Analysis Of Bubble Behaviour In The Channel Of A Petal-shaped Fuel Rod Bundle

With the continuous growth in demand for nuclear energy,there arises a need to further optimize nuclear reactors.Petal-shaped spiral rods,with their large body surface ratio and self-helix design,present a novel solution to enhance the power density of the reactor.However,the presence of bubbles around the fuel rods can significantly impact the thermodynamic characteristics of the flow channel,necessitating a thorough study of their behavior in the fuel assembly rod bundle channel.In order to study the kinetic characteristics of bubbles in the petal-shaped rod bundle channel,the bubble behavior of bubbles in static water in the rod bundle channel was simulated by the volume of fluid method(VOF)and compared with the moving bubbles in the circular rod bundle channel to obtain the trajectory,shape,and rise velocity of bubbles with different initial sizes,and the bubble behavior mechanism was analyzed by the flow field and pressure field around the bubbles.The results show that there is lateral movement of bubbles in the rising process.The smaller the bubble size is,the more obvious the lateral movement is,the easier the bubble touches the wall,and the bubble slides along the wall after touching the wall.The bubbles that did not touch the wall moved toward the center of the flow channel,and finally rose steadily along the center.The bubble shapes are spherical and ellipsoidal in the calculated size range.As the bubble gradually rises,the deformation of the bubble increases,the aspect ratio decreases and reaches stability after a period of time;as the bubble size increases,the bubble stabilization rate increases.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 motion can be judged by the pressure distribution around the bubble.Compared with the circular bar channel,the lateral displacement of bubbles in the petal-shaped fuel bar channel is larger and more likely to touch the wall,while the bubble shape changes less.By simulating the rising motion of the bubbles in the rod beam channel with different inlet flow rates,the trajectory,rising speed and shape changes of the bubbles with different initial sizes were obtained,and the flow and pressure fields around the bubbles were analyzed.The bubble motion law is summarized,and the rising motion and wall touching behavior of bubbles are analyzed and predicted.The results show that before the bubble touches the wall,with the increase of the initial size of the bubble,the transverse displacement of the bubble in the radius range of 0.5 mm-1.25 mm gradually increases,and the deformation degree of the bubble and its maximum rise velocity gradually decreases.The 1.5 mm radius bubble has a small transverse displacement due to its large size and contact with multiple fuel rods during the ascent.As the initial flow velocity of the fluid in the channel increases,the transverse displacement distance of the bubble increases and then decreases,and the aspect ratio of the bubble is not affected by the inlet flow velocity.The study of bubble upward motion can clarify the bubble behavior mechanism in the petal-shaped rod bundle channel,and lay the foundation for the study of two-phase flow and heat transfer in the channel.