Fluidelastic vibration of a tube array caused by two-phase flow has been reported in some papers. The critical flow velocity is usually estimated with a simple Connors-type criterion which is based on average flow velocity, average fluid mass density and damping in two-phase flow. However, there is no explanation why this simple criterion can be used or how the fluidelastic instability occurs by two-phase flow. This paper shows the experimental results on the fluidelastic vibration both by air-water two-phase flow and by steam-water flow in the condition of up to 7.0 MPa in pressure and to 284°C in temperature, and a new criterion based on an assumption of energy balance is here introduced using a “true” flow velocity. However, the comparison with the experimental data indicates that an intermittently rising slug speed, which has been introduced in Part 1, in slug or in froth flow region, should be used, and a modified new criteria for slug or froth flow region is derived. In addition, the new criteria is compared with the usual Connors-type criteria, which greatly depend on the estimation of the damping in two-phase flow. The agreement of both criteria is revealed to be in the vicinity of the variance of the unknown parameters in the usual criteria.

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