Helically coiled tube are widely used as the basic heat transfer elements in steam generators of the next generation reactors, such as HTR-PM (High Temperature Gas-cooled Reactor), IRIS (International Reactor Innovative and Secure) and SMART (System-integrated Modular Advanced Reactor), because of the advantages in reducing space, enhancing heat transfer, accommodating thermal stress and preventing two-phase flow instabilities. Owing to the presence of gravity and centrifugal force that being perpendicular to the main flow, two-phase flow in helically coiled tubes has different features with either vertical flow or horizontal flow. To ensure safety and reliability of the plant, it is necessary to carry out detail investigation on the two-phase flow phenomena and mechanisms in helically coiled tubes. However, less research has been carried out on this subject than on straight tubes. In this work, the upward air-water slug and plug flows in helically coiled tubes have been numerically analyzed based on the computational fluid dynamics (CFD) techniques. Three dimension models of helically coiled tubes with inner diameter of 16 mm, coil diameter of 0.1 and 0.4m, pitch of 0.08 and 0.16m are constructed, for which the structural meshes are generated by software ANSYS ICEM. The gas-liquid interface is captured by the volume of fluid (VOF) approach adopting geo-reconstruction scheme for interface interpolation, which is solved by a pressure-based transient solver in the commercial CFD software ANSYS FLUENT 14.5. Bubble chord length, slug/plug frequency, bubble velocity and void fraction under different superficial velocities have been investigated. The numerical results meet well with the pictures recorded by a high speed camera. It is revealed that in slug regime, the bubbles mainly migrate towards the top and inner wall of the tube due to the combined action of gravity and centrifugal force, leading to a highly asymmetrical internal phase distributions. Meanwhile, the secondary flow in the cross section introduced by the centrifugal force enhances the turbulence and prevents small bubbles to coalescent into enlarged bubbles. Accordingly the intermittent flow regime in helically coiled tubes is narrower than that in straight horizontal tubes. Furthermore, the influences of geometrical parameters on phase distribution characteristics are predicted. The results show that the bubble length will increase along with the increase of the coil diameter or the pitch of the helically coiled tube. And the bubble frequency will increase with the decreasing of the tube coil diameter.

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