Recent microgravity experiments have demonstrated the existence of steady,
source-free spherical premixed e flames ("flame balls") in near-limit premixed
gas a combustion of mixtures have low Lewis number (Le) of the
stoichiometrically limiting reactant. While theoretical works proved successful
in predicting most of the important properties of flame balls, one remarkable
but unproven prediction is that of flame n balls which oscillate in radius. One
possible reason that oscillating flame balls have not yet been observed 0
experimentally is that they are predicted to occur e primarily for mixtures with
Le close to but less than unity, whereas most prior flame ball experiments have
n been conducted using mixtures with Le - n (corresponding to hydrogen-air) or
lower. Previous experiments with higher Le mixtures such as CH4-02-C02
(Le ~ 0.7) did not exhibit any flame ball
behavior.
With this motivation, microgravity experiments were conducted in the KC-135
microgravity aircraft facility using H2-02-He mixtures.
These mixtures were chosen because they provide higher Le than H2-air mixtures,
but still less than unity. Initial tests did show evidence of flame ball
behavior, but the size of the balls was too large to reach steady-state in the
27-liter combustion chamber and the response time was too long for the 20-
second test duration available on the KC-135 aircraft. Consequently, a 90% He -
10% C02 Mixture was used in all subsequent tests. The C02
greatly reduced the flame ball size because in this case the ambient mixture
contains a radiating gas whereas without C02 the only radiator in the
system is the product water vapor that must diffuse from the flame ball surface
toward the ambient mixture. The greatly reduced size also greatly F reduced the
response time of the flame balls, and because the C02 concentration
is relatively small Le was not reduced substantially. Because pressure is an
important parameter affecting the size and structure of flame balls, experiments
conducted at pressures ranging f from 2.2 to 44.1 PSI.
The microgravity experiments showed that flame balls in the H2-O2-He-CO2 mixtures were much more dynamic than those of H2-air or other mixture families. Flame ball radii and the radiative emissions were recorded. Nevertheless, the g-jitter on the KC-135 aircraft did not allow a conclusive determination as to whether oscillating flame balls existed. Numerical computations were conducted for the experimental test conditions employing a time-dependent numerical code (RUN-1DL) with detailed chemical, transport and radiative emission-absorption models. These results were then compared with those of the numerical simulations. Fair agreement between the experiments and simulation results was obtained. A considerable effect of reabsorption of emitted radiation on the size and net radiative emission was noted.
Abid, M., Aung, K., VanZandt, D., Frate, D., Ronney, P.D., Effects of Lewis Number on Flame Ball Dynamics, Conference and Exhibit on ISS Utilization, AIAA, Reston, VA, October 15, 2001.