1. Karl J.L. Geisler, Ph.D. http://www.menet.umn.edu/~kgeisler
Buoyancy-Driven Two Phase Flow and Boiling Heat Transfer in Narrow Vertical Channels CFD Simulation of Two Phase Channel Flow
Karl J.L. Geisler, Ph.D.
© Karl John Larson Geisler 2007
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2. CFD Model 2-D FLUENT VOF multiphase simulation of channel flow
Evaluate convective enhancement mechanism
Estimated bubble parameters at selected operating point
DTsat = 12.3°C
Db = 0.78 mm
f = 59.3 Hz = (16.9 ms)-1
tg = 4.2 ms
N/A = /m2
Karl J.L. Geisler, Ph.D. January

3. liquid liquid phase volume fraction vapor
5 mm channel
View in Slide Show for animation
time in seconds
each frame = 5 ms
Karl J.L. Geisler, Ph.D. January

4. liquid liquid phase volume fraction vapor
0.7 mm channel
View in Slide Show for animation
time in seconds
each frame = 5 ms
Karl J.L. Geisler, Ph.D. January

5. liquid liquid phase volume fraction vapor
0.3 mm channel
View in Slide Show for animation
time in seconds
each frame = 5 ms
Karl J.L. Geisler, Ph.D. January

6. CFD Observations and Conclusions
Unconfined boiling heat flux nearly 50% due to enhanced convection
Disruption of thermal boundary layer by bubble motion
≈3x single phase natural convection
Narrow channels show higher mass flux, enhanced single phase convection below nucleation site
Sensible heat rise in 0.3 mm channel yields reduced heat flux compared to 0.7 mm channel
Maximum enhancement observed for 0.7 mm channel
0.7 mm channel only 20% better than unconfined
0.7 mm experiment 50–150% better
0.3 mm experiment 150–500% better
Enhanced liquid convection likely NOT dominant enhancement mechanism
Karl J.L. Geisler, Ph.D. January

7. CFD Background and Additional Results
For details, see:

8. Bubble Departure Diameter
Karl J.L. Geisler, Ph.D. January

9. Bubble Departure Frequency
Karl J.L. Geisler, Ph.D. January

10. Nucleation Site Density (1)
Karl J.L. Geisler, Ph.D. January

11. Nucleation Site Density (2)
Karl J.L. Geisler, Ph.D. January

12. Nucleation Site Density (3)
Karl J.L. Geisler, Ph.D. January

13. Latent Heat Contribution
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14. 2-D Bubble Volume
Karl J.L. Geisler, Ph.D. January

15. Vapor Generation Rate
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16. Vapor Inlet Mass Flux
Karl J.L. Geisler, Ph.D. January

17. Boiling parameter predictions for saturated FC-72 at atmospheric pressure (101 kPa)
Table F.1: Boiling parameter predictions for saturated FC-72 at atmospheric pressure (101 kPa) based on Eqs. (F.1)–(F.8).
Karl J.L. Geisler, Ph.D. January

18. Mikic and Rohsenow (1969) bubble growth rate correlation
Figure F.2: Mikic and Rohsenow (1969) bubble growth rate correlation.
For a spherical bubble growing in an infinite superheated pool, growth is initially inertia controlled, and bubble radius tends to increase linearly with time. Later, when bubble growth is limited by heat transfer rates, radius tends to increase as the square root of time (Carey, 1992).
Karl J.L. Geisler, Ph.D. January

19. CFD Model Geometry Figure F.3: Model geometry.
Karl J.L. Geisler, Ph.D. January

20. GAMBIT screen-shot of model geometry showing vertices, edges, and faces
Figure F.4: GAMBIT screen-shot of model geometry showing vertices, edges, and faces.
Karl J.L. Geisler, Ph.D. January

21. GAMBIT screen-shot showing mesh details in vicinity of vapor inlet
Figure F.5: GAMBIT screen-shot showing mesh details in vicinity of vapor inlet.
Karl J.L. Geisler, Ph.D. January

22. Comparison of temperature results from single phase numerical simulations
Figure F.20: Comparison of temperature results from single phase numerical simulations.
Karl J.L. Geisler, Ph.D. January

23. Velocity results for initial steady-state single phase solution
Figure F.8: Velocity results for initial steady-state single phase solution.
Karl J.L. Geisler, Ph.D. January

24. Nucleation site mass flux profiles
Figure F.9: Nucleation site mass flux profiles.
Karl J.L. Geisler, Ph.D. January

25. Phase contour plots at 4 ms time steps from the beginning of the VOF simulation through the first four bubble generations
Figure F.11: Phase contour plots at 4 ms time steps from the beginning of the VOF simulation through the first seven bubble generations, d = 5 mm.
Karl J.L. Geisler, Ph.D. January

26. Phase contour plots at 4 ms time steps from the beginning of the VOF simulation through the first four bubble generations
Figure F.11: Phase contour plots at 4 ms time steps from the beginning of the VOF simulation through the first seven bubble generations, d = 5 mm.
Karl J.L. Geisler, Ph.D. January

27. Velocity contour plot at end of VOF simulation, 5 mm channel
Figure F.19: Velocity contour plot at end of VOF simulation, d = 5 mm.
Karl J.L. Geisler, Ph.D. January

28. Inlet and outlet mass flow rates as a function of time, 5 mm channel
Figure F.12: Inlet and outlet mass flow rates as a function of time, d = 5 mm.
Karl J.L. Geisler, Ph.D. January

29. Heater top and bottom heat flux as a function of time, 5 mm channel
Figure F.14: Heater top and bottom heat flux as a function of time, d = 5 mm.
Karl J.L. Geisler, Ph.D. January

30. Two Phase Simulation Temperature Results Comparison
Figure F.23: Comparison of temperature results from two phase numerical simulations.
Red spots on 0.3 mm case are vapor.
Karl J.L. Geisler, Ph.D. January

31. Surface heat flux profiles for 5 mm channel single phase natural convection solution and VOF simulation results at t = 1.34 s
Figure F.18: Surface heat flux profiles for 5 mm channel single phase natural convection solution and VOF simulation results at t = 1.34 s.
Karl J.L. Geisler, Ph.D. January

32. Surface heat flux profiles
Karl J.L. Geisler, Ph.D. January

33. CFD Simulation Results Summary
Single phase results within 5% of Elenbaas
Maximum enhancement observed for 0.7 mm channel
Reduced enhancement for 0.3 mm contrary to experiment
0.7 mm channel only 20% better than unconfined
0.7 mm experiment 50–150% better
0.3 mm experiment 150–500% better
Enhanced natural convection NOT most likely dominant experimental enhancement mechanism
Karl J.L. Geisler, Ph.D. January