A Novel Semi-Visualizable Experimental Study of a Plate Gravity Heat Pipe at Unsteady State

An experimental study on a plate gravity heat pipe (PGHP) with inner cavity size of length 100 mm (X), width 2.5 mm (Y), and height 210 mm (Z) with acetone as the working fluid was carried out. The effects of heating power inputs (80–180 W) and fluid filling ratios (25%, 48%, and 55%) on the start-up temperature, start-up time, temperature difference, and relative thermal resistance on the Z-axis of the PGHP in a vacuum of 1 × 10−3 Pa were studied at unsteady state. Furthermore, the gas-liquid two-phase behavior of the interior working fluid of PGHP, and the coupling heat transfer behavior of the boiling liquid and the condensate were observed through a visualizable window under different experimental conditions. The results show that, with the increase of heating power input, the start-up temperature of the PGHP increases and the start-up time is shortened. The start-up temperature of the PGHP was around 33 ◦C and the start-up time was about 320 s at the heating power input of 120 W and working fluid filling ratio of 55%. The relative thermal resistance and the temperature difference on the Z-axis of the PGHP increase firstly and then decrease with the increase of heating power input at unsteady state. The complex gas-liquid two-phase behavior of the PGHP mainly includes: the formation and growth of bubbles, the merging and break up of bubbles, and the coupling heat transfer between boiling liquid and condensate, which demonstrate that the novel SVPGHP can be used to effectively study the heat transfer process of PGHP.