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

Rayleigh-Bénard convection is investigated with sulfur hexafluoride (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 ΔT 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(ΔT,ε) in both domains, with ε 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 ε→0.