Thermal radiation in Rayleigh-Bénard convection experiments

Abstract

An important question in turbulent Rayleigh-Bénard convection (RBC) is the effectiveness of convective heat transport, which is conveniently described via the scaling of the Nusselt number (Nu) with the Rayleigh (Ra) and Prandtl (Pr) numbers. In RBC experiments, the heat supplied to the bottom plate is also partly transferred by thermal radiation. This heat transport channel, acting in parallel with the convective and conductive heat transport channels, is usually considered insignificant and thus neglected. Here we present a detailed analysis of conventional far-field as well as strongly enhanced near-field radiative heat transport occurring in various RBC experiments, and show that the radiative heat transfer partly explains differences in Nu measured in different experiments. A careful inclusion of the radiative transport appreciably changes the Nu = Nu(Ra) scaling inferred in turbulent RBC experiments near ambient temperature utilizing gaseous nitrogen and sulphur hexafluoride as working fluids. On the other hand, neither the conventional far-field radiation nor the strongly enhanced near-field radiative heat transport appreciably affects the heat transport law deduced in cryogenic helium RBC experiments.

Direct measurements of the thermal dissipation rate in turbulent Rayleigh–Bénard convection

“…On the other hand, the maximum Rayleigh number in a large aspect ratio cell is much smaller than in a small aspect ratio cell with the same lateral dimension. This is the main reason why experiments [12][13][14][15] and numerical simulations [16][17][18][19][20] are mostly aimed toward aspect ratios of the order of one or even smaller. There are a few papers on flat geometries, but none of them deals directly with thermal dissipation rate.…”

Effect of boundary conditions in turbulent thermal convection (a)

“…The second group includes the actual physical properties of the working fluid as well as of construction materials of the RBC cell, such as thermal conductivity and heat capacity of plates (λ p ; c p ) and walls, the thermal conductivity of the electrical leads and, generally, the physical properties of the surrounding medium. Although various approaches to correct the raw data with respect to finite thermal conductivity of plates [10] and walls [11], parasitic heat leaks, adiabatic thermal gradient, thermal radiation [12] or non-OB effects [9,[13][14][15] have been attempted, it is very difficult if not impossible to fully eliminate all these factors. In order to single out and appreciate the role of BC on the RBC flow under study, it therefore seems the best to perform the experiment under the same conditions while changing the BC only.…”