The most frequently asked question about microgravity combustion is if and how a candle flame might burn in a weightless environment. As such, the candle flame was used to demonstrate the pivotal role gravity plays in simple combustion systems. These experiments addressed the characteristics and survivability of a classical diffusion flame in quiescent air in microgravity.
Immediately after ignition, the candle flame in the Shuttle tests was spherical and bright yellow (inferred from the video and verified by crew observations). After 8-10 seconds, the yellow, presumably from soot, disappeared, and the flame became blue and nearly hemispherical with a diameter of approximately 1.5 cm. These behaviors are consistent with the earlier, short-duration microgravity studies in aircraft (Carleton and Weinberg, 1989) and the NASA Lewis Research Center 5.2 second drop tower (Ross et al., 1991). After the ignition transient, the flame luminosity decreased continuously with time until extinction.
For the Mir experiments, immediately after ignition, the flames were very luminous and hemispherical, resembling the flames on the Shuttle. Unlike on the Shuttle, however, the yellow luminosity often lasted for minutes into the flame lifetime. This was due to the increased oxygen concentration in the Mir OS. The entire mass of wax melted within 90 seconds of ignition for the 5 mm diameter candles and within 6 minutes of ignition for the 10 mm diameter candles. These periods of time are much shorter than the period of time for complete melting of the candle at earth's gravity. The shape of the microgravity candle and wax looked. Small bubbles, presumably from air that had been trapped inside the wick, circulated inside the liquid wax. This motion was the result of surface tension gradients (temperature gradients) along the surface of the liquid. This molten ball of wax then became unstable, 'collapsed' suddenly and moved back along the candle holder as shown in Figure 3(c). This wax collapsed shortly (~ 10 sec)after all of the wax melted for the smaller candles. For the larger candles, the liquid ball of wax was stable for much longer (~ 180 sec) in some tests. After the collapse, the flame changed slowly until extinction.
For the Shuttle tests, extinction typically occurred between 40 and 60 seconds, except one very small flame that had a lifetime of 105 seconds. All of the candles in the Mir tests burned longer than the Shuttle candle flames. The flame lifetimes varied from over 100 seconds to over 45 minutes. For the Mir tests, the candles with the largest wick diameters had the shortest flame lifetimes and the candles with the smallest wick diameters had the largest lifetimes.
Each candle flame on the Shuttle oscillated spontaneously in the final 5 seconds. The flame traced symmetrically back and forth along the candle axis in each cycle. The top of the flame did not move during the oscillation. The base of the flame retreated and flashed back with a frequency of about 1 Hz with an amplitude that started small and grew until extinction. No oscillations occurred in any Mir tests with the smallest wick diameter (which was smaller than the wicks used in the Shuttle experiments). The flames in the tests with the two larger wick diameters, however, did oscillate before extinction. The oscillation frequency was similar to that in the Shuttle experiments, only for a much longer period of time, up to 90 cycles.
In the Mir experiments the lights in the glovebox were switched on after flame extinction, and a white, spherical cloud with a diameter 2-3 times that of the candle flame was present. This cloud is probably a mist of condensed wax droplets (and possibly water droplets) that formed while wax continued to vaporize after the flame extinguished. The formation of a flammable cloud served as an excellent reminder regarding fire safety in spacecraft: the hazardous event is not ended when the fire extinguishes because flammable material can continue for some time to issue from the source of the fire. Also, smoke detectors would be unable to sense these kinds of weak, non-smoking flames (but they are so weak as to be non-hazardous).
Analysis of the video recordings for the Shuttle experiments and the 35 mm photographs for the Mir experiments yielded both the flame diameter, D (maximum visible flame dimension perpendicular to the candle axis), and height, (maximum visible flame dimension parallel to the wick) as functions of time. For the Shuttle experiments, there was no consistent behavior of the flames with respect to H and D. For some the flame diameter and height sometimes increased with time and sometimes decreased with time. This was due to variations in the ignition procedure and in the initial ambient environment in the candlebox. The flame size in the Mir experiments always started small and increased with time, probably because of the more repeatable ignition source. Additionally, Figure 6 shows that the larger the wick size, the larger the quasi-steady flame size, as expected.
For both sets of experiments, the ratio H/D always decreased with time and was quite repeatable from experiment to experiment. While the values of the flame size of the Mir experiments were consistent with the Shuttle, the value of H/D is somewhat higher for the Mir tests. This latter observation is probably be due to the increased ambient oxygen concentration in the Mir tests.
A surprising result on the Shuttle experiments was the inability to sequentially ignite two, proximate candles oriented to face each other on a common axis. The crew attempted ignition with various wick separation distances (4-12 mm), ignition sequences, and igniter locations. After successfully lighting the first candle, the second candle could never be ignited; at no time was a stable flame(s) attained simultaneously on both. In one test, a single candle was lit and allowed to burn to extinction near an unlit second candle. Initially, the single candle flame was not affected by the presence of the second candle. With time, the flame grew closer until its tip was quenched by the wick of the unlit candle, immediately after which the surviving part of the flame rotated asymmetrically around the axis and then extinguished.
The results show that the candle flames in the Mir tests had much longer lifetimes than those on the Shuttle. The Shuttle candle flames extinguished because of a lack of oxygen in the vicinity of the flame. This was not due to oxygen depletion inside the sealed glovebox, but because of the restriction to oxygen transport due to the perforated polycarbonate box. Although the ambient oxygen mole fraction in the Shuttle tests was 0.217 and in the Mir tests the range was from 0.23 to 0.25 (and the Mir working volume was larger), these differences alone would not extend the lifetimes by a factor of 10 or more, as the experiments showed. Thus, the predominant reason for the observed increase in flame lifetime for the Mir tests was the diminished resistance to oxygen transport through the container. The Mir candle flames extinguished primarily because of a lack of fuel. When the wax 'collapsed' and moved back along the candle holder, it was not in close proximity to the wick. As a result, this fuel was unavailable for burning. For nearly all of the candles tested, there was a film of wax surrounding the candle holder after the flame extinguished.
Given a flame lifetime of up to 45 minutes, we believe that the gas-phase was quasi-steady, implying that the flame was steady over a time period much longer than any reasonable characteristic gas-phase transport time. The numerical model predicts that a steady-state candle flame will exist in a large ambient of air. The model results show that the time to reach quasi-steady behavior for the gas-phase region in the vicinity of the flame is less than 10 seconds. Further outside the flame the time to steady-state was seen to be on the order of 100 seconds, but the changes in the flame as gauged by the fuel vapor reaction rate contours are small during this time. This is in good agreement with previous drop tower and these space-based experiments . Visually, the candle flame reaches a quasi-steady size, shape and color within 10 seconds. The changes that occur during longer time-scales are initially a result of unsteady behavior in the solid/liquid phase, but eventually the changes are a result of decreasing the oxygen concentration in the sealed-glovebox volume.
In a normal gravity candle flame, the majority of the energy liberated by combustion is convected up and away from the candle flame. Thus only a small fraction of the total energy is conducted back to the candle to melt the wax. The conductive heat feedback from the candle flame more rapidly melts all of the wax in the microgravity candle tests than in earth's gravity, despite the lower flame temperature and mass consumption rate in microgravity. In addition, the wax does not drip as it does in normal gravity. The reason that the wax collapse occurs at different times, relative to complete melting, for the different candle diameters is because of the design of the candle holder. For the 5 mm diameter candle holder: as the wax melts, and the liquid wax contacts the relatively cold candle holder, surface tension forces pull the liquid along the candle holder. For the design of the 10 mm diameter candle holder: as the liquid wax melts, the liquid ball is 'pinned' to the edge of the candle holder. Theoretically, the liquid ball could remain pinned on the edge indefinitely. The liquid ball of wax in this position, however, is relatively unstable. Any disturbance that forces the liquid wax to move to the side of the candle holder, causes the ball to collapse. In the proposed experiment, we will re-design the candle holder such that this collapse does not occur.
Spontaneous oscillations are inherent to the near-extinction burning of these flames. Previously, we explained the flame oscillation before extinction as a flame base retreat and flashback mechanism. As the ambient oxygen concentration decreases, the flame oscillations are initiated when the flame base begins to retreat. Because of their thermal inertia, the liquefied wax and wick are still hot, so fuel vaporizes, and the fuel vapor and oxygen diffuse toward each other. Eventually a combustible mixture is formed and a rapid flashback of the flame occurs. This further depletes the ambient oxygen concentration, so that more of the base or weakest part of the flame (compared to the previous cycle) extinguishes, and the cycle repeats. The oscillations will continue, increasing in amplitude as the ambient oxygen is continuously depleted, until the ambient oxygen concentration becomes too low to sustain any part of the flame. The apparent dependence on wick diameter from the Mir experiments, implying a dependence on flame size, suggests that flame radiative losses may contribute to the onset of oscillations. The observation of these flame oscillations prompted theoretical work by Cheatham and Matalon (1996) to model the near-extinction behavior of spherical diffusion flames. The numerical model described in the Appendix also predicts the occurrence of these flame oscillations if the decrease in the ambient oxygen concentration is small enough. This step decrease in ambient oxygen concentration is similar to the gradual depletion of oxygen inside a sealed volume as the candle burns. The other findings from the model are described in the Appendix.
The inability to ignite two candles on the Shuttle was somewhat surprising at the time. The range of initial separation distances would produce near-optimal burning in normal gravity. Perhaps more significantly, aircraft-based testing, albeit limited, of the ignition procedure for two candles was successful. Since the candles were lighted sequentially, the first flame could have at least two undesirable effects on the unlit candle. First the heat from the first flame may have melted the wax of the unlit candle to the extent that the liquefied wax coated the wick of the unlit candle; in this case, ignition is much more difficult to achieve, a result observed firsthand by the crew members. Second, and probably more likely, the wicks in microgravity were within 1 flame diameter, so the oxygen around the unlit candle may have been sufficiently depleted prior to ignition to be unable to support a flame. The aircraft tests did not reveal similar behavior because the residual acceleration level was higher (so residual convective flow provided oxygen locally and causing the large, first flame to promote ignition) and because the time between lighting the candles was greater in the space-based mission. Since the USML-1 mission, these were further investigated via experiments by D. Dietrich in the 10 sec drop tower in Hokkaido, Japan and briefly on the Mir. To overcome both the deleterious possibilities, axially aligned candles were simultaneously ignited in the Hokkaido drop tests. At the same wick separation distance (about 1 cm) as in the shuttle tests, both candles were ignited and a merged, roughly elliptical flame was observed, whose temporal and spatial characteristics were still evolving (neither extinction nor steady state was seen) at the end of the drop. This occurred whether or not the candles were ignited in 1g and then dropped, or if they were ignited in microgravity. Dr. Lucid, with our approval, conducted very few two-candle experiments; she stated that she was most comfortable with the procedures for single candle experiments and preferred to continue with more of this type of experiment.
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