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Convection with a free surface
Most of you know that convection associated with density differences
(“hot air rises or cold air falls”). The next slide shows this. When a
layer of liquid is heated from below it will convect if the layer
becomes top heavy enough. This is almost like having a lot of
building blocks stacked on one another. They will fall once they
become tall enough. A tall liquid that’s heated tends to convect
because its potential energy is converted to kinetic energy.
Now there is another type of convection that can take place in the
absence of gravity. This one has nothing to do with the liquid being
top heavy. Surface tension depends on temperature and so if the
layer of liquid is subject to a temperature difference it would convect
if a slight disturbance in the temperature is given. High viscosity and
thermal conductivity helps dissipate this disturbance. The second
slide depicts this
Following this is a movie of a liquid on a petri dish heated from below.
Clearly the flow patterns seen are due both to density changes and
surface tension changes because this experiment was done on
earth with a thick liquid layer.
Physics of Convection
Buoyancy Driven Convection
• Density decreases as temperature increases
• Gravity acts to pull down more dense fluid
Gas
Liquid
Surface Tension Driven Convection
• As the surface tension decreases
with an increase in temperature, flow
occurs from hot to cold regions
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