Heat transfer and flow visualization of a two-phase closed thermosiphon using water, acetone, and HFE7100

The heat transfer and flow visualization of a two-phase closed thermosyphon (TPCT) were simultaneously investigated for three different working fluids: water, acetone, and HFE7100. A semi-cylindrical channel, 650 mm long with an inner diameter of 25 mm, was made of a copper block; further, the flat face of the channel was covered with glass for visual observation. Thermal and flow behaviors that varied for different working fluids were compared at filling ratios of 0.25 and 0.75 based on the volume of an evaporator section and at three inclination angles of 90 degrees, 45 degrees, and 10 degrees. The flow characteristics inside the thermosyphon were visualized using a high-speed camera through the front glass at the evaporator and condenser. A local dry-out was observed for water with an increasing heat flux, which rapidly decreased the boiling heat transfer coefficient. The entrainment was observed for HFE7100 at an inclination angle of 90 degrees owing to the increased shear stress between the vapor and the liquid condensate. The condensation heat transfer coefficient for water was higher than those for acetone and HFE7100 because the dropwise condensation was dominant for water, whereas filmwise condensation occurred for acetone and HFE7100. The condensation heat transfer coefficient for acetone was higher than that for HFE7100, which was confirmed by the thinner liquid film for acetone from the visual observation.