Split-type Mixing Vanes Research Articles

Experimental study of coherent structures downstream mixing vaned spacer grid of different inclination angles in a 5 × 5 rod bundle by TR-PIV

Experimental study of coherent structures downstream of a spacer grid with different inclination angles (20°, 25°, 30°) of split-type mixing vanes in a 5 × 5 rod bundle was conducted by time-resolved particle image velocimetry (TR-PIV) under Reynolds number from 6.6 × 103 to 3.96 × 104. The fluorinated ethylene propylene (FEP) tubes and water are used as the matched index of refraction (MIR) materials. The maximum standard uncertainty of time-averaged velocity is less than 2 % of bulk flow velocity (Wb), and that of Reynolds stress is less than 2 % Wb2.The secondary flow structures downstream spacer grid with mixing vanes of different inclination angles change with several similar patterns, including vortices shedding from mixing vanes and fast dissipation near spacer grid, counter-current shear-flow development and shear-flow induced vortices formation, shear-flow induced vortices decaying and dissipation. The development trends of spanwise time-averaged velocity, Reynolds stress, integral spatial length scales and integral temporal length scales are determined by coherent structures. With Reynolds number increasing, smaller coherent structures are generated and the time-averaged velocity, Reynolds stress, spatial and temporal length scales decrease. Larger inclination angle generates larger coherent structures, and spatial and temporal length scales increase with inclination angle increasing.The experimental findings provide valuable information for understanding sub-channel mixing phenomena caused by mixing vaned spacer grids. The experimental data can be used for sub-channel mixing model developments and validations of turbulent models.

Flow and Heat Transfer Simulation in a Complete Pressurized Water Reactor Fuel Assembly Using Wall-Modeled RANS

Abstract Reproduction of turbulent flow and heat transfer inside a pressurized water reactor (PWR) fuel assembly is a challenging task due to the complex geometry and the huge computational domain. Capability of a wall-modeled Reynolds-averaged Navier–Stokes (RANS) simulation approach has been examined, which had already been validated against the measurements of the MATiS-H experiment. The method is here expanded to a larger computational domain aiming to reproduce flow and thermal field in the entire PWR fuel assembly. Namely, in the first part of this study, wall-modeled RANS is performed in a relatively short section of the representative PWR fuel assembly containing one single mixing grid with an array of 15 × 15 fuel rods. Linear and nonlinear eddy-viscosity turbulence models have been applied; however no significant difference is observed in the predicted pressure drop in the fuel assembly. The obtained predictions revealed an interesting pattern of swirl flow as well as diagonal cross flow downstream the mixing grid, which is driven by the applied design of split-type mixing vanes. In the second part, the computational model is extended to a domain representative of a complete PWR fuel assembly with ten mixing grids, inlet and outlet sections. Pressure drop and flow field are analyzed together with the predicted temperature and potential hot spots. In spite of a relatively coarse spatial resolution of the applied approach, the wall-modeled RANS provided promising results at least for the qualitative prediction of the pressure, flow field, and location of hot spots.

LES and URANS study on turbulent flow through 3 × 3 rod bundle with spacer grid and mixing vanes using spectral element method

In this paper, the turbulent flow in rod bundles with spacer grid and mixing vanes in nuclear reactor core was studied using spectral element method with large eddy simulation (LES) and unsteady RANS (URANS) method. Combining with the radial periodical boundary conditions, the turbulent mixing phenomena in central subchannels were investigated, especially focusing on the flow field after the spacer grid and mixing vanes. Other than setting up long bare rod bundle to establish the fully developed turbulent flow, only a 2-cm-long bare rod bundle was extruded upstream the spacer grid and mixing vanes, while the fully developed laminar velocity profile was applied on the inlet surface for velocity boundary settings. The results indicate that both of the models could predict time-averaged velocity at 0.5Dh, 1Dh, 4Dh downstream mixing vanes properly, while the URANS model underestimate the root-mean-square (RMS) value of fluctuating velocity due to the inlet treatment effect. Furthermore, turbulent kinetic energy contour with transverse velocity vector was combined to investigate the behaviors of vortices formed by split-type mixing vanes, revealing the behavior of vortices for preliminary dynamic and mechanic analysis in design process of fuel assembly.

Experimental Study of Pressure Loss in a 5 × 5–Rod Bundle With the Mixing Vane Spacer Grid

Axial and lateral pressure loss in a 5 × 5 rod–bundle with a split-type mixing vane spacer grid was experimentally measured using differential pressure transmitters at different sub-channel Reynolds numbers (Re) and orienting angles. The geometrical parameters of the 5 × 5–rod bundle are as follows: they have the same diameter (D = 9.5 mm) and pitch (p = 12.6 mm) as those of real fuel rods of a typical pressurized water reactor (PWR), with a sub-channel hydraulic diameter (Dh) of 11.78 mm. The characteristics and resistance models of pressure loss are discussed. The main axial pressure loss is caused by the spacer grid, and the spacer grid generates additional wall friction pressure loss downstream of the spacer grid. The lateral pressure loss shows strong correlations with orienting angles and distance from the spacer grid. The lateral pressure loss shows a sudden burst in the mixing vanes region and a slight augmentation at z = 3Dh. After 3Dh, the lateral pressure loss decays in an exponential way with distance from the spacer grid, and it becomes constant quickly at z = 20Dh.

The numerical simulation and analysis of turbulent flow behavior in 5 × 5 fuel rod bundle with split-type mixing vane

In this research, the numerical simulation and analysis of flow behavior in 5 × 5 fuel rod bundle with consideration of spacer grid has been conducted. ANSYS FLUENT 19.0 with the consideration of the transition SST (Shear-Stress Transport) with SAS (Scale-Adaptive Simulation) turbulence model is used for the numerical simulation. The obtained simulation results are reasonable compared with the experimental data based on the comparison in the aspect of velocity distribution. The effect of the spacer grid on the flow behavior in the fuel rod bundle has also been investigated. The mixing vanes in the spacer grid can cause various strong flow branches; these flow branches develop and cause the complex flow behavior in the fuel rod bundle. Additionally, the secondary flow, the swirling factor and the mixing factor were investigated for the quantitative analysis of the spacer grid effect on the flow mixing behavior in fuel rod bundle.

Experimental investigation of turbulent flow characteristics in cross-flow planes of a [formula omitted] rod bundle with a spacer grid

This study summarizes experimental investigations of the flow field characteristics in the cross-flow planes of a 5×5 rod bundle with spacer grid and split-type mixing vanes. Under the isothermal and atmospheric conditions, the velocity fields in the cross-flow planes are obtained by applying the matched-index-of-refraction and time-resolved particle image velocimetry (TR-PIV) techniques for Reynolds numbers of Re1=14,000 and Re2=28,000. The cross-flow planes axially start from the tips of the mixing vanes of the spacer grid and end at 11.73Dh (Dh is the hydraulic diameter of the rod bundle). From the acquired TR-PIV velocity fields, flow statistics such as the in-plane mean velocity and vorticity, turbulent kinetic energy and Reynolds stress are computed and presented for four cross-flow planes of interest axially located at 0,2Dh,5Dh, and 11Dh. Two counter rotating vortices are observed near the spacer grid. After a change in elevation to z/Dh=5, the vortices merge into a single vortex centered in the middle of the subchannel. In addition, the turbulence flow characteristics at the elevations of interest are discussed through calculations of integral length scales and spectral analysis. No dominant frequency is observed, and the turbulence characteristics are strongly influenced by the proximity to the spacer grid. The mass transfer between the subchannels and the associated turbulent mixing flow rate is calculated for all elevations; hence, this study is a step toward improving the correlations currently used in subchannel codes. Results for secondary flow intensities, lateral velocity profiles and velocity contours are compared with large-eddy simulation results from the literature for Re1=14,000, and a reasonable agreement is observed.

Experimental study of cross flow and lateral pressure drop in a 5 × 5 rod bundle with mixing vane spacer grid

Cross flow and lateral pressure drop downstream a split-type mixing vane spacer grid in a 5 × 5 rod bundle were experimentally measured by particle image velocimetry (PIV) and differential pressure transmitter at the sub-channel Reynolds number (Re) of 13200. The geometrical parameters of the 5 × 5 rod bundle is as the same diameter (D = 9.5 mm) and pitch (P = 12.6 mm) as the real fuel rods of a typical pressurized water reactor (PWR), with sub-channel hydraulic diameter (Dh) 11.78 mm. With the aid of matched index of refraction (MIR) technique of transparent fluorinated ethylene propylene (FEP) tubes and water, the PIV measurements of cross flow covering all sixteen inner-sub-channels were conducted for cross sections at 1Dh, 4Dh, 8Dh and 16Dh downstream the spacer grid. With detailed error and uncertainty analysis, the uncertainty of the turbulent statistics is mainly determined by the out of plane error and sampling number error. The maximum standard uncertainty of statistical quantities normalized by the bulk velocity is less than 1%. The characteristics of cross flow were mainly determined by the mixing vanes of spacer grid in rod bundle. The mixing vanes generate strong cross flow and drastic turbulence flow downstream the spacer grid, enhancing the cross flow mixing and turbulent mixing between sub-channels. The lateral pressure drop was measured at 1Dh, 2Dh, 3Dh, 4Dh, 5Dh, 6Dh, 8Dh, 10Dh, 12Dh, 14Dh, 16Dh, 18Dh and 20Dh. The relative uncertainty level of lateral pressure drop is between 0.7% and 7.0%. Downstream the spacer grid, the lateral pressure drop increases from 1Dh to 2Dh. After 3Dh, the lateral pressure drop decays in exponential way with distance from spacer grid. After 16Dh, the lateral pressure drop almost keeps constant.

Solution verification of PANS model for a PWR fuel assembly

Partially averaged Navier-Stokes model is an energy based, filtered representation of the Navier-Stokes equations. As part of the bridging model paradigm family, it is possible to resolve the turbulent space and time scales of interest. The implicit filtering approach enables us to use the PANS model as a classical RANS model as well as large eddy simulation model or even as a direct numerical simulation, without changing the form of the closure model equations. The energy-based filter, unlike large eddy simulations, decouples the closure model relation from the computational grid size. This allow us to perform classical mesh-based refinement solution verification analyses. In the present study we will perform a solution verification study and model validation for a turbulent flow inside a 5 × 5 fuel bundle, with a single spacer grid and split-type mixing vanes. Grid convergence index analysis has been used to estimate the numerical discretization error for local and integral quantities of interest. A posteriori internal consistency criterion is presented in order to verify that the applied spatial discretization is commensurate with the externally imposed energy filter. The results of the PANS model will be compared with large eddy simulation results and PIV data available from literature.

PIV measurement of turbulent flow downstream of mixing vane spacer grid in 5×5 rod bundle

Axial and cross flow downstream a split-type mixing vane spacer grid in a 5×5 rod bundle are measured with particle image velocimetry (PIV) at sub-channel Reynolds number (Resc) of 6600, 13,200, 26,400 and 39,600. The cross flow in the sixteen inner sub-channels are obtained at five elevations, i.e. 3Dh, 5Dh, 10Dh, 15Dh and 20Dh downstream of the spacer grid. The axial flow measurement is conducted in a plane parallel to the bundle axis. The satisfactory convergence of turbulent statistics is achieved with the sampling number of 2000. The maximum standard uncertainty of statistical quantities normalized by the bulk velocity for all the test conditions is less than 0.6%. The flow characteristics and structures were determined by the mixing vanes located on the upper edge of support grids in rod bundle, and the flow downstream the spacer grid can be divided into the advection zone, the reorganization zone and the dumping zone. And the maximum lateral velocity decreases from 44% of the bulk velocity (Wb) at 3Dh to 16%Wb at 20Dh. The mixing vanes generate drastic turbulence flow downstream the spacer grid, and the lateral root mean square (RMS) fluctuating velocity decays in exponential way. The maximum lateral RMS fluctuating velocity decays rapidly from 20%Wb at 3Dh to 9%Wb at 20Dh. The maximum lateral Reynolds stress decreases from 0.6%Wb2 at 3Dh to 0.3%Wb2 at 20Dh. The normalized mean velocity and turbulent statistics show similarity for the Reynolds above 13,200, while the low Reynolds number effect at Re = 6600 is observed for the axial mean velocity and axial RMS fluctuating velocity.

Simulation of turbulent flow in MATIS-H rod bundle with split-type mixing vanes

Turbulent flow simulations of 5 × 5 rod bundle with the split-type mixing grid are presented. The simulations were performed by the Jožef Stefan Institute (JSI) in the frame of its participation in the OECD/NEA CFD Benchmark exercise. The experimental data from the MATIS-H benchmark experiment located at the Korean Atomic Energy Research Institute (KAERI) were used for posteriori comparison with simulation results. To predict the flow between the rods and mixing vanes, a complex, fully hexagonal flow-oriented mesh was constructed, taking into account the geometry of mixing vanes. High resolution numerical methods and a scale-resolving turbulence model was used in ANSYS CFX simulation. Time-averaged velocity profiles, profiles of velocity fluctuations and integral velocity circulation at the specified downstream locations show good agreement with experimental data. In addition, the simulated flow patterns over the entire rod bundle cross-section are presented and discussed.

Stereoscopic particle image velocimetry measurements of flow in a rod bundle with a spacer grid and mixing vanes at a low Reynolds number

In this study, we experimentally investigate the flow field characteristics in a fuel rod bundle with a spacer grid and split-type mixing vanes at a low Reynolds number of 14, 000. Time-resolved stereoscopic particle image velocimetry (TR-SPIV) measurements were performed in a matched-index-of-refraction facility, and taken downstream of the mixing vanes. Velocity measurements were performed along the vertical-horizontal planes in the inter-channels between the fuel rods. From the obtained TR-SPIV velocity vector fields, the full-field flow statistics such as the mean velocity and Reynolds stresses were computed. Moreover, spectral analysis, squared coherent functions and spatial-temporal velocity cross correlations were performed on the TR-SPIV velocity snapshots. A variety of integral length scales estimated from the two-point spatial cross-correlations have shown that the flow downstream of the mixing vanes was strongly anisotropic. Vortex shedding frequencies were found to contribute to the peaks of coherent functions computed from the fluctuating velocity. This work is the first experimental evidence confirming the findings and discussions from previous numerical studies about the anisotropy flow and a wide range of turbulence length scales generated by the mixing vanes.

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.

An investigation of the effect of split-type mixing vane on extent of crossflow between subchannels through the fuel rod gaps

Subchannels of a fuel bundle may have varying pressure drop characteristics due to the prevalence of different thermal hydraulic conditions. Diversion crossflow between two adjacent subchannels results from lateral pressure fluctuations between the two subchannels. Pressure fluctuations, may also be introduced by varying subchannel cross-sections. In addition to the contribution to crossflow by varying subchannel cross-section, flow deflectors such as mixing vanes attached to spacer grids contribute greatly to the extent of crossflow through gaps connecting adjacent subchannels.To establish the extent to which flow deflection by virtue of split-type mixing vane contribute to crossflow in rod bundles, computational fluid dynamic simulation using STAR-CCM+ was performed for flow of water at a Reynolds number of RE1=3.4×104 through a 5×5 rod bundle geometry for which the rod to rod pitch to diameter ratio was 1.33 and the wall to rod pitch to diameter ratio was 0.74. The two layer k-epsilon turbulence model with an all y+ automatic wall treatment function in STAR-CCM+ was adopted for an isothermal single phase flow through the geometry with imposed cyclic periodic interface boundary condition of fully developed flow type at flow inlet and outlet boundaries.The objectives were to primarily investigate the extent of predictability of the physical problem by the computational fluid dynamic (CFD) code as a measure of reliability of the models employed. Another objective is to assess through comparative analysis the effect of split type mixing vanes on crossflow through gaps connecting adjacent subchannels.Validation of simulation results with experimental data showed good correlation of mean flow parameters with experimental data whiles turbulent fluctuations deviated largely from experimental trends. Generally, the agreement of simulation results with data obtained from the experimental investigation confirmed the suitability of the CFD code, STAR-CCM+ to analyze the physical problem considered. Relative to the non-linear standard k-epsilon and the k-Omega models, the two layer realizable k-epsilon turbulence model employed was unable to appreciably predict turbulent velocity fluctuations. Flow diversions at the position of the mixing vanes were strengthened greatly and observed to be due to the contribution of flow sweeping by the split-type mixing vanes. As observed from the magnitude of diversion and directed crossflow, the mixing vanes contributed largely to the extent of crossflow through the gaps which should result in improved distribution of flow properties between subchannels.

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.

CFD Analysis for the Turbulent Flow Distribution in a Fuel Assembly with the Split-Type Mixing Vanes by Using the Advanced Scale-Resolving Turbulence Models

In general, the turbulent flow inside PWR (Pressurized Water Reactor) fuel assembly depends on the mixing vane configuration and the pattern of the mixing vane arrangement on the strap of the spacer grid. In this study, in order to examine the turbulent flow structure inside fuel assembly with the split-type mixing vanes, simulations were conducted with the commercial CFD (Computational Fluid Dynamics) software, ANSYS CFX R.14. Two different types of turbulence models, i.e. SAS (Scale-Adaptive Simulation)-SST (Shear Stress Transport) and DES (Detached Eddy Simulation), were used. The predicted results were compared with the measured data from the MATiS-H (Measurement and Analysis of Turbulent Mixing in Subchannels-Horizontal) test facility. Although there were locally differences between the prediction and the measurement, ANSYS CFX R.14 predicted the time averaged velocity field in the reliable level. The predicted horizontal and vertical velocity components were more in agreement with the measured data than the axial velocity component. There was no significant difference in the prediction accuracy of both turbulence models.

Correlation for cross-flow resistance coefficient using STAR-CCM+ simulation data for flow of water through rod bundle supported by spacer grid with split-type mixing vane

Mass transfer by diversion cross-flow through gaps is an important inter-subchannel interaction in fuel bundle of power reactors. It is normally due to the lateral pressure difference between adjacent sub-channels. This phenomenon is augmented in the presence of flow deflectors and is referred to as, directed cross-flow. Diversion cross-flow carries the momentum and energy of flow and hence affects the velocity and temperature profile in the rod bundle. The resistance to cross-flow in the transverse momentum equations is specified by the cross-flow resistant coefficient which is the subject of concern in the present study.In order to obtain data to correlate cross-flow resistance coefficient, computational fluid dynamic simulation using STAR-CCM+ was performed for flow of water at the bundle Reynolds number of Re1=3.4×104 through a 5×5 rod bundle geometry supported by spacer grid with split mixing vanes for which the rod to rod pitch to diameter ratio was 1.33 and the rod to wall pitch to diameter ratio was 0.74. The two layer k-epsilon turbulence model with an all y+ automatic wall treatment function in STAR-CCM+ were adopted for an isothermal single phase (water) flow through the geometry. The objectives were to primarily investigate the extent of predictability of the experimental data by the computational fluid dynamic (CFD) simulation as a measure of reliability on the CFD code employed and also apply the simulation data to develop correlations for determining resistance coefficient to cross-flow.Validation of simulation results with experimental data showed good correlation of mean flow parameters with experimental data whiles turbulent fluctuations deviated largely from experimental trends. Generally, the agreement of simulation results with data obtained from the experimental investigation confirmed the suitability of the CFD code, STAR-CCM+ to analyze the physical problem considered. Relative to the non-linear standard k-epsilon and the k-Omega models, the two layer realizable k-epsilon turbulence model employed was unable to appreciably predict turbulent velocity fluctuations.Cross-flow resistance coefficients were observed to be dependent not only on the ratio of lateral to axial flow velocity but also on the hydraulic diameter of the interconnected sub-channel with larger cross-section, the centroid to centroid distance and the gap width. The coefficient is also dependent on the dynamics of the flow in the neighborhood of specific gaps. These dynamics were attributed to the behavior of secondary flows in the sub-channels and especially across the gaps. Profiles of cross-flow resistant coefficient showed a decreasing trend as the ratio of lateral to axial flow velocity increased.

The Experimental Study on Mixing Characteristics in a Square Subchannel Geometry with Typical Flow Deflectors

The lateral flow structures in a subchannel geometry were carefully examined in an enlarged 5 × 5 rod bundle array by using a two-color LDA system. The detailed measurement data of lateral velocities in subchannels have been obtained as fabricating the rod bundle array 2.6 times larger than the real bundle size. Two types of flow deflectors were selected for the test: a conventional split-type mixing vane and a swirl-type vane that had been designed to enhance the swirling flow in a subchannel. Experiments were performed in a cold test loop facility at KAERI. The experimental conditions were normal pressure (1.5 bar) and temperature (35°C). The average axial velocity was maintained at 1.5 m/s and was equivalent to Re = 50,000. From the experimental results, specific flow characteristics for each vane types were observed.