In opposed-flow flame spread over solid fuels, an indefinite 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, and, therefore, 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 times (residence time to chemical time), known as 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 these correlations explain the behavior observed in the experiments there is considerable spread in the correlations found in literature despite the use of several variations of the definition of the Damkohler number. With all the progress made in this area, it is still not possible to predict the blow-off extinction velocity for a given fuel at any given oxidizer condition. In this work we present new flame spread data over ashless filter paper acquired in a eight meter tall vertical steel chamber in which the sample is moved at a command velocity to create a desired opposing flow. The developing boundary layer over the fuel sample and the relative humidity in the chamber are shown to have a significant effect on the measured spread rate and it does not correlate at all with opposing flow. Once the data is adjusted for humidity, and an effective flow velocity that incorporates the developing boundary layer is substituted for opposing velocity, the correlation is shown to improve drastically. Given the importance of the boundary layer development, much of the data in literature that do not mention the development length must be cautiously used.
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