Preparation of cobalt oxide and tin dioxide nanofluids and investigation of their thermophysical properties

Nanofluids are a new generation of heat transfer fluids in many industries. The stability of nanoparticles in the base liquid is one of the main challenges in the industrial applications of nanofluids, which depends on various factors such as pH, concentration, size and morphology of nanoparticles. In this study, cobalt oxide (Co3O4) and tin dioxide (SnO2) nanoparticles were synthesized for nanofluid preparation. Nanoparticles were dispersed in base fluid using surfactants, the change of acidity and ultrasonic vibration. The structural properties of nanoparticles were characterized by Fourier transformation infrared (FTIR), X-Ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The synthesized nanoparticles stabilized with sodium dodecyl sulfate (SDS) have been used for preparation of water (25%)/ethylene glycol (75%) (both are eco-friendly)-based nanofluids. The highest stability was observed at pH of 7.5 and 8 for SnO2 and Co3O4 nanofluids, respectively. To improve the dispersion of particles, the best ultrasonication time of 450 and 360 min was obtained for SnO2 and Co3O4 nanofluid, respectively. The effects of particles concentrations ranging from 0.1 to 0.5 wt% at various temperatures of 25, 45, and 65 °C were investigated on thermophysical properties of nanofluids. Results show that both density and viscosity of samples decrease with an increase in temperature and a decrease in particle mass fraction. In addition, the specific heat capacity of nanofluids goes up with increasing temperature for all samples. We found that, the thermal conductivity of the nanofluids increased non-linearly with mass fraction and temperature, compared to the base fluid. The highest thermal conductivity enhancement for nanofluids using Co3O4 and SnO2 nanoparticles was achieved to be 59% and 72.5%, respectively.