Thermal conductivity and viscosity measurements of ethylene glycol-based Al2O3 nanofluids

Abstract

The dispersion and stability of nanofluids obtained by dispersing Al2O3 nanoparticles in ethylene glycol have been analyzed at several concentrations up to 25% in mass fraction. The thermal conductivity and viscosity were experimentally determined at temperatures ranging from 283.15 K to 323.15 K using an apparatus based on the hot-wire method and a rotational viscometer, respectively. It has been found that both thermal conductivity and viscosity increase with the concentration of nanoparticles, whereas when the temperature increases the viscosity diminishes and the thermal conductivity rises. Measured enhancements on thermal conductivity (up to 19%) compare well with literature values when available. New viscosity experimental data yield values more than twice larger than the base fluid. The influence of particle size on viscosity has been also studied, finding large differences that must be taken into account for any practical application. These experimental results were compared with some theoretical models, as those of Maxwell-Hamilton and Crosser for thermal conductivity and Krieger and Dougherty for viscosity.

Figure 1

SEM image of S1 dry Al₂O₃nanopowder at two magnifications. a × 5,000; b × 60,000.

Figure 2

Comparison of thermal conductivity values. Values obtained in this work (filled circle, water; empty circle, EG) and several literature values for water (filled triangle [38]; filled square [37]; filled diamond [41]; downturned triangle [36]) and EG (ex [52]; empty diamond [39]; empty square [42], empty triangle [38]).

Figure 3

Experimental measured thermal conductivity. Alumina nanofluids in EG versus volume fraction concentration at different temperatures: 283.15 K (filled circle); 303.15 K (empty circle), and 323.15 K (downturned triangle).

Figure 4

Enhancement in the thermal conductivity at 303.15 K. Alumina nanofluids as a function of the volume fraction of nanoparticles. Solid line, Prediction of Maxwell model of Equation 1.

Figure 5

Dynamic viscosities for both Al₂O₃/EG nanofluids versus temperature. S1 samples (a) and S2 samples (b). Experimental points at different volume fractions: EG (filled circle), 0.005 (empty circle), 0.010 (filled diamond), 0.015 (empty diamond), 0.021 (filled square), 0.031 (empty square), 0.048 (filled triangle), 0.066 (empty triangle), Vogel-Fulcher-Tammann equation (solid line).

Figure 6

Enhancement of viscosity increase for alumina nanofluids as a function of volume fraction of nanoparticles. S1 (diamond) and S2 (triangle) samples. Prediction of Einstein equation (broken solid line), Equation 3 with N = 1 (dashed line), Equation 5, considering variable a_a/a ratio (dashed-dot line), and Equation 5 considering constant a_a/a ratio (solid line).