Simulations of Fully-Flexible Fuel Bundle Response due to Turbulence Excitation

This work presents a numerical model for a fully-flexible CANDU fuel bundle to predict the vibration response due to turbulence excitation. The model includes 37 fuel elements and two endplates. The contact between system components such as fuel-to-fuel and fuel-to-pressure tube is modeled using the single point contact method (SPC). A range of flow velocities was examined, and the associated impact forces and work rates were calculated. In addition, the stresses on the endplates due to vibration of the fuel elements were predicted.

Phase Relation of Forces Between Multiple Bends

Multiphase flow induced vibrations are difficult to calculate. This is in part the prediction of the force frequency spectrum at a single bend, part the influence of bends on the multiphase flow behavior and therefore the prediction of the multiphase flow characteristics between bends. In this paper, the evolution of the forces between subsequent bends are discussed including the phase relations of the forces on the different bends. For annular flow, the forces between the different bends are incoherent and are more or less random. However, for stratified and especially slug flow, the forces remain coherent up to high frequencies. This means in a frequency response analysis of a piping structure, the transport velocity of the multiphase structures must be taken into account.

Towards Understanding Two-Phase Flow Induced Vibration of Piping Structure With a U-Bend

The current work investigates two-phase flow induced vibrations in 90° U-bend. The two-phase induced vibration of the structure was investigated in the vertical, horizontal and axial directions for various flow patterns from bubbly flow to wavy and annular-dispersed flow. The void fractions at various locations along the piping including the fully developed void fraction and the void fraction at the entrance of the U-bend were fully investigated and correlated with the vibration amplitude. The results show that the excitation forces of the two-phase flow in a piping structure are highly dependent on the flow pattern and the flow conditions upstream of the bend. The fully developed void fraction and slip between phases are important in modelling of forces in U-bends and elbows.

Experimental Investigation of Fluid-Elastic Vibration of Square Array Tube Bundle in Two Phase Flow

The in-plane (in-flow) fluid-elastic instability (in-plane FEI) of triangular tube arrays caused tube-to-tube wear indications as observed in the U-bend regions of tube bundles of the San Onofre Unit-3 steam generators[1]. Several researches revealed that the in-plane FEI is likely to occur in a tightly packed triangular tube array under high velocity and low friction conditions, while it is not likely to occur in a square array tube bundle. In order to confirm the potential of steam-wise fluid-elastic instability of square arrays, the critical flow velocity in two-phase flow, (sulfur hexafluoride-ethanol) which simulates steam-water flow, was investigated. Two types of test rigs were prepared to confirm the effect of the tube diameter and tube pitch ratio on the critical velocity. In both rigs, vibration amplitudes were measured in both in-flow and out-of-flow directions in various flow conditions. In any case, in-flow fluid elastic instability was not detected. Based on the results of the tests, it is concluded that the flow interaction force is small for concern to occur the fluid-elastic instability in the in-flow direction of the square tube bundles of steam generators.

The Effect Mechanism of Hydrodynamic Factors on Naphthenic Acid Flow-Induced Corrosion

Hydrodynamic factors are the important factors affecting the flow-induced corrosion of naphthenic acid. The effect mechanisms of hydrodynamic factors such as flow velocity, flow pattern, erosion angle and multiphase flow, etc. on the flow-induced corrosion of naphthenic acid are analyzed comprehensively, and the effect mechanisms of critical hydrodynamic parameters such as surface shear stress and near-wall turbulence intensity, etc. on naphthenic acid corrosion are explained. It is pointed out that in the flow-induced corrosion system of naphthenic acid, hydrodynamic factors such as flow velocity, flow pattern, erosion angle and multiphase flow, etc. influence the erosion intensity and mass transfer process generally by changing the magnitude of surface shear stress and near-wall turbulence intensity, thus affecting the severity of corrosion.

Experimental Investigation on Flow Field Characteristics by Drag Reducing Agent Additives in Stirred Vessel

In this paper, experimental investigation on two oil-soluble DRAs were carried out in stirred vessel by standard six-blade Rushton, based on the application of particle image velocimeter (PIV). Two DRAs (1# and 2#) with different concentration from 3 ppm to 50 ppm were added into diesel respectively, and speed of impeller speed was set 400 rpm. Flow field characteristics including turbulence intensity, turbulent kinetic energy, energy dissipation rate influenced by those additives in stirred vessel were study. It was found that inhibition effect of turbulence intensity of the two DRAs is not obvious with concentration below 10 ppm. However, when concentration is above 10 ppm, turbulence inhibition effect become more obvious. Under low concentration, 1# has better turbulence inhibition effect in area near impeller, while 2# has better turbulence inhibition effect under high concentration. When the two DRAs are under the same concentration of 50ppm, turbulent flow energy and energy dissipation rate are obviously reduced.

Experimental Investigation of the Flow-Induced Vibrations of a Rod Cluster Control Assembly Inside Guides With Enlarged Gaps

In order to better understand the mechanisms of fretting wear damage of guide cards in some Pressurized Water Reactor (PWR) Nuclear Power Plant (NPP), an experimental investigation is undertaken at the Magaly facility in Le Creusot. The test rig consists of a complete Rod Cluster with eleven Guide Cards, submitted to axial flow inside a water tunnel. In order to mimic the effect of fretting wear, the four lower guide cards have enlarged gaps, so that the Control Rods are free to oscillate. The test rig is operated at ambient temperature and pressure, and Plexiglas walls can be arranged along its upper part, and a series of camera records the vibrations of the control rods above and below the guide cards. The vertical flow velocity is in the range of a few m/s. Beam-like pinned-pinned modes at about 5 Hz are observed, and oscillations of several mm of the central rods are measured, which come along with impacts at the higher flow velocities. A simple non-linear calculation reveals that the main effect of the impacts between Control Rods and Guide Cards is an increase of the natural frequency of the rods by about 10%. Furthermore, as the vibration spectra collapse remarkably well with the flow velocity, the experiments prove that turbulent forcing is responsible for the large oscillations of the control rods, no other mechanism being involved.

Tunable EOS Material Model in the Simulation of Pulsed Mercury Spallation Target Vessel

A pulsed spallation target is subjected to very short (∼1μs) but intense loads from repeated proton pulses. The effect of this pulsed loading on the stainless-steel target module that contains flowing mercury target material is difficult to predict. Different simulation approaches and material models for the mercury have been tried. To date the best matching simulation to the experimental data was obtained by an equation of state (EOS) material model with a specified tensile cutoff pressure, which simulates the cavitation threshold [1]. The inclusion of a threshold to represent cavitation was a key parameter in achieving successful predictions of stress waves triggered by the high energy pulse striking the mercury and vessel. However, recent measurements of strain responses of target modules showed that significant discrepancy between the measured strain and simulated value with the EOS mercury model still exists. These differences grow to irreconcilable values when non-condensable helium gas is intentionally injected into the flowing mercury. A novel EOS mercury model embedded into ABAQUS VUMAT has been investigated in this project, which introduces the concept of proportional, integral, and derivative (PID) control into the mercury EOS model. By tuning the new introduced PID parameters (Kp, Ki and Kd), we replace the specified cutoff pressure with an adjustable spring-damper-like material behavior which may better match the complex dynamics of the mercury and helium mixture. This approach is expected to reduce the gap between measured and simulated vessel strain responses. Primitive application of this tunable EOS mercury model on prototypic shape experimental target has demonstrated its capability and potential of improving mechanical behavior of EOS mercury with cutoff pressure considered.

A Hybrid Model to Analyze the Fluid-Structure Interaction Phenomenon of a Relief System and Experimental Validation

As one essential component of a pressurized system, a relief valve is used to guarantee the pressure within a prescribed range. But in practical engineering, pressure fluctuation caused by the operation of a relief valve will travel along the pipeline and couple with the motion of the valve, which might result in malfunction of the valve and the system. In order to investigate the fluid-structure interaction (FSI) phenomenon, a hybrid model combining the method of characteristics (MOC) and computational fluid dynamics (CFD) method is proposed. In the hybrid FSI model, the characteristics of pressure resource is modeled using the performance curves, the compressible gas transmitting in the pipe is calculated by one-dimensional MOC, and the air flow in the valve as well as the valve motion is simulated by a two-dimensional CFD model. To validate the hybrid model, 1:1 scaled test rig is conducted. The compared results show that the hybrid model not only can accurately capture the pressure fluctuation in straight pipeline induced by the closure of the valve but also can accurately predict the forms of the valve motion.

Investigation on Fluidelastic Instability Accompanied by Wake Shedding With a Time-Domain Model

As the most dangerous flow-induced vibration (FIV) mechanism, fluid-elastic instability is always accompanied by the wake shedding. If both of the two FIV mechanisms are considered, fluid forces in this condition can be quite complex. In this paper, a time-domain model based on unsteady flow theory was used to investigate the fluid-elastic instability in a rotated triangular tube array. The vortex shedding forces were simplified as harmonic forces. Computational fluid dynamics (CFD) was used to get the fluid force coefficients with vortex shedding. The model was established by a finite element code with MATLAB software and simulation results agreed with the experiment results. The results showed the critical instability velocity can be influenced by vortex shedding forces, and double peaks can be found in the frequency spectrum of displacements of tubes. The time-domain displacements showed the phases had been impacted by the shedding and periodic characteristic was found in the displacements results. The model can also be adopted in fluid-elastic instability analysis in other tube arrangements and flow conditions.