CFD Analysis And Application On Boiling Heat Transfer And Two-phase Flow In Rod Bundles

Critical Heat Flux(CHF)is an important parameter which is related to the economy and safety of nuclear reactor.It is necessary to investigate the boiling heat transfer and two-phase flow in rod bundle channel of nuclear reactor,as well as the CHF value.By defining proper CHF value,it is helpful to improve the design,ensure the reactor safety and increase the total power level.Nowadays,the main research methods of boiling heat transfer and two-phase flow in nuclear reactor include experimental method and numerical simulation.The main research points of experiment are the macroscopic characteristics of two-phase flow or the microscopic characteristics of bubble movement and wall boiling heat transfer.This method can also be used to provide the experimental data for the validation of theory models.During the numerical simulation by use of system or subchannel codes,CHF is always determined by associating two-fluid model with CHF Lookup Table or CHF empirical formulations to realize the rapid calculation of macroscopic parameter of nuclear reactor system.Computational Fluid Dynamics(CFD)method can analyze the influence of geometric feature of small scale region on the characteristics of flow and heat transfer in reactor core.So the CFD method can be used to investigate the boiling heat transfer and two-phase flow phenomenon of complex structure and calculated the CHF value in boiling flow.However,the research of flow and heat transfer is still insufficient in the rod bundle channel with spacer grids and mixing vanes.It is also difficult to determine the applicability of grid resolution and turbulence models and the proper models related to flow and heat transfer.So based on upward vertical pipe channel and rod bundle channel,a detailed and comprehensive analysis is developed for the CFD analysis of boiling heat transfer and two-phase flow.It includes the applicability research of grid resolution and turbulence models,application research of interphase force models and wall boiling models,the effect of solid structure on boiling crisis phenomenon.Firstly,grid scheme and turbulence models are investigated in this paper.Grid is distributed based on the thickness of first layer in the near-wall region.After analyzing the CFD calculation of different grid distributions,turbulence models and wall functions,the application scheme is proposed for CFD analysis of boiling crisis.In order to well predict the position of sharp increase of temperature and peak value in the boiling crisis phenomenon,the first layer grid should be set in the logarithmic layer of the wall boundary layer and the dimension less number Y~+on the near-wall region should be larger than 60.In addition,the realizable k-?model and standard wall function should be used among all the turbulence models.Secondly,applicability of different interphase force models is determined.After theory analyzing,CFD calculations with different interphase force models are applied in three experimental data which include subcooled boiling flow in rod bundle channel,subcooled boiling flow and boiling crisis experiment in vertical heating pipe.Results are compared with experimental data and an application scheme is developed for the interphase force models which include drag force,lift force,wall lubrication force,et al.The appropriate boundary condition is high pressure larger than 4.5Mpa and the mass flux between 1000kg/m~2s and1500kg/m~2s.After that,combined the results of grid resolution,turbulence models and interphase force models,the paper also proposes the application scheme related to two-phase flow in the CFD analysis of boiling heat transfer and two-phase flow.Thirdly,research on application of wall boiling model and key parameter models is developed in this paper.CFD calculations with different key parameter models are applied in three experimental data which include subcooled boiling flow in rod bundle channel,subcooled boiling flow and boiling crisis experiment in vertical heating pipe.Results are compared with experimental data and the application scheme is proposed for the key parameter models in CFD simulation of boiling crisis in rod bundle channel.Unal model for bubble departure diameter and Lemmert-Chawla model for nucleate site density can be used to simulate the boiling flow accurately.In addition,the derivation of force balance model is provided for the calculation of bubble departure diameter in the CFD simulation.In subcooled boiling flow,the force balance model can predict the axial distribution of wall temperature and average void fraction accurately.But the results in boiling crisis experiment show a large error that the wall temperature is larger than experimental data.Furthermore,three influenced parameters are analyzed which include bubble contact diameter,contact angle and friction velocity.Result shows that small bubble departure diameter is generated by use of small bubble contact diameter and contact angle or large friction velocity.This will reduce the average void fraction in channel and increase the wall superheat.At last,the effect of solid structures is investigated on the boiling crisis phenomenon.Based on upward vertical pipe and rod bundle,the research focuses on the influence of heating pipe and cladding in the flow conditions of single phase,subcooled boiling and boiling crisis.Results show that the azimuthal heat conduction in cladding can decrease the peak value of wall temperature.The heating source should be set according to the experiment in the CFD calculation.Furthermore,the effects of spacer grid and mixing vane on boiling crisis are investigated in 2×2 and 5×5 rod bundle channels.Although they bring some flow resistance,the flow disturbance of these two structures decreases the void fraction in near-wall region and improves the heat transfer coefficient.The peak value of wall temperature increases along with the number of spacer grids.The accumulation of bubbles happens in the downstream of mixing vane along with the increasing of bending angle.After considering all the results of research on spacer grids and mixing vanes,this paper proposes the implemented strategy and setting scheme of temperature measuring point for boiling crisis experiment to gain an accurate CHF value.Considering all the aspects,the CFD analysis is investigated based on boiling crisis experiment in rod bundle channel.The calculation scheme includes research on grid resolution scheme,application scheme of flow related models and wall boiling models.Comparison is discussed between the experimental data and simulation data of CHF value to validate the applicability of each scheme in boiling crisis calculation of rod bundle channel.The error is around 20%.It provides reference for the experiment and calculation of boiling crisis in rod bundle channel,as well as the flow and heat transfer analysis of spacer grids.