Analysis of the two-phase flow, heat transfer, and instability characteristics in a loop thermosyphon

Numerical modeling based on volume of fluid (VOF) method is conducted and experimentally validated for the two-phase flow and heat transfer in a loop thermosyphon for wide filling ratios. The results show that, under the 87% filling ratio, the loop is dominated by bubbly flow and sub-cooled flow boiling, with sensible heat transfer enhanced by bubble pumping effect, which is beneficial for wobbling, tilting, and mobile applications to avoid evaporator dry-out. Under the 64% filling ratio, geyser boiling instability occurs. A train of bubble-liquid intermittently passes the loop summit under the collaboration of buoyancy, gravity, viscous, and surface tension forces, causing intense flow and temperature oscillations, enhancing both latent and sensible heat transfer. Under the 38% filling ratio, the characteristic flow patterns are churn flow, mist flow, and film condensing, leading to highly efficient latent heat transfer.