Experimental analysis of the start-up characteristics in a pulsating heat pipe with different filling ratios under vertical vibration excitations

Pulsating heat pipes (PHPs) are inevitably exposed to external vibration disturbances in practical applications. An in-depth understanding of the start-up behavior of PHPs is critical for improving their thermal responsiveness. This work experimentally examined the effect of vertical vibration amplitudes (0.2–1.0 mm) and frequencies (10–50 Hz) on the start-up performance of a water-based two-turn PHP with varying filling ratios (35–75 %). The start-up process of the PHP was elucidated by analyzing the transient oscillation characteristic of wall temperatures and the motion behavior of vapor-liquid slugs. Furthermore, the capillary hysteresis resistance induced by external vibrations was quantitatively analyzed by tracking the meniscus position and morphology. Results indicate that the start-up time of the PHP increases as the vibration amplitude rises. This is because an asymmetry deformation of menisci causes the difference in the contact angles on both sides of a liquid slug, thereby producing an additional capillary hysteresis resistance that impedes the fluid motion. Moreover, a high filling ratio can mitigate the adverse effect of vibration amplitudes on the start-up time by suppressing the meniscus deformation. Conversely, the start-up time of the PHP shortens as the vibration frequency increases, which is most pronounced under the filling ratio of 55 %. Additionally, the vibration frequency and amplitude exhibit a competing effect. The high-frequency and low-amplitude vibration (50 Hz, 0.2 mm) makes the greatest enhancement in the start-up time, but may increase the start-up temperature at a lower filling ratio of 35 %. Under the vibration conditions of this work, the PHP with a filling ratio of 55 % exhibits the optimal start-up performance. The start-up time and temperature reduced by 20.1 % and 11.0 %, respectively, compared to the non-vibration condition.