Nanofluids are suspensions of ultra-fine nanoparticles or tubes resulting in the nanofluids having thermal conductivities significantly higher than those of the conventional base fluid alone. The purpose of this study is to evaluate the thermo-physical properties, including thermal conductivity, viscosity, electrical conductivity and above all, convective heat transfer coefficient, of de-aggregated and fully functionalized diamond nanofluids. The microstructural characterization of nanodiamond powder has been done using X-ray diffraction, energy dispersive X-ray spectroscopy and transmission electron microscopy. Results suggest that there is an optimum point at which maximum thermal enhancement is obtained with few adverse effects from risen viscosity. The results also indicate up to a 27% enhancement in the heat transfer coefficient of the nanofluid with a particle loading of only 0.05 wt.%.

Forced convection cooling is an effective method in thermal management that relies mainly on dissipating heat by pumping heat transfer fluid (HTF) through the heat source. In this paper, we investigate the thermal properties enhancement of dielectric water as the HTF. To enhance the properties of the HTF, microencapsulated phase change materials (MPCM) will be added to the base fluid. The MPCMs are composed of phase change material (PCM) encapsulated with shell materials. The PCM inside the capsules may undergo a phase change. This leads to a significant heat gain and release. The numerical model is developed to solve for continuity, momentum, and heat transfer equations using the finite volume method. The behavior of the MPCM slurry in curved channels, generates unique patterns due to different viscosity values and the centrifugal forces. Our preliminary numerical data on MPCM slurry through planar spiral coil heat exchangers show the new patterns of velocity and heat transfer curves. The current paper studies the steady condition of laminar flow at different boundary conditions. The velocity and temperature profiles, heat transfer data with different mass fractions of MPCM additives to the base fluid, and their heat removal capabilities are quantified and discussed in detail.

In this article, first, graphene oxide nanosheets were synthesized in-house according to the modified Hummers method, and these nanosheets were used to prepare graphene oxide nanofluids at two concentrations. Then the thermophysical properties of nanofluids were characterized using X-ray diffraction analysis, a scanning electron microscope, and UV–Vis spectrophotometry. The particle size distribution was investigated using dynamic light scattering. Then, a fundamental study was conducted on the thermal-hydraulic characteristics of graphene oxide nanofluids flowing through a straight copper tube. An experimental setup was developed to find the heat transfer characteristics and pressure drop of nanofluids in the test section consisting of a copper tube with constant heat flux. The flow regimes and associated pressure drop and heat transfer characteristics at varying flow rate were investigated at three different heat flux conditions of 7.4, 9.1, and 12.6 kW/m². Due to the increase in viscosity, flowrate and Reynolds number decreased from 0.01 to 0.1 wt% of graphene oxide nanofluids at constant pump frequency. Experimental data obtained for water were validated with the findings from the literature, and the correlations were formulated for the Nusselt number and Reynolds number by considering the multiple regression analysis. The convective heat transfer coefficient for graphene oxide at 0.01 wt% was higher when compared to graphene oxide at 0.1 wt% and water. The variation of Nusselt number with the heat flux and velocity was insignificant.

Parabolic trough collectors (PTCs) are essential for solar thermal energy systems, and their thermal efficiency can be significantly enhanced using turbulators and nanofluids. This numerical study introduces three novel fin-spiral turbulator configurations (4, 7, and 10 blades) to enhance heat transfer within the absorber tube. Additionally, three nanofluid types including water-based single-walled carbon nanotubes (SWCNT), cupric oxide (CuO), and a hybrid SWCNT-CuO, at concentrations of 1%, 3%, and 5% were evaluated. The simulations, conducted in ANSYS-FLUENT under steady-state turbulent flow conditions, revealed that the 10-blade turbulator improved the heat transfer coefficient by 12.25% compared to a plain tube, while the hybrid SWCNT-CuO/water nanofluid exhibited a 24.8% increase in thermal conductivity compared to the base fluid. Furthermore, a maximum pressure drop increase of 44% was observed for the hybrid nanofluid at 5% volume concentration and a Reynolds number of 12,500. The study also demonstrated that the Performance Evaluation Criterion (PEC) improved by 15.6% for the hybrid nanofluid compared to CuO/water nanofluid. These findings highlight the effectiveness of combining fin-spiral turbulators and hybrid nanofluids to optimize the thermal and hydraulic performance of PTC systems.