Mixed convection heat transfer in heated tubes has been studied extensively in the past decades, which is widely used in various industrial fields such as cooling of a nuclear reactor core. The secondary flow, which is induced by buoyancy force, has been found in previous research to have profound influence on the heat transfer difference on circumferential position and occurrence of heat transfer deterioration in horizontal heated channel. Therefore, understanding the secondary flow velocity field has important implications to prevent heat transfer deterioration and ensure the safe operation of nuclear power plants. Numerical methods have been adopted in literature to analyze the complex interaction between secondary flow and heat transfer deterioration. However, to the knowledge of the author, experimental measurement of secondary flow in the radial cross-section of a horizontal tube does not exist.

In this paper, a novel measurement method, which combines the transparent heating and PIV (Particle Image Velocimetry) technology, has been adopted to experimentally investigate the secondary flow velocity field on the radial cross-section in a horizontal heated tube. The heat transfer deterioration mechanism is revealed through analysis of the distribution of secondary flow along circumference direction at low mass flow rates and high heat flux conditions. We found that the buoyancy force lead the hot fluid to rise along the tube wall from bottom to top. While the secondary flow is most intensive near the middle of the interface, the secondary velocities are high at the bottom of the cross-section, where the tube wall is well cooled by cold fluid descends from the central part of the cross-section. Near the top of the tube wall, the secondary velocities are very small and the thermal acceleration effect makes the fluid rise. As a result, the mixed convection of top and center part of cross-section is weak and heat is primarily transferred by conduction, which leads the occurrence of thermal stratification of fluid. Consequently, the thermal accumulation of fluid in the top leads to heat transfer deterioration. Moreover, thermal properties differences between Freon (FC-72) and water, especially the Prandtl number (Pr), make the occurrence of heat transfer deterioration much easier for FC-72 than water with same working conditions.

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