Heat transfer characteristics of a flat plate pulsating heat pipe with microstructures for unidirectional thermal-to-mechanical energy conversion

The phase change of a liquid into vapor results in a sharp volume expansion, generating a powerful driving force, which is fundamental for the operation of pulsating heat pipes (PHPs). However, the performance of PHPs is hindered by instability arising from the lack of directional control over the expansion process. In this work, we propose a PHP incorporating thermal-to-mechanical energy conversion (TMC) microstructures, which direct the TMC by controlling the expansion in desired directions, thereby more efficiently utilizing the driving force. Two designs for enhancement of TMC, i.e., one featuring a vapor actuator microstructure (VA-PHP) and another with diameter-variant channels (DVC-PHP), were fabricated and experimentally investigated under low inclination angles and horizontal orientations (0°, 1°, 5°, and 9°). The experimental results indicated that the incorporation of the TMC microstructures into the PHPs facilitated more efficient startup, enhanced unidirectional circulation, and reduced the dependence on the inclination angle. Notably, while the conventional PHP failed to operate at small inclination angles, the VA-PHP achieved stable operation at 1°, and the DVC-PHP further maintained stable operation even at 0°. The robust performance of the proposed PHPs at low inclination angles demonstrates the potential of TMC microstructures to overcome orientation-dependent limitations, thereby delivering a practical solution for improving the effectiveness of PHPs.