Numerical investigation on the mixed convection and heat transfer of supercritical water in horizontal tubes in the large specific heat region

Numerical simulation is performed in the present study to get further insight into the mechanism of heat transfer phenomena of water in ∅32 × 3 mm horizontal smooth tubes under supercritical pressures. Both the heat transfer enhancement (HTE) and heat transfer deterioration (HTD) in the so-called large specific heat region (LSHR) of supercritical fluids are analyzed based on the numerical results. The governing equations of the fluid are solved on fixed three-dimensional grid systems, and the RNG-k–ε model with enhanced wall treatment method is employed to handle the coupled wall-to-fluid heat transfer. The numerical results are compared with the corresponding experimental data, and a good agreement is achieved, indicating good accuracy and reliability of the numerical method used in this study. Variation of the tube inner wall temperature and the local heat transfer coefficient with a few parameters, such as the local water enthalpy, the water mass flow rate and the heat flux on the tube wall, are obtained under various conditions. It is showed that in the large specific heat region (LSHR), there exists strong non-uniformity in the circumferential distribution of the tube inner wall temperature, and as well, the heat flux on the inner tube wall are distinctly non-uniform along the tube wall circumference. Further analysis of the numerical data shows that the above-mentioned non-uniformity is mainly due to the rapid change in fluid properties, which results complex secondary flows and mixed convection in the supercritical fluid. Based on the numerical results, a critical value is obtained for the parameter Gr/Re2.7, which is believed to be a key parameter affecting the secondary flow in the LSHR. In the present study, the critical value for Gr/Re2.7 is determined to be 1.5 × 10−5, above which the HTD phenomenon may be observed.

► Strong non-uniformity is obvious, especially with the cases of high heating loads.
► Thermal acceleration and buoyancy play important roles to non-uniformity.
► Evolution of secondary vortices is closely related to heat transfer deterioration.
► Local heat transfer deterioration phenomenon appears if Gr/Re2.7 > 1.5 × 10−5.