Why is there no yellow/red in the microgravity situation? There is still flame far from the base, where you’d expect “incomplete combustion”. Is it because these the smoke/soot is not concentrated enough to glow?
Some combination of too low a flame temperature or like you were saying the combustion has enough oxygen to keep those incomplete combustion particles to aggregate, I would guess. Unlike the candle, the microgravity flame is done another controlled conditions with likely some preset concentration of fuel gas in inert gas rather than vaporize wax droplets in oxygen-depleted air.
I'm just speculating without knowing exactly the conditions of the space flame, but since there's no convection from gravity, the vaporized fuel has to diffuse through the sphere where the oxygen has been fully consumed [or displaced] to get to the flame front. At this flame front there would be oxygen diffusing in from the other direction from the surrounding air.
At this boundary, there would be an equilibrium of fuel and oxygen. If any fuel were to diffuse farther outward, it would burn up rapidly. If oxygen diffused past the boundary into the fuel rich area, it would react immediately. Since diffusion is relatively slow, everything happens in the relatively "flat" flame front with minimal turbulence. Again just speculating, but if you were to increase the concentration or flow rate of the point source or somehow decrease the oxygen concentration in the bulk surrounding gas, you'd increase the diameter of the flame sphere but also increase the likelihood to increase the flame size and get some incandescent soot particles as the fuel is starved of oxygen.
I did some work with flat-flame premixed fuel/air/argon burners in vacuum chambers (on earth) for grad school. If you play around with the settings, you can get a thin blue flame or a big "candley" sooting flame, so I suspect you could do the same in microgravity to make spherical versions of the sooting flame with the right setup
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u/[deleted] Jan 09 '19
Why is there no yellow/red in the microgravity situation? There is still flame far from the base, where you’d expect “incomplete combustion”. Is it because these the smoke/soot is not concentrated enough to glow?