Oscillatory two-phase flow dynamics in capillary tubes under microgravity conditions: Numerical modeling and qualitative analysis of the flow structures

Abstract

This work examines the oscillatory motion of gas–liquid structures in $P$ulsating $H$eat $P$ipes (PHP), which is critical for designing passive cooling systems for microgravity applications. Accurately capturing gas–liquid volume fraction behavior is crucial for understanding the mechanisms driving the break-up and coalescence of the gas plugs. Experimental data obtained from the ZARM drop tower facility in Bremen, Germany, were used to validate numerical simulations conducted with OpenFOAM v2106, employing the $V$olume $O$f $F$luid (VOF) method. In experiments, ethanol was utilized as the working under two distinct initial vapor bubble configurations. A boundary condition enforcing the oscillatory behavior of the velocity vector was implemented in the simulations. The results demonstrate high of accuracy in reproducing the observed flow structures, providing a qualitative comparison between the algebraic VOF method and experimental observations. The simulations successfully captured the oscillatory dynamics of two-phase structures, offering valuable insights into vapor bubble behavior in microgravity. While heat transfer was not included in the present analysis, these findings are a foundation for future studies integrating thermal-flow processes. This preliminary analysis advances the understanding of PHP behavior in microgravity and highlights pathways for more comprehensive modeling efforts.