Jan 29, 2010

NASA/Marshall Space Processing Headline: Chuck - heat transfer

simulation of convection

Computer simulations depict the differences in how a warm, fluid moves on Earth (below) and in space (top). The color differences indicate variations in density, with the density dropping as color shifts from blue to yellow to orange.

Thus, the pictures show that on Earth, warm fluid rises to the top and curling back down to stir the entire volume, while in space the warm area grows slowly by conduction and diffusion.

How heat moves

In school, most of us learned that heat moves by conduction, convection, and radiation.

The first two are mechanical -- objects have to be in direct contact with each other. The third is not. And there is a fourth method that we usually are not taught.

Conduction involves solid objects touching each other. Heat is just atoms and molecules in motion. In a solid, they vibrate in place and bump each other, so heat transfers slowly.

Convection involves liquids flowing in a container. Because the atoms and molecules in a liquid are not locked in place, they can expand outward - get more elbow room - as they bump into each other. That makes the hot fluid less dense than the cold fluid, and the cold fluid slides under, pushing the hot fluid to the top.

Radiation involves electromagnetic radiation, light waves, being emitted by one body and absorbed by another, like holding your hand in the sunlight.

The weightlessness of space means that convection does not occur since gravity's effects are reduced to about 1/1,000th (or less) the strength of gravity on Earth. Since nothing is heavier or lighter, nothing falls and everything floats.

Diffusion is the kind of heat transfer which becomes important. The hot atoms and molecules will slowly push their way into the crowd of colder atoms and molecules and gradually spread the heat over a wide area. It is kind of like a fragrance filling a room.

We usually are not taught about diffusion as a heat transfer method because most of our experience is on Earth where gravity and convection rule. But in space, it becomes an important factor in calculating how long an experiment might take.

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