Síntese e caracterização de nanofluidos para aplicação em sistemas térmicos

Abstract

The goals of this work were to study the synthesis and characterization of the thermophys ical properties of various types of nanofluids in order to evaluate their application in thermal exchange systems. For this, nanofluids of aqueous base, base oil and ethylene glycol base were prepared and studied, containing six types of nanoparticles: TiO2, Ag, Cu, MWCNT, diamond and graphene. Hybrid nanoparticles (Diamond-Nickel, MWCNT-Ag, graphene-Ag) and functionalized surface MWCNT-COOH, Di-COOH and TiO2-PVA were also produced. For the structural characterization of the nanoparticles, X-ray diffraction (XRD), Raman spectroscopy and infrared (FTIR) techniques were used, as well as transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images. The synthesis of the nanofluids consisted of the application of the "two-step" method, using sonication processes and high pressure homogenization to disperse the nanoparticles, using volumet r ic concentrations ranging from 0.00125% to 0.1%, and mass concentrations of 0.05 to 1.0 wt%, in base fluids. The thermophysical properties (thermal conductivity and viscosity) were measured experimentally in a temperature range of 10° to 60° C and an increase in both the thermal conductivity and the viscosity were observed with the increase in the volumet r ic fraction of nanoparticles and with the temperature. The stability of the nanofluids was evaluated by the sedimentation observation method. In general, nanofluids without stabilizers remained stable for a few weeks. The experimental results, obtained for thermal conductivity, revealed a maximum increase of 10.67% for diamond nanofluids in mineral oil with oleic acid as stabilizing agent. The maximum increase in viscosity was obtained for nanofluidos of MWCNT, being 1.6 times greater than the viscosity of the water. The results, of thermal conductivity, viscosity and specific mass, were still compared with the classic models of the literature. Thus, the studies of the synthesis and characterization of nanofluids revealed an increase of the thermal conductivity with the dispersion of nanoparticles being a promising alternative as fluids applied in processes of thermal changes.