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 was employed, and the heat transfer experiments were conducted in three orientation modes: horizontal, vertical, and sideways orientation modes. The wick structure was a sintered copper powder, and the working fluid was water. The heat pipe had a small gap between the wick structure and the container wall. In each orientation mode, a condenser section of the heat pipe was water-cooled with a cooling jacket, and an evaporator section was heated with a heater. Fluid-flow and phase-change phenomena in the heat pipe were captured by using a video camera, and the temporal changes in the temperatures of the heat pipe were obtained by using thermocouples. Due to the additional working fluid, liquid slugs were 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 demonstrated that the dynamic and static liquid slugs were distributed more effectively in the horizontal orientation mode. Thus, the thermal resistance of the heat pipe tended to be smaller and the maximum heat input to the heat pipe was larger in the horizontal orientation mode than in the other two orientation modes. The experimental results also confirmed that the small gap was effective in the horizontal orientation mode. The additional working fluid was stored in the gap, which increased the thermal performance of the heat pipe.