Demonstrating steady burning for small flat materials in microgravity in a quiescent ambient

Abstract

The likelihood of steady burning in microgravity is examined with Burning Rate Emulation (BRE) using a gas burner with a flat 25 mm diameter porous surface with two embedded heat flux sensors. The data are greatly expanded from a previous work. The fuel mixture used in this analysis are ethylene and ethylene diluted with nitrogen resulting in a range of heats of combustion from 12 to 47.2 kJ/g. An analytical solution of these flames was utilized to compute their steadiness, temperatures at flame sheet, and sizes. 103 flames were ignited in an ambient with nominal oxygen mole fractions of 0.21, 0.265, 0.34 and 0.40. 49 of those flames burned for the entire duration of the test before the fuel supply was terminated, while the remainder self-extinguished. The flames attained at least 90% of their asymptotic heights for all 49 fuel terminated flames. An estimated critical steady flame temperature ranging from 1100 to 1200 K was observed segregating the sustained and self-extinguished flames. Flame temperatures for the 49 terminated flames were above the critical temperature range indicating sustained steady burning, whereas below they self-extinguished. The measured flame sizes, and radiative fractions are reported at the endpoint of the terminated tests. An empirical correlation of the radiative fraction based on radiation theory with Ω, a dimensionless parameter, was used in prediction for real fuels at steady burning in microgravity. A flammability diagram, as a plot of the emulated fuel heat of gasification with its burning mass flux, for the range of steady data, is presented comparing how real fuels might burn steadily in normal and microgravity conditions. The results suggest that fuel mass burning rates would be about 3 to 4-times higher in 1-g. The heats of gasification and combustion of some real fuels are presented and compared against the burner flames.