Study On Heat Transfer Characteristics Of Subcooled Boiling For Fusion Reactor Divertor

The safety and reliability of the divertor in ITER fusion reactor is affected by the high heat flux coming from the plasma.It is necessary to discharge the high heat flux from the divertor.The aim of this dissertation is to study an excellent heat transfer component under the conditions of high-temperature and high-pressure in the divertor.The coolant is water.With the methods of the numerical simulation and experimental analyses,the dissertation carried out the studies of the heat transfer and flow characteristics of subcooled water flow boiling in the tube with twisted tape inserts under the conditions of asymmetric high-temperature and high-pressure in the divertor.The heat transfer and flow characteristics of the tube with twisted tape inserts were compared with those of the smooth tube and screw tube.(1)The simulation analyses of the heat transfer and flow characteristics in the subcooled flow boilingBy using the Fluent software,the dissertation analyzed and compared the heat transfer enhancement of the subcooled flow boiling in the tube with twisted tape inserts,screw tube and smooth tube.The Eulerian multiphase model was coupled with the Non-equilibrium boiling model.The grid independence was analyzed.The Eulerian equation was solved by the finite volume method.Under the different heat fluxes,the heat transfer regions,thermodynamics,void fraction and the temperature distribution of the smooth tube were analyzed;The temperatures of the wall and the components,pathlines,pressure drop and void fraction of the tube with twisted tape inserts,smooth tube and screw tube were simulated and compared.The effect mechanisms of the different twist ratios and the tape width ratios on the heat transfer and flow characteristics in the tube with twisted tape inserts were conducted.The research results are as follows:The points of ONB and FDB appeared earlier and earlier with the increase of the heat flux.Under the working conditions of the dissertation,the heat transfer regions included the following three parts:single phase forced convection,partial nucleate boiling and fully developed boiling.Compared with the outer tungsten and the middle Oxygen-Free High-Conductivity(OFHC)Copper,the inner CuCrZr tube was damaged firstly with the increase of the heat flux;Twisted tape of the tube with twisted tape inserts and helical fins of the screw tube induced the swirl flow.The swirl flow may improve the turbulence intensity and postpone the departure from nuclear boiling(DNB).While the helicoidal fin in the screw tube mixed the near-wall flow field,the twisted tape in the tube with twisted tape inserts mixed the bulk flow field.Considering the heat transfer and flow characteristics of the screw tube,the tube with twisted tape inserts and smooth tube,the tube with twisted tape inserts had the best heat transfer characteristics.With the decrease of the twisted ratio,the Nusselt number and the friction factor in the tube with twisted tape inserts increased.When the twisted ratio in the tube with twisted tape inserts was equal to 4,its thermal performance factor was the highest among the different twist ratios.With the increase of the tape width ratio in the tube with twisted tape inserts,the Nusselt number and the friction factor increased.When the tape width ratio was equal to 0.29,the tube with twisted tape inserts had the highest thermal performance factor value.(2)The experimental study of the heat transfer and flow characteristics of the subcooled flow boilingThe dissertation introduced the experimental system of the subcooled flow boiling,and the structure of the cooling tubes,the measuring method of the experimental parameters and data processing.The experimental researches of the tube with twisted tape inserts and the smooth tube were carried out under the conditions of different heat fluxes and mass fluxes.The affecting mechanisms of different heat fluxes and mass fluxes on the heat transfer and flow characteristics of subcooled flow boiling were conducted;The heat transfer and flow characteristics of the subcooled flow boiling in the tube with twisted tape inserts were compared with those of the smooth tube.The experiment results showed that:The influence of heating entrance effect existed in the subcooled flow boiling of the smooth tube.The characteristics of the heat transfer enhancement became better with the increase of the heat flux and mass flux.The heat flux had a little effect on the pressure drop.The heat transfer of the tube with twisted tape inserts whose twisted ratio were equal to 4 and 6 were compared with those of the smooth tube.The tube with twisted tape inserts whose twisted ratio was equal to 4 had the best heat transfer characteristics.(3)The evaluation of the critical heat flux(CHF)correlationsThis dissertation introduced the handling methods of the data about the critical heat flux.According to the experimental data between the heat transfer and flow characteristics and occurrence mechanism of the critical heat flux,the dissertation carried out the influence of the thermodynamics and mass flux on the critical heat flux of subcooled flow boiling under the conditions of asymmetrical high temperature and high heat flux in the water-cooling divertor.The research results were as follows:As the thermodynamics decreased or mass flux increased,the critical heat flux became larger,which postponed the critical heat flux.Under the conditions of high temperature and high pressure,the dissertation collected the experimental data source of the critical heat flux,and predicted and assessed the following empirical correlations of the critical heat flux of smooth tube and the tube with twisted tape inserts:Bowring correlation,the correlation of the critical heat flux in the Soviet Union and the modified Tong-75 correlation.Among these correlations,the modified Tong-75 correlation(smooth tube)had the best predictable performance.(4)The analyses of heat stressThe dissertation conducted the analyses of heat stress of the tube with twisted tape inserts,the screw tube and the smooth tube of the monoblock in the divertor.The secondary stresses in the tungsten armor tile and the CuCrZr tube of the tube with twisted tape inserts were the smallest compared with those of the smooth tube and the screw tube.In the tube with twisted tape inserts,the three kinds of secondary stresses in the tungsten armor tile were both in the range of the 3Sm allowable stress values.The three kinds of secondary stresses in the CuCrZr tube both exceeded the allowable stress values of 3Sm.But they met the Bree-diagram criterion.