Effects of tube-support clearance and preload on in-plane fluidelastic instability of tube arrays

Abstract

The effects of tube-support clearance and preload on In-Plane (IP) or streamwise Fluidelastic Instability (FEI) in tube arrays are investigated in this paper.First, experiments were conducted on an array of straight tubes fixed at one end and supported by flat bars in the out-of-plane (OOP) or transverse direction, at the other end. The array was made of half inch (12.7 mm) outer diameter tubes, arranged in a parallel triangular configuration with a pitch-to-diameter ratio P/D of 1.5. Tube-support clearance and preload were varied to study the precise effect of these parameters on the IP FEI behaviour of the tube array. The tests were conducted in single-phase air and water flows as well as two-phase flow. For each setup and volumetric quality, the flow rate was incrementally increased and the tube vibration response monitored until the tube bundle became unstable. The IP FEI threshold was found to be constant or only slightly increasing with decreasing support gaps in liquid and two-phase flows. As for the effect of preload, the IP FEI threshold was found to be nearly constant for preloads lower than 0.1 N and linearly increasing with preload beyond 0.1 N in two-phase flows. In single-phase liquid flow, the effect of preload was much more significant as a small preload was enough to suppress the IP FEI. In liquid flow, it was also found that, for centred tubes and at a flow velocity much higher than the IP FEI threshold, the motion of the tubes switched from the streamwise (IP) to the transverse (OOP) direction. It appeared that FEI of the second transverse mode caused the first IP mode to become stable.Second, a time-domain, nonlinear, numerical model was developed to simulate fluidelastic instability of the tube bundle, including the effects of the loose supports. The focus of this numerical study was the ability of the model to accurately represent the effects of the tube-support clearance and preloads. The results of the model were consistent with the experimental results: the variation of the instability threshold versus tube-support gap was found to be small and the trend of the results versus preload was generally linear beyond a certain threshold. The agreement between the predictions of the model and the experimental results of a loosely supported, multi-span, U-bend tube bundle subjected to two-phase Freon was found to be very good (within 22%), demonstrating the capability of the model to simulate IP FEI of a realistic tube bundle.