Effect of preferential flexibility angle on fluidelastic instability of a rotated triangular tube bundle

Fluidelastic instability is the most important vibration excitation mechanism for heat exchanger, or nuclear steam generator tube bundles. It leads to very high vibration amplitudes and may cause short term failure by fatigue or wear. In nuclear power plants, steam generator U-tubes are susceptible to fluidelastic instability because of the high velocity two-phase flow in the U-tube region. Thus the region of concern in Steam Generators (SG) is the upper most U-bend region where the flow crosses a large number of tubes which can cause significant hydraulic resistance. This hydraulic resistance forces the flow to change direction. As a result the flow leaves the tube bundle at different angles of attack varying from 0° to 90°. From a fluidelastic instability point of view, the tube bundle is excited by oblique cross flow. The purpose of this work is to examine the instability phenomena in a rotated triangle tube bundle subjected to oblique single phase cross flow.; A wind tunnel study was conducted to determine the effect of the flexibility angle on the cross-flow induced fluidelastic instability on a rotated triangular tube bundle. The array consists of unidirectionally flexible tubes. The tubes supports in the bundle are designed with capability of rotation in order to provide different angles of attack in four steps, from 0 to 90 degrees. Fluidelastic instability results are in agreement with what was expected. The results show that fluidelastic instability is strongly dependent on the angle of attack. The results also show that, generally, the elimination of bundle flexibility in the direction transverse to the flow, greatly affects the stability behavior of the array. However in a few cases, changing the angle of the attack does not affect the critical velocity for instability, because the effective number of flexible tubes remains constant, in spite of the reduction of tube bundle flexibility.; The quasi-static theory presented by Price & Paidoussis (1986 & 87) is used as the basis for the development of a stability model for a single flexible cylinder surrounded by rigid cylinders and subject to oblique cross-flow. A reasonable degree of success has been achieved with this quasi-static type analyses to predict fluidelastic instability critical flow velocity. Experimental results are used to validate theoretical model applications.