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Study on a single, straight-tube pulsating heat pipe with an open end (Function and role of the liquid film surrounding vapor plug)

A single, straight-tube pulsating heat pipe (SST-PHP) with an open end that was proposed by the authors’ research group provides a high heat transport rate of 75 W with an effective thermal conductivity of 40 kW/(m・K) due to a self-excited, large amplitude, periodical oscillation of a single vapor plug. Liquid film surrounding the vapor plug is pushed back to the heating section to spread over the wick for each shrinkage of the plug, which maintains the heating section near the saturation temperature of the working liquid. In the present paper, first, the variation of the amount of liquid contained in the heating section during the oscillation is examined through the measurement of the vapor volume in the parts of the pipe. The dynamic inflow/outflow of the liquid film to/from the heating section and its relation to the occurrence of the dry-out are discussed. Secondly, the temperature variation across the liquid film that flows out along with the vapor plug into the heat transport tube is estimated with reference to the analytical solution of the temperature distribution in a liquid column oscillating with the same amplitude as that of the vapor plug. The effect of the heat transfer from the tube (temperature drop) reaches the location of the liquid film surface with a time delay around a quarter of the oscillation period. It would imply that the condensation of vapor on the film surface begins at around the maximum growth of the vapor plug, as a result, leading to the large-amplitude oscillation.