Experimental study on the thermal performance of a pulsating heat pipe inspired by the cactus spines under inclined and vibrational conditions

Pulsating heat pipes (PHPs) inevitably operate under inclined orientations and vibrational disturbances in practical applications, making it crucial to improve their thermal performance and operational adaptability in such conditions. This study proposed a novel 2-turn water-based PHP with diameter and wettability gradient characteristics (DWG-PHP), inspired by the directional transport of droplets on cactus spines. The effects of inclination angles (0–90°) and vibration amplitudes (0–0.8 mm) on thermal performance were experimentally examined under different heat loads and filling ratios, with comparison to a conventional PHP (CPHP). Furthermore, the enhanced mechanism of the DWG-PHP was revealed by the combination of flow visualization and theoretical analysis. Results indicate that the DWG-PHP exhibits a lower thermal resistance under inclined and vibrational conditions. The heat transfer enhancement mechanism originates from the directional growth of bubbles induced by asymmetric capillary resistance, as well as the enhanced fluid motion caused by Laplace pressure differences and surface free energy gradients. The thermal performance of the DWG-PHP is less sensitive to variation in inclination angles than that of the CPHP. When the inclination angle reduces from 90° to 0°, the thermal resistance of the DWG-PHP with a filling ratio of 55% increases by only 3.8% at a heat load of 20 W. Moreover, the DWG-PHP is capable of operating stably even at an inclination angle of 0° under different heat loads and filling ratios. Under a vibration frequency of 20 Hz, the DWG-PHP exhibits the most significant enhancement in heat transfer performance, with the thermal resistance reduced by 14.3% compared with that of the CPHP at a vibration amplitude of 0.8 mm and a filling ratio of 35%.