Experimental validation of a correlation capturing the boundary layer effect on spread rate in the kinetic regime of opposed-flow flame spread

Abstract

In opposed-flow flame spread over thermally thin solid fuels, an increase in flow velocity eventually leads to flame extinguishment. While the chemical time is independent of the flow velocity, the residence time of the oxidizer at the flame leading edge is inversely proportional to the flow velocity. A competition between the two leads to a situation where finite-rate kinetics dominates the flame spread behavior, leading to blow-off extinguishment. The ratio of the two competing, the Damkohler number, captures this finite-rate effect and has been used to correlate the non-dimensional spread rate with opposing flow velocity and ambient oxygen level. Although qualitatively successful, these correlations show considerable spread in the data despite the use of several variations of the definition of the Damkohler number. One possible reason could be the neglect of the developing boundary layer in front of the flame by the experimentalists. In this work we present new flame spread data over ashless filter paper acquired in an eight meter tall vertical steel chamber, in which a sample is moved at desired velocities to create a relative opposing flow. The developing boundary layer over the fuel sample is shown to have a significant effect on the measured spread rate and flame shape that does not correlate with opposing flow velocity if an Oseen flow approximation is used. Once the data is adjusted for the developing boundary layer using an effective flow velocity formula obtained from scaling arguments and refined through computational results, the correlation improves drastically, establishing the importance of the boundary layer in the kinetic regime. Much of the thermally thin fuel spread rate data available in literature does not mention the development length and must be cautiously used.