A wide range of engineering industrial applications require both the thermal and optical efficiencies of the system to be maximized with a reasonable low penalty for the friction factor and subsequently low losses in pressure. Among the family of concentrated solar power systems, parabolic trough collectors (PTCs), which have recently received significant attention, face similar challenges. The current work presents an extensive review of the PTC systems comparing recent and past technologies, which are widely being used to improve and enhance the thermal and optical efficiencies. Furthermore, the techniques used for single and two‐phase flow modeling in numerical simulations, design variables, and experimental processes have been discussed in detail. The article also presents different numerical methods and analytical approaches of implementing the nonuniform solar distribution with different design parameters. Four main technologies are comprehensively addressed to effectively enhance the thermal performance of the PTCs; changing working heat transfer fluids, replacing the working fluids by nanofluids (single and hybrid) that have higher thermal–physical properties than those of base working fluids, inserting different tabulators with various design configurations, and finally combining the advantages of nanofluids and swirl generators in the same application. The article also critically summarizes the studies investigating the enhancement of thermal performance: use of novel design of PTCs and passive heat transfer enhancement techniques. Finally, a wide range of numerical and experimental studies are proposed for the future work related to the aforementioned main technologies.

In this study, we numerically investigated the effect of swirl inserts with and without nanofluids over a range of Reynolds numbers for parabolic trough collectors with non-uniform heating. Three approaches were utilized to enhance the thermal-hydraulic performance—the variation of geometrical properties of a single canonical insert to find the optimized shape; the use of nanofluids and analysis of the effect of both the aforementioned approaches; the use of swirl generators and nanofluids together. Results revealed that using the straight conical strips alone enhanced the Nusselt number by 47.13%. However, the use of nanofluids along with the swirl generators increased the Nusselt number by 57.48%. These improvements reduced the thermal losses by 22.3% for swirl generators with nanofluids, as opposed to a reduction of only 15.7% with nanofluids alone. The investigation of different swirl generator designs showed various levels of improvements in terms of the overall thermal efficiency and thermal exergy efficiency. The larger swirl generator (H30mm-θ30°-N4) with 6% SiO₂ nanofluids was found to be the optimum configuration, which improved the overall collector efficiency and thermal exergy by 14.62% and 14.47%, respectively.

Changing the heat transfer fluid (HTF) is a viable approach to study the corresponding effect on the thermal and hydraulic performances of parabolic trough collectors (PTC). Three categorized-types of pure fluids are used in this study; water, Therminol^® VP-1 and molten salt. The parametric comparison between pure fluids is also studied considering the effect of various inlet fluid temperatures and different Reynolds ( $Re$ ) numbers on the thermal performance. Two low-Reynolds turbulence models are used; Launder and Sharma (LS) k-epsilon and Shear Stress Transport (SST) k-omega models. In order to assess the performance of each fluid, a number of parameters are analyzed including average Nusselt ( $Nu$ ) number, specific pressure drop distributions, thermal losses, thermal stresses and overall thermal efficiency of the PTC system. Results confirmed that changing the working fluid in the PTC enhances the overall heat transfer thereby improving thermal efficiency. For a temperature-range of (320–500) K, the Therminol^® VP-1 performed better than water, resulting in higher $Nu$ numbers, lower thermal stresses and higher thermal efficiencies. On the other hand, for the common temperature-range, both Therminol^® VP-1 and molten salt preformed more or less the same with Therminol^® VP-1 case depicting lower thermal stresses. The molten salt is thus the best choice for high operating temperatures (up to 873 K) as it does not depict any significant reduction in the overall thermal efficiency at high temperatures; this leads to a better performance for the Rankine cycle. For the highest tested Reynolds number for an inlet fluid temperature of 320 K, a comparison of heat transfer performance (Nusselt number) and the overall thermal efficiency between Therminol^® VP-1 and water showed that Therminol^® VP-1 is the best candidate, whereas the molten salt is the best choice for a higher inlet temperature of 600 K. For example, at an inlet temperature of 320 K, the Nusselt number and overall thermal efficiency of therminol VP-1 were 910 and 49% respectively as opposed to 443 and 38% for water. On the other hand, at the higher inlet temperature of 600 K, these two parameters (Nusselt number and overall thermal efficiency) were recorded as 614 and 41 % for molten salt and 500 and 39 % for Therminol^® VP-1.

The present work describes the airflow characteristics upstream and downstream of the circular orifice numerically using the standard k-ε model and RSM and experimentally employing hot-wire and Pitot tube devices to measure the flow velocities through a tube. An expectation has to bear in mind; the RSM performed well and predicted results in a good level of agreement against the experimental measurements found in preceding studies and the present experimental data as well. In terms of flow physics point of view, velocity and turbulent kinetic energy distribution are presented. Moreover, turbulent viscosity, Reynolds stresses (u², v², w²) are also considered. Furthermore, the contours of velocity, static pressure and turbulent kinetic energy are presented beside the wall shear stress and y+ profiles are illustrated to show the flow physics as well as to compare the results of two different turbulence models considered in this study employing Reynolds of 30000 and the ratio of orifice diameter to the pipe diameter of 0.5.