Influence of buoyancy on heat transfer to water flowing in horizontal tubes under supercritical pressure

The present paper was devoted to clarifying the effect of buoyancy on heat transfer characteristics of supercritical pressure water flowing in horizontal tubes. Experiments were conducted in horizontal tubes with inner diameters of 26 mm and 43 mm. Operating conditions included mass fluxes of 300-1000 kg/m2 s, heat fluxes up to 400 kW/m2, and a pressure of 25 MPa. Based on the experimental data, the difference of heat transfer characteristics between the top and bottom surfaces of a horizontal tube was analyzed. It was found that the buoyancy effect makes the low density hot water gather at the top surface of the horizontal tube; hence, heat transfer condition is deteriorated and wall temperature is increased. The effects of mass flux, flow direction and tube diameters on the heat transfer characteristics of water in horizontal tubes were discussed in detail. Jackson and Petukhov buoyancy criterions were selected to judge the buoyancy effect in horizontal tubes. The results showed that when the buoyancy effect is significant, both criterions are able to predict the onset of buoyancy effect. Furthermore, numerical simulations were carried out by using Fluent solver, aiming to extending experimental research on buoyancy effect.