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Boiling Water in Zero-G
One of my teachers this semester is working on a long-term project which, among other things, studies the way water boils in zero gravity.
Apparently, in normal gravity, as the heat is conducted to the water, bubbles of steam form and lift off the surface. (Quite apparently, actually; if you've ever boiled water, you may have seen this happen.) This mechanism is what transfers the heat so quickly from the edge to the main body of the fluid. However, if the temperature of the wall of the container is above a critical point, you begin to get 'film boiling', in which the entire layer adjacent to the hot wall is gaseous. Since gases conduct heat poorly compared to water, the rate of heating at this point is likely to be lower than that at the lower temperature. (This 'film boiling' phenomenon is what makes drops of water skitter around so easily on hot frying pans.) Thus, the best heating occurs when the evaporating water has time to float off the surface to be replaced with more liquid.
However, at zero-g, the evaporated water doesn't lift off. Buoyancy works because the floating object is less dense than the fluid containing it, and thus the fluid falls under it, pushing it up. In zero-g, nothing falls, so nothing floats.
Fortunately, the experiments thus far seem to show that there is another process that can allow for rapid heat transfer, even in zero-g. If the bulk liquid temperature (the temperature far away from the wall) is low relative to the boiling point, and if the wall temperature is in the proper (narrow) range, a large steam bubble will form against the wall, surrounded by smaller bubbles. When these smaller bubbles reach a certain size, they merge with the big bubble, and simultaneously the big bubble releases heat through conduction into the main body of the fluid. There is still significant heat transfer because there is still a significant amount of liquid adjacent to the wall, yet there is also boiling.
I like this research a lot. I guess it's partially because zero-g is always cool, thanks to science fiction, but it's also because I like stories about the mechanics of phenomena, and this one's a good story.
Apparently, in normal gravity, as the heat is conducted to the water, bubbles of steam form and lift off the surface. (Quite apparently, actually; if you've ever boiled water, you may have seen this happen.) This mechanism is what transfers the heat so quickly from the edge to the main body of the fluid. However, if the temperature of the wall of the container is above a critical point, you begin to get 'film boiling', in which the entire layer adjacent to the hot wall is gaseous. Since gases conduct heat poorly compared to water, the rate of heating at this point is likely to be lower than that at the lower temperature. (This 'film boiling' phenomenon is what makes drops of water skitter around so easily on hot frying pans.) Thus, the best heating occurs when the evaporating water has time to float off the surface to be replaced with more liquid.
However, at zero-g, the evaporated water doesn't lift off. Buoyancy works because the floating object is less dense than the fluid containing it, and thus the fluid falls under it, pushing it up. In zero-g, nothing falls, so nothing floats.
Fortunately, the experiments thus far seem to show that there is another process that can allow for rapid heat transfer, even in zero-g. If the bulk liquid temperature (the temperature far away from the wall) is low relative to the boiling point, and if the wall temperature is in the proper (narrow) range, a large steam bubble will form against the wall, surrounded by smaller bubbles. When these smaller bubbles reach a certain size, they merge with the big bubble, and simultaneously the big bubble releases heat through conduction into the main body of the fluid. There is still significant heat transfer because there is still a significant amount of liquid adjacent to the wall, yet there is also boiling.
I like this research a lot. I guess it's partially because zero-g is always cool, thanks to science fiction, but it's also because I like stories about the mechanics of phenomena, and this one's a good story.