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. http://www.menet.umn.edu/~kgeisler © Karl John Larson Geisler 2007 SOME RIGHTS RESERVED This work is licensed under the Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ or send a letter to Creative Commons, 543 Howard Street, 5th Floor, San Francisco, California, 94105, USA. Karl John Larson Geisler is the owner of this work and retains copyright, but this license allows you to download, copy, use, and distribute it for noncommercial purposes as long as you do not alter, transform, edit, or build upon the work. This license is not intended to reduce, limit, or restrict any fair use or academic fair use rights. Any of the license terms may be waived and/or modified with written permission from Karl John Larson Geisler. You may verify that this copy is unaltered by comparison with the original, available at http://www.menet.umn.edu/~kgeisler or by contacting the author at kgeisler@me.umn.edu

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 = 96354 1/m2 Karl J.L. Geisler, Ph.D. January 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

CFD Background and Additional Results For details, see: http://www.menet.umn.edu/~kgeisler/Geisler_PhD_Dissertation.pdf

Bubble Departure Diameter Karl J.L. Geisler, Ph.D. January 2007 http://www.menet.umn.edu/~kgeisler

Bubble Departure Frequency Karl J.L. Geisler, Ph.D. January 2007 http://www.menet.umn.edu/~kgeisler

Nucleation Site Density (1) Karl J.L. Geisler, Ph.D. January 2007 http://www.menet.umn.edu/~kgeisler

Nucleation Site Density (2) Karl J.L. Geisler, Ph.D. January 2007 http://www.menet.umn.edu/~kgeisler

Nucleation Site Density (3) Karl J.L. Geisler, Ph.D. January 2007 http://www.menet.umn.edu/~kgeisler

Latent Heat Contribution Karl J.L. Geisler, Ph.D. January 2007 http://www.menet.umn.edu/~kgeisler

2-D Bubble Volume Karl J.L. Geisler, Ph.D. January 2007 http://www.menet.umn.edu/~kgeisler

Vapor Generation Rate Karl J.L. Geisler, Ph.D. January 2007 http://www.menet.umn.edu/~kgeisler

Vapor Inlet Mass Flux Karl J.L. Geisler, Ph.D. January 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

CFD Model Geometry Figure F.3: Model geometry. Karl J.L. Geisler, Ph.D. January 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

Nucleation site mass flux profiles Figure F.9: Nucleation site mass flux profiles. Karl J.L. Geisler, Ph.D. January 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler

Surface heat flux profiles Karl J.L. Geisler, Ph.D. January 2007 http://www.menet.umn.edu/~kgeisler

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 2007 http://www.menet.umn.edu/~kgeisler