The ignition delay and subsequent downward flame spread over a finite-length PMMA slab with the effect of radiation included in a two-dimensional wind tunnel are investigated using the opposed flow velocity as a parameter. The gas and solid phase temperatures, preheat length and heat flux are considered in order to examine the flame ignition and spread characteristics. The numerical results reveal that the ignition delay time increases with the opposed flow velocity. However, the flame spread rate varies with the opposed flow velocity in a non-monotonic manner that can be identified as two distinct regimes with a peak value in between. The flame spread rate reaches a maximum at V_g = 32cm/s, and then falls, regardless of whether the flow velocity is increasing or decreasing. For V_g < 32cm/s, the flame behaviors are dominated by oxygen transport. For V_g > 32cm/s, the flame stretch effect controls the flame behavior. Additionally, this work demonstrates that the effect of radiation delays the flame ignition and reduces both the flame strength and the corresponding spread rate. The effects of the opposed flow temperature and thickness of the solid fuel are also investigated. The predicted results indicate that a higher opposed flow temperature or a thinner solid fuel facilitates the ignition and accelerates flame spread. The effect of opposed flow velocity eventually overcomes that of opposed flow temperature as the flow speed is further increased.
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