Quantitative infrared image analysis of simultaneous upstream and downstream microgravity flame spread over thermally thin cellulose fuel in low speed forced flow

Abstract

Quantitative image analysis of infrared (IR) measurements are compared to visible color images for simultaneous upstream and downstream flame spread over thermally thin cellulose samples in low speed forced flow in microgravity. These results are a unique set of data that provide a wealth of information to guide future modeling efforts. Test conditions at ambient pressure were conducted in 21–50% O2 by volume (N2 balance) and forced flow velocities from 2 cm/s to 20 cm/s. The tests conditions encompass strictly upstream flame spread to simultaneous upstream and downstream flame spread. Steady upstream flame spread is observed while the downstream flame only begins to spread at the highest flow velocities due to the oxygen shadow cast by the upstream flame (oxygen depleted region downstream of the flame). Blackbody surface temperature gradients for the upstream flame are an order of magnitude larger than the downstream gradients. Upstream preheat lengths decrease with flow while downstream preheat lengths increase. Downstream pyrolysis lengths reach steady state for most cases and increase with convective heating while upstream pyrolysis lengths increase with time. Surface thermocouple (TC) histories were non-dimensionalized to obtain a characteristic surface temperature profile using new scaling analyses. Applying the ellipticity of the leading edge to the scaling reveals the local flow velocity at the leading edge is on the order of diffusive velocities and the flame thermal expansion acts as a significant barrier to divert the flow. A surface energy balance reveals that the peak heat flux for both upstream and downstream flames increases with oxygen concentration and forced flow velocity. The upstream flame heat flux is equally divided between fuel heat up and surface radiation. The downstream heat flux goes almost exclusively to surface radiation. Comparisons of these results to previous applicable research are provided throughout the text and generally agree well.