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Exergy efficiency, Nusselt number, friction factor, pressure drop, thermal and frictional entropy generation of water-based nanodiamond + Fe3O4 nanofluid flow in a tube and with various coiled wire inserts have been studied experimentally under turbulent and constant heat flux boundary conditions. The experiments were conducted in the Reynolds number range from 2000 to 22,000, particle concentrations of 0.05%, 0.1% and 0.2% and coiled wire inserts of different $$p/d$$ values of 3.67, 2.34 and 1.00, respectively. Results indicate that at 0.2% vol. and Reynolds number of 20,095, without coiled wire inserts, the heat transfer coefficient, Nusselt number, friction factor, pressure drop and pumping power are enhanced to 44.36%, 29.55%, 11.1%, 29.58% and 39.49% over the base fluid data. Similarly, at 0.2% vol. and Reynolds number of 20,095, with coiled wire inserts of $$p/d$$ = 1, the heat transfer coefficient, Nusselt number, friction factor, pressure drop and pumping power are further enhanced to 107.19%, 66.36%, 38.84, 64.44% and 76.54% over the base fluid data without inserts. The thermal entropy generation is decreased to 30.80% and it is further decreased to 46.34% at 0.2% vol. and Reynolds number of 20,095 with coiled wire inserts of $$p/d$$ = 1. The exergy efficiency of water is 18.95%, and it is increased to 24.06% for 0.2% vol. and it is further increased to 51.85% for 0.2% vol. and Reynolds number of 20,095 with coiled wire inserts of $$p/d$$ = 1. The study indicates that the hybrid nanofluids with coiled wire inserts are guaranteed choice for augmenting the exergy efficiency of flow through tube.

An experimental investigation was conducted aiming to determine the heat transfer, friction and instantaneous collector thermal efficiency of a thermosyphon (natural circulation) solar water heating system using as working fluids of water and the Cu/H2O nanofluid. The Cu nanoparticles were synthesized using the chemical reduction method and characterized by the x-ray diffraction and transmission electron microscopy techniques. The stable Cu/H2O nanofluid was prepared for the volume concentrations of 0.1% and 0.3%. The empirical correlations developed for Nusselt number and friction factor for the Cu/H2O nanofluid fit the experimental data with a deviation of less than ±3.5% and ±2.5%, respectively. The results of present experimental investigation were presented at various Reynolds number and particle volume concentrations under thermosyphon conditions. The comparison indicates that the heat transfer enhancement obtained with the Cu/H2O nanofluid for the thermosyphon is higher than that for the plain tube collector and increases with the increase of particle volume concentration. The overall thermal performance of the thermosyphon increases when the operating fluid is Cu/H2O nanofluid as compared to water.

The study is aimed to understand the heat transfer coefficient and thermal entropy generation analyses of plate heat exchanger by using water-based nanodiamond nanofluids. The experiments were conducted in the volume concentration range: 0 ≤ ϕ ≤ 1.0%, the Reynolds number range: 140 ≤ Re ≤ 610, the mass flow rate range: 0.05 ≤ m ≤ 0.183 kg / s , and the Peclet number range from 895.78 ≤ Pe ≤ 3882.72, respectively. The effect of Reynolds number, Peclet number and particle volume loadings of nanodiamond nanofluids on heat transfer characteristics and entropy generation has been investigated. Water and nanodiamond nanofluids were considered as hot and cold medium in the plate heat exchanger, respectively, for the experimental study. The study reveals considerable augmentation in heat transfer coefficient and Nusselt number with an increase of nanofluid particle loadings. The study showed 32.50%, 55.47%, 35.11%, 22.80%, and 18.93% enhancements in overall heat transfer coefficient, heat transfer coefficient, Nusselt number, pressure drop and pumping power compared to base fluid at a Reynolds number of 526.37 at ϕ = 1.0%. Improved effectiveness, number of transfer units and exergy efficiency of 14.41%, 32.81% and 19.72% was observed at ϕ = 1.0% and at a Reynolds number of 526.37 against base fluid data. The thermal entropy and friction entropy generation was showed decreasing and increasing trends respectively, in the measured particle volume loadings. The Bejan number and entropy generation number demonstrated the roles of heat transfer and friction factor in the entropy generation. From the experimental results a new Nusselt number and friction factor correlations were proposed.

The collector efficiency, heat transfer, energy, and environmental emissions of water -based nanodiamond (ND) nanofluids circulate in flat plate solar collector (FPSC) has been analyzed experimentally. The analysis was performed at 0.2%, 0.4%, 0.6%, 0.8% and 1.0% particle concentrations, the stability and thermo-physical properties were also analyzed. The thermal conductivity and viscosity enhancements are 22.86% and 79.16% at particle loading of 1.0% against water data at 60 °C. The water and 1.0 vol% of ND/water nanofluids in collector shows efficiencies of 53% and 74%, respectively, which leads to 39.62% enhancement. The collector area was decreased to 28.66% with the use of 1.0 vol% of nanofluid against water data. The collector weight is decreased to 35.81 kg, at 1.0% particle loading, whereas the collector with water weighed 50 kg. The embodied energy of 1.0 vol% of nanofluid is decreased to 1039.51 MJ, whereas, the collector with water its embodied energy is 1451.4 MJ. The CO2 emissions were noticed as 249.98 kg by using 1.0 vol% of nanofluid in FPSC. Additionally, the Nusselt number and heat transfer is augmented to 32.31% and 52.33% at 1.0% vol% with a friction factor penalty of 1.26-times at a Reynolds number of 12,766 against water data. The obtained data is validated with literature and the general form of regression equations was developed in order to evaluate the Nusselt number and friction factor.