Alexandria Digital Research Library

Turbulent Rayleigh-Benard Convection of Low Prandtl-Number Gases

Hogg, James Douglas
Degree Supervisor:
Guenter Ahlers
Place of Publication:
[Santa Barbara, Calif.]
University of California, Santa Barbara
Creation Date:
Issued Date:
Physics, General and Physics, Fluid and Plasma
Dissertations, Academic and Online resources
Degree Grantor:
University of California, Santa Barbara. Physics
Ph.D.--University of California, Santa Barbara, 2012

Experimental results are presented for heat transport and flow dynamics in turbulent Rayleigh-Benard convection using pressurized gases with Prandtl numbers around 0.7 and gas mixtures with Prandtl numbers between approximately 0.17 and 0.7, in two different sized apparatuses. The large apparatus had a cell of height 0.5 m and diameter 0.25 m, and reached a maximum Rayleigh number of about 1013. The small apparatus had a cell of height 0.93 cm and diameter 10 cm, and reached a maximum Rayleigh number of about 108.

The heat transport in the large apparatus, after applying corrections for the wall current and an asymmetry in the top and bottom thermal boundary layers, showed reasonable agreement with other recent experimental and computational results, and relatively poor agreement with the Grossmann-Lohse (GL) model. There was no evidence of a transition to a new regime at a Rayleigh number around 1011, as had been reported by an earlier group. A large-scale circulation in the bulk fluid was also observed for all Rayleigh numbers studied.

In the small apparatus, the heat transport of the pure gases agreed moderately well with the GL model. The gas mixtures showed an enhanced heat transport, as expected from the coupling between temperature gradients and concentration gradients. A boundary-layer model to predict the heat transport enhancement showed qualitative agreement with the measured results, but predicted a smaller magnitude of enhancement than was observed.

A shadowgraph optical apparatus provided flow visualization from the top of the small apparatus. The Reynolds number of the flow was measured from these images using the Elliptic Approximation, and agreed well with the GL model prediction. Spatial power spectra of the shadowgraph images showed peaks at small wavelengths which decreased with increasing Rayleigh number. Only at the highest observed Rayleigh numbers were large-scale structures comparable in size to the height of the cell observed.

Physical Description:
1 online resource (256 pages)
UCSB electronic theses and dissertations
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Inc.icon only.dark In Copyright
Copyright Holder:
James Hogg
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