As part of his visit, Prof.  Bruno D’ Aguanno from CIC Energigune, Parque Tecnológico, Miñano, Álava, Spain will give a seminar on Wednesday the 6th of April, at 2 pm (room ENG209 in the Engineering Hub, building 13 on the university map). Please join us in attending this interesting talk on nanofluids.

On the current understanding of thermal properties of nanofluids at high temperature

The fast growing industrial and technological sectors connected with the use of concentrated solar power, CSP, and with the re-use of waste heat are requesting increasing and increasing research efforts on the development of new materials with enhanced thermal energy storage and conservation properties, as well as with enhanced thermal energy transport properties. The two quantities that largely control the mentioned properties are the heat capacity, C_P, and the thermal conductivity, 𝜅. The most promising materials with enhanced thermal properties appear to be stable colloidal suspensions of nanometer-sized particles in various liquid solvents which, currently, are named as “nanofluids”.

Experimentally, many results are available on nanofluids made by suspending silica, titania and alumina nanoparticles in molten alkali nitrates, alkali carbonates, chlorides and fluorides. Typical nanoparticle sizes are in the range ∼ 7-20 nm, nanoparticle concentrations are in the range ∼ 0.5-2 weight%, while the exploited temperature range goes up to 800 ºC. As compared to the heat capacity of the pure suspending medium, C_P,pf , the experimental value of the heat capacity of the nanofluid, C_P,nf , shows a variety of behaviours, going from decreases to large increases, with enhancements, in the liquid phase, up to C_P,nf =2.24 C_P,pf , despite the fact that ideal mixing models are always showing a C_P,nf decrease, C_P,nf < C_P,pf .

Several interpretations of the deviations of C_P,nf from the ideal behaviour have been proposed. All are based on the building of a macroscopically large layering of liquid molecules on the surface of the nanoparticles, with a corresponding modification of the bulk properties of the suspending medium. However, no one of these interpretations is based on firm theoretical and experimental findings. The SEM images in the top figure show an example of large-scale structure induced by the nanoparticles, while the bottom figure shows a schematic representation of the energy storage mechanism arising from the semi-solid molecules at the interface nanoparticle/suspending medium.

In this talk, a detailed and critical review of the main experimental and theoretical results on nanofluids at high temperature will be presented, together with the open and challenging problems. In addition, a strategy to approach a clear understanding of the phenomenology at the basis of the C_P,nf behaviour will be proposed and illustrated. Such a strategy will include the use of a set of experimental techniques, such as Differential Scanning Calorimetry, Adiabatic Calorimetry, Dynamic Light Scattering and Small Angle Neutron Scattering, and a set of theoretical approaches including Integral Equation Theory of liquids, and simulation techniques, such as classical and first principle Molecular Dynamics.