The Effect of Boundary Layer on Blow-Off Extinction in Opposed-Flow Flame Spread over Thin Cellulose: Experiments and a Simplified Analysis

Study of flame spread plays a crucial role in our understanding of fires with direct implications to fire safety. Flame-spread in an opposing flow configuration and relative blow-off extinction in the kinetic regime have been studied for the last few decades. It is known that the extinction velocity is related to the Damköhler number, the ratio of residence time to the combustion time at the flame leading edge. It is also well known that the behavior of the flame is affected by the presence of the boundary layer. However, there is no experimental evidence in literature to quantify the boundary layer effect. In this work we experimentally establish the effect of boundary layer development length on the extinction velocity, opposed flow velocity of the oxidizer at which the blow-off extinction occurs, for flame spread over thin fuels. Using a vertical combustion tunnel, a large number of downward flame spread experiments over thin ashless filter paper are conducted for an opposed flow velocity range of 40 cm/s to 100 cm/s. The extinction length, the distance from the sample leading edge at which the blow-off extinction occurs, is shown to be directly related to the opposing flow velocity. A correlation between the two based on scaling analysis and on an empirical law reveals that blow-off extinction occurs at a constant effective velocity. This simple conclusion can have implications in future refinement of Damköhler number correlations for blow-off extinction.