Experimental and numerical investigations on the influence of buoyancy on supercritical water heat transfer characteristics in inclined smooth tubes

Abstract

The influence of the varying buoyancy effects on heat transfer characteristics was experimentally and numerically investigated. Different pressures, heat fluxes, and inclination angles were selected to alter the buoyancy effect. Operating parameters were varied (Pressure varied from 25 MPa to 29 MPa, mass flux varied from 800 kg/(m2·s) to 2294 kg/(m2·s), heat flux varied from 500 kW/m2 to 714 kW/m2, and the inclination angle varied from 0° to 90°). The comparison of heat transfer characteristics between 2000 kg/(m2·s) and 2294 kg/(m2·s) was conducted by experiments, showing that the average heat transfer coefficients at G = 2000 kg/(m2·s) are better than those at G = 2294 kg/(m2·s) due to the different buoyancy effects. The comparison between 650 kW/m2 and 714 kW/m2 showed that the heat transfer characteristics improve with the increase in heat flux under a high mass flux condition. Moreover, a 3-D numerical model was built for predicting the heat transfer characteristics in inclined tubes. The influence of buoyancy effects induced by the different inclination angles on the heat transfer characteristics was investigated numerically. The numerical results showed that the buoyancy effect is evident at G = 800 kg/(m2·s). The buoyancy effects reflect in the velocity of secondary flow, fluid temperature, and temperature gradient of boundary layers. The influence of pressure on the heat transfer characteristics reflects in the radial distributions of properties. The temperature and temperature gradient are higher at the bottom of the 75° inclined tube and the top of the 15° inclined tube, resulting in a decline in the heat transfer coefficients. Finally, the ratio of vorticity and Reynold’s numbers (Se/Re) was adopted as a criterion to predict the buoyancy effects in the inclined tubes, and the results showed that Se/Re is suitable for evaluating buoyancy effect.