Forced forward smoldering experiments in microgravity

Results from two forward forced-flow smolder tests on polyurethane foam using air as oxidizer conducted aboard the NASA Space Shuttle (STS-105 and STS-108 missions) are presented in this work. The two tests provide the only presently available forward smolder data in microgravity. A complimentary series of ground-based tests were also conducted to determine, by comparison with the microgravity data, the effect of gravity on the forward smolder propagation. The objective of the study is to provide a better understanding of the controlling mechanisms of smolder for the purpose of control and prevention, both in normal- and microgravity. The data consists of temperature histories from thermocouples placed at various axial locations along the fuel sample centerline, and of permeability histories obtained from ultrasonic transducer pairs also located at various axial positions in the fuel sample. A comparison of the tests conducted in normal- and microgravity indicates that smolder propagation velocities are higher in microgravity than in normal gravity, and that there is a greater tendency for a transition to flame in microgravity than in normal gravity. This is due primarily to the reduced heat losses in the microgravity environment, leading to increased char oxidation. This observation is confirmed through a simplified one-dimensional model of the forward smolder propagation. This finding has important implications from the point of view of fire safety in a space-based environment, since smolder can often occur in the forward mode and potentially lead to a smolder-initiated fire. (C) 2004 Elsevier Inc. All rights reserved.