Yuanzineng kexue jishu (Oct 2023)
Numerical Study on Rising Behavior of Single Bubble in Petal-shaped Rod Bundle Channel
Abstract
Bubble behavior has a significant impact on the characteristics of two-phase flow and heat transfer. To investigate the dynamics of bubbles in a petal-shaped rod bundle channel, the behavior of bubbles in static water in the rod bundle channel was simulated using the volume of fluid method (VOF) and compared with the behavior of moving bubbles in a circular rod bundle channel. The trajectory, shape, and rise velocity of bubbles with different initial sizes were obtained and analyzed to understand the mechanism of bubble behavior through the flow and pressure fields around the bubbles. The results show that bubbles exhibit lateral movement during the ascent process. The smaller the bubble size, the more pronounced the lateral movement, making it more likely for the bubble to touch the wall surface. After touching the wall surface, the bubble slides along the wall surface. Bubbles that do not touch the wall move towards the center of the flow channel and finally rise steadily along the center. The calculated size range produces spherical and ellipsoidal bubble shapes. Before the bubble touches the wall, the bubble deformation increases and the aspect ratio decreases as the bubble gradually rises, reaching stability after a period of time. While after the bubble touches the wall, the bubble adheres to the wall and its shape changes to semi-circular and its aspect ratio decreases rapidly. The stabilization rate of bubble increases as the size of the bubble increases. In the first stage, the bubble velocity rises rapidly from zero and the wake begins to form. In the second stage, the bubble velocity rises slowly and the wake changes asymmetrically at both ends of the bubble. In the third stage, the bubble velocity decreases rapidly if it touches the wall, and the shape of the wake changes near the wall. If the bubble does not touch the wall, the bubble velocity will fluctuate around the stable velocity and the flow field will stabilize, and the wake will be generated and shed in a continuous cycle. The direction of bubble movement can be determined by the pressure distribution around the bubble. Compared to the circular rod channel, the bubbles in the petal-shaped fuel rod channel have a greater lateral displacement and are more likely to touch the wall, while the bubble shape is less variable. The study of bubble upward motion can clarify the bubble behaviour in the petal-shaped rod bundle channel and provide a basis for revealing the two-phase flow and heat transfer mechanism.
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