Fluid-Structure Interaction

Fluidelastic Instability Modeling of Loosely Supported Multispan U-Tubes in Nuclear Steam Generators

[+] Author and Article Information
Marwan Hassan

Flow-Induced Vibrations Laboratory
School of Engineering
University of Guelph
Guelph, ON, N1G 2W1, Canada
e-mail: mahassan@uoguelph.ca

Atef Mohany

Fluid-Sound-Structure Interaction Laboratory
Automotive Center of Excellence
University of Ontario Institute of Technology
Oshawa, ON, L1H 7K4, Canada
e-mail: Atef.Mohany@uoit.ca

1Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received October 30, 2011; final manuscript received March 6, 2012; published online December 5, 2012. Assoc. Editor: Jong Chull Jo.

J. Pressure Vessel Technol 135(1), 011306 (Dec 05, 2012) (9 pages) Paper No: PVT-11-1193; doi: 10.1115/1.4006854 History: Received October 30, 2011; Revised March 06, 2012

Steam generators in nuclear power plants have experienced tube failures caused by flow-induced vibrations. Two excitation mechanisms are responsible for such failures; random turbulence excitation and fluidelastic instability. The random turbulence excitation mechanism results in long-term failures due to fretting-wear damage at the tube supports, while fluidelastic instability results in short-term failures due to excessive vibration of the tubes. Such failures may require shutdowns, which result in production losses, and pose potential threats to human safety and the environment. Therefore, it is imperative to predict the nonlinear tube response and the associated fretting-wear damage to tubes due to fluid excitation. In this paper, a numerical model is developed to predict the nonlinear dynamic response of a steam generator with multispan U-tubes and anti-vibration bar supports, and the associated fretting wear due to fluid excitation. Both the crossflow turbulence and fluidelastic instability forces are considered in this model. The finite element method is utilized to model the vibrations and impact dynamics. The tube bundle geometry is similar to the geometry used in CANDU steam generators. Eight sets of flat-bar supports are considered. Moreover, the effect of clearances between the tubes and their supports, and axial offset between the supports are investigated. The results are presented and comparisons are made for the parameters influencing the fretting-wear damage, such as contact ratio, impact forces, and normal work rate. It is clear that tubes in loose flat-bar supports have complex dynamics due to a combination of geometry, tube-to-support clearance, offset, and misalignment. However, the results of the numerical simulation along with the developed model provide new insight into the flow-induced vibration mechanism and fretting wear of multispan U-tubes that can be incorporated into future design guidelines for steam generators and large heat exchangers.

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Fig. 1

Velocity distribution in the U-bend region for the central plane of a CANDU steam generator at 100% power [23]

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Fig. 2

Flow cell concept for the fluidelastic instability model

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Fig. 3

Tube-support model

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Fig. 4

U-bend flow cell model

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Fig. 5

Crossflow velocity distribution along a typical steam generator U-bend tube (extracted from Fig. 1)

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Fig. 6

Mode shapes: (a) first in-plane mode (4.2 Hz), (b) second in-plane mode (10.4 Hz), (c) third in-plane mode (19.9 Hz), (d) first out-of-plane mode (58.1 Hz), (e) second out-of-plane mode (89.1 Hz), (f) third out-of-plane mode (96.4 Hz)

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Fig. 7

Linear tube response

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Fig. 8

Tube response for a clearance of 0.2 mm at various velocity ratios: normal impact forces at FBS 5 (a) 0.3, (d) 1.7, (g) 2.3; lift response spectra (b) 0.3, (e) 1.7, (f) 2.3; drag response spectra (c) 0.3, (f) 1.7, (i) 2.3

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Fig. 9

Effect of the radial support clearance (Cr) on the tube response for a support axial offset of Ca = 20 mm: (a) rms lift response, (b) rms impact force, (c) normal work rate

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Fig. 10

Effect of the support axial offset (Ca on the tube response for a radial clearance Cr = 0.01 mm: (a) rms lift response, (b) rms impact force, (c) normal work rate

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Fig. 11

Clearance distribution at the supports

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Fig. 12

Effect of the radial support clearance (Cr) on the impact force: (a) effect of Cr2 on impact at support 2, (b) effect of Cr7 on impact at support 7, (c) effect of Cr15 on impact at support 16, (d) effect of Cr9 on impact at support 10

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Fig. 13

Work rate histogram: (a) support 7, (b) support 8, (c) support 9, (d) support 10



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