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Date of Award

7-1999

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Supervisor

Dr. David S. Weaver

Co-Supervisor

Dr. Ross L. Judd

Abstract

The U-bend region of nuclear steam generators tube bundles have suffered from two-phase cross flow induced vibrations. Tubes in this region have experienced high amplitude vibrations leading to catastrophic failures. Turbulent buffeting and fluidelastic instability have been identified as the main causes. Previous investigations have focused on flow regime and two-phase flow damping ratio. However, tube bundles in steam generators have vapour generated on the surface of the tubes, which might affect the flow regime, void fraction distribution, turbulence levels and tube-flow interaction, all of which have the potential to change the tube vibration response. A cantilevered tube bundle made of electric cartridge heaters was built and tested in a Freon-11 flow loop. Tubes were arranged in a parallel triangular configuration with pitch to diameter ratio of 1.48. The bundle was exposed to two-phase cross flows consisting of different combinations of void from two sources, void generated upstream of the bundle and void generated at the surface of the tubes. Tube tip vibration response was measured optically and void fraction was measured by a gamma densitometry technique. It was found that the ratio of tube vibration amplitude in the transverse direction was reduced by a factor of 8 for void fraction generated at the surface of the tubes only, when compared to the response observed under the upstream only void generation case. The main explanation for this effect is a reduction in the correlation length of the turbulent buffeting forcing function. Theoretical calculations of the tube vibration response due to turbulent buffeting under the same experimental conditions predicted a similar reduction in tube amplitude. The void fraction for the fluidelastic instability threshold in the presence of tube bundle void fraction generation was higher than that for the upstream void fraction generation case. The first explanation of this difference is the level of turbulent buffeting forces to which the tube bundle is exposed. Increased values of turbulence will lower the void fraction for instability. The second explanation is related to the flow regime. In this study, it was clear that flow regime for bundle void generation was at all times bubbly and homogeneous, while the upstream void fraction generation cases showed a clear tendency to churn flow. A change in flow regime from bubbly to churn flow will produce the same effect as an increase in turbulence buffeting levels, and hence it seems difficult with the present knowledge to distinguish between the two causes. In as much as turbulence levels are related to flow regime, it is essential to have a clear knowledge of the flow regime in steam generators in order to predict the fluidelastic instability threshold of the tubes.

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