Natural convection heat transfer of supercritical binary fluid in a long closed vertical cylinder

We study the natural convection heat transfer of supercritical N₂-Ar binary fluid in a closed vertical cylinder with an aspect ratio of 27 in the Rayleigh number range of 9.0 × 10¹¹-2.0 × 10¹⁴. The cylinder is heated at the lower part and cooled from the top surface. The composition of the binary fluid and the average temperature of the cold part of the cylinder are varied to investigate the heat transfer characteristics of natural convection. Three different regimes are found with the increase of Ra, i.e., Laminar thermal boundary layer (TBL) regime, Transition regime, and Ultimate regime, with the two critical Rayleigh numbers determined experimentally to be Ra_crit1 ≈ 4.0 × 10¹³ and Ra_crit2 ≈ 1.14 × 10¹⁴, respectively. The scalings of natural convection in the Laminar TBL regime and Ultimate regime are Nu ∝ Ra^0.22 and Nu ∝ Ra^1/3, respectively. When natural convection experiences near-critical temperature range, the drastic variations of the thermo-physical properties lead to an anomalous phenomenon where an inflection appears in the Nu-Ra correlation with an increase in heat transfer rate, and the corresponding temperature of the inflection point becomes higher for the fluid with higher critical temperature. The binary components in the fluid show no apparent effect on the heat transfer characteristics of natural convection except affecting the corresponding temperature of the inflection point in the present study, and the binary fluid behaves as a pseudo-pure fluid (an assumed pure fluid with the same thermo-physical properties as the corresponding binary fluid).