Heat Studies in Space Can Yield Earth-Bound Benefits
Microgravity laboratories in the International Space Station are being used to understand the basic science of heat transfer and how it can improve terrestrial power systems.
Space exploration has benefitted scientific research in a variety of ways. The stringent requirements of space exploration have resulted in innovation and advances in electronics, aerodynamics, materials development, and medicine. Beyond the obvious technologies used to develop cell phones, Velcro, fuel cells, and solar panels, the micro-gravity provided by laboratories in space enables research and development and explorations into basic science impossible on Earth.
International Space Station. Image used courtesy of NASA
Launched in 1998, the International Space Station (ISS) orbits 248 miles (400 kilometers) above the Earth’s surface. The ISS provides a microgravity research laboratory for studies in life sciences, physical sciences, remote sensing, technology development, and education. Much of this research has real-world applications back on Earth.
Of particular interest are microgravity studies of fluid physics. Researchers have found that the loss of buoyancy-driven convection that exists in a microgravity environment allows them to examine the underlying mechanisms that exist within such physical systems. Buoyancy has a major influence on convection (the movement of particles) and depends upon the effects of gravity. In a microgravity environment, these buoyancy effects are largely eliminated, allowing scientists to study the underlying effects that influence fluid motion.
On Earth, the motion of air over a device is one of the primary ways electronics keep cool. When the air is very thin at high altitudes or nonexistent in the vacuum of space, air can’t remove heat, so other methods must be employed. One way is to conduct heat away from the source and dump it into radiators that release it by radiation. Conducting heat away from its source can be accomplished using a heat pipe.
A heat pipe is a two-phase, heat-transfer device that uses the evaporation and condensation of a working fluid to transfer heat efficiently. Heat pipes are found in a variety of applications, including electronics cooling, satellites, and space rovers, as well as in Earth-bound electronic devices.
Heat pipe system in a laptop. Image used courtesy of Wikimedia Commons by Kristoferb
On the surface, at the hot end of the heat pipe, the volatile working fluid is turned into a vapor, which absorbs heat from the surface. The vapor that forms travels along the heat pipe to the cold end, where it condenses back into a liquid, releasing the heat it has absorbed. The liquid returns to the hot end of the heat pipe, either through gravity, centrifugal force, or capillary motion, and the process repeats, helping to keep the source of the heat (which might be an electronics device or solar-heated surface) from being damaged by overheating.
Video used courtesy of ACT
Heat pipes work efficiently as thermal conductors. A heat pipe can conduct as much as 100 kilowatts per meter Kelvin (kW/(m-K), a measure of thermal conductivity. By comparison, copper, known for its heat conduction, can conduct around 0.4 kW/(m-K).
Studying in Microgravity
Microgravity provides a unique environment for studying heat pipes. In the absence of gravity, the buoyant forces that drive convection are eliminated, and the surface tension of the working fluid is reduced. This allows researchers to study the fundamental physics of heat pipe operation without the interference of gravity.
An ISS National Laboratory-sponsored investigation has been launched to study the vapor-liquid interfaces of organic mixtures used in heat pipes. The investigation is being led by researchers at Rensselaer Polytechnic Institute, with support from Tec-Masters, Inc. The findings from this investigation could lead to the development of more efficient and reliable heat pipes for a wide range of applications.
One of the key areas of research in microgravity heat pipes is the behavior of the vapor-liquid interface. The interface is the boundary between the liquid and vapor phases of the working fluid, and it plays a critical role in heat transfer.
Some of the key findings from microgravity studies of heat pipes include:
- The absence of gravity in microgravity causes the vapor-liquid interface to take on a variety of shapes depending on the flow conditions and the properties of the working fluid.
- Heat pipes can operate effectively in microgravity, but their performance is affected by the reduction in surface tension.
A Range of Applications
By understanding the basic mechanisms that are involved in thermal conduction and heat pipes, it is proving possible to develop more effective heat transfer systems. Compact electronic devices continue to shrink in size while generating more power, and as devices get smaller, heat becomes a bigger problem. This is not only true for electronic devices in space but for those finding their way into electric vehicles and control systems for electrical power grids and distributed energy sources, such as microgrids using renewables. It’s another way the technologies developed for space have found terrestrial applications.