CFM-1 Experiment Results
Initial and Quasi-Steady Flame Behavior
The crew conducted about 10 single-candle experiments during the STS-50, USML-1 space shuttle mission. Immediately after ignition, the candle flame was spherical, with a bright yellow core. After 8-10 seconds, the yellowpresumably from sootdisappeared and the flame became blue and nearly hemispherical, with a diameter of approximately 1.5 cm (slightly more than half an inch). These behaviors are consistent with the earlier, short-duration studies in aircraft, the NASA Glenn Research Center's 5.2-second drop tower and Japan's JAMIC 10-second drop tower.
The microgravity candle flame differs from a normal gravity flame in size, shape, color and flame structure (please see the photo on the right). The microgravity flame has a larger flame standoff than that of a normal gravity candle flame (at the base). The width of the flame-standoff implies a weaker heat feedbackfrom the flame and a smaller wax burning rate.
The nearly spherical nature of the microgravity flame suggests that the flame is providing heat to the wick. This is unlike normal gravity where only a portion of the vaporized fuel reacts in the vicinity of the wick; the rest of the fuel vapor is swept downstream by buoyant convection and reacts in the plume region. Thus, the flame structure of these two flames is different.
In normal gravity, the gas-phase structure of the candle flame resembles that of a downward propagating diffusion flame over a thin solid. Models of the later system show that the most intense reaction zone (highest reaction or heat release rate per unit volume) is close to the bottom of the flame near the wick. This region serves to stabilize the rest of the flame and provides the largest heat feedback to the wick for fuel vaporization.
Candle Flame Structure in Microgravity
The candle flame structure in microgravity is different. Its nearly spherical shape resembles the droplet flame in microgravity. The visible candle flame disappears, however, at the base because of heat loss to the candle wax for the microgravity flame. Because of this quenching, the local reactivity does not have the same spherical symmetry as a droplet flame (the near-extinction behavior may be different). Contrary to normal gravity, the highest reactivity in microgravity exists at the top of the wick and diminishes in strength toward the bottom because of the quenching by the wax.
Analysis of the video footage from the shuttle experiment yielded the flame diameter, D, and height, H, as a function of time. The results show that some flames reached steady-state with respect to both diameter and height. For others the flame diameter and height increased with time and for still others decreased with time. This test-to-test variation of the temporal behavior of D and H is probably due to variations in wick/liquid initial conditions resulting from the ignition process. The size of the flame is determined by not only the gas phase but by the size of the evaporating surface of the wick, and the magnitude of the heat loss to the solid/liquid wax. The last two parameters are determined to a large extent by the initial condition of the wick/liquid. The initial condition varied from test to test because the ignition was manual in the shuttle experiment and, therefore, not reproducible, .
Normalizing H by D provides more of a measure of the flame shape than its absolute size. For nearly all flames the ratio of 2H/D was 1.8 early in the flame lifetime, and gradually decreased during the test, until extinction when the value was about 1.3.
The decrease in 2H/D throughout the flame lifetime occurs primarily from changes in H. Because the glovebox is a sealed volume, the candle burns in an atmosphere of continuously decreasing oxygen content. As a result the local reactivity decreases everywhere in the flame as a function of time. The flame then retreats (H decreases) as the local reactivity falls below a critical value required for a luminous flame. The reactivity at the base of the flame will decrease below this critical value first since the reactivity is always the lowest there.
A common question is why the microgravity candle flame color is blue while normal gravity candle flames in the same atmosphere are sooty (as determined by the yellow color). There are three possible explanations for this. They are:
The second possibility, however, is unlikely since visual observation shows no black soot trace in either this series of tests or in low pressure normal gravity experiments (where the flame is also blue). Although the oxygen leakage can contribute to less soot formation in our candle experiment, it is probably not enough to eliminate soot altogether by itself. Near the top of the flame, the mixture should still be fuel rich even with oxygen leakage.
The suppression of soot formation in the microgravity candle flames is most likely the result of the reduced diffusion flame temperature. The measured maximum flame temperature in a visually similar, reduced-pressure candle flame is around 1530 K. Other near-limit microgravity diffusion flames which have configurations not favoring oxygen leakage also are completely blue. These near-limit flames, according to theory, should also have low flame temperatures.