Dynamics of evaporation in a single, straight-tube pulsating heat pipe

The dynamics of vapor flow in a pulsating heat pipe consisting of a single, straight tube (SST-PHP) is investigated using a hot-wire anemometric technique. The proposed SST-PHP generates stable and periodic oscillation of a single vapor plug with a large stroke that transports the heat of 75 W with an effective thermal conductivity of 40 kW/(m·K) at horizontal orientation. Vapor produced in the heating section flows into the heat transport tube and condenses on a liquid film formed between the vapor plug and the tube wall to transport heat from the heating section to the heat transport tube. The measured flow rate of vapor varies drastically during one oscillation cycle, i.e., increases to approximately ten times the molar rate of increase of vapor in the vapor plug during the growth period while remaining relatively low during shrinkage. The evaporation rate in the heating section, which is evaluated from the vapor flow rate, significantly exceeds the vapor generation rate that corresponds to the heat transport rate during the growth of the vapor plug. The condensation rate varies similarly so as to catch up with the evaporation rate with a small phase delay in accordance with the oscillation motion of the vapor plug. The storage and release of the sensible heat at the liquid film resulting from the variation in the saturation temperature due to the pressure oscillation are suggested to play a significant role in the dynamics of vapor generation and the large-amplitude oscillation.