Dynamic transition and instability of the underexpanded vapor jet submerged in liquid lead-bismuth eutectic

When a steam generator tube rupture occurs in a lead-cooled fast reactor, a high-pressure vapor jet is injected into the surrounding liquid lead-bismuth eutectic (LBE), posing significant structural damage risks. This study employs large eddy simulation to explore the flow dynamics of a compressible, underexpanded submerged jet from injection to quasi-steady state. Interestingly, the results reveal the dynamic process of jet mode transition at a fixed nozzle pressure ratio (NPR). A two-stage transition in jet mode is characterized by a distinct discontinuity in pressure and velocity along the jet axis. The transition is correlated with the dynamic ratio of nozzle pressure to temporally averaged gas pressure, showing a critical threshold at a ratio of approximately 4. A theoretical model is developed to predict the mean vapor pressure based on vapor bubble geometry, complemented by a time-dependent predictive formula. This model enables efficient estimation of average vapor pressure and jet mode transition time under various NPRs, offering a practical alternative to resource-intensive numerical simulations. The gas–liquid interface interaction leads to pronounced oscillations in the jet shear layers, driven by elevated turbulence kinetic energy and asymmetric pressure distributions. Spectral analysis shows that while these oscillations peak in intensity within the turbulent region, their dominant frequency is notably lower than upstream fluctuations. Furthermore, increased NPR amplifies the amplitudes of oscillations, underscoring its role in modulating interfacial instability. These findings advance the mechanistic understanding of interactions between vapor and LBE in reactor accidents and provide predictive information to mitigate steam generator risks.