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. K E Y W O R D Sheat transfer enhancements, nanofluids, parabolic trough solar collectors, solar thermal energy, tabulators, thermal and optical performances

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A CFD study of flow quantities and heat transfer by changing a vertical to diameter ratio and horizontal to diameter ratio in inline tube banks using URANS turbulence models Link to publication record in Manchester Research Explorer Citation for published version (APA): Abed, N., & Afgan, I. (2017). A CFD study of flow quantities and heat transfer by changing a vertical to diameter ratio and horizontal to diameter ratio in inline tube banks using URANS turbulence models. AbstractThis paper reports the effect of changing the aspect ratio on the heat transfer and flow quantities over in-line tube banks. Two types of in-line arrangements were employed; square and non-square configurations. The models that were examined are a standard k-ε model, SST k-ω model, v2-f model, EB k-ε model and EB-RSM model. The closer results to the experimental data and LES were obtained by the EB k-ε and v2-f models. For the square pitch ratios, the solution has faced a gradual change from a strong asymmetric to asymmetric and then to a perfect symmetry. The strong asymmetric solution was found by the very narrow aspect ratio of 1.2. However, the behaivour of cases of 1.5 and 1.6 became less strong than that predicted in the case of 1.2. In the larger aspect ratio of 1.75, the flow behaviour is seen to be absolutely symmetric for all variables under consideration except Nusselt number. For the very large pitch ratio of 5, the flow has recorded maximum distributions for all parameters on the windward side of the central tube with a perfect symmetric solution around the angle of 180° while the vortex shedding frequency has recorded minimum value and the Strouhal number; therefore, has given the smallest value. However, for the non-square pitch ratio of constant transverse distance, the solution is still asymmetric for all parameters with merely one stagnation at the angle of 52° at the case of the 1.5X1.75 while by increasing the longitudinal distance to 2 and 5, the solution provided a comprehensive symmetry for all variables with two vortices are fully developed mirrored in shape on the leeward side of the central tube. On the contrary, for the non-square pitch ratio of constant longitudinal distance, the flow of the case of 1.75X1.5 provided two stagnation locations at around 52° and 308° with a very similar solution to the case square ratio of 1.75 for all variables whereas by increasing the transverse distance to 2 and 5, the solution recorded was not perfectly symmetric resulting in two different vortices and one stagnation position located at the leading edge of

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 ( R e ) 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 ( N u ) 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 N u 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.

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This paper presents three dimensional numerical simulations of parabolic trough collectors (PTC) based on two low-Reynolds eddy viscosity turbulence models, namely; Launder and Sharma k-epsilon and k-omega SST models. For the simulations, water was used as the Heat Transfer Fluid (HTF) with four different nanoparticles; Al2O3, TiO2, CuO and Cu. Different volume fractions () of the nanoparticles were investigated for various Reynolds (Re) numbers with uniform heat flux. Results showed that the overall performance of the system is more sensitive to changes in the thermal properties of nanofluid than the thermal properties of the HTF. At a volume fraction of 6% and a Re number of 70,000, the Nusselt number (Nu) enhancement of nanofluids TiO2-water, Al2O3-water, CuOwater and Cu-water were found to be 21.5%, 20.2%, 18.11% and 15.7% respectively while the performance evaluation criteria (PEC) were 1.214, 1.2, 1.18 and 1.155 respectively.

The smoothed particles hydrodynamics (SPH) technique has been used to treat some problems occurred in the free surface flow particularly dam-breaking flow using the open-source SPH code through the NAG FORTRAN compiler and the result presentation was done by using the Para view visualization software. Several methods are employed in order to solve the experienced problems. The most important features have been achieved from this work are: the density filtering method is used to treat pressure fluctuation and the operation time has decreased. Moreover, changing the boundary condition has provided two important things; preventing the particles from penetration the walls and also decreasing the operation time even more. Furthermore, changing the smoothing length from the optimum case (h=1.3 Δx) to (h=1.5 Δx), it shows a few pressure fluctuations and an increase in operation time with no wave propagation. The second change is from (h=1.3 Δx) to (h=1.0 Δx) the pressure noise rose a little bit further but the operation time decreased. The third change is from (h=1.3 Δx) to (h=0.5 Δx) which led to increasing the pressure oscillations and the solution accuracy has been decreased. In spite of decreasing the influence radius, the operation time has increased. Finally, there is no difference has been achieved from changing the pressure formulation in the shape of frames except the operation time has reduced.