تحسين الأداء الحراري للمجمع الشمسي ذي القطع المكافئ باستخدام معرقلات اسطوانية لتحلية المياه

Enhancing the Thermal Performance of Parabolic Trough Solar Collector Using Cylindrical Turbulators for Water Desalination

ABSTRACT
Water is necessary for human survival; individuals consume (10-30 L) of clean water daily for drinking, cooking, and other household activities. Contaminated water is not only unsanitary but also dangerous to one’s health. The main aim of this study is to investigate this problem and improve freshwater productivity by developing a solar desalination unit (SDU) containing a solar still (SS) and a parabolic trough collector (PTC) that has a positive effect on cumulative productivity. Improving the heat transfer coefficient of the PTC is the key point of this study, which is performed by inserting a newly designed hollow cylindrical turbulators inside the parabolic trough receiver (PTR). The designed cylindrical turbulators inside a PTR are expected to improve the heat transfer coefficient and heat transfer area between a PTR and water as a heat transfer fluid (HTF) compared with other turbulators reported in the literature (twisted tape, rods, fins, etc.). The work was carried out numerically and experimentally under the climatic circumstances of Babylon city, Babil (32.77° N, 44.29° W), at the College of Engineering/Al-Musayab from 9:00 to 15:00, during selected days from October 2023 to May 2024.
Numerically, the best design for the reflector of PTC was selected using the Parabola Calculator tool 2.0 then the resulted Excel sheet introduced into the ANSYS FLUENT 20.0 R-2 to perform the simulation. The PTC and cylindrical turbulators were designed and simulated using Solidworks and computational fluid dynamics (CFD) tools, respectively. The PTC is designed with dimensions of 2.00 m in length and 1.00 m in width. The related reflector is made of lined sheets of aluminium, and the tubes are made of stainless steel used to absorb heat. They have an outer diameter of 0.051 m and a wall thickness of 0.002 m. The cylindrical turbulators are coupled axially and also radially joined to the interior of the PTR. Eighty different internal cylindrical receivers are examined and compared with the plain tube. The lengths, thicknesses, and numbers of turbulators are tested at varied values, and all these tubes are examined under a constant inlet temperature of 300 K and mass flow rates in the range of (0.6-1.0 kg/s). The developed model is validated with experimental correlations for plain tube from the literature review. The simulation findings demonstrated that the addition of hollow cylindrical turbulators may greatly enhance the uniformity of the temperature gradient between the wall of the PTR and the water. The optimum model of the cylindrical turbulators was selected to be a length of 0.040 m, a thickness of 0.002 m, and a number of 15 turbulators. This model recoded the Nusselt number ratio of 1.79, the friction factor ratio of 1.10, and the highest performance evaluation criteria 1.73. Moreover, the outlet water temperatures of the plain tube and modified tube are increased from 300 K to 325.5 K and 339.7 K, respectively. Based on the numerical findings, the optimum model of the hollow cylindrical turbulators was considered for the experimental work.
Experimentally, the outlet water temperatures recorded 61.5°C and 57.0°C for the modified PTR (tube with novel cylindrical turbulators) and the plain tube (tube without novel cylindrical turbulators), respectively. The heat gain reaches 1053.9 W in the modified PTR and 903.3 W in the plain tube. Compared to the PTC with a plain tube, the PTC with modified PTR showed 38% enhancement in thermal efficiency. It is concluded that the experimental results are in a good agreement with the numerical results of the optimal model of cylindrical turbulators. Also, the PTC performance is evaluated by comparing the experimental and numerical outcomes: the outlet temperature, useful heat, and thermal efficiency for modified PTR and plain tube. Finally, the productivity of freshwater from the modified SDU (MSDU) (SS+PTC contained cylindrical turbulators) improved by 89% and 15% compared to the conventional SS and SDU, respectively.