Lesson 16BOILING HEAT TRANSFER

DESCRIBE the process that occurs in the following regions of the boiling heat transfer curve:

a. Nucleate boilingb. Partial film boilingc. Film boilingd. Departure from nucleate boiling (DNB)e. Critical heat flux

Boiling• Nucleate Boiling

• Bulk Boiling

• Film Boiling

• Critical Heat Flux

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.

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.

Film Boiling• Partial 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.

Boiling Heat Transfer Curve

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.

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)