Numerical study on the effects of cylindrical roughness on heat transfer performance and entropy generation of supercritical carbon dioxide in vertical tubes

Abstract

In view of heat transfer deterioration(HTD), rough tubes may help improve heat transfer performance and reduce irreversibility simultaneously. The effects of the tube with cylindrical roughness (CRT) on heat transfer and entropy generation of S-CO2 are studied numerically. The considered variables are the protrusion's axial spacing(l/L), height(e/d), width(w/d) and circumferential angle(α/2π), in the range of l/L = 1/50∼1, e/d = 0∼0.0656, w/d = 0∼0.153 and α/2π=1/12∼1. The results show CRT helps improve heat transfer, the heat transfer coefficient increases about 20% as e/d, w/d increases or l/L, α/2π decreases. Regarding irreversibility, on the one hand, CRT helps entropy generation drop in the HTD area. With e/d, w/d increasing or l/L, α/2π decreasing, entropy generation in the HTD area decreases. On the other hand, CRT causes entropy generation to rise in the non-HTD area. In the non-HTD area, with l/L increasing, entropy generation increases, with e/d, w/d increasing or α/2π decreasing, entropy generation initially increases and then essentially remains constant. The overall entropy generation through the whole tube is the combination of these two areas. In some ranges, the heat transfer coefficient and overall entropy generation of CRT are both better than those of smooth tubes. Because the disturbance effect of protrusions promotes the generation and diffusion of turbulent kinetic energy, thereby alleviating heat transfer deterioration. This study may provide references for S-CO2 heat transfer enhancement and the design of related components.