A candle flame in zero gravity (as on the space station) is indeed quite different than a candle flame down here on Earth. In zero gravity the flame is spherical, blue, and centered on the candle wick. On Earth a candle flame is teardrop-shaped, yellow, and mostly well above the wick. As you suspected, the lack of gravity means that there is no convection, which accounts for the very different shape. But what explains the different color?
In the familiar candle flames we see on Earth, the bright yellow-white color is primarily due to incandescent carbon particles. In other words, the rising particles of soot are so hot that they give off this light. The candle on the space station has no such incandescent soot. But why is this? Does the flame produce no soot, or is the flame cooler than on Earth? In fact, both of these factors are true.
According to Dr. Forman Williams, the principal scientific investigator for the Flame Extinguishment Experiment on the space station: “In microgravity hot gases don’t rise, so an entirely different process, called molecular diffusion, drives flame behavior.” The resulting flame is not only spherical, it is also cooler than on Earth. Without the airflow caused by convection, there is much less mixing of fuel vapors and air — which greatly reduces the rate of burning. In zero gravity, these two types of gases come into contact through slow diffusion instead of rapid convection. “Cool flames burn at the relatively low temperature of 500K to 800K,” says Williams. (That translates to a range of 440 to 980 degrees Fahrenheit.) “Normal flames produce soot, CO2, and water. Cool flames produce carbon monoxide and formaldehyde.” So there it is — a cool flame without soot! And therefore a candle flame on the space station produces no incandescence, resulting in a pale blue flame instead of a bright yellow-white flame.
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