Numerical Researches On Single-and Two-phase Flow And Heat Transfer In Rod Bundles

Rod bundle is a kind of narrow straight and non-circular complex channels which is widely used in reactor core and other industrial equipment.The thermal-hydraulics behavior of the rod bundle differs greatly from the conventional pipe and plane plate.The rod bundle is a high efficiency and economy thermal-hydraulics equipment for its compact structure and high effective heat transfer area.The bundle size of the fuel rod in Pressurized Water Reactor(PWR)is always large and there are always several complex mixing vane grid spacers in axial direction,which exert great influence on the fluid flow and heat transfer.The simplified model experiment model and system code were the main methods in nuclear industry.Recently,the computational fluid dynamics(CFD)has been used extensively in nuclear industry for its well-known benefits along with the development of the computer science and the numerical solving technique.Further investigation into the numerical methods and the application characteristics of the numerical models makes great sense for the contribution to apply CFD to thermal-hydraulics research and structural optimization design in reactor core.The fluid flow in reactor core is single-phase flow in normal operating conditions.However,sometimes in order to improve the output-power or in accidental conditions,the two-phase flow and phase-change heat transfer might occur in PWR core.The paper investigated a series of issues on the appropriate numerical methods and treatments applied to the numerical research in reactor core system by the methods of numerical simulations,theoretical analysis,model establishments and contrastive analysis comprehensively and deeply.The main content research and results are presented as follows.Firstly,for the single-phase and heat transfer,since the different or even contrary conclusions were obtained by different researchers from different points of view in the research of numerical treatments(turbulence model,near-wall treatments,near-wall mesh types),this paper investigated the influence of the numerical treatments on the numerical results comprehensively from several aspects of fluid flow,heat transfer and resistance properties.The reasons of the phenomenon that different or even contrary conclusions were obtained by different researchers were established.In order to reduce the computational amount in the mixing vane rod bundle research,the effectiveness of applying periodic boundary conditions to a central two-subchannel model was validated by comparing the velocity profiles to the experimental and other CFD data.Based on the validated two-subchannel model,the application characteristics of the numerical treatments applied to the mixing vane rod bundle were investigated.Research found that the numerical treatments showed different application characteristics in bare and mixing vane rod bundles.Based on the native nature of the numerical treatments and their application characteristics in rod bundles,the optimal simulation methods for engineering application and theoretical study have been given respectively,which guarantees the high reliability and computational efficiency synchronously.Later,for two-phase flow and phase-change heat transfer,the numerical simulations methods of subcooled boiling in pipes,triangle and square arrayed rod bundle were validated.Based on the validated wall boiling model,the influence of three key sub-models(bubble departure diameter,wall nucleation site density and bubble detachment frequency)and two essential non-drag force models(lift force and turbulent dispersion force)on numerical results was investigated by comparing the numerical results to the experiment data and empirical formulation.The analysis results provides references for the choices of sub-models and model parameters.As an example,the paper investigated the influence of mixing vane and its slop angle on two-phase flow and heat transfer in mixing vane rod bundle based on the numerical simulation with wall boiling model.The optimal mixing vane slop angle was recommended.Finally,three methods were used or proposed to solve the difficult problems in the numerical simulation of mixing vane rod bundle caused by the large amount of unstructured tetrahedral meshes.The methods are domain-divided solving technique(DDST),polyhedral meshes application and forced swirling momentum source(FSMS),respectively.DDST effectively solves the large computational complexity caused by the mixing vane spacer grids along the axial direction.The whole rod bundle domain is divided in several sub-domains along the axial direction and each of the sub-domains is simulated separately.DDST effectively reduces the computational amount of a single simulation and thus the bundle size could be larger in the domain.The paper clarified the key technique for the successful implementation of DDST.The polyhedral meshes show higher computational efficiency and better convergence properties for the same reasonable accuracy compared to tetrahedral meshes.The concept of FSMS was presented and the mechanism-mathematics model was established.The analysis results demonstrated that the FSMS reproduce the swirling flow and complex heat transfer well downstream of the mixing grid in the bare rod bundle.As an example,a numerical simulation of a typical 17×17 fuel assembly with mixing vane spacer grid was performed at reasonable computational cost by using DDST and polyhedral meshes.The comparison results of 5x5 and 17×17 shows that the bundle size exert great influence on thermal-hydraulics performances of mixing vane rod bundle.