Downward Flame Spread In an Opposed Forced Flow

Abstract

Rates of flame spread for the downward propagation of flames over vertical cylinders of polymethylmethacrylate (PMMA) of 2.54, 0.64, and 0.16 cm in diameter were measured in an opposed forced convective air flow with free stream velocities ranging from 2 to 70 cm/sec. It was found that for all fuel thicknesses, the flame spread rate is independent of the opposed flow if the velocity of the flow is smaller than that induced by natural convection by the flame, For larger flow velocities, the spread rate decreases as the velocity of the opposed flow increases. It was also found that profiles of surface temperature ahead of the flame present similar characteristics with the profiles remaining unchanged for low opposed flow velocities and becoming steeper with the temperature gradients near the flame front increasing as the opposed flow velocity increases. A simplified theoretical model is developed to account for the effect of an external opposed flow on the flame spread process. It is shown in the model that the principal effect of the opposed gas flow is to decrease the thickness of the thermal boundary layer which in its turn decreases the residence time, resulting in an inverse power functional dependence between the spread rate and the magnitude of the external flow velocity.