Experimental investigation and model development for the thermal conductivity of glycerol-based nanofluids

Abstract

Glycerol has historically been used as an antifreeze fluid to facilitate heat transfer in the automotive and air conditioning and refrigeration industries. It has also been used as a lubricant in the processing of food and the production of pharmaceuticals and cosmetics. Although a lot of work has been done recently to evaluate the potential to enhance heat transfer using nanoparticles mixed with a base fluid to form a nanofluid, no work has been done on using glycerol as a base fluid. Therefore the purpose of this study was to investigate the effect of nanoparticle volume fraction, nanoparticle size and temperature on the thermal conductivity of stable glycerol-based nanofluids. Two types of metal oxide nanoparticles were considered namely MgO and ?-Al2O3. The particle sizes of the MgO ranged from 21 nm to 119 nm and for the ?-Al2O3 it ranged from 31 nm to 134 nm. The thermal conductivities were determined by experimental measurements and with analytical and empirical models. The thermal conductivity measurements were taken at temperatures ranging from 20?C to 45?C, for nanofluids prepared at volume fractions ranging from 0.5% to 4%. The nanofluids were prepared with a two-step method that included ultrasound mixing to ensure the nanoparticles were fully dispersed and deagglomerated in the glycerol. The experimental results showed that both the ?-Al2O3-glycerol and MgO-glycerol nanofluids had substantially higher thermal conductivity than the base fluid. It was also found that at room temperature, the effective thermal conductivity remains almost constant for at least 50 hours. The maximum thermal conductivity enhancement for the ?-Al2O3-glycerol nanofluids was observed for a 4% volume fraction to be 19.5% for a nanoparticle size of 31 nm. For the MgO-glycerol nanofluids the maximum thermal conductivity enhancements were also for a volume fraction of 4%, however, the enhancement was 18% for a particle size of 21 nm. Furthermore, the thermal conductivities as function of nanoparticle size, volume fraction and temperature, of the two nanofluids were investigated. It was found that the thermal conductivities of the ?-Al2O3-glycerol nanofluids were significantly more dependent on particle size than the MgO-glycerol nanofluids. Furthermore, it was found that no equations exist at present that can accurately predict the thermal conductivity of glycerol based nanofluids and therefore new empirical equations correlations were developed.