| The basic structure of a Pressurized Water Reactor(PWR)core is a rod bundle channel.Under some accident conditions or in an integrated reactor,natural circulation conditions are an important operating mode of a nuclear reactor system.Therefore,understanding the flow characteristics of the rod bundle channel in natural circulation system is very important for nuclear reactor safety.Especially under ocean conditions,due to the effects of additional inertial forces and periodic changes in the spatial position of nuclear power plant.Under those conditions,the flow rate and frictional resistance coefficient in the rod bundle channel will fluctuate periodically,which will affect the operation and safety of the nuclear power plant.In this paper,the in-depth analysis of the flow characteristics of the rod bundle channel in natural circulation system is carried out through experimental and numerical studies.By developing a one-dimensional user code,the coupling between user code and STAR-CCM+ under steady-state and rolling conditions are successfully implemented.A numerical study of the flow characteristics of the rod bundle channel in the natural circulation system has been successfully realized and is described in this paper.The experimental study on the flow characteristics of the rod bundle channel under vertical conditions found that the frictional resistance coefficient of a rod bundle channel under natural circulation is related to the physical properties of fluid and the size of the rod bundle channel.According to different flow regions,the relationship between the frictional resistance coefficient and Reynolds number is discussed separately,and the correlation function of the frictional resistance coefficient with Reynolds number in the laminar region and the transition region is presented.After considering the influence of fluid properties on the frictional resistance coefficient,a kinematic viscosity correction factor is introduced to modify the frictional resistance coefficient correlation obtained previously.Finally,the prediction correlation function of the frictional resistance coefficient is obtained.After verification,the prediction effect of the correlation is good,and the relative error with the experimental data is within ±5%.The experimental study of the flow characteristics of the rod bundle channel under rolling conditions was carried out.It is shown that the frictional resistance coefficient of the rod bundle channel under natural circulation conditions varies with the rolling conditions: the larger the rolling amplitude or frequency,the larger the average value of the frictional resistance coefficient,and the greater the fluctuation of the coefficient.The frictional resistance coefficient is also affected by the fluid property parameters.Specifically the coefficient will decrease,and its fluctuation will also decrease with the higher the fluid temperature in the channel,and the faster the fluid velocity,which means the larger the heating power of the rod bundle channel and the steady state Reynolds number.Similarly,by introducing the kinematic viscosity correction factor,the correlation function of instantaneous frictional resistance coefficients under different rolling conditions and heating conditions was developed.After comparing with a large amount of experimental data,it is shown that the correlation function can be widely applicable in predicting the instantaneous frictional resistance coefficient under rolling conditions for both laminar and transition flow,and the prediction results are in good agreement with experimental values.Based on the three-dimensional Computational Fluid Dynamics(CFD)numerical study of the flow characteristics of the rod bundle channel under vertical and inclined conditions,the fully developed section position of laminar and turbulent fluids in the heated rod bundle channel under steady state conditions was determined.The fluid in the development section was analyzed in detail.The resistance characteristics of the fluid along the axial direction of the rod bundle channel was studied,and the flow field along the heating channel was analyzed.In addition,the frictional resistance coefficient of the rod bundle channel under vertical conditions was studied in a wide range of Reynolds number.Divisions are made according to different characteristics of frictional resistance coefficients in different Reynolds number.It was found that there is no obvious difference between the frictional resistance coefficient under inclined conditions and vertical conditions.It can be considered that the influence of the inclined condition on the resistance characteristics of the rod bundle channel can be ignored.In order to explore the natural circulation flow characteristics in the rod bundle channel under steady-state and rolling conditions,a simulation method was used to couple the three-dimensional CFD software STAR-CCM+ with one-dimensional user code.The three-dimensional software was used to simulate the rod bundle channel,and the one-dimensional self-developed user code simulated the other parts in the natural circulation loop except the rod bundle channel in order to realize the simulation of the entire natural circulation system.The user source code was compiled and linked into a Dynamic Link Library(DLL)and then loaded into STAR-CCM+,so that data could be exchanged between the two during the coupling solution process,thereby realizing the coupling between user code and STAR-CCM+.For the rolling condition,the dynamic mathematical model was established in the user code and a method of adding rotational motion to STAR-CCM+ was used to realize the coupling of the natural circulation system.In order to prove the accuracy and rationality of the coupled simulation,simulations using only CFD was performed under the same conditions.Comparing the results of natural circulation coolant flow velocity and rod bundle channel pressure drop obtained from these two numerical simulations with experimental results,it was found that the coupled simulation results are closer to the experimental results than the CFD simulation results.That indicates that STAR-CCM+coupled with user code can more accurately predict the natural circulation flow characteristics in a rod bundle channel and this coupled method is feasible.In addition,a coupled simulation can be performed independently of the experiment,and its practicability is better than the CFD simulation.A coupled simulation can obtain the refined flow field in the rod bundle channel under natural circulation conditions.It can further analyze the resistance characteristics in the rod bundle channel and can be used for the research of heat transfer and other issues.This paper introduces the coupled method of STAR-CCM+ and user code in detail,which provides reference for multi-scale coupling research of natural circulation systems.This paper presents the results of three coordinated activities to better understand the natural coolant flow characteristics of the rod bundle channel.First the results of an experimental study of the flow characteristics with emphasis on the analysis of the frictional resistance coefficient is presented.The result of that analysis was the development of a heretofore unknown kinematic viscosity correction factor.The universality of the frictional resistance coefficient calculation formula is enhanced.Secondly,the flow regularity in a heating rod bundle channel is clarified by numerical simulation.Finally,the research method of three-dimensional CFD software STAR-CCM+ and one-dimensional user defined code is used to realize the research of the rod bundle channel under natural circulation condition,which can expand the experimental scope and reduce the experimental dependence for future research on natural circulation. |
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