Mixing Vane Grid Research Articles

Modeling of spacer grid mixing effects through mixing vane crossflow model in subchannel analysis

Spacer grids, especially those with mixing vanes or other mixing devices in the rod bundle, have great influence on local flow fields in the reactor core. This mixing effect has been demonstrated by both experimental and numerical simulations, however, the spacer grids’ effect on flow field is not well reflected in most subchannel codes that are currently used as main instruments in calculating local conditions and safety analysis for the nuclear industry. This paper focuses on improvement of the mixing vane crossflow model in subchannel analysis for simulating the mixing effects of spacer grids. The distributed resistance method (DRM) is applied, and the source terms associated with the spacer grids are added to momentum equations to account for the mixing vane grids’ influence on local flow fields in rod bundles (U.S. patent, application number: 61841961, confirmation number: 8224; U.S. patent, application number: 62375023, confirmation number: 2306; Chinese patent, application number: 201611179395.1). The improved DRM model is used to analyze the thermal hydraulic performance in the 5×5 rod bundle. The calculation results are compared with experimental data and CFD simulation results respectively. The subchannel calculation results are also compared with those from the conventional code without the DRM model. The flow characteristics, such as the crossflow direction, the crossflow values at grid zone, and the decay trend of the crossflow downstream of the mixing vane grid are analyzed. The improved mixing vane crossflow model is assessed by comparison analysis, and some conclusions are obtained.

Review of the correlation developments and a new concept based on mixing mechanism for heat transfer enhancement of spacer grids

Abstract Spacer grids could cause heat transfer enhancement both at the spacer grid regions and downstream of the spacer grids as a result of mixing promoted by the spacer grids in the rod bundle. This phenomenon has been demonstrated by many experiments, and several correlations have been developed based on these experimental data. This paper gives a review of the grid-enhanced heat transfer correlation developments in single phase flow. Following the exploration of the correlation development history, a predictive formulation of grid-enhanced heat transfer in single phase flow is established taking into account the effect of both swirl flow and crossflow. With emphasis on modeling of the mixing mechanism associated with the mixing vane grid, the new correlation could better reflect the physical process of the heat transfer augmentation, while a large number of experimental data are needed to determine the coefficients of the new correlation.

Study on effects of mixing vane grids on coolant temperature distribution by subchannel analysis

Abstract Mixing vane grids (MVG) have great influence on coolant temperature field in the rod bundle. The MVG could enhance convective heat transfer between the fuel rod wall and the coolant, and promote inter-subchannel mixing at the same time. For the influence of the MVG on convective heat transfer enhancement, many experiments have been done and several correlations have been developed based on the experimental data. However, inter-subchannel mixing promotion caused by the MVG is not well estimated in subchannel analysis because the information of mixing vanes is totally missing in most subchannel codes. This paper analyzes the influence of mixing vanes on coolant temperature distribution using the improved MVG model in subchannel analysis. The coolant temperature distributions with the MVG are analyzed, and the results show that mixing vanes lead to a more uniform temperature distribution. The performances of split vane grids under different power conditions are evaluated. The results are compared with those of spacer grids without mixing vanes and some conclusions are obtained.

CFD analysis on mixing effects of spacer grids with different dimples and sizes for advanced fuel assemblies

Abstract The thermal hydraulic characteristics of a mixing vane grid are largely dependent on the structure of key components, such as strip, spring, dimple, weld nugget, as well as the mixing vane configuration. In this paper, several types of spacer grids with different dimple shapes are modeled under subcooled boiling conditions. Prior to the application of CFD on the dimple shape analysis, the mixing effects of spacer grids were studied. After the dimple shape analysis, the side channel effect is discussed by comparing the simulation results of a 3 × 3 and a 5 × 5 spacer grid. The two phase flow CFD models in this study are validated through simple geometry showing that the calculated void fraction is in good agreement with the experimental data. The dimple comparison result shows that varying dimple structures can result in different temperatures, lateral velocities and void fraction distributions downstream of the spacer grids. Comparison of two sizes of spacer grids demonstrate that the side channel generates different flow distribution pattern in the center channel.

Computational fluid dynamics (CFD) round robin benchmark for a pressurized water reactor (PWR) rod bundle

This study examined the capabilities and limitations of steady Reynolds-Averaged Navier–Stokes (RANS) approach for pressurized water reactor (PWR) rod bundle problems, based on the round robin benchmark of computational fluid dynamics (CFD) codes against the NESTOR experiment for a 5×5 rod bundle with typical split-type mixing vane grids (MVGs). The round robin exercise against the high-fidelity, broad-range (covering multi-spans and entire lateral domain) NESTOR experimental data for both the flow field and the rod temperatures enabled us to obtain important insights into CFD prediction and validation for the split-type MVG PWR rod bundle problem. It was found that the steady RANS turbulence models with wall function could reasonably predict two key variables for a rod bundle problem – grid span pressure loss and the rod surface temperature – once mesh (type, resolution, and configuration) was suitable and conjugate heat transfer was properly considered. However, they over-predicted the magnitude of the circumferential variation of the rod surface temperature and could not capture its peak azimuthal locations for a central rod in the wake of the MVG. These discrepancies in the rod surface temperature were probably because the steady RANS approach could not capture unsteady, large-scale cross-flow fluctuations and qualitative cross-flow pattern change due to the laterally confined test section. Based on this benchmarking study, lessons and recommendations about experimental methods as well as CFD methods were also provided for the future research.

PIV experiment research of lateral flow field in 2 × 2 rod bundles channel with mixing vane grids

PIV experiment research of lateral flow field in 2 × 2 rod bundles channel with mixing vane grids

Numerical investigation and modeling of two-phase flow sweeping in rod bundles with mixing vane grid spacers

Grid spacers with mixing vanes are used in nuclear fuel assemblies in order to increase the lateral cross-flows between sub-channels that homogenize the temperature distribution within the fuel rod bundle. For the thermal-hydraulic design of such fuel bundles, sub-channel analysis codes are used in order to predict the mass and temperature distribution. These analysis codes compute sub-channel averaged values and contain empirical correlations that account for the effects of microscopic flow phenomena, such as wall friction, turbulent mixing or heat transfer. Experimental investigations show a strong swirling flow in the vicinity of a mixing vane grid spacer. No model was found in literature that accounts for the swirling flow effect on the lateral cross-exchange of mass, momentum or energy. Systematic CFD investigations of the flow in a 5×5 rod bundle containing a single grid spacer with split-type mixing vanes were performed with single-phase and two-phase flow conditions with void fractions ranging between αg=0%and15%. A cubic non-linear k–ε model was used to simulate the typical rod bundle flow effects, such as secondary flow and non-isotropic turbulence. A detailed analysis of the forces and flows in the gap showed that mixing vane induced swirling flow in the sub-channels strongly affects the inter-sub-channel cross-flow. A new model was developed, based on the CFD simulation results, that predicts the flow sweeping between the sub-channels. This model was implemented into the sub-channel analysis code COBRA-FLX™ that was developed and is used by AREVA GmbH. The new model significantly improves the predictions for the cross-flow and mass flow distribution within the rod bundle in the vicinity of the mixing vane grid spacer.

Hydraulic benchmark data for PWR mixing vane grid

The purpose of the present study is to present new hydraulic benchmark data obtained for PWR rod bundles for the purpose of benchmarking computational fluid dynamics (CFD) models of the rod bundle. The flow field in a PWR fuel assembly downstream of structural grids which have mixing vane grids attached is very complex due to the geometry of the subchannel and the high axial component of the velocity field relative to the secondary flows which are used to enhance the heat transfer performance of the rod bundle. Westinghouse has a CFD methodology to model PWR rod bundles that was developed with prior benchmark test data. As improvements in testing techniques have become available, further PWR rod bundle testing is being performed to obtain advanced data which have high spatial and temporal resolution. This paper presents the advanced testing and benchmark data that has been obtained by Westinghouse through collaboration with Texas A&M University.

Computational Fluid Dynamics Prediction of Grid Spacer Thermal-Hydraulic Performance with Comparison to Experimental Results

The thermal-hydraulic performance of a nuclear reactor fuel assembly grid spacer is predicted using computational fluid dynamics. The modeled flow domain exploits the periodicity of the spacer and is separated into a bare bundle and grid region to maintain a manageable model size. An iterative process is used to couple the segregated flow domains to arrive at a converged solution. The grid spacer is a 7 × 7 mixing vane grid representative of an actual pressurized water reactor grid. Pressure drop and rod wall temperature predictions for steady-state operation are computed. The results show excellent agreement with experimental data. The agreement in these results demonstrates the usefulness of the method presented as a design tool for nuclear fuel manufacturers and as a prediction tool for off-design operating conditions such as simulated accident scenarios.

Modelling of PWR cladding corrosion in mixed core situations

In this paper a thermal-hydraulic model for cladding corrosion recently developed in ABB Atom and used in the CORPRO3.0 code is presented. The features of the model are a subchannel geometry which consists of a 3 × 3 matrix of rods, and modelling of coolant cross-flow and coolant enthalpy mixing. The CORPRO3.0 thermal-hydraulic model is benchmarked against the TORC code, which is a 3D code for analysing the thermalhydraulics of a reactor core. In addition, results of model calculations are compared with corrosion data obtained in mixed core situations, i.e. situations where the fuel assemblies in the core have different designs (e.g. different grid and nozzle designs). Fuel assembly components in assemblies of different designs usually have unequal flow resistances. These differences result in transverse pressure gradients, which in turn increase the lateral flow velocity and thus affect the cociant mass flow rate distribution. Two different situations where this type of mismatch between fuel assemblies in the Ringhals 3 core have occurred are studied in this paper. In the first case a reload batch of fuel assemblies, with Zircaloy mixing vane grids, inserted in a core where the resident fuel assemblies have Inconel mixing vane grids is considered. In the second case cladding tubes from the same manufacturing lot that have been irradiated for the same period of time but have been situated in fuel assemblies with Zircaloy mixing vane grids of different designs are considered. The results manifest the capability of the CORPRO3.0 code to model the effects of flow resistance on cladding corrosion.

Rod Bowed to Contact Departure from Nucleate Boiling Tests in Coldwall Thimble Cell Geometry

Departure from nucleate boiling (DNB) tests were conducted to measure the effect of a rod bowed to contact in a coldwall thimble cell of a pressurized water reactor. The tests were conducted using electrically heated 4×4 rod bundles with 4,270mm heated length and with axially and radially non-uniform heat flux, locating mixing vane grids on a 660 mm spacing. One of central heater rods was bowed into contact at the topmost grid span where most of DNB were observed in the unbowed geometry test. The tests were resulted in no effect of a rod bowed to contact at low pressure (105 and 127 ata) but considerable effect at high pressure (148 and 169 ata). The reduction of DNB heat flux was given by a function of rod average heat flux and pressure.