Heat transfer deterioration and circumferential wall temperature inhomogeneity in a helically coiled tube carrying supercritical water: An intensive numerical simulation

Abstract

Analysis of the flow and heat transfer characteristics of supercritical water (SCW) become complicated by the extensive changes in the physical properties of SCW near its pseudo-critical line and by the secondary flow stemming from the buoyancy and centrifugal force in a helically coiled tube (HCT). To address the heat transfer deterioration (HTD) of SCW in HCTs, we investigated the distribution of circumferential wall temperatures and heat transfer coefficients of SCW in a vertical HCT under the HTD condition via numerical simulation. Wall temperature was highest on the inner side, and the local heat transfer coefficient was the lowest. With increasing heat flux, the wall temperature of each circumferential position around the tube rose, while the inner wall temperature rose the most. To explain the mechanism of HTD, the rapid decrease of the specific heat and thermal conductivity near the inner side played a certain role in deteriorating the heat transfer. Further, the buoyancy effect, and the thermal acceleration effect could be used to quantitatively assess the HTD condition. The threshold values of the dimensionless number Bu describing buoyancy effect and the dimensionless number Ac describing thermal acceleration effect were found to be 3.0 × 10−6 and 1.28 × 10−6, respectively. The concept of circumferential wall temperature inhomogeneity (CWTI) was used to quantify differences in local wall temperature around the HCT. CWTI significantly increased with increasing heat flux. At low heat flux levels (qw = 300–700 kW m−2), the CWTI decreased at first, and then increases. When the heat transfer deteriorated with increasing heat flux, the trend of CWTI reversed. These results indicated that differences in circumferential wall temperature became larger under HTD conditions. Increasing pressure reduced the physical property changes in the HCT, leading to more uniform wall temperatures and a smaller CWTI. Under a constant ratio of heat flux to mass velocity, increasing the mass velocity weakens the heat transfer and even triggers HTD. In contrast, increasing pressure suppresses HTD.