Nanofluids have emerged as an addition for thermal management and energy conversion applications. The dispersion of a small amount of solid nanoparticles occasionally leads to an unexpected enhancement in the specific heat of the dispersant fluid. This effect has technical, economic, and social significance, and for that, it has received a lot of attention from applied research, but the associated physical and chemical phenomena explaining this phenomenon are yet to be described. We report here a combined experimental and theoretical investigation of nanofluids consisting of palladium nanoplates in a typical heat transfer oil used for concentrating solar power. Their specific heat per unit volume is found to be maximally enhanced at intermediate nanoparticle concentrations, at all temperatures. This is consistent with the phenomenological description provided by the mesolayer model. Density functional theory calculations of adsorption energies and diffusion/desorption activation barriers reveal a strong interaction between the base fluid molecules and palladium surfaces, leading to a nanofluid model where the metal particles are decorated by a static layer of organic molecules. Such layering is potentially responsible for the anomalous enhancement on the thermal properties of the nanofluid, such as the specific heat. Our contribution with this work is a first step toward a complete understanding on the structure and properties of nanofluids using ab initio molecular simulation techniques rather than phenomenological descriptions only.

No abstract

No abstract

No abstract

No abstract

Aiming for the introduction of stability requirements in nanofluids processing, an interface-based three-step method is proposed in this work. It is theory-based design framework for nanofluids that aims for a minimum tension at the solid-liquid interface by adjusting the polar and dispersive components of the base fluid to meet those of disperse nanomaterial. The method was successfully tested in the preparation of aqueous nanofluids containing single-walled carbon nanotubes that resulted to be stable and to provide good thermal properties, i.e. thermal conductivity increases by 79.5% and isobaric specific heat by 8.6% for a 0.087 vol.% load of nanotubes at 70 °C. Besides, a system for these nanofluids was modelled. It was found to be thermodynamically consistent and computationally efficient, providing consistent response to changes in the state variable temperature in a classical Molecular Dynamics environment. From an analysis of the spatial components of the heat flux autocorrelation function, using the equilibrium approach, it was possible to elucidate that heat conduction through the host fluid is enhanced by phonon propagation along nanotubes longitudinal axes. From an analysis of the structural features described by radial distribution functions, it was concluded that additional heat storage arises from the hydrophobic effect.

No abstract

Nanofluids are systems with several interesting heat transfer applications, but it can be a challenge to obtain highly stable suspensions. One way to overcome this challenge is to create the appropriate conditions to disperse the nanomaterial in the fluid. However, when the heat transfer fluid used is a non-polar organic oil, there are complications due to the low polarity of this solvent. Therefore, this study introduces a method to synthesize TiO2 nanoparticles inside a non-polar fluid typically used in heat transfer applications. Nanoparticles produced were characterized for their structural and chemical properties using techniques such as X-ray Diffraction (XRD), Raman spectroscopy, Transmission Electron Microscopy (TEM), Fourier Transform Infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The nanofluid showed a high stability, which was analyzed by means of UV-vis spectroscopy and by measuring its particle size and ζ potential. So, this nanofluid will have many possible applications. In this work, the use as heat transfer fluid was tested. In this sense, nanofluid also presented enhanced isobaric specific heat and thermal conductivity values with regard to the base fluid, which led to the heat transfer coefficient increasing by 14.4%. Thus, the nanofluid prepared could be a promising alternative to typical HTFs thanks to its improved thermal properties and high stability resulting from the synthesis procedure.