Fluid-elastic instability of a tube bundle in two-phase cross-flow using measured quasi-static forces

Although a great number of the process heat exchangers operate in two-phase flow, the complex nature of the flow makes the prediction of fluidelastic instability a challenging problem yet to be fully solved. In the nuclear steam generator, because of the high velocity of the two-phase mixture, the U-bend region is very susceptible to fluidelastic instability. In the work reported here, the U-bend region is modeled by a parallel triangular tube bundle.; Fluidelastic instability in two-phase flow is conventionally investigated using air-water mixtures. Since production and control of air-water mixtures is simpler and cheaper in comparison to the other types, air-water flow is chosen for the present study. The selection has permitted taking advantage of the existing flow loop and the available dynamic test data points obtained at the same test section.; The air-water two-phase flow loop of the Fluid-Structure Interactions Laboratory at Ecole Polytechnique de Montreal was used for the quasi-static force measurements. The test section of three columns of full cylinders bounded with sidewalls with fixed half cylinders was used to perform quasi-static measurement of forces. The quasi-static fluid force-field was measured in a rotated-triangle tube bundle (P/D=1.5 and D=30mm) for a series of void fractions and flow velocities in water and air-water flow. The forces were found to be strongly dependent on void fraction, flow rates and relative tube positions.; The quasi-static fluid force variations obtained in the present test section differ from those reported earlier using air in the same configuration with different pitch to diameter ratio and tube diameter. Although the two-phase flow results found at high void fractions are similar to those reported in air flow, the sharp changes in the lift coefficient in the cross-flow direction are absent in the present tube array. Interestingly, for void fractions below 40%, the variation of central cylinder lift coefficient in the cross-flow direction is the inverse of that in air flow. This means that for low void fractions, two- phase flow instability may be governed by the variation of force coefficients while for higher void fractions fluidelastic instability is mostly governed by time delay effects.; The fluid force field was then employed along with the quasi-steady model originally developed for single phase flows, to model the two-phase flow problem. Since the present tube bundle is not really infinite, introducing small modifications in the orieinal formulation to account for different phase angles was inevitable.; The variation of the most important fluid force coefficients with the Re number been emphasized in the analysis. Taking into consideration these variations when evaluating stability at very low velocity (or Reynolds numbers) the "premature" prediction of instability in the quasi-static model is explained (as opposed to changes in gap velocity originally used to explain unreasonably narrow unstable regions).; Having one dynamic test data made it is possible to go beyond the quasi-steady model and to find the first order of the decay function using quasi-unsteady model. Using the approximate formula of decay function applying the dynamic data points and fluid force coefficients obtained experimentally, the first order of the decay function has been calculated for different void fractions.; The present work uncovers some of the complexities of the fluid force field in two-phase flows. The data are invaluable since they are the necessary inputs to the class of quasi-static, quasi-steady and quasi-unsteady fluidelastic instability theoretical models. This database opens a new research avenue on the feasibility of applying quasi-steady models to two-phase flow.