Study On Subcooled Flow Boiling Bubble Departure Characteristics Of Petal-shaped Fuel Rods

The implementation of the carbon peaking and carbon neutrality strategy has promoted the development of nuclear energy and other clean energy sources.Breakthroughs in technologies such as small nuclear reactors are the main direction of nuclear energy development.Petal-shaped fuel rods are an important tool for developing small reactors by improving the performance of nuclear reactors through structural optimization.In this paper,I use numerical simulation and mechanistic modeling to study the behavior of vapor bubbles in the rod bundle channel of petal-shaped fuel rod assemblies,obtain the characteristics of vapor bubble behavior under different heating conditions,analyze the vapor bubble departure behavior and its influencing factors,and construct as well as verify the vapor bubble departure dynamic model.This study will help to understand the boiling heat transfer characteristics of petal-shaped fuel rods and lay the foundation for their industrial applications.A two-dimensional simulation study of the behavior of subcooled boiling vapor bubbles of petal-shaped fuel rods was carried out based on the VOF method coupling phase change model,and the behavior of vapor bubbles under uniform heat flux,uniform wall temperature,and non-uniform wall temperature heating conditions was calculated and analyzed.The results show that the vapor bubbles under uniform heat flux and uniform wall temperature go through the life process of generation,growth,lift-off,and condensation,and there is no significant lift-off behavior under non-uniform wall temperature.There are differences in the generation time,number and shape of vapor bubbles under different heating conditions.Vapor bubble generation induces flow,promotes vapor bubble growth and enhances heat transfer effect.Vapor bubble mergence increases local flow velocity and improves heat transfer efficiency.Meanwhile,under different heating conditions,vapor bubbles are always generated at the center of the concave arc of the fuel rod first,so the location of the initial vapor bubbles in the 3D simulation is determined.Three microlayer models of initial thickness,time-varying and wall superheat are compared and analyzed by combining relevant experimental data,and the results show that the time-varying model has better computational accuracy.Based on the VOF method,the time-varying microlayer model and the phase change model are applied to construct a three-dimensional simulation numerical method of vapor bubble behavior.The results show that the bubble departure behavior mainly includes shape adjustment,growth,sliding,and departure.Bubble departure makes the flow disturbed and forms vortices near the heated wall after the bubble.The surface heat flux on the local of the bubble is asymmetrically distributed,and the peak value is about four times of that in the main area.The bubble departure diameter increases with the increase of wall superheat,and decreases with the increase of mainstream flow velocity and subcooling.Based on the Klausner force balance model,the centrifugal force and secondary flow drag force generated by the spiral structure of the petal-shaped fuel rod are introduced,and the empirical coefficients such as bubble growth constants are determined based on relevant experimental studies to construct a dynamic model for the departure of boiling bubbles in the subcooled flow of petal-shaped fuel rods,and validate as well as analyze it.The results show that the model proposed in this paper has higher accuracy in predicting the departure diameter of subcooled boiling vapor bubbles in petal-shaped fuel rods compared with the Tolubinsky &Kostanchuk model and the Klausner model,whose MRE = 21.36%,and the model proposed in this paper can predict the departure and lift-off diameters of vapor bubbles with different parameters in different literature,so the model has certain applicability and accuracy.