Investigation of Buoyancy Effects on Supercritical CO2 Heat Transfer Away From Pseudocritical Temperature

Abstract Buoyancy effects on heat transfer in supercritical CO2 flows are an interesting phenomenon that can either enhance or decrease the amount of expected heat transfer. This paper presents experimental and numerical results to investigate diminishing buoyancy effects on heat transfer in supercritical CO2 flows away from the pseudocritical temperature. The focus of analysis here is the circumferential variation of heat transfer rather than circumferentially averaged heat transfer to investigate local effects. In this study, a circular tube with an internal diameter of 9.4 mm was tested under a fixed mass flux of 180 kg/m2 s and a constant heat flux of 44 kW/m2. The Reynolds number, based on test conditions, ranged from 50,000 to 80,000, varying significantly along the tube due to pronounced changes in fluid properties as a function of local bulk temperature. A complementary numerical study was conducted to extend the analysis beyond the experimental range of pressures and temperatures. At a bulk temperature of 200 °C and an inlet pressure of 85 bar, the maximum circumferential variation in the Nusselt number was 10%. This indicates substantial buoyancy effects on heat transfer, even at temperatures above the pseudocritical point. The diminishing influence of buoyancy on heat transfer with increasing temperature is effectively captured by the ratio of modified Grashof numbers, as adopted from previous studies in the literature.