Experimental research on the turbulent convection heat transfer of supercritical pressure CO2 in a serpentine vertical mini tube

Convection heat transfer characteristics of supercritical pressure CO2 in vertical straight tubes were investigated experimentally and numerically extensively by the researchers. However, in some practical applications, the channel may not be straight but rather serpentine. The integrated effects of buoyancy and centrifugal forces on the convection heat transfer behavior of supercritical fluids in serpentine tubes need to be studied thoroughly. This paper presents experimental investigations of the turbulent convection heat transfer of supercritical pressure CO2 in a serpentine vertical mini tube with an inner diameter of 0.953 mm and curvature diameter of 8.01 mm for various inlet Reynolds numbers, heat fluxes, and flow directions. Infrared temperature measurement was used to measure the continuous distribution of the wall temperature. The effects of variations in thermophysical properties, integrated effect of buoyancy force and centrifugal force were analyzed through comparison the heat transfer performance between the upward and the downward flow in serpentine tube and straight tube under similar experimental conditions. It was found that the heat transfer performance was better in the serpentine tube than in the straight tube because of the secondary flow attributable to centrifugal forces. At relative low Bo∗, the heat transfer in the serpentine vertical tube for downward flow performed better than upward flow due to the effect of gravitational buoyancy on the intensity of turbulence. At relative high Bo∗, in contrast with the existing research on turbulent convection heat transfer of supercritical fluid in straight vertical tubes, the turbulent convection heat transfer in the serpentine vertical tube for upward flow performed better than downward flow and no heat transfer deterioration occurred in the serpentine vertical tube.