Transient Flame Growth and Spread Processes over Thin Solids in Concurrent Low-Speed Flows in Microgravity - a Comparison between Large and Small Sample Sizes

摘要

A previously developed three-dimensional transient numerical model was used to simulate concurrent flame growth and spread processes over thin composite fabrics (25% fiberglass and 75% cotton) in a microgravity environment. The simulations considered three sample sizes: 2×10cm, 5×30cm, and 41×94cm. A low-speed flow at 20cm/s was used. The samples were ignited from the upstream end and were allowed to bum for a sufficient period of time. The simulation results showed that, for all sample sizes, the flame base reached a quasi-steady spreading state with a limiting flame length. For the small samples, the pyrolysis length increased asymptotically to the limiting length. For the large sample (41×94cm), the pyrolysis length overshot before reaching the limiting length. This overshooting phenomenon was also reported in the SAFFIRE project, an ongoing NASA-led international effort where a large-scale thin sample was burned under a similar configuration and flow condition. In this work, detailed profiles of the gas and solid phases, including flow patterns, species concentrations, temperature, heat release rate, solid burning rate, and solid density, were examined. Comparisons between different sample sizes and the effect of sample size on flame growth and spread processes were discussed.