Thermal Radiation in Rayleigh-B\'{e}nard Convection Experiments.

An important question in turbulent Rayleigh-B\'{e}nard convection (RBC) is the effectiveness of convective heat transport, which is conveniently described via the scaling of the Nusselt number (${\rm{Nu}}$) with the Rayleigh (${\rm{Ra}}$) and Prandtl (${\rm{Pr}}$) 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 ${\rm{Nu}}$ measured in different experiments. A careful inclusion of the radiative transport appreciably changes the ${\rm{Nu}}={\rm{Nu}}({\rm{Ra}})$ 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.