Thermal and flow pattern analysis under confinement and vapor superficial velocity conditions in a two-phase closed thermosiphon: A numerical study

This study presents a comprehensive numerical investigation of flow patterns and thermal behavior in a Two-Phase Closed Thermosiphon (TPCT) using water as the working fluid. Numerical simulations were conducted in STAR-CCM+ employing the Lee phase-change model to examine boiling and condensation phenomena across fifteen TPCT configurations with varying inner diameters and heat inputs, while maintaining constant initial pressure and fluid properties. The analysis identifies distinct flow regimes, including slug, geyser, churn, and bubbly flows, as well as transitional and periodic structures that exhibit combined characteristics of multiple regimes. The results indicate that the confinement number ( C o ) and the dimensionless superficial vapor velocity ( J V ∗ ) govern the transition boundaries between these regimes, which are influenced by variations in fluid properties under the operating boundary conditions. Spatiotemporal distributions of vapor volume fraction ( α V ), wall temperature ( T ), local heat transfer coefficient ( H T C ), and heat flux ( q ′ ′ ) reveal a strong coupling between phase-change dynamics and thermal performance. The contributions of convection, evaporation, and quenching to the overall H T C were quantified, with instantaneous peak values exceeding 9000 W/m 2 ⋅ K in boiling-dominant regions. Frequency-domain analysis of pressure fluctuations confirms the presence of characteristic oscillations for each flow regime, where low-frequency peaks correspond to slug and Taylor bubble formation, while high-frequency peaks are associated with nucleation events. These findings provide deeper insight into complex flow transitions and heat transfer mechanisms in TPCTs, supporting the design optimization of two-phase cooling systems for data center applications. • Distinct flow regimes (slug, geyser, bubbly, and churn) and five periodic transitional combinations were identified. • The pressure in the system, driven by heat input and operating conditions, was found to control flow regime transitions and vapor generation. • Flow regimes strongly affected heat transfer, wall temperature uniformity, and pressure oscillations. • The combined spatiotemporal and spectral analyses provide a comprehensive understanding of phase change, flow transitions, and heat transfer behavior.