BOILING HEAT TRANSFER
As system temperature increases or system pressure drops, the bulk fluid can reach saturation
conditions. At this point, the bubbles entering the coolant channel will not collapse. The bubbles
will tend to join together and form bigger steam bubbles. This phenomenon is referred to as bulk
boiling. Bulk boiling can provide adequate heat transfer provided that the steam bubbles are
carried away from the heat transfer surface and the surface is continually wetted with liquid
water. When this cannot occur film boiling results.
When the pressure of a system drops or the flow decreases, the bubbles cannot escape as quickly
from the heat transfer surface. Likewise, if the temperature of the heat transfer surface is
increased, more bubbles are created. As the temperature continues to increase, more bubbles are
formed than can be efficiently carried away. The bubbles grow and group together, covering
small areas of the heat transfer surface with a film of steam. This is known as partial film
boiling. Since steam has a lower convective heat transfer coefficient than water, the steam
patches on the heat transfer surface act to insulate the surface making heat transfer more difficult.
As the area of the heat transfer surface covered with steam increases, the temperature of the
surface increases dramatically, while the heat flux from the surface decreases. This unstable
situation continues until the affected surface is covered by a stable blanket of steam, preventing
contact between the heat transfer surface and the liquid in the center of the flow channel. The
condition after the stable steam blanket has formed is referred to as film boiling.
The process of going from nucleate boiling to film boiling is graphically represented in Figure
13. The figure illustrates the effect of boiling on the relationship between the heat flux and the
temperature difference between the heat transfer surface and the fluid passing it.