Experimental study of thermophysical properties of MWCNT and graphene coolant nanofluids for automotive application

Abstract

The present study reports on an experimental investigation regarding thermal conductivity and dynamic viscosity of multi-walled carbon nanotubes (MWCNT) and graphene coolant nanofluids at four volumetric concentrations (0.025%, 0.050%, 0.1% and 0.15%), with focus on automotive applications. The base fluid was composed of a mixture of water–ethylene glycol (70:30). The nanofluids were prepared using the high-pressure homogenization method, which presents long-term stability. Thermal conductivity of nanofluids was measured using the transient hot bridge method, at temperatures ranging from 20 to 50 °C and dynamic viscosity from 20 to 80 °C using a rotational viscometer. Experimental results revealed that nanoparticle concentration and temperature significantly affect the thermophysical properties. Increases in thermal conductivity were found of up to 7.7% for MWCNT nanofluid at a 0.15% concentration. Graphene nanofluids presented thermal conductivity similar to MWCNT, with its highest augmentation found at 5.9% for a 0.1% volumetric concentration. MWCNT nanofluids presented discrete increments on viscosity relative to base fluid for temperatures lower than 50 °C (up to 9.2%) and more pronounced increments at higher temperatures (29.7% at 80 °C). Graphene nanofluids’ viscosity was close to base fluid over the entire analyzed temperature range. The maximum increase was 4.7% for 0.05% concentration at 30 °C, and viscosity was lower than base fluid for some conditions, with a maximum decrease of 3.9%. The experimental results on thermal conductivity and viscosity were compared with proposed correlations found in the literature and showed good agreement. Finally, the increase in thermal conductivity combined with similar viscosity compared to base fluid indicates that the use of nanofluids, as an alternative coolant for automotive applications, is certainly feasible.