Effects of insert design and optimization on the performance of parabolic trough receivers with inserted absorbers

To address the non-uniform heat flux density characteristics of parabolic trough solar collectors (PTSCs), an innovative insert (composed of vortex generator, VG) layout scheme is introduced in this study. Under turbulent conditions, an analysis is conducted on the thermal performance of three different placement strategies (uniform distribution, UD, directional distribution-1, DD-1, and directional distribution-2, DD-2), including the calculation of Nusselt numbers, drag coefficients, and thermal enhancement factors. The results indicate that VG can induce paired vortices, and the position, intensity, and quantity of vortices are closely related to the shape of VG. Based on the direction of vortex flow, the flow field is divided into collision area (CA) and pushing area (PA). In the CA, vortices collide with each other, consuming turbulent energy and reducing local heat transfer efficiency. In the PA, the direction of vortice motion is opposite, which can achieve efficient local heat efficiency. In the three VG cases, the DD-2 configuration can induce non-uniformly distributed high-intensity mixed vortices and further achieve the demand for heat transfer enhancement in the area of high heat flux density through the ejection and sweeping movements of these vortices. In all investigations, DD-2 configuration can achieve an improvement in heat transfer rate ranging from 1.5 to 2.12. In terms of heat transfer performance ($\psi$), the ψ value of DD-2 is 1.37 at N = 4 and Re = 20,000. Besides, the thermal performance of the tube with insert is analyzed through entropy generation.