Study on liquid film formed with liquid column oscillation in pulsating heat pipe (Measurement of liquid film thickness using forced oscillation system)

Abstract

In order to gain a better understanding of the latent heat transfer mechanism in pulsating heat pipes (PHPs), this study experimentally investigated the thickness characteristics of a liquid film that forms during oscillations of the liquid column within PHPs. Accordingly, the liquid column was oscillated sinusoidally under different oscillating conditions to simulate the flow phenomena. A circular tube with an inner diameter of 2 mm was used as the test channel; ethanol and FC-40 were used as working liquids. The average thickness of the liquid film formed due to the liquid column oscillations on the channel wall was determined by comparing the oscillation amplitudes of the tip of the liquid column obtained with and without the liquid film. It was confirmed that the average thickness of the liquid film varied based on the average capillary number for each working liquid. The time-average velocity of the tip of the liquid column, which was derived based on the assumption that the tip of the liquid column oscillates sinusoidally, was used as the characteristic velocity to calculate the average capillary number. The experimental results were compared with various correlations proposed in previous studies. The results show that the acceleration of the oscillating liquid column affected the liquid film thickness for a high average capillary number. For this high average capillary number, the rate of increase in liquid film thickness with the average capillary number decreased, and the liquid film thicknesses during the liquid column oscillation approached those under the steady condition. Furthermore, the liquid film thicknesses for the vertical channel were compared with those for the horizontal channel, and it was made clear that the effect of gravity on liquid film thickness is significant. Thus, the thickness characteristics of a liquid film during the liquid column oscillation within PHPs were clarified.