Natural circulation will only occur if the
correct conditions exist. Even after natural circulation
has begun, removal of any one of
these conditions will cause the natural circulation to stop.
The conditions for
natural circulation are as follows.
1. A temperature difference exists (heat
source and heat sink exists).
2. The heat source is at a lower elevation
than the heat sink.
3. The fluids must be in contact with each
other.
There must be two bodies of fluid at
different temperatures. This could also be one body of fluid
with areas of different
temperatures. The difference in temperature is necessary to
cause a density
difference in the fluid. The density difference is the
driving force for natural circulation flow.
The difference in temperature must be
maintained for the natural circulation to continue. Addition
of heat by a heat source must exist at the high temperature
area. Continuous removal of
heat by a heat sink must exist at the low temperature area.
Otherwise the temperatures would eventually
equalize, and no further circulation would occur.
The heat source must be at a lower elevation
than the heat sink. As shown by the example of the
balloon, a warmer fluid is less dense and will tend to rise,
and a cooler fluid is more dense and
will tend to sink. To take advantage of the natural movement
of warm and cool fluids, the heat
source and heat sink must be at the proper elevations.
The two areas must be in contact so that flow
between the areas is possible. If the flow path is obstructed
or blocked, then natural circulation cannot occur.
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