Analysis of Void Oscillations in Flow Boiling Based on Eulerian-Eulerian Two-fluid Model

Void oscillations may occur during numerical simulations of flow boiling and may reduce the accuracy and reliability of predicted two-phase flow characteristics. Understanding the conditions that promote such oscillations and clarifying the underlying mechanisms are important for improving the reliability of computational simulations of flow boiling. This study investigates the occurrence of void oscillations in flow boiling and examines the effects of operating conditions and turbulence modeling on this phenomenon. Particular attention is given to the relationship between vapor generation, turbulence production, and the spatial evolution of void fraction along the heated channel. Numerical simulations were performed based on the typical operating conditions of the Bartolomei experiment. The two-phase flow was described using the Eulerian-Eulerian two-fluid model. The wall boiling model was applied to account for evaporation at the heated wall. Simulations were carried out by varying inlet temperature, heat flux, pressure, and mass flux. The axial position of the void fraction turning point was analyzed to evaluate the tendency of void oscillations under different conditions. In addition, the wall heat flux partitioning curves were examined to determine whether the oscillatory behavior was related to the wall boiling model. Four turbulence models were compared, including the standard k-ε model (SKE), the realizable k-ε model (RKE), the standard k-ω model (SKO), and the shear stress transport k-ω model (SSTKO). The results show that the occurrence of void oscillations is strongly affected by working conditions. The turning point moves toward the inlet as inlet temperature or heat flux increases, while it shifts downstream as pressure or mass flux increases. These trends indicate that void oscillations are more likely to appear under relatively high inlet temperature, high heat flux, low mass flux, and low pressure conditions. The analysis of the wall heat flux partitioning curves indicates that the oscillatory behavior is not caused by an incompatibility of the wall boiling model. The comparison of turbulence models shows that turbulence modeling has a significant impact on the prediction of void oscillations. Among the tested models, the SSTKO model significantly reduces oscillatory behavior and produces smoother void fraction distributions. Mechanism analysis indicates that strong vapor generation occurs under relatively high inlet temperature, high heat flux, low mass flux, and low pressure conditions. The evaporation process causes local volumetric expansion and flow acceleration. As a result, the local velocity gradient increases and turbulence production becomes stronger. The enhanced turbulence production may trigger a positive feedback process that leads to the rapid growth of turbulent kinetic energy and ultimately induces void oscillations. These findings provide useful guidance for improving the stability and reliability of numerical simulations of flow boiling. The results also suggest that the appropriate selection of turbulence models is essential for obtaining physically consistent predictions of void fraction distribution in flow boiling.