Flow In Rod Bundle Research Articles

Comparative Analysis of CTF and Trace Thermal-Hydraulic Codes Using OECD/NRC PSBT Benchmark Void Distribution Database

The international OECD/NRC PSBT benchmark has been established to provide a test bed for assessing the capabilities of thermal-hydraulic codes and to encourage advancement in the analysis of fluid flow in rod bundles. The benchmark was based on one of the most valuable databases identified for the thermal-hydraulics modeling developed by NUPEC, Japan. The database includes void fraction and departure from nucleate boiling measurements in a representative PWR fuel assembly. On behalf of the benchmark team, PSU in collaboration with US NRC has performed supporting calculations using the PSU in-house advanced thermal-hydraulic subchannel code CTF and the US NRC system code TRACE. CTF is a version of COBRA-TF whose models have been continuously improved and validated by the RDFMG group at PSU. TRACE is a reactor systems code developed by US NRC to analyze transient and steady-state thermal-hydraulic behavior in LWRs and it has been designed to perform best-estimate analyses of LOCA, operational transients, and other accident scenarios in PWRs and BWRs. The paper presents CTF and TRACE models for the PSBT void distribution exercises. Code-to-code and code-to-data comparisons are provided along with a discussion of the void generation and void distribution models available in the two codes.

On the choice of correlations for calculating the heat transfer coefficient in binary gas mixtures

Analytical review of numerous Russian and foreign sources of information on convective heat transfer to single-phase flows in circular pipes and rod bundles is represented. Formulas for calculation of heat transfer in circular pipes and rod bundles for different flow regimes of a binary gas mixture are proposed.

Experimental benchmark data for PWR rod bundle with spacer-grids

In numerical simulations of fuel rod bundle flow fields, the unsteady Navier–Stokes equations have to be solved in order to determine the time (phase) dependent characteristics of the flow. In order to validate the simulations results, detailed comparison with experimental data must be done. Experiments investigating complex flows in rod bundles with spacer grids that have mixing devices (such as flow mixing vanes) have mostly been performed using single-point measurements. In order to obtain more details and insight on the discrepancies between experimental and numerical data as well as to obtain a global understanding of the causes of these discrepancies, comparisons of the distributions of complete phase-averaged velocity and turbulence fields for various locations near spacer-grids should be performed. The experimental technique Particle Image Velocimetry (PIV) is capable of providing such benchmark data. This paper describes an experimental database obtained using two-dimensional Time Resolved Particle Image Velocimetry (TR-PIV) measurements within a 5×5 PWR rod bundle with spacer-grids that have flow mixing vanes. One of the unique characteristic of this set-up is the use of the Matched Index of Refraction technique employed in this investigation to allow complete optical access to the rod bundle. This unique feature allows flow visualization and measurement within the bundle without rod obstruction. This approach also allows the use of high temporal and spatial non-intrusive dynamic measurement techniques namely TR-PIV to investigate the flow evolution below and immediately above the spacer. The experimental data presented in this paper includes explanation of the various cases tested such as test rig dimensions, measurement zones, the test equipment and the boundary conditions in order to provide appropriate data for comparison with Computational Fluid Dynamics (CFD) simulations. Turbulence parameters of the obtained data are presented in order to gain insight of the physical phenomena.

Development of a subchannel void sensor and two-phase flow measurement in 10 × 10 rod bundle

An accurate and detailed experimental database is crucial for modeling the multidimensional two-phase flow and for validating the numerical calculation results. In particular, a two-phase flow in the rod bundle flow channel is so complicated that it is difficult to measure a multidimensional flow structure. Based on the available reference, a point-measurement sensor for acquiring void fractions and bubble velocity distributions do not infer interactions of the subchannel flow dynamics, such as a cross flow and flow distribution, etc. In order to acquire multidimensional two-phase flow in a 10×10 rod bundle with an o.d. of 10mm and length of 3110mm, a new sensor consisting of 11×11 wire and 10×10 rod electrodes was developed. The electrical potential in the proximity region between the two wires creates a void fraction in the central subchannel, like a so-called wire-mesh sensor. A unique feature of the devised sensor is that the void fraction near the rod surface can be estimated from the electrical potential in the proximity region between one wire and one rod, meaning the additional 400 points of void fraction and phasic velocity in the 10×10 rod bundle can be acquired. The devised sensor demonstrates multidimensional flow structures, i.e. void fraction, phasic velocity, sauter mean diameter and interfacial area concentration distributions. Acquired data exhibit complexity of two-phase flow dynamics in a rod bundle flow channel, such as coalescence and the breakup of bubbles in transient phasic velocity distributions.

Single-phase computational fluid dynamics applicability for the study of three-dimensional flow in 5′ 5 rod bundles with spacer grids

Spacer grids play an important role in pressurized water reactor (PWR) fuel assembly in that they have significant influence on the thermal-hydraulic characteristics of the reactor core. But so far, the numerical studies are performed without regarding dimple and spring of spacer grids, just considering mixing vane. Moreover, these studies use k-ɛ turbulence model without considering the suitability of the other turbulence models upon the different spacer grids flow. A study is carried out to understand the 3-D single-phase flow in AFA-2G 5×5 rod bundles with spacer grids based on numerical method. In order to investigate the suitability of different turbulence models, k-ɛ model and k-ω model, the influence of different parts of spacer grid on the fluid flow is also predicted. By using second-order upwind scheme, hybrid grids technique, and improved SIMPLEC algorithm, the Reynolds averaged mass conservation and momentum conservation equations are solved, and the pressure and velocity field of flow are obtained. The numerical simulation results are compared with experiment results and the agreement is satisfactory. The simulation results show the influences of the spring, dimple and mixing vane, and the different characteristics of the k-ɛ model and k-ω model. Comparing with the experiment results, the simulation results suggest that the k-ω model is suitable for the simulation of the rod bundle flow with spacer grids; the spring and dimple are the main causes of the pressure loss in the spacer grid channel. The friction coefficient of the channel with spring and dimple is 1.5 times the coefficient of the channel with the vane. These results are beneficial to enhance the simulation ability of spacer grids flow and optimization design ability of spaces grid.

B223 ロッドバンドル内液流の乱流特性(OS9 熱・流動)

Validation of turbulence models is not an easy task mainly due to lack of experimental data on turbulent flows in rod bundles. In this study, we therefore measured distributions of velocity and turbulence intensities of water flow in a 2x2 rod bundle by using a stereoscopic PIV system. The diameter and pitch of the rods were 20 and 25 mm, respectively. Area-averaged velocity was 2 m/s. Local time-averaged velocity and turbulent intensities were obtained from 10,000 instantaneous velocities at each measurement point. Experimental results showed that horizontal components of turbulence intensity in the cross-section are anisotropic near the rod gap. Since eddies larger than the gap width cannot exist in the rod gap, the turbulence spectrum takes low values in the rod gap compared with center region of subchannel. To the contrary, difference in the turbulence spectrum of eddies smaller than the gap width is small between the rod gap and the center of subchannel.

Measurements of turbulent flows in a 2×2 rod bundle

Abstract Turbulence models such as the k – ɛ model have been used in numerical predictions of turbulent flows in rod bundles. However, the validation of the turbulence models is not an easy task mainly due to the lack of experimental data on turbulent flows in rod bundles. In this study, we therefore measured three-dimensional velocity and turbulence intensities of water flows in a 2 × 2 rod bundle by using a particle image velocimetry (PIV) system. The diameter and pitch of the rods were 20 and 25 mm, respectively. The mean velocity ranged from 1 to 2 m/s. To measure the velocity distributions in the subchannels, the rod in the measurement region was made of fluorinated ethylene propylene (FEP) resin, the refractive index of which is almost the same as that of water. The time-averaged velocity and turbulent intensities at each measurement point were obtained from 10,000 instantaneous velocities. Time-series velocity distributions were also acquired to understand the motion of turbulence eddies in the subchannel. The experimental results indicate that the axial mean velocity along the line normal to the rod in the inner subchannel agrees well with the one-seventh power law, and that the velocity fluctuation in the direction normal to the rod wall becomes weak in the rod gap. Numerical simulations using the k – ɛ models were carried out to examine their applicability to the turbulent flow in the rod bundle. They gave reasonable predictions for the distribution of the axial velocity in the rod bundle and the flow rate in each subchannel.

Numerical simulations of developing flow and vortex street in a rectangular channel with a cylindrical core

Abstract Three-dimensional, unsteady simulations of developing turbulent flows in a rectangular channel containing a cylindrical rod have been performed to investigate their sensitivity to the choices of boundary conditions and turbulence models. Among all methods, large eddy simulations, employed in a downstream sub-domain of the channel as part of the segregated hybrid model, reproduced most accurately the experimental results. However, unsteady Reynolds-averaged Navier-Stokes (URANS) simulations with a Reynolds stress model appear to be also an acceptable choice for approximate rod bundle analyses, making fairly accurate predictions at a much lower computational cost. In agreement with previous findings, steady RANS simulations are not recommended as a low-cost substitute of URANS for flows in tightly packed rod bundles. The URANS simulations were found to be insensitive to inlet turbulence specification and to be more accurate when a uniform inlet velocity was specified rather than a fully developed inlet velocity distribution. Developing flow simulations were found to be preferable to simulations with a streamwise-periodic boundary condition. Finally, unsteady inviscid (Euler) simulations with a fully developed initial velocity distribution predicted the onset of gap instability, but were otherwise found to be unsuitable for practical analysis of rod bundle flows.

Reprint of: Rod bundle vortex networks, gap vortex streets, and gap instability: A nomenclature and some comments on available methodologies

This note introduces the following terms, which apply to large-scale, vortical, coherent structures, appearing in flows through tightly packed rod bundles and other channels that have narrow gap regions: the term gap instability describes the mechanism of generation of such structures in the gap region in laminar and turbulent flows; the term gap vortex street describes the chain of counter-rotating vortices which from on either side of a single gap; and the term rod bundle vortex network describes the set of mutually dependent gap vortex streets that appear in rod bundles and other channels with multiple gaps. Moreover, this notes summarizes the capabilities and limitations of available CFD methods for simulating flows in rod bundles.

The flow and heat transfer of turbulent pulsating flow in rod bundles in rolling motion

The flow and heat transfer of turbulent pulsating flow in rod bundles in rolling motion is investigated. The simulation results are firstly validated with Direct Numerical Simulation results. The effect of spanwise and transverse additional forces due to rolling motion is weak. The effect of velocity oscillation could be divided into three parts, the Reynolds number, velocity oscillating period and oscillating velocity Reynolds number. As for these three parameters, only the contribution of oscillating velocity Reynolds number is obvious.

Numerical simulation of the coherent structure and turbulent mixing in tight lattice

The turbulent flow and transverse mixing in triangular rod bundles is investigated numerically. The calculation results were firstly validated with experimental data. Then the effects of Reynolds number, Prandtl number and pitch to diameter ratio on the coherent structure and turbulent mixing are analyzed. The coherent structure exists in a very wide range of Reynolds number in tight lattice. The effect of Prandtl number on the coherent structure and turbulent mixing is more limited than that of Reynolds number. The effect of pitch to diameter ratio on the coherent structure is most significant. The critical pitch to diameter ratio for this lattice is next to 1.03. As the pitch to diameter ratio decrease from a big number to the critical value, the coherent structure becomes more significant which is benefit to the local flow and heat transfer. But as the pitch to diameter ratio continues decreasing from this critical value to a very small value, the coherent structure becomes weaker and weaker, and the local flow and heat transfer also become more and more terrible.

Rod bundle vortex networks, gap vortex streets, and gap instability: A nomenclature and some comments on available methodologies

This note introduces the following terms, which apply to large-scale, vortical, coherent structures, appearing in flows through tightly packed rod bundles and other channels that have narrow gap regions: the term gap instability describes the mechanism of generation of such structures in the gap region in laminar and turbulent flows; the term gap vortex street describes the chain of counter-rotating vortices which from on either side of a single gap; and the term rod bundle vortex network describes the set of mutually dependent gap vortex streets that appear in rod bundles and other channels with multiple gaps. Moreover, this notes summarizes the capabilities and limitations of available CFD methods for simulating flows in rod bundles.

The flow and heat transfer characteristics of turbulent flow in typical rod bundles in ocean environment

The flow and heat transfer characteristic of turbulent flow in typical 4 and 7 rod bundles in ocean environment is investigated theoretically. In ocean environment, the periodic variation of secondary flow in 7 rod bundles is not obvious. Because of the velocity oscillation, there is a periodic heat accumulation on the tube wall. And the restriction of the channel wall on the rolling motion is considerable. In 7 rod bundles, because of the restriction of the channel wall, the effect of the additional force perpendicular to flowing direction is limited, and the turbulent flowing and heat transfer is mainly determined by the axial turbulent intensity and inlet velocity. However, in the 4 rod bundles, the restriction of the channel wall is small. The effect of the additional force perpendicular to flowing direction on the flowing and heat transfer is significant. And the additional force perpendicular to flowing direction can also affect the Reynolds stress.

Large Eddy Simulation of Turbulence in a Rod Cluster

To assess the accuracy of large eddy simulation (LES) predictions for a flow in a rod bundle, analyses were performed with different parameters of a constant-coefficient Smagorinsky LES model for a flow in a square-pitch rod bundle, and model predictions are compared with experimental data. The parameters considered are the grid structure, the value of the Smagorinsky constant, the damping of the eddy viscosity, and the size of the channel geometry. Because LES simulations are computationally very demanding, for adequately accurate predictions the grid structure needs to be well optimized in terms of cell size, aspect ratio, and cell orthogonality. The use of hanging nodes can significantly reduce the number of cells without a significant penalty on the accuracy of predictions. For this flow, the change in the value of the Smagorinsky constant from 0.14 to zero did not have a drastic effect on predictions. Although, overall, Lilly damping gave slightly better predictions than van Driest damping, both damping functions gave similar predictions. The LES predictions for the mean axial velocity, for the fluctuating velocity component in the main flow direction, and for the Reynolds stresses are in very good agreement with the experimental measurements. There is also good agreement between predictions and measurements for the wall shear stress, but there is a significant discrepancy between predictions and measurements for the fluctuating velocity components in the lateral directions (u and v).

E204 2×2ロッドバンドル内乱流の速度分布(OS9 熱・流動(試験))

Validation of turbulence models is not an easy task mainly due to lack of experimental data on turbulent flows in rod bundles. In this study, we therefore measured distributions of velocity and turbulence intensities of water flows in a 2x2 rod bundle by using a PIV system. The diameter and pitch of the rods were 20 and 25mm, respectively. The mean velocity ranged from 1 to 2m/s. The local time-averaged velocity and turbulent intensities were obtained from 10,000 instantaneous velocity data at each measurement point. Time resolved PIV measurements were also carried out to obtain the turbulence spectra. As a result, the following conclusions were obtained: (1) The fluctuation of velocity component normal to the rod wall is weaker in the low wave number region than that of the perpendicular component in horizontal plane in the rod gap. (2) The distribution of Reynolds stress on the centerline of the test section has the trend similar to the gradient of stream wise velocity. This implies the validity of the eddy viscosity assumption.

Simulation of turbulent flow and heat transfer in channels between rod bundles

The turbulent flow and heat transfer in triangular rod bundles are investigated theoretically with CFD code FLUENT. The unsteady Reynolds Stress Model is adopted as turbulence modeling. The wall function is used for near wall boundary layer. The calculation results were in agreement with experimental data. The effects of the Reynolds number and pitch to diameter ratio on the flow and heat transfer in the lattice are significant. The traditional theoretical models could not predict the flow and heat transfer in the lattice. The P/D = 1.03 is a critical point. In this case, the flow and heat transfer in the lattice is the most desirable and most efficient, and the nuclear power could also reach its maximum. The variation of large scale coherent structure with pitch to diameter ratio is consistent with the variation of the Nusselt number with pitch to diameter ratio.

CFD Analysis of Turbulent Flow in Typical Rod Bundles in Rolling Motion

The influence mechanism of rolling motion on the flowing and heat transfer characteristics of turbulent flow in typical four rod bundles is investigated with FLUENT code. The flowing and heat transfer characteristics of turbulent flow in rod bundles can be affected by rolling motion. But the flowing similarity of turbulent flow in adiabatic and non-adiabatic can not be affected. If the rolling amplitude is big or if the rolling period is small, the radial additional force can make the parameter profiles and the turbulent flowing and heat transfer change greatly. And the frictional resistance coefficient and heat transfer coefficient can not be solved by the correlations in steady state. In rolling motion, as the pitch to diameter ratio decrease, especially if it is less than 1.1, the flowing and heat transfer of turbulent flow in rolling motion change significantly.

Void Fraction in a Four by Four Rod Bundle under a Stagnant Condition

In the case of a hypothetical failure of a residual heat removal (RHR) systems under mid-loop operation, vapor generated in a reactor core forms two-phase flow in a stagnant liquid and rises the water level in the core. The vapor flows into a steam generator through a hot leg, and condenses in the steam generator. Since the flow rate of vapor from the reactor core to the hot leg depends on the water level and the void fraction α in the reactor core, the reliable analysis of the RHR failure cannot be carried out without accurately estimating the void fraction in the reactor core. Although a number of studies on void fractions in two-phase flows in rod bundles have been carried out, there are few experimental data on void fractions in rod bundles under the stagnant condition. Void fractions in four by four rod bundles under the stagnant condition were measured for a wide range of gas volume fluxes to examine the validity of available void correlations. Flow patterns were visualized by using a high-speed video camera to examine the effects of flow pattern on the void fraction. As a result, the following conclusions were obtained: (1) Dependence of the void fraction on the gas volume flux JG changed at JG ≅ 1.5 m/s due to the flow pattern transition. (2) Murase's correlation agreed well with the void fraction in the two kinds of rod bundles having different dimensions under the stagnant condition.

A second order turbulence model based on a Reynolds stress approach for two-phase boiling flow and application to fuel assembly analysis

High-thermal performance PWR (pressurized water reactor) spacer grids require both low-pressure loss and high critical heat flux (CHF) properties. Numerical investigations on the effect of angles and position of mixing vanes and to understand in more details the main physical phenomena (wall boiling, entrainment of bubbles in the wakes, recondensation) are required. In the field of fuel assembly analysis or design by means of CFD codes, the overwhelming majority of the studies are carried out using two-equation Eddy Viscosity Models (EVM), especially the standard K– ɛ model, while the use of Reynolds Stress Transport Models (RSTM) remains exceptional. The simulation of swirling flow generated by the mixing vanes plays an important role for the prediction of the CHF for the fuel assemblies. For this reason, according to Mimouni et al. (2008b, 2009b), rotation effects and RSTM model are more specifically addressed in the paper. Before comparing performance of EVM and RSTM models on fuel assembly geometry, we performed calculations with simpler geometries, the DEBORA case and the ASU-annular channel case. ASU-annular channel case has already been addressed in Mimouni et al. (2008b, 2009b). Then, a geometry closer to actual fuel assemblies is considered. It consists of a rectangular test section in which a 2 × 2 rod bundle equipped with a simple spacer grid with mixing vanes is inserted. The influence of the turbulence model on target variables linked to CHF limitation will be discussed. Moreover, the sensitivity to the mesh refinement will be particularly examined. The study of this case is a further step towards the modelling of the two-phase boiling flow in real-life grids and rod bundles.

Non-intrusive experimental investigation of flow behavior inside a 5 × 5 rod bundle with spacer grids using PIV and MIR

The validity of the simulation results from computational fluid dynamics (CFD) is still under scrutiny. Some existing CFD closure models for complex flow produce results that are generally recognized as being inaccurate. Development of improved models for complex flow simulation requires an improved understanding of the detailed flow structure evolution with dynamic interaction of the flow multi-scales. Thus, the goal of this work is to contribute to a better understanding of presupposed and existent events that could affect the safety of nuclear power plants. The fundamental phenomena of fluid flow in rod bundles with spacer grids can be elucidated by using state-of-the-art measurement techniques. This study aims to develop an experimental data base with high spatial and temporal resolution of fluid flow velocity inside a 5 × 5 rod bundles with spacer grids. The full-field detailed data base is intended to validate CFD codes at various temporal-spatial scales. Measurements are carried out using dynamic particle image velocimetry (DPIV) technique inside an optically transparent rod bundle utilizing the matching index of refraction (MIR) approach. This work presents full field velocity vectors and turbulence statistics for a rod bundle under single phase flow conditions.