Experimental investigation of buoyancy effects on the heat transfer of supercritical carbon dioxide in a vertical tube.

• The heat transfer characteristics of supercritical CO 2 are studied experimentally. • The mechanism of heat transfer deterioration is analyzed. • Two buoyancy criteria are assessed and compared. • A new heat transfer correlation with a buoyancy parameter is proposed. This study experimentally investigates the effect of buoyancy on the heat transfer characteristics of supercritical CO 2 in a vertical tube of 7.74 mm inner diameter. The experiments are performed for inlet temperatures ranging from 14 to 33 °C, pressure from 7.8 to 10 MPa, heat flux from 29.0 to 60.3 kW/m2, and mass flux from 138.3 to 512.1 kg/m2·s. The influences of heat flux, mass flux, pressure, inlet temperature, and flow direction on the heat transfer are analyzed with an emphasis on the buoyancy effect. The results show there is only one wall temperature peak at moderate heat flux and two peaks at high heat flux. The heat transfer under different mass fluxes is completely different due to the buoyancy effect. Current experimental data differ significantly from widely used empirical correlations. Therefore, a new correlation with a buoyancy parameter is developed, which provides better prediction accuracy of the Nussult number. [ABSTRACT FROM AUTHOR]