Opposed-flow spreading flames: Effect of sub-atmospheric pressure on spread and burning rates

Flame spread over solid fuels is a canonical problem in fire science, due to its direct implications on material flammability and importance in fire development. Flames in a microgravity environment can behave very differently than on Earth, posing additional risks for spaceflight life safety. Due to the difficulties associated with microgravity testing, sub-atmospheric pressures in ground-based experiments have been proposed to approximately replicate the burning behavior of solid fuels in reduced gravity conditions because of similar effects on heat and mass transfer mechanisms from the flame to the solid. However, the roles played by gravity and pressure vary with the flame spread configuration. In opposed flame spread, the solid fuel is heated by the flame ahead of its leading edge, and this process is strongly affected by the ambient conditions. In this work, we consider flames spreading over acrylic samples exposed to a forced flow of 20 cm/s, and pressures between 30 and 100 kPa. Flame characteristics such as spread rate, standoff distance, and length are obtained from the video analysis of the experiments and compared at different pressures. Mass burning rates are calculated from the samples weight measured before and after the experiments. Additionally, gas emissions measured during the experiments are used to estimate the heat release rate of the spreading flames. Results show a decreasing non-monotonic behavior of flame length, spread rate, and mass burning rate with reducing pressure. The comparison of the heat release rate obtained from the measured emissions and the estimated mass burning rate, suggests that chemical kinetics is not driving the decrease in flame spread rate observed at low pressures. These results could provide more information to guide future Earth-based flammability testing of materials for spacecraft applications. This research was supported by NASA Grant NNX12AN67A.