Numerical study of the heat transfer and flow stability of water at supercritical pressures in a vertical tube

The subcritical steam generators in the current High-Temperature-Reactor Pebble-Bed Module, HTR-PM, in China can be replaced by supercritical steam generators to better coordinate the reactors and the supercritical steam turbines to improve thermal efficiencies with no phase change at supercritical pressures. This study numerically analyzed the heat transfer and the flow instabilities of supercritical pressure water. Five turbulence models were evaluated to model the heat transfer with a constant wall heat flux for steady-state calculations with the results showing that the RNG k-ε model with the enhanced wall treatment gave the best results. This model was then used in transient calculations with the wall heat flux increasing linearly with time. The flow and heat transfer characteristics and the velocity and turbulence fields at various times were used to analyze the influences of flow rate variations, thermophysical property variations, buoyancy and flow acceleration. The effects of the fluid inlet enthalpy and pressure drop along the tube on the flow instabilities were also studied.