In-flow fluidelastic instabilities of tube bundles subjected to a two-phase flow.

Fluidelastic instability is the most important vibration excitation mechanism for heat exchanger, or steam generator type of tube bundles. It is so because of the very high vibrations amplitude that it can induce to the tubes which can lead to rapid failure by fatigue or wear. Also, unlike vibrations induced by vortex shedding (vortex-induced vibrations), fluidelastic instability is not a self limiting phenomenon: amplitude of vibrations does continue to increase with velocity past the critical onset of the instability. This is shown on Figure 1 for the case of single-phase flow.

Figure 1 – Vibrations amplitude as a function of flow pitch velocity for a flexible cylinder in a rigid cluster (taken from Pettigrew et al. 1991). The cylinder is free to vibration in the cross-flow direction.

In nuclear power plant steam generators, U-tubes are very susceptible to undergo fluidelastic instability because of the high velocity of the two-phase mixture flow in the U-tube region and also because of their low natural frequencies in their out of plane modes. In nuclear power plant steam generator design, flat bar supports have been introduced in order to restrain vibrations of the U-tubes in the out of plane direction (see Figure 2). Since those supports are not as effective in restraining the in-plane vibrations of the tubes, there is a clear need to verify if fluidelastic instability can occur for a cluster of cylinders preferentially flexible in the flow direction.

Figure 2 – Typical nuclear power-plant steam generator with anti-vibration bars (AVB) or flat bar supports.

This text gives some details on the research that was conducted in my master thesis. The purpose of this research was to verify experimentally if fluidelastic instability occurs for a cluster of cylinders preferentially flexible in the flow direction in two-phase (air-water) flow. This work was perform at Ecole Polytechnique de Montréal in Canada in the BWC/AECL/NSERC industrial research chair of fluid and structure interaction under the supervision of prof. Michel J. Pettigrew.

The experiments were performed in an air-water loop. The loop consist of a 30 l/s variable speed water pump, a magnetic flow meter, a 250 l/s air supply system, a 2500 l tank, a test section and connecting pipes as shown in Fig. 1.
Figure 3 – Two-phase flow loop used in the BWC/AECL/NSERC industrial research chair of fluid and structure interaction at Ecole Polytechnique de Montréal.
The test section, shown in Figure 4, has a flow area of 0.038 m2 (0.2m x 0.19m). It includes twelve rigid tubes and seven flexible tubes in a rotated triangular configuration. Each tube has a diameter of 0.038 m and the pitch to diameter ratio (P/D) is 1.5. Half tubes have been added to the lateral walls of the test section to better simulate a real tube bundle. The test section has been designed so that it is possible to switch the position between flexible tubes and rigid tubes. This offers the possibility to experiment with different configurations of flexible and rigid cylinders. So far, four configurations have been tested: a single flexible tube inside a rigid array (Fig. 5a), a cluster of flexible tubes that are all placed in the middle of the test section referred to as the central cluster case (Fig. 5b), a single flexible column (Fig. 5c) and two partially flexible columns (Fig. 5d).

Figure 4 – Test section

Figure 5 - Configurations of flexible tubes tested within the test section: (a) single flexible tube, (b) central cluster, (c) single flexible column, (d) two-partially flexible columns. Flow direction is upward.

Several important conclusions were drawn from the results of this research. First of all, fluidelastic instability does occur for a bundle of cylinders preferentially flexible in the flow direction in two-phase flow. Second, the observed instability for the tubes flexible inflow occurred at higher flow velocities than axisymetrically flexible tubes. Third, no instability was observed for the case of the single flexible column. This is different than what was observed in air flow (reported by Mureithi et al. 2005). It was concluded that in high void fraction two-phase flow, flexible tubes must be located on at least two adjacent columns inside the bundle for fluidelastic instability to occur. Detailed information on this study can be found in Violette et al. 2006.



Fluidelastic instability of an array of tubes preferentially flexible in the flow direction subjected to two-phase cross flow.Violette R., Pettigrew M.J., Mureithi N.W., Journal of Pressure Vessel Technology – Transaction of the ASME, v. 128 (1), pp. 148-159, 2006.

Fluidelastic Instability Tests on an Array of Tubes Preferentially Flexible in the Flow Direction.
Mureithi, N.W., Zhang, C., and Pettigrew, M.J., Journal of Fluids and Structures, v. 21(1), pp. 75-87, 2005.

Fluidelastic Instability of Heat Exchanger Tube Bundles: Review and Design Recommendations
Pettigrew, M.J., Taylor, C.E., Journal of Pressure Vessel Technology, Vol. 113, pp.252-256, 1991.