Microgravity: A Teacher's Guide with Activities, Secondary Level, Activity 7, p. 35-37.
Microgravity Science and Applications Division: Office of Space Science and Applications
Education Division: Office of Human Resources and Education
National Aeronautics and Space Administration
Publication EP-280, July 1992
To illustrate the effects of gravity on the burning rate of candles
A candle flame is often used to illustrate the complicated physio-chemical processes of combustion. The flame surface itself represents the location where fuel vapor and oxygen mix at high temperature and with the release of heat. Heat from the flame melts the wax (typically at C20 to C35 hydrocarbon) at the base of the exposed wick. The liquid wax rises by capillary action up the wick, bringing it into closer proximity to the hot flame. This close proximity causes the liquid wax to vaporize. The wax vapors then migrate toward the flame surface, breaking down into smaller hydrocarbons enroute. Oxygen from the surrounding atmosphere also migrates toward the flame surface by diffusion and convection. The survival and location of the flame surface is determined by the balance of these processes.
In normal gravity, buoyancy-driven convection develops due to the hot, less dense combustion products. This action has several effects: (a) the hot reaction products are carried away due to their buoyancy, and fresh oxygen is carried toward the flame zone; (b) solid particles of soot form in the region between the flame and the wick and are convected upward, where they burn off, yielding the bright yellow tip of the flame; (c) to overcome the loss of heat due to buoyancy, the flame anchors itself close to the wick; (d) the combination of these effects causes the flame to be shaped like a teardrop.
In the absence of buoyancy-driven convection, as in microgravity, the supply of oxygen and fuel vapor to the flame is controlled by the much slower process of molecular diffusion. Where there is no "up" or "down", the flame tends toward sphericity. Heat lost to the top of the candle causes the base of the flame to be quenched, and only a portion of the sphere is seen. The diminished supply of oxygen and fuel causes the flame temperature to be lowered to the point that little or no soot forms. It also causes the flame to anchor far from the wick, so that the burning rate (the amount of wax consumed per unit time) is reduced.
Birthday candles (several)
Balance beam scale (0.1 gm or greater sensitivity)
Clock with second-hand or stopwatch
Small pan to collect dripping wax
Faraday, M., (1988), The Chemical History of a Candle, Chicago Review Press.
Cornia, R., (1991), "The Science of Flames," The Science Teacher, v58n8, p. 43-45.
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