Influence of the liquid plug on the heat transfer performance of the ultra-thin flat heat pipe

Under extreme size, the ultra-thin flat heat pipe will form a liquid plug connecting the upper and lower surfaces due to the size effect and surface tension, and the liquid plug occupies and separates the vapor flow path. To study the influence of the liquid plug on the heat and mass transfer performance of the ultra-thin flat heat pipe, the ultra-thin flat heat pipe with a thickness of 1 mm and a filling rate of 30% is theoretically analyzed and visualization research. The results show that the liquid plug divides the vapor flow path into two separate sections, obstructing the continuous vapor flow. And the two sections are evaporated and condensed, respectively. The liquid plug will affect the temperature gradient distribution. In the hot end bubble, the temperature gradient decreases in the region where the condensing phase change occurs and increases in the region adjacent to this. The liquid plug needs to be pushed from the evaporator to the condenser during the heating process, resulting in an increased start-up time for the ultra-thin flat heat pipe. At low heating power of 4 W and 8 W, the liquid plug leads to the wick of the evaporator being completely covered by the liquid plug or liquid film, and the thermal resistance is higher than that without the liquid plug. When the heating power rises to 12 W and 16 W, the evaporator enters the nucleated boiling stage, and the liquid plug will replenish the working fluid reflux in the wick, and the thermal resistance decreases compared to the case without the liquid plug.