As flow-induced damage is a major concern for designers and operators of industrial heat exchangers and steam generators, some methodologies have been recently proposed to obtain a more accurate estimation of the critical flow velocity. These methodologies are based on unsteady semi-analytical models for fluid-dynamic forces, associated with dimensionless fluid force coefficients generally obtained from experiments. The present paper deals with new experiments performed on a square in-line tube bundle, subjected to two-phase water-freon cross-flow. A large range of void fractions and flow velocities has been investigated, up to fluidelastic instability threshold. Experimental results are analyzed first to derive evolution of frequency, damping ratio and tube motion amplitude evolution versus flow velocity and void fraction. Special attention is then focused on the determination of two-phase fluidelastic coefficients. Either homogeneous, slip or drift-flux models are used to describe two-phase flow characteristics and to dimensionalize the fluidelastic force coefficients. As a final step, the two-phase fluidelastic coefficients are compared to previously published data.

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