Experimental study on Rayleigh–Bénard convection during supercritical phase transition of carbon dioxide

Supercritical fluid is a kind of special fluid under extreme conditions (temperature and pressure are above the critical point). The Rayleigh–Benard (RB) convection of supercritical fluid driven by buoyancy is a new nonlinear thermal convection system. Its buoyancy does not conform to the Boussinesq approximation. Under the action of temperature difference, the physical properties of RB fluid show severe distortion near the critical point, accompanied by abundant flow and phase transition coupling processes. In this experiment, a transparent sapphire pressure vessel capable of carrying supercritical carbon dioxide (SCO₂) was designed to establish RB convection of supercritical fluid under the effect of vertical temperature gradient. The flow structure and supercritical phase transition process under different temperature differences were observed. The velocity field of “atomized” droplets was calculated by image cross-correlation algorithm. In the experiment, platinum resistance temperature measurement was used to accurately control the temperature of the upper and lower ends of the container, and the evolution of various flow modes and velocity fields in the linear cooling process was studied. In the linear cooling process, SCO₂ goes through three typical processes: supercritical flow, transcritical flow and gas liquid two-phase flow. The strong coupling of the transcritical phase transition with buoyancy convection results in the heterogeneous unsteady flow of supercritical carbon dioxide RB convection. It shows that the supercritical RB convection is extremely sensitive to temperature difference, and the larger the temperature difference is, the more intense the convection in the supercritical domain is. With the decrease of temperature, the atomized droplets condense to form abundant structure of multi-layer flows, which finally turn to the two-phase flow.