Tomomichi Nakamura

Two-Phase Flow-Induced Vibration of Parallel Triangular Tube Arrays With Asymmetric Support Stiffness

Laboratory experiments were conducted to determine the flow-induced vibration response and fluidelastic instability threshold of model heat exchanger tube bundles subjected to a cross-flow of refrigerant 11. Tube bundles were specially built with tubes cantilever-mounted on rectangular brass support bars so that the stiffness in the streamwise direction was about double that in the transverse direction. This was designed to simulate the tube dynamics in the U-bend region of a recirculating-type nuclear steam generator. Three model tube bundles were studied, one with a pitch ratio of 1.49 and two with a smaller pitch ratio of 1.33. The primary intent of the research was to improve our understanding of the flow-induced vibrations of heat exchanger tube arrays subjected to two-phase cross-flow. Of particular concern was to compare the effect of the asymmetric stiffness on the fluidelastic stability threshold with that of axisymmetric stiffness arrays tested most prominently in literature. The experimental results are analyzed and compared with existing data from literature using various definitions of two-phase fluid parameters. The fluidelastic stability thresholds of the present study agree well with results from previous studies for single-phase fiow. In two-phase flow, the comparison of the stability data depends on the definition of two-phase flow velocity.

Investigation of In-Flow Fluidelastic Instability of Square Tube Arrays Subjected to Air Cross Flow

In-flow instability of tube arrays is a recent major issue in heat exchanger design since the event at a nuclear power plant in California [1]. In our previous tests [2], the effect of the pitch-to-diameter ratio on fluidelastic instability in triangular arrays is reported. This is one of the present major issues in the nuclear industry. However, tube arrays in some heat exchangers are arranged as a square array configuration. Then, it is important to study the in-flow instability on the case of square arrays.The in-flow fluidelastic instability of square arrays is investigated in this report. It was easy to observe the in-flow instability of triangular arrays, but not for square arrays. The pitch-to-diameter ratio, P/D, is changed from 1.2 to 1.5. In-flow fluidelastic instability was not observed in the in-flow direction. Contrarily, the transverse instability is observed in all cases including the case of a single flexible cylinder. The test results are finally reported including the comparison with the triangular arrays.Copyright

An Analysis of In-Flow Fluidelastic Instability Based on a Physical Insight

Fluidelastic vibration of tube arrays caused by cross-flow has recently been highlighted by a practical event. There have been many studies on fluidelastic instability, but almost all works have been devoted to the tube-vibration in the transverse direction to the flow. For this reason, there are few data on the fluidelastic forces for the in-flow movement of the tubes, although the measured data on the stability boundary has gradually increased.The most popular method to estimate the fluidelastic force is to measure the force acting on tubes due to the flow, combined with the movement of the tubes. However, this method does not give the physical explanation of the root-cause of fluidelastic instability.In the work reported here, the in-flow instability is assumed to be a nonlinear phenomenon with a retarded or delayed action between adjacent tubes. The fluid force acting on tubes are estimated, based on the measured data in another paper for the fixed cylinders with distributed pressure sensors on the surface of the cylinders. The fluid force acting on the downstream-cylinder is assumed in this paper to have a delayed time basically based on the distance between the separation point of the upstream-cylinder to the re-attachment point, where the fluid flows with a certain flow velocity.Two models are considered: a two-cylinder and three–cylinder models, based on the same dimensions as our experimental data to check the critical flow velocity. Both models show the same order of the critical flow velocity and a similar trend for the effect of the pitch-to-diameter ratio of the tube arrays, which indicates this analysis has a potential to explain the in-flow instability if an adequate fluid force is used.Copyright

Study on In-Flow Fluidelastic Instability of Circular Cylinders Caused by Air Cross Flow (Triangular Arrays)

The in-flow instability of cylinder arrays corresponds to the in-plane instability of U-bend tubes in steam generators. This rarely occurring phenomenon has recently been observed in a nuclear power plant in U.S.A. For this reason, the importance of studying this instability has recently increased.The fluidelastic instability of a cylinder array caused by cross-flow was found to easily occur in air-flow and hardly in water-flow in our previous report. The present report introduces the results of this phenomenon in several patterns of triangular cylinder arrays in air-flow. The pitch spacing between cylinders is one of the parameters, which varies from P/D = 1.2 to 1.5, for a five-by-five cylinder array. The instability is examined both in the in-flow direction and in the transverse direction. The test cylinders are supported with thin plates to move in one direction. The number and the location of the flexibly supported cylinders are the other parameters.Differences between the instability in the in-flow and in the transverse direction are found. Among these differences the most important is the fact that the fluidelastic instability has not been observed for a single flexible cylinder in the in-flow direction, although it is observed in the transverse direction.However, the in-flow instability can be estimated with the Connors’ type formula as in the transverse direction.Copyright

Study on Power Generation Using Unstable Energy by In-Flow Fluidelastic Vibration

In-flow instability of tube arrays is one of recent major issues to be examined in the heat exchanger design since the event at a nuclear power plant in California [1]. In our previous tests [2], the effect of the pitch-diameter ratio in triangular arrays is reported. This is one of the present major issues in the nuclear industry.On the other hand, there are some trials [3] to produce the electric power by the phenomenon of the flow-induced vibration. They are mainly using the vortex shedding, but in this report the power generation by the unstable vibration is investigated. It is spotlighted the combination between the unstable vibration and the power generation whether the limit cycle exists or not. This report introduces the result on the existence of the limit cycle.Many engineers have attempted to produce electric power using semi-conductor devices, but they could not obtain enough power. Then, the device using the electro-magnetic system is used here. The in-flow instability has a small effect on the fluid flow compared with the usual fluidelastic vibration in the transverse direction, because the in-flow movement does not disturb the flow path compared to the transverse fluidelastic vibration.The coil-magnet system is set on the top of the flexible cylinder cantilevered from the bottom. A three by three array of cylinders in a triangular pattern is set in a small wind tunnel. When the flow velocity increases, the cylinder array becomes unstable in the in-flow direction as similarly to the transverse direction, and it produces the electricity in the coil. However, as the power generation means energy dissipation, resulting in increase of the system damping, the effect on the instability is examined.At the end of this report, an analytical solution is introduced to explain the measured results. It is successfully completed to show the existence of the limit cycle.Copyright

Design and Maintenance in an IAEA Technical Guidelines on Fluid-Structure Interaction

In nuclear power plants, fluid structure interactions (FSI) occurring in component systems can cause excessive forces or stresses to the structures resulting in mechanical damages that may eventually threaten the structural integrity. FSI in the guidelines includes flow-induced vibration, water hammer, and pipewhip. It can also include movement, deformation, or fracture of equipments by tsunami etc. They can be issues of design and maintenance. Authors cannot find complete guidelines to correspond to the FSI phenomena which can be important in the design and maintenance of nuclear power plants. Based on the background, International Atomic Energy Agency (IAEA) has drafted guidelines on FSI. This paper summarizes general description of FSI as well as design and maintenance against FSI.Copyright

Damping estimation of a loosely supported single U-bend tube used in a steam generator

An evaluation method for estimating the damping of loosely supported single U-bend tube used in a steam generator colliding with a support plate is proposed. First, we performed experimental modal analysis and obtained natural frequencies, modes and damping without collision by Impulse Modal Test and then analysed natural frequencies, modes and damping with collision employing FEM analysis taking account of the collision force. In modelling the characteristics of the collision force, we applied Bijlaards model for the spring constant and assumed hysteresis. After that, we performed experiments for measuring the damping ratio by changing the gap size and the support plate position. Comparison between calculated and experimental results is made which shows good agreement. Experimentally observed fact showing damping coefficient increases with the initial amplitude is well explained by theoretical model.

Study on In-Flow Vibration of Cylinder Arrays Caused by Cross Flow

The authors have studied the in-flow vibration phenomena of cylinder arrays caused by cross-flow in the low Reynolds number range around Re=800. This Reynolds number range has been studied because it is the range where symmetric vortex shedding occurs. This report is our first trial to study the in-line fluidelastic vibration of cylinder arrays. In initial tests, the flow velocity was increased up to the maximum achievable level by the test equipment. However, it was found that the array’s cantilever tube supports resulted in large static tube deflections due to static drag forces. The cylinder array tube supports have therefore been replaced by thin plates supported at both ends. The cylinders are set to be flexible both in the streamwise direction and the direction transverse to the flow. The obtained results of these two patterns are also compared with previous cantilevered data. The origin of the observed vibrations whether a self-induced mechanism or vortex shedding is discussed in detail.Copyright

Development of Flexible Liquid Damper

The “Flexible Liquid Damper” (FLD) is a new concept of a vibration control device. The damper is composed of only two parts, a flexible ball and liquid filling in the flexible ball. The damper design is simple, but the mechanism underlying its significant damping capacity has not been clarified. Two kinds of tests are conducted to investigate the characteristics of the damper in this study. One is the sinusoidal sweep test, while the other is the free decay test. First, several parameters of the damper are investigated, such as its size, material and the properties of the liquid itself. As a result, it is found that the case of around 50% volume is the most effective rather than the case where the ball is 100% filled with liquid. Moreover, other conditions have some effect, such as the wall thickness of the ball, and the liquid viscosity. Both the free vibration and the sinusoidal sweep tests show its great damping capacity. The damping effect obtained in free vibration tests is higher than that by the sinusoidal sweep tests. In some test cases, two dominant peaks are observed; the peaks only appear in the case of some combinations of the damper parameters. An attempt is made to analyze the damping mechanism using the general-purpose computational tool “ANSYS”, by creating an analytical model of the damper to simulate the test results. From the numerical results, it is found that the vibration control mechanism of this damper is mainly similar to the mechanism of the dynamic vibration absorber. However, the numerical results also show that the damper is not a simple dynamic vibration absorber and may include some other energy dissipation or energy transfer mechanism.Copyright

A Study on Pairs of Stream-Wise Flexible Circular Cylinders in Cross Flow

The importance of the in-flow oscillation of a single cylinder in cross-flow has been in the spotlight since the accident in a FBR-type reactor. In-flow oscillations can also be observed in heat exchanger tube arrays. Previous reports show some interesting phenomena on the oscillation of cylinder arrays. In this paper, detailed observations on the effect of the pitch ratio for pairs of cylinders, in parallel and in tandem, is highlighted in the range of low flow velocities, where each cylinder can move only in a given direction. The motion of the cylinders is measured by attached strain gages and by a high-speed digital video camera.Copyright