Experimental investigation of buoyancy effects on local heat transfer of supercritical pressure CO2 in horizontal semicircular tube

To estimate the characteristics of local heat transfer in the channel of a printed circuit heat exchanger (PCHE), a semicircular channel (tube) was manufactured to replicate the channel shape of the PCHE. The heat transfer in a horizontal semicircular tube has different behaviors at the top and bottom walls. Buoyancy and flow acceleration have substantial influence on the convective heat transfer of a semicircular tube with supercritical pressure fluids. They are induced by a drastic variation in the thermophysical properties near the critical point, especially density. Thus, in this study, to investigate the effect of buoyancy and flow acceleration on the heat transfer of supercritical CO2, an experiment was conducted in a horizontal semicircular tube. The tube test section had a hydraulic diameter of 4.73 mm, width of 7.75 mm, depth of 3.88 mm, and a heated length of 1.0 m. The experimental variables were mass flux (70–200 kg/(m2·s)) and heat flux (14.6–50.7 kW/m2) with constant inlet temperature (30 °C) and pressure (7.7–7.8 MPa). The temperature of the outer wall was measured at four positions (top center (TC), top side (TS), bottom side (BS), and bottom center (BC)) in the cross section of the tube. The stratification of the temperature along the wall in the cross section of the semicircular tube was verified by the experimental results. The heat transfer deterioration at the top region (TC and TS) was greater than that at the bottom region (BS and BC). This difference in heat transfer deterioration was caused by the upward push of the buoyancy of the low-density fluid along the inner wall in the cross section of the tube.