Correlation Based Fluidelastic Analysis of Tube Arrays 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 of the fluid forces is presented. The model is applied to fluidelastic instability analysis of a long span heat exchanger tube model. It is shown 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. Lastly, partial correlation is found to eliminate multiple instability boundaries at low values of the mass-damping parameter; this is a practically significant result.