1. Boiling Heat Transfer Source:
Vishwas V. Wadekar, HTFS, Aspen Technology
J.P. Holman

2. Boiling Heat Transfer Definitions/Terminology
T surface > Tsat of liquid  boiling may occur and heat flux depends on T
Pool boiling process
 heated surface is submerged below a free surface of liquid.
Subcooled or local boiling  Tof liquid < Tsat
Saturated or bulk boiling  Tof liquid = Tsat

3. Two Modes of Heating

5. Region I = Single phase No bubbles, wall superheat too low
Motion of fluid near surface = free convection currents
Liquid near heated surface = superheated slightly, when it rises to liquid surface, it evaporates.
Calculation uses free convection relations.

6. Coefficient increases with Temp excess
Region II
Nucleate boiling
Bubbles begin to form on surface of a wire and dissipated in liquid after breaking away from surface.
This region indicates the beginning of nucleation boiling
Coefficient increases with Temp excess

7. Coefficient increases with Temp excess
Region III
Nucleate boiling
Tx increases, bubbles form more rapidly and rise to surface of liquid and dissipated.
Coefficient increases with Temp excess

8. Region IV
transition boiling
Bubbles form so rapidly and they blanket the heating surface and prevent the inflow and of fresh liquid from taking their place.
Bubbles coalesce and form vapor film (cover the surface)
Film cause thermal resistance due to reduction in heat flux.
Film boiling region: this region is transition region (from nucleate to film boiling)
The film is unstable.

9. Region V Vapor film at wall Stable film boiling
Surface temperature is high to maintain stable film boiling.

10. Region VI
Heat loss from surface is the result of thermal radiation.

11. Point a  wire is unstable, small increase in T  Critical heat flux
Point b  this temp. is higher than melting Temp. of wire (cause of burnout results)
If maintain at point a  partial nucleate boiling and unstable film region

12. p = pv-pl

13. Bubbles
pv , Tv
pl , Tl
If Tv = Tsat and Tl < Tsat  heat conducted out of bubble and vapor condense  bubble collapse
If Tl > Tv  a metasatable condition  bubble growth after leaving the surface

14. Copper rod heated and immersed in isopropanol
free convection boiling  nucleate boiling  film boiling

15. Boiling of methanol on a horizontal steam-heated copper tube
Nucleate boiling
q/a = kW/m2
Temp excess = 37C
Transition boiling
q/a = kW/m2
Temp excess = 62C
Film boiling
q/a = 40.9 kW/m2
Temp excess = 82C

16. Calculation of boiling heat transfer
Nucleate pool boiling : Rohsenow
This eqn. can use for geometries other than horizontal wire.
Geometry is not a strong factor in determining heat flux for pool boiling.

18. Vapor-liquid surface tension for water

19. Heat flux data for water boiling on a platinum wire (numbers in parentheses are pressure in MN/m2)

22. Example
A heated brass plate is submerged in a container of water at atmospheric pressure. The plate temperature is 242F. Calculate the heat transfer per unit area of plate.

23. Forces convection boiling occurred when surface Temp > Tsat of liquid
This equation is applicable to forced convection where the bulk liquid temp. is subcooled (local forced convection boiling)

24. For fully developed nucleate boiling independent of flow velocity or forced convection effects
For low pressure boiling water
For high pressure boiling water

25. Peak heat flux for nucleate pool boiling
Zuber equation:

26. Simplified relations for boiling heat transfer with water

28. For forced convection local boiling inside vertical tubes:
Valid for atm
p is pressure in Mpa

29. Example: Water at 5 atm flows inside a tube of 2
Example: Water at 5 atm flows inside a tube of 2.54 cm diameter under local boiling conditions where the tube wall temperature is 10C above the saturation temperature. Estimate the heat transfer in a 1.0 m length of tube.