The Woodruff School

SUMMARY

Turbulent thermal convection is a crucial part of heat transport in several important natural and engineering flows. Large scale natural systems such as the Earth's atmosphere---its oceans as well as the interior---and the interior of stars such as the Sun, are all affected to various degrees by thermal convection. The simplified physical model used to understand this ubiquitous heat transport mechanism is the Rayleigh-Bénard convection (RBC), which is a fluid flow driven by a temperature difference between the top and bottom plates of an experimental cell with adiabatic sidewalls. Despite the long history of the subject and the recent progress in theoretical, numerical and experimental domains, many questions remain unresolved. A fundamental question concerns the heat transfer scaling in highly turbulent convective flows and in particular regarding the transition to the asymptotic regime of enhanced heat transfer. Other questions are related to the flow structures at these extreme flow parameters, and in particular about the existence of irregular polygonal structures that resemble those observed just above the onset of convection.

This project aims to explore RBC at extreme parameters (at Rayleigh numbers that will exceed the record-holding values of 𝑅𝑎~10^17 of "Turbulent convection at very high Rayleigh numbers", J.J. Niemela's helium experiments) by using liquid or gaseous nitrogen close to its coexistence curve, from 77K at 1 atm to its critical point (about 126K and 34 bar/500psi) in a unique multi-purpose facility at Georgia Institute of Technology.

The specific technical objectives are to:
1. Construct a Rayleigh-Bénard convection facility for experimentation at Rayleigh numbers up to 10^15 for a test cell of aspect ratio 1 and 10^17 for a test cell of aspect ratio 0.2. Validate the newly constructed facility across the classical regime of thermal convection.

2. Perform experiments to measure heat transfer and local temperature fluctuation to understand the effect of Rayleigh number on the heat transfer rate. The experiments are performed for different test cell geometry (aspect ratio) and for Rayleigh numbers up to 10^17. These experiments are performed to test the existence/validity of the ultimate regime.

SUBJECT:	Ph.D. Proposal Presentation

BY:	Stephen Johnston

TIME:	Tuesday, June 25, 2019, 1:00 p.m.

PLACE:	Love Building, 109

TITLE:	Rayleigh-Bénard convection at high Ra - Facility design and initial exploration of the existence of an Ultimate Regime

COMMITTEE:	Dr. Devesh Ranjan, Chair (ME) Dr. Mostafa Ghiaasiaan (ME) Dr. Yogendra Joshi (ME) Dr. Michael Schatz (PHYS) Dr. Katepalli R. Sreenivasan (ME) Dr. Pui-Kuen Yeung (AE)

SUMMARY Turbulent thermal convection is a crucial part of heat transport in several important natural and engineering flows. Large scale natural systems such as the Earth's atmosphere---its oceans as well as the interior---and the interior of stars such as the Sun, are all affected to various degrees by thermal convection. The simplified physical model used to understand this ubiquitous heat transport mechanism is the Rayleigh-Bénard convection (RBC), which is a fluid flow driven by a temperature difference between the top and bottom plates of an experimental cell with adiabatic sidewalls. Despite the long history of the subject and the recent progress in theoretical, numerical and experimental domains, many questions remain unresolved. A fundamental question concerns the heat transfer scaling in highly turbulent convective flows and in particular regarding the transition to the asymptotic regime of enhanced heat transfer. Other questions are related to the flow structures at these extreme flow parameters, and in particular about the existence of irregular polygonal structures that resemble those observed just above the onset of convection. This project aims to explore RBC at extreme parameters (at Rayleigh numbers that will exceed the record-holding values of 𝑅𝑎~10^17 of "Turbulent convection at very high Rayleigh numbers", J.J. Niemela's helium experiments) by using liquid or gaseous nitrogen close to its coexistence curve, from 77K at 1 atm to its critical point (about 126K and 34 bar/500psi) in a unique multi-purpose facility at Georgia Institute of Technology. The specific technical objectives are to: 1. Construct a Rayleigh-Bénard convection facility for experimentation at Rayleigh numbers up to 10^15 for a test cell of aspect ratio 1 and 10^17 for a test cell of aspect ratio 0.2. Validate the newly constructed facility across the classical regime of thermal convection. 2. Perform experiments to measure heat transfer and local temperature fluctuation to understand the effect of Rayleigh number on the heat transfer rate. The experiments are performed for different test cell geometry (aspect ratio) and for Rayleigh numbers up to 10^17. These experiments are performed to test the existence/validity of the ultimate regime.