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A solar heat pipe collector was designed and fabricated to study its performance of the outdoor test condition. The thermal performance of the wickless heat pipe solar collector was investigated for pure water and nanofluid with varied range of CNT nanofluid concentration (0.15%, 0.45%, 0.60%, and 1% by volume) and various tilt angles (20 deg, 32 deg, 40 deg, 50 deg, and 60 deg). CNT nanoparticles with diameter 10–12 nm and 0.1–10 μm length are used in the present experimental investigation. The optimal value of CNT nanofluid concentration for better performance is obtained from the investigation. The thermal performance of the heat pipe solar collector with CNT nanofluid is compared to that of pure water.

Nanofluids are suspensions of metallic or nonmetallic nanopowders in base liquid and can be employed to increase heat transfer rate at various applications. In the present study, forced convective heat transfer in an Al 2 O 3 /water nanofluid has experimentally been compared to that of pure water in automobile radiator. Five different concentrations of nanofluids in the range of 0-1.0 vol. % have been prepared by the addition of Al 2 O 3 nanoparticles into the water. The test fluid flows through the automobile radiator consisted of 33 vertical tubes with elliptical cross section and air makes a cross flow inside the tube bank with constant speed. The test fluid flow rate has been changed in the range of 3 l/min to 8 l/min to have fully turbulent regime. Obtained results demonstrate that increasing the fluid circulating rate can improve the heat transfer performance. The application of the nanofluid with low concentration can enhance heat transfer efficiency up to 40-45% in comparison with pure water. The increase in heat transfer coefficient due to presence of nanoparticles is higher than the prediction of single phase heat transfer Dittus Boelter correlation used with nanofluid properties. These results can be implemented to optimize the size of an automobile radiator.

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In this study the thermal performance of three different wickless heat pipe solar collectors were investigated by using pure water, watersurfactant and CNT-water nanofluid for different coolant mass flow rates and tilt angles (20 0 , 31.5 0 , 40 0 , 50 0 and 60 0 ). First collector uses only pure water, the second one utilizes CNT nanoparticle of diameter 10-12 nm and 0.1-10 μm length with water as a base fluid. While the third collector uses water with 150 ppm 2-ethyl-hexanol as coolant for the investigation. Experiments were carried out for the three collectors under the same experimental conditions. It is observed that the performance of the solar collector that uses 2-ethyl-hexanol surfactant with water as coolant is better compared to solar collectors that utilize pure water and CNT/water nanofluid.

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