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Heat Transfer in Microgravity

We have seen that the dominant form of heat loss is determined by the local conditions such temperature differences, fluid flow and the presence or absence of good conductors. In space, such as onboard the International Space Station, the situation is in some ways simpler since we do not have to worry about the effects of gravity. In other ways it is more complex as we are faced with challenges that are not encountered on Earth.

Take the example of a satellite. As it operates it will most likely be generating heat internally through electrical heating from its equipment. In addition the side facing the Sun will be heated to a high temperature and at least part of the shaded side will be facing a Universe at about 2.7 Kelvin. For Earth orbiting satellites a large area will also be absorbing infra-red radiation from the Earth’s surface, at around 300 Kelvin. The only way for the satellite to control its average temperature is by radiation, since neither conduction nor convection will work in a vacuum. Satellites will therefore be carefully designed to radiate surplus heat away into space. In addition heat needs to be transferred from the hotter areas of the satellite to the cooler. Heat pipes are very useful in this role, particularly for smaller satellites. For a large assembly such as the International Space Station heat pipes will not be able to transfer sufficient heat and so active heat transfer systems (essentially space going fridges) need to be used to pump heat to radiator panels

Inside the ISS the situation is even more complex as it contains very sensitive and vulnerable equipment, namely astronauts! Humans usually control their heat loss by the evaporation of perspiration which is then carried away by air currents. In freefall however there are no natural convection currents, also the surrounding air is a very poor conductor and the astronauts’ temperatures are not high enough for radiation to be effective. The result is that the crew would overheat rapidly (as well as hot drinks taking an age to cool down). However even before this happened the crew would be in extreme danger of suffocating in their own personal cloud of carbon dioxide as normally, warm CO2 rich exhaled air would dissipate due to thermal convection currents. The solution is to equip the ISS with forced convection, driven by fans. This makes the ISS a rather noisy place to be. One benefit however is that if you lose anything on the ISS you can almost certainly find it on the air intake grill, where it has been carried by the circulating air.

 

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