Heat transfer deterioration in helically coiled tubes with supercritical water: A numerical study on the impact of cross-sectional shapes

Supercritical water flow in helically coiled tubes (HCTs) offers a promising solution for high-efficiency, compact heat exchangers in advanced power systems. This study conducts a numerical investigation of wall temperature and heat transfer coefficient ( h ) in circular HCT (C-HCT), horizontally elliptical HCT (HE-HCT), and vertically elliptical HCT (VE-HCT) at 25 MPa. Numerical analysis reveals that local heat transfer coefficient ( h local ) distributions exhibit distinct sensitivities to cross-sectional shapes: the order of h local at the top of the inner wall of the tube is HE-HCT > C-HCT > VE-HCT, whereas at the inner side of the inner wall of the tube, the order is VE-HCT > C-HCT > HE-HCT, with the h local at the inner side of VE-HCT being 41.1% higher than C-HCT at high heat flux. As bulk fluid temperature or mass flux increases, centrifugal force dominates, and temperature and velocity distributions become increasingly symmetrical, and the location with the lowest h local moves inward. In terms of the average h on the cross-section, C-HCT outperforms HE-HCT and VE-HCT across the investigated bulk fluid enthalpy range, demonstrating its superior overall heat transfer efficiency. At high mass flux, although VE-HCT exhibit a 63.1% lower circumferential wall temperature inhomogeneity compared to C-HCT, indicating improved temperature uniformity, this comes at the expense of heat transfer performance. The average h on the cross-section for VE-HCT is only 71% of that for C-HCT. These results could provide essential theoretical insights and actionable recommendations for the design of high-performance heat exchangers in the lead-cooled fast reactors.