Research Papers: Fluid-Structure Interaction

Fluid-Elastic Instability of Rotated Square Array U-Tubes in Air-Water Flow

[+] Author and Article Information
In-Cheol Chu

Thermal Hydraulics Safety Research Division, Korea Atomic Energy Research Institute, 1045 Daedeokdaero, Yuseong-gu, Daejeon 305-353, Koreachuic@kaeri.re.kr

Heung June Chung, Chang Hee Lee

Thermal Hydraulics Safety Research Division, Korea Atomic Energy Research Institute, 1045 Daedeokdaero, Yuseong-gu, Daejeon 305-353, Korea

J. Pressure Vessel Technol 131(4), 041301 (Jun 12, 2009) (8 pages) doi:10.1115/1.3148187 History: Received January 03, 2008; Revised February 11, 2009; Published June 12, 2009

Fluid-elastic instability characteristics of a U-tube bundle were experimentally investigated in air-water two-phase flow. A total of 39 U-tubes were arranged in a rotated square array with a pitch-to-diameter ratio of 1.633. Vibration responses of four U-tubes were measured with three-axis accelerometers. Two sets of experiments were performed to investigate the onset of fluid-elastic instability, and the damping and hydrodynamic mass of the U-tube. The experiments were performed for a void fraction of 70–95%. Fluid-elastic instability was clearly observed in an out-of-plane mode vibration. The effect of a primary side flow on the vibration of U-tube was investigated separately. The damping ratio of the present U-tube was higher than the damping ratio of the cantilever tubes in the literature. The hydrodynamic mass of the U-tube was generally in accordance with the hydrodynamic mass of the cantilever tubes in the literature. The instability constant (K) of the Connors equation was assessed with a simplified effective gap velocity, and the fluid-elastic instability constant was 8.5.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Schematic view of the test facility

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Figure 2

U-tube bundle test section: a front view (a) and a plane view (b)

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Figure 3

Sectional diagram of U-tube arrangement in the horizontal part and the location of the instrumented U-tubes. Instrumented U-tubes are marked solid.

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Figure 4

Vibration mode shapes of row 5 U-tube obtained from modal test in air: OP first mode (a), IP first mode (b), OP second mode (c), and IP second mode (d). OP: out of plane and IP: in plane.

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Figure 5

rms displacements of row 4, column 4 U-tube (a) and row 5, column 3 U-tube (b). Rectangular, circular, and triangular symbols represent the void fractions of 80%, 85%, and 90%, respectively. Solid, hollow with “+,” and hollow symbols represent the OP first, IP first, and IP second mode vibrations, respectively.

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Figure 6

Power spectral density function of row 5, column 3 U-tube in the OP mode vibrations (a), and in the IP mode vibrations (b) at the void fraction of 70–95%

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Figure 7

Total damping ratios of the present U-tube in the OP and IP first mode vibrations. The total damping ratios of Pettigrew (4) are compared together, which were averaged over drag and lift directions in cantilever tube bundle.

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Figure 8

Hydrodynamic mass ratio of the present U-tube in the OP first mode vibration. The hydrodynamic mass ratios of Pettigrew (4) are compared together.

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Figure 9

Assumptions on the flow velocity distribution for the evaluation of the effective velocity. Illustrated is the nonuniform velocity distribution that the round part of the U-bend region had two times higher velocity than the horizontal part.

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Figure 10

Photographic observation of two-phase flow direction along the horizontal and round parts of the right half side of the U-tubes. Void fraction was 70% and homogeneous gap velocity was 4.0 m/s.

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Figure 11

Results of the fluid-elastic instability of the present U-tube. Damping ratio, hydrodynamic mass, tube frequency of OP first mode vibration, and effective velocity with uniform velocity distribution were used.




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