Time Domain Simulation of the Vibration of a Steam Generator Tube Subjected to Fluidelastic Forces Induced by Two Phase Cross Flow

Abstract

Having previously verified the quasisteady model under twophase flow laboratory conditions, the present work investigates the feasibility of practical application of the model to a prototypical steam generator (SG) tube subjected to a nonuniform twophase flow. The SG tube vibration response and normal workrate induced by tubesupport interaction are computed for a range of flow conditions. Similar computations are performed using the Connors model as a reference case. In the quasisteady model, the fluid forces are expressed in terms of the quasistatic drag and lift force coefficients and their derivatives. These forces have been measured in twophase flow over a wide range of void fractions making it possible to model the effect of void fraction variation along the tube span. A full steam generator tube subjected to a nonuniform twophase flow was considered in the simulations. The nonuniform flow distribution corresponds to that along a prototypical steamgenerator tube based on thermalhydraulic computations. Computation results show significant and important differences between the Connors model and the twophase flow based quasisteady model. While both models predict the occurrence of fluidelastic instability, the predicted preinstability and post instability behavior is very different in the two models. The Connors model underestimates the flowinduced negative damping in the preinstability regime and vastly overestimates it in the post instability velocity range. As a result the Connors model is found to underestimate the workrate used in the fretting wear assessment at normal operating velocities, rendering the model potentially nonconservative under these practically important conditions. Above the critical velocity, this model largely overestimates the workrate. The quasisteady model on the other hand predicts a more moderately increasing workrate with the flow velocity. The workrates predicted by the model are found to be within the range of experimental results, giving further confidence to the predictive ability of the model. Finally, the twophase flow based quasisteady model shows that fluidelastic forces may reduce the effective tube damping in the preinstability regime, leading to higher than expected workrates at prototypical operating velocities.