Experimental investigation of natural convection in a supercritical binary fluid

We experimentally investigate natural convection of a supercritical nitrogen/argon (0.9/0.1 in molar fraction) binary fluid in a bottom-heated cavity with an aspect ratio of 2.5 in the present study. We obtain the development process of natural convection by the holographic interferometry technique, which is divided into three phases: Stable thermal boundary layer (TBL) phase, Developing phase, and Stable flow phase. After thermal perturbation applied at the bottom, the TBL is thickened and then loses stability and thermal plumes are generated, which signifies the onset of natural convection. Thereafter, natural convection gradually develops to a stable state. As the heat input increases, the experimental results show that the convection grows more intense and spreads deeper into the bulk of the fluid. The TBL in supercritical binary fluid is hydrodynamically more unstable and the natural convection develops faster than the case of a pseudo-pure fluid due to the existence of the Soret effect (SE) and the Dufour effect (DE). The SE and DE are analyzed by comparing the temperature variation in the bulk of a supercritical nitrogen/argon binary fluid with the case of its pseudo-pure fluid counterpart, showing that they enhance the heat transfer in the fluid and further accelerate the development of natural convection.