Rayleigh-Bénard Convection with Phase Change Close to the Critical Point

Rayleigh-B\'enard convection is investigated with sulfur hexafluoride (${\mathrm{SF}}_{6}$) in the vicinity of its critical point. In the supercritical domain, direct measurements of the heat flux $Q$ as a function of the temperature difference $\mathrm{\ensuremath{\Delta}}T$ are consistent with the usual scaling laws of single-phase turbulent convection. Along the liquid-vapor coexistence curve, heat fluxes are dramatically enhanced by condensation and boiling. Optical measurements are performed to document the size and velocity of the bubbles. We report $Q(\mathrm{\ensuremath{\Delta}}T,\ensuremath{\epsilon})$ in both domains, with $\ensuremath{\epsilon}$ the dimensionless distance to the critical point. Critical scaling laws are observed that can guide the development of theoretical models. In addition, this documents a system of diverging heat transfer coefficient, i.e., in which a significant heat flux can be achieved with an arbitrarily small temperature difference as $\ensuremath{\epsilon}\ensuremath{\rightarrow}0$.