Local heat transfer coefficients and friction factors in a U-type compound parabolic solar thermal collector tube operating as a thermosyphon

U-type external compound parabolic concentrators (XCPCs) are non-imaging solar collectors designed for low to medium-temperature applications (60 °C to 300 °C). The XCPC collector consists of an aluminium compound parabolic reflector and an evacuated tube receiver with a direct flow U-shaped absorber tube. While extensive research from the literature has focused on reflector designs and optical components, the heat transfer characteristics and flow patterns inside the absorber tube have not been thoroughly investigated. The flow dynamics inside the absorber tube are unique due to various boundary conditions. These include a non-uniform heat flux around the tube, regions of secondary and developing flow and recirculation flows at the inlet, outlet and through multiple bends. Therefore, this study aimed to determine the heat transfer coefficients and friction factors in an XCPC absorber tube operating as a thermosyphon. The local Nusselt numbers and local friction factors were obtained numerically in ANSYS Fluent 2023R2. It was found in the results that the heat transfer coefficient and friction factors vary significantly downstream of the tube inlet and change near the U-bend of the absorber tube.