Hollow cylindrical geometries represent critical and commonly encountered geometry in spacecraft systems appearing in tubes, conduits, wire housing, and structural elements. Understanding flame spread over hollow cylindrical fuels is highly relevant for the application of space fire safety. This work presents experimental study on opposed flow flame spread over thin hollow cylindrical cellulosic fuel of diameters varying from 10 to 49 mm in microgravity environment. To understand the effect of flow on flame spread, experiments are conducted in low convective opposed flow conditions ranging from 10 to 30 cm/s for different hollow cylindrical fuel diameters at oxygen concentration of 21% and 1 atm pressure. In the microgravity environment, the flame length and the flame spread rate are seen to increase with increase in hollow cylindrical fuel diameter over the flow range studied here. The flame spread rate exhibited a non-monotonic trend with flow speed, for flow of large diameter whereas a monotonic increasing trend is noted for small diameters. A simplified analysis is carried out to arrive at an expression for flame spread rate over thin hollow cylindrical fuels. The analysis shows that the radiation exchange from the hot char to the inner surface of hollow virgin fuel (solid to solid radiation) and overall equivalence ratio dictates flame spread rate trend with fuel diameter. This study addresses the existing knowledge gap on flame propagation along hollow cylindrical surfaces, where strong flame interaction and oxygen starvation are competing each other inside cylinder. This advances the fundamental understanding of flame spread in complex geometries. These insights are particularly valuable for identifying atypical fire behaviors in space environments.

In the present study, opposed flow flame spread over several fuel configurations of thin cellulosic fuels are investigated experimentally in normal gravity and microgravity environments. The fuel is configured in different shapes, namely, planar, hollow cylindrical (circular duct), C channel, and L channel, with the help of specifically designed fuel sample holders. The flame spread phenomena are examined for each configuration in both normal gravity and microgravity environments under ambient conditions of 21% oxygen and 1 atm. pressure. The microgravity experiments are conducted using a 2.5 s drop tower facility. The flame spread rates are measured at various opposed flow speeds. The effective flow speed accounts for the induced reference buoyant flow speed and externally imposed flow. The flame spread rates for each configuration are plotted against the effective flow speed ranging from 10 cm/s to 40 cm/s. While there is a nonmonotonic increasing-decreasing flame spread rate trend with respect to the effective opposed flow speed for all configurations, the flame spread rate can vary significantly with changes in the configuration. The C-channel configuration shows the highest flame spread rate compared with the other configurations of the same scale and identical experimental conditions. The effect of fuel size on the flame spread rate is also investigated for the duct configuration. The flame spread rate is noted to increase with the increase in fuel diameter.

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.