Visualization Study on Flow Pattern Evolution and Heat Transfer Characteristics of a Two-Phase Loop Thermosyphon Under High Heat Flux with Various Filling Ratios

To investigate the flow pattern evolution and phase-change characteristics of a two-phase loop thermosyphon under high heat flux conditions with different filling ratios, a visual experimental study was conducted. The filling ratio ranged from 36.8% to 92.3%, with a maximum heat flux of 240 W/cm2. The results indicate that at a filling ratio of 81.6%, bubbly flow is observed under low heat flux (30–60 W/cm2), with a bubble detachment size of approximately 2 mm. When the heat flux increases to 90–180 W/cm2, intermittent boiling occurs, and the flow pattern cycle shifts from bubbly flow to single-phase flow and back to bubbly flow, with the period shortening from 5.92 s to 4.92 s. At heat fluxes ≥ 200 W/cm2, intermittent boiling disappears, transitioning to stable high-velocity subcooled flow boiling, and the detachment size decreases from 1951 μm to 762 μm. At a filling ratio of 65.3%, bubbly and slug flow are observed under low heat flux (30–120 W/cm2). Intermittent boiling appears at heat fluxes ≥ 150 W/cm2, characterized by a cycle of “churn flow–bubbly flow–single-phase flow–bubbly flow–churn flow”. This intermittent boiling persists under high heat flux (200–220 W/cm2) without transitioning to stable flow boiling. At a filling ratio of 36.8%, large-scale slug bubbles (maximum 37 cm) form under 30–60 W/cm2, transitioning to churn flow at 90–120 W/cm2. Instantaneous dryout occurs at 150 W/cm2, followed by complete dryout at 180 W/cm2. The visualization results reveal the critical heat flux for flow pattern transitions and the periodic characteristics of intermittent boiling under different filling ratios.