Experimental study of the effects of flow acceleration and buoyancy on heat transfer in a supercritical fluid flow in a circular tube

Abstract

Experiments on turbulent heat transfer by supercritical CO2 in a vertical upward flow were conducted in a tube with an inner diameter of 4.5 mm. The experiments were performed for bulk fluid temperatures ranging from 29 to 115 degrees C, pressures ranging from 74.6 to 102.6 bar, local wall heat fluxes ranging from 38 to 234 kW/m(2), and mass fluxes ranging from 208 to 874 kg/m(2) s. The wall temperature distributions were significantly influenced by wall heat flux and mass flux. The wall temperature had a noticeable peak value when the wall heat flux was moderate and the mass flux was low. To determine the buoyancy and flow acceleration effects on heat-transfer characteristics, the ratios of the Nusselt numbers obtained from the experimental data and from a reference correlation are compared with Bo* and q(+) along the test section. To analyze the changes in the shear stress distribution due to flow acceleration and buoyancy effects, the ratios of the shear stress reduction to wall shear stress due to flow acceleration and buoyancy effects are derived from approximate considerations. A new heat-transfer correlation is proposed, which could be used to predict heat transfer phenomena in a vertical upward flow of a supercritical fluid. The correlation is assessed by comparison with various experimental data. (C) 2010 Elsevier B.V. All rights reserved.