Convection - Experiments

Turbulent non-Oberbeck Boussinesq convection

For simplicity, most theoretical and numerical studies on convection assume Oberbeck-Boussinesq (OB) conditions, which can be briefly summarized as - Density ρ, Isobaric heat capacity c_p, thermal diffusivity κ, viscosity ν and isobaric expansion coefficient α are independent of temperature T and pressure p, while only for the buoyancy one assumes a that the density depend linearly on the temperature with coefficient α. In many natural and industrial convection systems, however, temperature differences are very large and thus the OB-assumption is violated.

We study Rayleigh-Bénard Convection systems beyond OB approximation using Sulphur Hexafluoride (SF₆) near its critical point, where material parameters vary significantly with temperature and pressure. We focus on the two unique characteristics of such system: the deviation of the centre temperature T_c
from mean temperature T_m due to the broken symmetry, and its relation to the change in global heat transport efficiency Nu/Nu_OB. There are two goals of the study: 1. to generalized current theories and understandings on the OB system to real-world fluids 2. to eliminate NOB effects in the search of ultimate states.

Phase diagram of SF6. Color code represents the Prandtl number. The green line marks the phase boundary between liquid and gas. The green dot marks the critical point (Tcrit=45.5 degC, Pcrit = 37.6 bar). We take measurements along isolines of Pr (black curve) and adjust Δ (black horizontal lines), such that also Ra stays constant during the experiment. — Phase diagram of SF₆. Color code represents the Prandtl number. The green line marks the phase boundary between liquid and gas. The green dot marks the critical point (T_crit=45.5 degC, P_crit = 37.6 bar). We take measurements along isolines of Pr (black curve) and adjust Δ (black horizontal lines), such that also Ra stays constant during the experiment.

Phase diagram of SF₆. Color code represents the Prandtl number. The green line marks the phase boundary between liquid and gas. The green dot marks the critical point (T_crit=45.5 degC, P_crit = 37.6 bar). We take measurements along isolines of Pr (black curve) and adjust Δ (black horizontal lines), such that also Ra stays constant during the experiment.

See also:
Weiss, S., He, X., Ahlers, G., Bodenschatz, E., & Shishkina, O. . Bulk temperature and heat transport in turbulent Rayleigh–Bénard convection of fluids with temperature-dependent properties. Journal of Fluid Mechanics, 851, 374-390 (2018).