Observations of Methane and Ethylene Diffusion Flames Stabilized Around a Blowing Porous Sphere Under Microgravity Conditions

This paper presents the experimental and theoretical results for expanding methane and ethylene diffusion flames in microgravity. A small porous sphere made from a low-density and low-heat-capacity insulating material was used to uniformly supply fuel at a constant rate to the expanding diffusion flame. A theoretical model which includes soot and gas radiation is formulated but only the problem pertaining to the transient expansion of the flame is solved by assuming constant pressure infinitely fast one-step ideal gas reaction and unity Lewis number. This is a first step toward quantifying the effect of soot and gas radiation on these flames. The theoretically calculated expansion rate is in good agreement with the experimental results. Both experimental and theoretical results show that as the flame radius increases, the flame expansion process becomes diffusion controlled and the flame radius grows as gamma t. Theoretical calculations also show that for a constant fuel mass injection rate a quasi-steady state is developed in the region surrounded by the flame and the mass flow rate at any location inside this region equals the mass injection rate