Thermal driving head is the force that
causes natural circulation to take place. It is caused by the difference in density
between two bodies or areas of fluid.
Consider two equal volumes of the same type
of fluid. If the two volumes are not at the same temperature,
then the volume with the higher temperature will also have a
lower density and, therefore,
less mass. Since the volume at the higher temperature will
have a lower mass, it will also
have less force exerted on it by gravity. This difference in
the force of gravity exerted on the
fluid will tend to cause the hotter fluid to rise and the
colder fluid to sink.
This effect is seen in many places. One
example of this is a hot air balloon. The force causing
a hot air balloon to rise is a
result of a difference in density between the hot air inside
the balloon and the
cooler air surrounding it.
Heat added to the air in the balloon adds
energy to the molecules of air. The movement of the air
molecules increases and the air molecules take up more space.
The air molecules inside the balloon
take up more space than the same amount of air molecules
outside the balloon. This means
the hot air is less dense and lighter than the surrounding
air. Since the air in the balloon is
less dense, gravity has less effect on it. The result is that
the balloon weighs less than the surrounding
air. Gravity pulls cooler air down into the space occupied by
the balloon. The downward
movement of the cooler air forces the balloon out of the
space previously occupied, and
the balloon rises.