Effect of Preferential Flexibility Direction on Fluidelastic Instability of a Rotated Triangular Tube Bundle

In operating shell-and-tube heat exchangers, tube vibration induced by cross-flow can be a serious problem. The region of concern in steam generators is the upper most U-bend region where the flow crosses a large number of tubes, which also cause significant hydraulic resistance. This hydraulic resistance forces the flow to change direction. From a fluidelastic instability point of view, the tube bundle is excited by oblique cross-flow. A secondary consequence of change in flow direction is a change in the flexibility direction of the tubes relative to the oncoming flow direction at different locations within the U-bend region. It is this somewhat simpler problem that is studied in this work. The effect of array flexibility direction on the fluidelastic instability phenomenon in a rotated-triangular tube bundle is investigated for single phase flow as a starting point. The study consists of both experiments and theoretical analysis of a simplified single-flexible-tube array. Experimental tests are conducted in a wind tunnel on a reconfigurable tube bundle. The results show that fluidelastic instability is strongly dependent on the flexibility angle. The results also show that, generally, the elimination of bundle flexibility in the direction transverse to the flow has a strong stabilizing effect on the tube bundle. The effect is, however, nonlinearly related to flexibility angle. In the second part of this work, the quasi-steady fluidelastic analysis is adapted for a single tube (within a rigid array), flexible in a single but arbitrary direction relative to the flow and subjected to cross-flow. The fluid-force expressions are rewritten to account for an arbitrary tube flexibility direction relative to the approaching flow. In the process, a simplified, flexibility direction dependent, one degree-of-freedom equation is obtained. The model is then evaluated against measured experimental data. This evaluation shows that the predicted critical flow velocity for fluidelastic instability is in qualitative agreement with experimental results, at least in the trend on the effect of varying the flexibility angle. At the same time, the model sheds some light on the role played by the flexibility angle in determining the overall fluid-structure damping underlying the observed stability behavior.