1. Lesson 16 BOILING HEAT TRANSFER
DESCRIBE the process that occurs in the following regions of the boiling heat transfer curve:
a. Nucleate boiling
b. Partial film boiling
c. Film boiling
d. Departure from nucleate boiling (DNB)
e. Critical heat flux

2. Boiling
Nucleate Boiling
Bulk Boiling
Film Boiling
Critical Heat Flux

3. Nucleate Boiling
The most common type of local boiling encountered in nuclear facilities
Steam bubbles form at the heat transfer surface and then break away and are carried into the main stream of the fluid - enhances heat transfer because the heat generated at the surface is carried directly into the fluid stream.
Desirable - the energy created at the heat transfer surface is quickly and efficiently "carried" away.

4. Bulk Boiling
Bulk fluid reaches saturation conditions as system temperature increases or system pressure drops
Bubbles entering the coolant channel will not collapse - will tend to join together and form bigger steam bubbles.
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.

5. Film Boiling Partial Film Boiling Film Boiling
Occurs when the pressure of a system drops or the flow decreases
Bubbles cannot escape as quickly from the heat transfer surface.
As the temperature continues to increase, bubbles grow and group together, covering small areas of the heat transfer surface with a film of steam.
Film Boiling
Steam patches on the heat transfer surface act to insulate the surface making heat transfer more difficult.
As heat transfer surface area increases, the temperature of the surface increases while the heat flux from the surface decreases.
Affected surface becomes covered by a stable blanket of steam, preventing contact between the heat transfer surface and the liquid in the center of the flow channel.

6. Boiling Heat Transfer Curve

7. Boiling Heat Transfer Curve (cont.)
Natural Convection and Nucleate Boiling regions
As heat flux increases, the temperature difference (surface to fluid) does not change very much.
Better heat transfer occurs during nucleate boiling than during natural convection.
Partial Film Boiling Region
Heat flux increases,
Partial film boiling occurs.
Characterized by an increase in temperature difference and a decrease in heat flux.
Film Boiling
Increase in temperature difference causes total film boiling, in which steam completely blankets the heat transfer surface.

8. Departure from Nucleate Boiling (DNB)
Critical Heat Flux
Departure from Nucleate Boiling (DNB)
The point of transition from nucleate boiling to film boiling
Occurs suddenly and the temperature difference increases rapidly if the heat flux is increased (see dashed line on BHT curve)
Heat flux associated with DNB is commonly called the critical heat flux (CHF)