Flame spread over thin circular ducts

In this work opposed flow flame spread over thin cellulosic circular ducts is investigated in normal gravity and microgravity environments for the first time. The experiments are conducted under different opposed flow speeds on circular ducts of diameter 10 mm, 19 mm and 38 mm and for comparison corresponding planar fuels of width of 10 mm, 20 mm and 40 mm are chosen respectively. All the microgravity tests are conducted using a 2.5 s drop tower facility. Over the matrix of test in the present study, the flame spread rates for circular ducts are higher compared to the planar fuel of corresponding widths. Unlike planar fuels where the effect of fuel width is small, the flame spread rate significantly increases with the increase in duct size in both normal gravity and micro- gravity. The flame spread rate over ducts exhibits a non-monotonic trend with flow speed which can also be seen in planar fuels. However, the variation with flow is much more significant especially at large diameters. The duct configuration also shows significant change in flame shape and size between normal gravity and micro- gravity environments. An analytical model is developed to predict flame spread rate over planar as well as circular ducts. The model successfully captures the flame spread rate trends with flow speed as well as fuel diameter.