Dynamic Interaction Between a Straight Tube and an Anti-Vibration Bar

Fretting-wear is a known problem in steam generator U-tubes. These tubes are supported by flat bars called anti-vibration bars (AVB) in the plane of the U-bend. Clearances between the tubes and the bars are designed to be minimal, but cumulative tolerances and manufacturing variations may lead to clearances larger than expected. Large clearances may result in ineffective support leading to in-plane and out-of-plane motion causing fretting-wear and impact abrasion. In the present work, the problem is investigated with a single two span tube, an anti-vibration bar at mid-span and a local excitation force. The dynamic behavior of a tube with simple supports at both ends and an anti-vibration bar at mid-span is characterized. The influence of clearance, preload and tilt of the support on the dynamics of the tube are investigated experimentally. The results indicate that the fretting-wear work-rate is very low with preloads, reaches a maximum around a zero clearance and diminish again for larger clearances. The tilt of the anti-vibration bar in our experiments seems to change the dynamic behavior of the tube.

Numerical Estimation of Fluidelastic Instability in Staggered Tube Arrays

This study investigates the unsteady flow and the resulting fluidelastic forces in a tube bundle. Numerical simulations are presented for normal triangle tube arrays with pitch-to-diameter (P/d) ratios of 1.35, 1.75, and 2.5 utilizing a 2-dimensional model. In this model a single tube was forced to oscillate within an otherwise rigid array. Fluid forces acting on the oscillating tube and the surrounding tubes were estimated. The predicted forces were utilized to calculate fluid force coefficients for all tubes. The numerical model solves the Reynolds-Average Navier-Stokes (RANS) equations for unsteady turbulent flow, and is cast in an Arbitrary Lagrangian-Eulerian (ALE) form to handle mesh the motion associated with a moving boundary. The fluidelastic instability (FEI) was predicted for both single and fully flexible tube arrays over a mass damping parameter (MDP) range of 0.1 to 200. The effect of the P/d ratio and the Reynolds number on the FEI threshold was investigated in this work.

Fluid Flow Field in the Annulus of Boiling Water (BWR) Nuclear Reactors Under Normal and Accident Conditions

This paper presents a theoretical study of the velocity field in the annulus formed between the Reactor Pressure Vessel (RPV) and the shroud of a Boiling Water Reactor (BWR) under normal and accident flow conditions. Simplified geometry and an ideal irrotational flow are assumed to solve the problem using velocity potentials.

Coupling a Detonation Shock Dynamics Code to an Eulerian Hydrocode

In this technical effort we have incorporated the Detonation Shock Dynamics (DSD) methodology into the Eulerian hydrocode CTH by coupling it to a DSD code called WAVETRACKER. Based on some preliminary work, we developed a strategy for coupling these two codes. Both CTH and WAVETRACKER are stand-alone codes. In the coupled code, CTH calls WAVETRACKER as a subroutine. Interface routines are developed to pass geometry, mesh and boundary information from CTH to WAVETRACKER. Additional parameters required by WAVETRACKER are read directly from a user input file. Results in the form of a burn table are interpolated and passed back from WAVETRACKER to CTH.

An Experimental Investigation of High Level Vibration on the Residual Heat Removal Line of a Pressurized Water Reactor

This paper presents the results of a scaled aero acoustic test that modeled a side branch resonance observed in the residual heat removal suction line of a large pressurized water reactor. Resolution of the acoustic resonance was sought by detuning the eddy shedding frequency from the fundamental side branch acoustic mode. The specific physical modifications and their ability to detune the coupled system are presented.

Water Hammer Likely Cause of Large Oil Spill in North Sea

On December 12, 2007, the second largest oil spill in the history of Norwegian oil exploration occurred on StatoilHydro’s Statfjord Alpha platform. The spill was caused by a snapped 20″ oil off-loading hose. Thorough investigations by StatoilHydro [1] and by the Norwegian authorities [2] revealed the chain of events that led to this incident. One of the links in this chain was the unintended fast closure of the shuttle tanker’s bow loading valve during off-loading. This closure initiated a pressure surge in the oil off-loading system. As part of the internal investigation by StatoilHydro, TNO carried out a water hammer analysis of the entire oil off-loading system, including the off-loading hoses to the seabed and further subsea piping up to the platform. These simulations revealed that high pressures could occur in the oil off-loading system due to fast closure of the bow loading valve followed by multiple reflections at diameter changes. The maximum pressures were more than 100 bar above the normal operating pressure of 10 bar. The diameter changes were introduced into the oil off-loading system to maximize the off-loading capacity. The results of the water hammer analysis provided the missing link between the fast closure of the valve and the damaged hose and also showed that this damage most likely occurred within 0.5 second after the closure of the valve. Based on the results of this analysis, also other oil off-loading systems are being reinvestigated to prevent a similar incident to occur in the future.

Azimuthal Behaviour of Self-Excited Diametral Modes of Internal Cavities

The aerodynamic excitation of ducted cavity diametral modes, which are inherently antisymmetric acoustic modes, by the oscillation of the axisymmetric free shear layer gives rise to complex flow-sound interaction mechanisms, in which the acoustic diametral modes do not possess a preferred azimuthal orientation. The azimuthal behaviour of this self-excitation mechanism is investigated experimentally. The study is performed for axisymmetric shallow cavities in a duct for a range of cavity length to depth ratio of L/d = 1 to 4, and for Mach numbers up to 0.4. Three pressure transducers flush mounted to the cavity floor are used to determine the acoustic mode amplitude and orientation. The excited acoustic modes are classified into spinning, partially spinning and stationary diametral modes. An analytical model based on the superposition of two orthogonal modes with 90° temporal phase shift is developed to reproduce the spinning and the partially spinning diametral modes. The developed model clarifies the observed complex behaviour of the azimuthal modes.

On the Modelling of Noise Generation in Corrugated Pipes

The offshore oil and gas industry uses pipes which are made flexible by means of a corrugated construction. Those pipes that run from a platform to the seabed are known as risers while those that connect two pipes on the seabed are known as jumpers. Gas flowing within the pipe interacts with the corrugations and generates noise. This noise is of concern because it is of sufficient amplitude to cause pipework vibration with the threat of fatigue and pipe breakages. This paper examines the conditions that give rise to the large noise levels. The conditions for the onset of noise are investigated using an eigenvalue approach which involves the effect of damping due to losses from the pipe boundaries and pipe friction. The investigation which is conducted in terms of reflection conditions shows why only few of the very many possible natural frequencies are selected. The conditions for maximum noise response are also investigated using a non-linear model of vortex shedding. Here an approach is developed in which the net power generated along a single wavelength is calculated.

Experimental Study on the Frequency of a Free Surface Fluctuation in a Vessel With or Without an Internal Structure

An experimental study has been performed to investigate the frequency of a free surface fluctuation in a vessel with and without an internal structure. Water flows in from the bottom nozzle and flows out at the side wall nozzles. There are two dominant frequency regions which are generated by a standing wave and a jet. In the standing wave region, the frequency is well described by f(4πdV/g)1/2 = 1.07 in a circular vessel. The frequency generated by a jet can be described by a dimensionless period and Froude number according to its fluctuation stability. In the case of a vessel with an internal structure, it needs a geometry factor which is described by a vessel diameter to a hydraulic diameter ratio in a standing wave region.

Simulation of Tube Bundle Vibrations: A Numerical 3D-Method

The shell-side cross-flow in tubular heat exchangers may cause vibrations leading to failure within hours or in long term. Design is still based on half-empirical correlation, based on the equation of Connors (1978). Overdesign (Kassera 1996) and singular cases of damage (Fischer and Strohmeier, 2002) are the result. Therefore a structural model for the tube motions has been developed further and coupled to the commercial flow simulation code ANSYS CFX. The predictive capability of such coupled methods is limited by the flow simulation. Still simplifications or modeling are needed, especially for turbulence. The paper starts with an overview of modeling assumptions used so far. In addition to simulations of flow around rigid circular cylinders (Reichel and Strohmeier, 2008) LDA-measurements of flow through rigid glass-bundles have been compared to flow simulations. The sample of results presented below demonstrates that, besides level of fluctuations is predicted far too low, the overall velocity distribution on the shell side is predicted well by the SST turbulence model (Menter 1994), making URANS models like SST worth a try, if mainly flow forces are needed. To capture the tube dynamics an Euler-Bernoulli beam model of Fischer (2001), discretized by central differences in space and Newmark’s method (1959) in time, has been extended and implemented into ANSYS CFX. Calculations will be presented, showing that simulations of initially deflected tubes almost perfectly match analytic predictions. To adapt the numerical grid for the flow calculations to the tube displacements, the code inherent standard methods at large displacements resulted in negative volumes and solver failure. Therefore the standard methods have been replaced by own routines for grid deformation. Even for grids with fine near wall resolution, this method is able to cope with large displacements. Finally, coupled simulations are conducted of a single cylinder and of a cantilevered tube bundle in cross flow. For the single cylinder amplitudes are extremely overpredicted as long as 2D-modeling is used. 3D-modeling shows a phase shift of vortex shedding along the cylinder, which results in noticeably lower tube deflections. But, using the SST-model, amplitudes are still higher than measured. A model extension for laminar to turbulent transition leads to further improvement. The same holds for the tube bundle. Onset velocity of instability is predicted too low, amplitudes are too high. Modeling transition and large scale 3D-effects moves results closer to experimental observations. Further improvements are expected taking small scale 3D-effects into account by introducing more grid layers along the tubes.