FLUID FLOW AND HEAT TRANSFER CHARACTERISTICS OF A CENTERED-WICK HEAT PIPE WITH ADDITIONAL WORKING FLUID: EXPERIMENTAL STUDY WITH VISUALIZATION

This article describes the experimental study of the centered-wick heat pipe with additional working fluid. A semi-transparent heat pipe is employed, and the heat transfer experiments are conducted in three orientation modes: horizontal, vertical, and sideways. The wick structure is a sintered copper powder, and the working fluid is water. The heat pipe has a small gap between the wick structure and the container wall. In each orientation mode, a condenser section of the heat pipe is water-cooled with a cooling jacket, and an evaporator section is heated with a heater. Fluid-flow and phase-change phenomena in the heat pipe are captured by using a video camera, and the temporal changes in the temperatures of the heat pipe are obtained by using thermocouples. Due to the additional working fluid, liquid slugs are found in vapor flow channels. The liquid slugs can be categorized into two types: a dynamic liquid slug and a static liquid slug. The experimental results demonstrate that the dynamic and static liquid slugs are distributed more effectively in the horizontal orientation mode. Thus, the thermal resistance of the heat pipe tends to be smaller and the maximum heat input to the heat pipe is larger in the horizontal orientation mode than in the other two orientation modes. The experimental results also confirm that the small gap is effective in the horizontal orientation mode. The additional working fluid is stored in the gap, which increases the thermal performance of the heat pipe.