Experimental investigation on validity of buoyancy parameters to heat transfer of CO2 at supercritical pressures in a horizontal tube

Heat transfer in a horizontal tube represents different behaviors at top and bottom walls. Buoyancy and bulk flow acceleration, which are induced by drastic variation of properties, especially density, are major reasons for such a difference. In this study, in order to investigate the effect of buoyancy on heat transfer of supercritical CO2, the validities of buoyancy parameters including BuC and BuJ which is a modified form of BuC were experimentally examined in the horizontal tube. The test section had an inner diameter of 7.75 mm and a heated length of 0.91 m. The experimental variables were mass flux (64.1-250.5 kg/(m2 s)) and heat flux (3.1-25.9 kW/m2) with constant inlet temperature (30°C) and pressures (75.87-77.35 bar). As a result, the heat transfer transition from enhancement to deterioration occurs earlier at the top wall than at the bottom wall because hot and low-density fluid moves toward the top wall by buoyancy. The previously suggested thresholds of 10−3 for BuC and 10 for BuJ are merely applicable to determine the existence of buoyancy. We concludes that BuJ shows the improved prediction compared to BuC. While, bulk flow acceleration is closely related to the heat transfer behavior along the flow direction rather than buoyancy.