Evaluation of burning rate in microgravity based on the fuel regression, flame area, and spread rate

The fuel burning rate and heat-release rate (HRR) play key roles in determining the fire intensity and hazard. On Earth, the burning rate of a condensed fuel is normally measured by the mass loss, but in microgravity, the impossibility of measuring the weight loss with a balance makes the measurement of burning rate challenging. This work proposes three methods to quantify the burning rate of condensed fuels in microgravity by measuring (i) the regression rate of the fuel surface, (ii) the spread rate of the flame leading edge, and (iii) the flame-sheet area, which all rely on video imaging of the flame or fuel surface geometry. The accuracies of these methods are quantified first in the ground-based tests with representative fuels, 1) solid candle and PMMA rods with diameters from 8 to 15 mm, 2) liquid fuels including propanol, hexane, and kerosene, and 3) the methane and propane gases. Results show that the burning rate obtained optically by tracking the flame leading edge and the fuel regression were less accurate due to strong sensitivity to camera resolution and background light. Comparatively, measuring the flame-sheet area is easier and gives more accurate results, and microgravity PMMA-rod flame (BASS-II project in the International Space Station) show that the fuel mass flux across the flame sheet is almost constant (0.15 mg/cm²-s) for a given fuel configuration and environment. This work offers a useful way to measure fuel burning rate and HRR in spacecraft and provides a path for the performance-based spacecraft fire safety design.