Flow visualization and heat transfer performance of annular thermosyphon heat pipe

To clearly understand the operation phenomena Of thermosyphon heat pipes, current study experimentally investigated the visualization of the operation and limit conditions for a water-filled thermosyphon as well as its thermal performance. The flow patterns and behaviors at the evaporator region are essential for understanding the heat transfer mechanism of heat pipes. Flow visualization was conducted during operation and limiting condition using a high-speed camera. In this study, the flow regime and heat transfer were observed during changes in the flow path, wick and fill ratio. In the thermosyphon, bubbly flow at low heat loads, slug and churn flow at high heat loads, and churn-annular flow at limiting condition were visualized. In the annular thermosyphon, generated bubbles rapidly coalesced; therefore, slug flow occurred at low heat flux, without bubbly flow. The Gr numbers of the thermosyphon and annular thermosyphon were 2.1 x 10(6)-1.99 x 10(7) and 3.5 x 10(4)-1.5 x 10(6), respectively. Therefore, the shear viscosity force was more dominant than buoyancy force in the annular thermosyphon. At the operation limit, a thin liquid film following dry-out was observed at peak surface temperature. When the cross-sectional area of the evaporator was reduced by 40.3%, the operational limit of the capillary heat pipe decreased by 10.2-26.3%, while that of thermosyphon heat pipe decreased by 22.1-44.2%. Experimental results were utilized to describe these effects on the performance of the thermosyphon heat pipes according to changes of cross-sectional area of the flow path, fill ratios, working fluid, and mesh wick without changing the equivalent heated diameter. The results from this study can be used for predicting the heat transfer performance and operation limit of the structure-inserted heat pipe systems. (C) 2017 Elsevier Ltd. All rights reserved.