The unsteady flow model for the prediction of fluidelastic instability in tube arrays has been employed successfully to single phase cross-flow. Stability boundaries as well as inter-tube vibration mode patterns can be predicted.

Single phase flow studies are a precursor to the ultimate practical problem of fluidelastic instability in heat exchangers where the flow consists of two co-existing phases. In principle, existing unsteady fluid force models can be applied to two-phase flow induced vibration. However, the models become intractably complex for the case of U-bend tubes in two-phase flow. In single phase flow, the fluid forces may be considered to be fully correlated along the tube span. This is not the case for two-phase flow. For tube lengths much longer than the typical linear dimension of the two phase flow unit structure (slug, bubble etc.), forces along the tube span are only partially correlated.

In this paper, an unsteady flow model taking into account partial spanwise correlation is presented. The model is applied to fluidelastic analysis of a long span heat exchanger tube. Preliminary results show that the effect of partial correlation is to reduce the effective net fluid forces which thus raises the effective instability threshold. Another important result is the introduction of additional coupling between modes that would otherwise be uncoupled in the fully correlated case. This has important implications for heat exchanger tubes which have closely spaced modes.

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