Experimental studies of turbulent Rayleigh-Bénard convection, in a rotating frame and with a liquid-vapor two-phase fluid
Department of Physics, University of California, Santa Barbara
I will introduce two experiments of turbulent Rayleigh-Bénard convection. We conducted measurements of the Nusselt number Nu and the Reynolds number Re in a cylindrical cell of aspect ratio 1 and rotated about a vertical axis at a rate &Omega. When &Omega increased both the Nu and the Re experienced a sharp transition at a Rossby number Ro*=2.5. As results of the Ekman pumping, Nu increased but Re decreased after the transition. At large rotating speeds Nu decreased strongly as predicted by the Taylor-Proudman theorem. I will also present the result that in a rotating frame the large-scale circulation in turbulent Rayleigh-Bénard convection precessed in a retrograde direction with a constant speed due to the Coriolis force.
Below the critical point (CP) at Pc, Tc liquid and vapor co-exist along a line T&phi(P) in the temperature-pressure plane. When a fluid at P < Pc is heated from below and the resulting temperature difference &Delta T = Tb - Tt (Tb and Tt are the temperatures at the bottom and top of the sample respectively) straddles T&phi, then liquid drops condense at the top and vapor bubbles rise from the bottom. By virtue of the latent heat of vaporization this process will contribute strongly to the effective conductivity &lambdaeff of the sample. We measured &lambdaeff using ethane (C2H6) close to but below the CP using a constant &Delta T and varying the temperature fraction parameter &phi =0.5+(Tb + Tt-T&phi)/&Delta T. In the condensation regime 0.5 &le &phi < 1.0, &lambdaeff increased linearly with decreasing &phi. The heat current density through the sample was independent on &Delta T . In the boiling regime 0 &le &phi < 0.5, measurements on the heat transport showed hysteresis. I will show shadowgraph images of both the condensation and the boiling processes that gave qualitative information about the heat transfer mechanics.