Heat transfer in cryogenic helium gas by turbulent Rayleigh-Bénard convection in a cylindrical aspect ratio one cell

0436513 - ÚPT 2015 AR eng A - Abstrakt
Urban, Pavel - Hanzelka, Pavel - Musilová, Věra - Králík, Tomáš - La Mantia, M. - Srnka, Aleš - Skrbek, L.
Heat transfer in cryogenic helium gas by turbulent Rayleigh-Bénard convection in a cylindrical aspect ratio one cell.
27th International Conference on Low temperature Physics (LT27). Abstracts Book. Buenos Aires: Institute of Physics of Buenos Aires, 2014. s. 90.
[LT27. International Conference on Low Temperature Physics /27./. 06.08.2014-13.08.2014, Buenos Aires]
Grant CEP: GA ČR GA14-02005S
Institucionální podpora: RVO:68081731
Klíčová slova: Rayleigh-Bénard convection * heat transfer efficiency * cryogenic helium
Kód oboru RIV: BK - Mechanika tekutin

Heat transfer efficiency of cryogenic turbulent Rayleigh-Benard convection (RBC) of 4He gas is investigated in a cell 0.3 m both in diameter and height. The Nusselt number Nu(Ra, Pr) scaling is investigated in the range of Rayleigh number Ra from 1E6 up to 1E15, with the Prandtl number Pr varying from 0.7 up to 15. For Ra from 7.2E6 up to 1E11 we observed Nu(Ra) scaling with power exponent 2/7. Up to Ra 1E12, our data could be treated as Oberbeck-Boussinesq. For Ra higher than 1E12, the heat transfer efficiency becomes affected by non Oberbeck-Boussinesq (NOB) effects, causing asymmetry of the top and bottom boundary layers. For Ra from 1E12 up to 1E15, we observed Nu(Ra) scaling with power exponent 1/3 if Nu and Ra are evaluated based on the working fluid properties at directly measured bulk temperature, Tc. In contrast, if the mean temperature is determined as arithmetic mean of the bottom, Tb, and top plate temperatures, Tt, Nu(Ra) with power exponent n displays spurious crossover to higher n that might be misinterpreted as a transition to the ultimate Kraichnan regime. The second step of our analysis is to ignore the NOB effects affecting the top half of the RBC cell. We replace it by the inverted nearly OB bottom half, which leads to the effective temperature difference, DTeff = 2(Tb−Tc), and to effective Nu and Ra values. We discuss the effective heat transfer efficiency, showing no tendency of crossover to the ultimate regime.
Trvalý link: http://hdl.handle.net/11104/0240241