Modelling of fluidelastic instability in tube bundles under two-phase bubbly flow conditions

Abstract

One of the major factors affecting the integrity of tube bundles in a steam generator undergoing excessive vibrations is fluidelastic instability. Currently, a comprehensive model for predicting fluidelastic instability in tube bundles under two-phase cross flow has not been developed. Accurate design guidelines on fluidelastic instability in tube bundles is crucial to avoid tube failure due to tube vibrations. Therefore, this study presents a model for the fluidelastic instability (FEI) of a two-phase flow in a normal square tube bundle. The model focuses on the bubbly flow regime of a two-phase flow. This complex flow across the bundle is simplified as a one-dimensional inviscid flow in channels. The liquid is treated as a continuous phase while the gas bubbles are treated as compressible dispersed particles. The motion of each individual bubble is tracked in time domain simulations. Bubble-to-bubble interaction, bubble break-up, and bubble coalescence mechanisms are considered in the current model. The continuity equation is solved using the local instantaneous void fraction. The momentum equation is then solved to calculate the fluid forces on the tube. Using this technique, it was possible to calculate the change in the flow density around the tube. Prediction of the stability threshold showed very promising results when compared with the experimental data.