Numerical study on heat transfer characteris tics of supercritical water in straight and helical tubes

The subcritical steam generators in high-temperature gas-cooled reactor pebble-bed modules can be replaced by supercritical steam generators to improve thermal efficiency. The axial and circumferential nonuniform flow and heat transfer characteristics of supercritical water in straight and helical tubes with varying inclinations (0-90 degrees) are simulated to investigate the combined effects of tube structure, flow direction, thermophysical properties, property variations, buoyancy, and centrifugal forces. With heat fluxes of 233-930 kW/m2 and inlet mass fluxes of 225 or 1260 kg/(m2 s) under a working pressure of 24.5 MPa, the friction coefficients, turbulent kinetic energy, secondary flow, temperature, and heat transfer coefficients are studied in the cross section of variable structure tubes. The wall temperature and heat transfer coefficients of four regions are compared in straight and helical tubes; the maximum heat transfer coefficient appeared before the pseudo-critical point. The heat transfer coefficients are the greatest at the bottom of the straight tubes, where fluidity was high, and at the outer of the helical tubes, where the fluid velocity was high under centrifugal force. The secondary flow in the helical tubes causes a nonuniformity in the wall temperature, causing the circumferential maximum wall temperature to move.