The fluidelastic instability threshold of tube arrays can be estimated using measured unsteady fluid force. It is usually assumed that the fluid force is completely correlated along the tube axis; this may be true in single phase flow. However, the flow in the two-phase flow is no longer steady, in space or in time. Thus, the correlation of the fluid force acting along the tube axis should be introduced. There are very few measured data for the correlation of the fluid force in two-phase flow, and it is difficult to deduce from measured time-history fluid data, such as the void fraction or the flow velocity. In this report, the average correlation length is derived from critical flow velocities of U-bend tubes in several two-phase flow conditions, based on an approximate theory for the two-phase flow condition when the unsteady fluid force for a short span model has been measured. As a result, the average correlation length gives the critical flow velocities of each U-bend tube. The result gives a good explanation why the critical factor in the U-bend tube is larger than that in short span models. This method can be a new estimation of the fluidelastic instability of U-bend tubes in two-phase flow conditions.

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