Let's say you're boiling water to make pasta. As you watch, vapor in the form of bubbles rises up through the liquid. You wonder, "What's happening with all those bubbles? What role does gravity play in boiling?" Scientists have asked the same questions, particularly when it comes to boiling in a microgravity environment.
The Microheater Array Boiling Experiment (MABE) was an investigation into how boiling behavior changes under different gravity levels. By studying boiling in space, scientists were almost able to eliminate gravity in order to understand its role along with the other heat transfer processes.
Bubble formation is a good method to cool a hot surface, because it takes a lot of energy to convert liquid to vapor. Because bubbles are lighter than the surrounding liquid, when they grow to a certain size gravity causes them to detach from the surface, allowing fresh liquid to slip under them and make new bubbles.
However, there is a maximum amount of heat that can be removed, which is called the critical heat flux. At this point, the heater is covered with so much vapor that it starts to prevent the liquid from getting to the hot surface. Whether it is a computer chip or a nuclear reactor, this condition can destroy the heater if left unchecked because it causes the temperature of the heated surface to rise dramatically. Determination of the critical heat flux in microgravity is essential for designing reliable cooling systems for spacecraft.
Scientists studied boiling at different gravity levels in earlier ground-based studies using aircraft flying in a parabolic, or roller-coaster, path that put the experiments in free-fall to simulate microgravity. From those experiments, researchers were able to predict how boiling behaves in space. However, the vibrations from the aircraft engines, weather, machinery, people and other factors resulted in a small amount of residual gravity, or g-jitter. This g-jitter caused the bubbles to dance around on the surface just enough to alter the results.
To refine the model with minimal g-jitter, it was necessary to conduct the MABE experiments on the International Space Station. Using data from over two hundred boiling tests aboard the space station, the heat transfer during boiling was determined more accurately than was possible during the parabolic aircraft flight tests.
"We did a lot of experiments on the aircraft, but the aircraft bounces around producing residual gs on the order of one hundredth of Earth's gravity," said Professor Jungho Kim, MABE principal investigator from the University of Maryland, College Park. "We came to some conclusions about how the boiling would behave at these low-gravity levels and came up with some models and correlations, but we weren't really sure if we could extend the results to the very low g-levels encountered by spacecraft. The great benefit of MABE is that it allowed us to obtain really clean low-gravity data and use it to correct the model."
MABE's updated model accurately predicted the experimental microgravity data to within ±20 percent. Published in the August 2012 issue of the American Society of Mechanical Engineers' Journal of Heat Transfer, the article "Pool Boiling Heat Transfer on the International Space Station: Experimental Results and Model Verification" detailed the results of the investigation.
Experiments revealed that boiling could be divided into two regimes: Buoyancy Dominated Boiling (BDB) and Surface tension Dominated Boiling (SDB). BDB is common on Earth. It is what you see when you boil water for your pasta. Typically, as liquid is heated and vaporizes into a bubble, the bubble grows as it is held onto the surface by surface tension forces. As it becomes larger, the density difference between the vapor bubble and surrounding liquid results in larger buoyancy forces, pushing the bubble off the bottom of the pot so it rises through the water. Liquid rushes in behind the bubble, works its way to the bottom, and the process of heating and boiling repeats.
At lower gravity levels, the boiling behavior is controlled by SDB. A single bubble covers a large portion of the total heater surface. The bubble's size is determined by vaporization of liquid, mergers with smaller vapor bubbles that surround it, condensation of vapor at the top of the bubble and surface tension of the liquid.
"With a refined model, you could allow for more miniature electronics that could be cooled in low-g," said John McQuillen, MABE project scientist at NASA's Glenn Research Center in Cleveland. "Getting the heat out and cooling these electronics is important. There's something called heat density or power density of these electronics, which is one of the limiting factors that keep us from making them smaller and smaller. A better understanding about heat transfer can enable us to make them smaller."
Smaller is certainly better when it comes to hardware planned for space exploration, since reduced mass and size free up valuable cargo and living space. A better understanding of bubbles and heat transfer will help produce better cooling systems and higher-powered electronics that can be used in space, on the moon, on Mars, or even on Earth.
The same heat transfer approach used in space can be applied to developing microelectronics on Earth. Circulating water through channels that are too small can simulate the same behavior seen in microgravity. As a result, bubble and heater sizes are limited. However, MABE's results may help designers overcome these limitations. When it comes to cooling components in computers or machinery, designers could apply MABE's data to produce better, smaller systems.
NASA's Glenn Research Center