Void fraction intermittency and its role in two-phase fluidelastic instability in nuclear steam generators

Steam generators in nuclear power plants have experienced tube failures driven by flow-induced vibrations, primarily driven by random turbulence and fluidelastic instability (FEI). Turbulence typically causes long-term fretting wear at tube supports, while FEI can trigger rapid failure due to large-amplitude vibrations. In nuclear steam generators where two-phase flow exists, prediction of FEI remains challenging due to the unsteady and intermittent nature of the flow. Existing models largely rely on homogeneous equilibrium assumptions, which suppress phase slip and void-fraction intermittency, leading to misprediction of instability thresholds and inability to reproduce experimentally observed growth–decay vibration behavior. This study presents a time-domain, control-volume-based formulation for two-phase FEI that explicitly resolves phase slip and spatially distributed void-fraction intermittency. The inter-tube flow field is discretized into variable-width control volumes with locally evolving void fraction, and void fraction is represented as a stochastic, time-resolved field governed by experimentally derived probability distributions, enabling intermittent flow structures to be represented while preserving conservation of mass and momentum. The formulation captures intermittent reductions in effective mass-damping that transiently violate mean stability conditions, producing the growth–decay vibration behavior observed experimentally but absent in the homogeneous model. Validation against experimental data demonstrates accurate prediction of FEI thresholds across a wide range of mass-damping parameters and void fractions, as well as realistic reproduction of time-domain response characteristics in intermittent flow regimes. The present work establishes a physically grounded, computationally efficient framework for predicting two-phase FEI in tube arrays and provides a direct mechanistic link between void-fraction intermittency and instability onset and evolution.