Observations of long duration microgravity spherical diffusion flames aboard the International Space Station

Spherical diffusion flames of gaseous fuels supported on a porous spherical burner are investigated experimentally in microgravity for long durations aboard the International Space Station (ISS). Experimental results are compared with a transient numerical model. The experiments involve normal spherical diffusion flames (i.e., fuel flowing into oxidizer) burning ethylene in an oxygen/nitrogen mixture at atmospheric pressure. The stoichiometric mixture fraction and adiabatic flame temperature are varied by diluting the fuel with nitrogen and varying the oxygen concentration in the ambient. The diagnostics consist of color video, thin-filament pyrometry for gas temperature, thermocouple measurements of burner temperature, and photomultiplier tube measurements of broad and narrow-band flame emission. Burn times range from 28 – 336 s. All flames grow monotonically after ignition. Peak gas temperature generally decreases with time. Radiative extinction is observed when, at constant reactant flow rate, the flame expands until radiation heat loss approaches the chemical heat release. Before a flame experiences complete extinction, it may enter an oscillatory mode in which part of the flame partially extinguishes and then reforms. The oscillations grow in magnitude until complete extinction. Sufficiently small flames with low radiation losses exhibit quasi-steady behavior, but burner heating is significant, and the experimental duration is dictated by the temperature limit of the porous sphere.