Study on the effects of oscillation characteristics on heat transfer mechanisms in pulsating heat pipes

Abstract

A correlation between oscillation characteristics and the thermal performance in a pulsating heat pipe (PHP) is derived, and based on the correlation, a novel channel design that can effectively improve the thermal performance of a PHP is proposed. To do this, first, a merit number that represents the heat transfer mechanisms inside a PHP is theoretically derived and experimentally verified. The heat transfer mechanisms are defined as the ratio of latent heat to sensible heat, and the merit number is expressed using the amplitude and the frequency. For experimental verification, a PHP with overall dimensions of 28×67.5×1.6 mm3 is fabricated. Then, ethanol is used as the working fluid and the filling ratio is fixed at 45% by volume. The amplitude and the frequency are obtained through the high-speed imaging technique through the glass side while latent heat and sensible heat are obtained through the high-speed infrared imaging technique through the silicon side. Finally, using the verified merit number, a correlation that can predict the thermal performance within a 20% error is derived through linear regression of experimental data reported in the earlier works. Based on the derived correlation, a passive check valve without a subsidiary channel is proposed to effectively increase the thermal performance of a PHP. To do this, topology optimization is employed to maximize the diodicity (Di) of the check valve. The proposed passive check valve consists of a pair of hook-shaped structures and has a Di value of 2.2. A series of experiments are conducted to determine how effective the proposed passive check valve is in improving the thermal performance and oscillation characteristics of PHPs. The overall dimensions of PHPs are 53.5×80×1.5 mm3. A total of twenty passive check valves are implemented into the entire channel at the location where the condenser section and the adiabatic section meet. HFE-7000 is used as the working fluid and the filling ratio is fixed at 48% by volume. First, a series of experiments is conducted on PHPs with a single-diameter channel in both vertical orientation and horizontal orientation. The experimental results show that the proposed passive check valves significantly improve the thermal performance of PHPs: 48% and 81% reduction in the thermal resistance in the vertical orientation and the horizontal orientation, respectively. Then, a dual-diameter channel with widths of 1 mm and 3 mm is employed to widen the relatively narrower operating range in the horizontal orientation compared to that in the vertical orientation. In PHPs with the dual-diameter channel, it is experimentally confirmed that the passive check valves effectively improve the thermal performance over the wide operating range regardless of the orientation of PHPs: 33% and 34% reduction in the thermal resistance in the vertical orientation and the horizontal orientation, respectively. In addition, it is revealed that implementing the passive check valves and the dual-diameter channel together can lead to 66% and 90% performance improvements of the conventional PHPs in the vertical orientation and the horizontal orientation, respectively. It is found through high-speed photography that this enhanced performance occurs because the passive check valves significantly increase the time-averaged volumetric fraction of vapors in the condenser section by 5 times from 0.09 to 0.45. This stems from the fact that the passive check valves effectively make the fluid flow 3.5 times more preferentially in one direction over the other.