flame spread, are concerned herein.PMMA samples with thickness ( δ) of 2, 3, 4, 5, 7, 9 mm were employed including both thermally-thinand thermally-thick conditions. A wind tunnel was used to provide the uniform concurrent airflow rangingfrom 0.3 m/s to 1.2 m/s in 0.3 m/s intervals. Flame characteristics and basic parameters to quantifythe flame steady spread process have been studied, including flame spread rate (FSR), equivalent pyrolysislength as well as heat flux feedback in both pyrolysis and preheating zones. Results show that FSRdecreases with sample thickness, being more sensitive to sample thickness for ceiling flame configurationthan that for floor flame configuration. Equivalent pyrolysis length increases with sample thicknessand ceiling flame’s equivalent pyrolysis length is larger than floor flame’s. Convection feedback is dominantunder concurrent airflow regardless of the flame configuration, airflow velocity or sample thickness.In pyrolysis zone, the total heat flux to fuel surface of ceiling flame configuration is larger than thatof floor flame configuration for thin samples due to buoyancy, but it decreases rapidly as sample thicknessincreases. In preheating zone, the heat flux decreases in power law for ceiling flame configurationwhile a piece-wise fitting is found for floor flame configuration, which is interpreted based on their inherentflame characteristic difference hence the thermal boundary layer conditions between these twoconfigurations. To analyze the sample thickness effect on concurrent flame spread, a characteristic solidtemperature (T*) is proposed by theoretical analysis, as an essential parameter to be used in the concurrentflame spread model to represent the coupling effect of heat input in gas phase and in-depth heatloss into the solid phase (or namely thermal inertia controlled by a characteristic thermal penetrationdepth). The calculated non-dimensional characteristic solid temperature ( θ) shows to be less sensitiveto flame configuration and concurrent airflow velocity for relatively thicker samples because the thermalpenetration depth will tend to approach a limit. A general trend of its variation with sample thickness ( δ)can be employed to predict the heat loss into solid phase and its predictive capability is verified by theexperimental results. This work advances the fundamental understanding of the sample thickness effecton concurrent flame spread mechanism.