Stability and Thermal Conductivity of Metallic-nanofluids

Abstract

Nanofluid or nanoparticle-fluid is a fluid containing particles that are small enough to remain in suspension over time that sometimes exhibit exceptional properties. While considerable work has been reported on the thermal conductivity of such mixtures based on oxides, inert powders and non-corrosive fluids, there are only few studies on metallic-nanofluids. In addition, there is a need to produce metallic-nanofluids with excellent thermal conductivity with good long term stability, and understand the heat transfer mechanism of these fluids. Therefore, the present work explores both the stability and thermal conductivity of metallic-nanofluids. Fine particles of stainless steel were selected and were dispersed into pure water or ethylene glycol. As some previous studies have shown the strong relationship between the particle dispersion stability and fluid thermal conductivity, the first step was producing stable nanofluids. Amongst the parameters that are known to influence the dispersion of particles in fluid, it was found that controlling the fluid pH helps to stabilise the mixtures more than the addition of surfactants which is important in obtaining ζ-potentials that are large enough to sustain a significant repulsion between like particles in the fluid. For 0.017 wt% stainless steel-water fluids, pH 11 was the optimal stability condition and the thermal conductivity enhancement was also highest. This work forms the foundation of future studies on the properties of such mixtures, especially for heavy metallic particles. The second step was to reveal the relationship between the stability and thermal conductivity at various particle concentrations. Previous studies have shown that the thermal conductivity of fluids containing nanoparticles increased with the particle volume fraction. However, the rate of enhancement and the linearity have been hard to predict. To determine the complete particle dispersion at various concentrations that has not been deeply discussed before, the particle molar absorptivity was used. Under the complete particle dispersion, the thermal conductivity enhancement showed linear relationship with particle volume fraction when stainless steel particles were dispersed into both water and ethylene glycol. Thermal conductivity enhancement with stainless steel particles in water was 20% and 15% with 0.003 vol.% of particles in water and ethylene glycol, respectively. This is comparable with other metallic-nanofluids which were previously reported, and show that obtaining good particle dispersion is necessary to benefit the particle with high thermal conductivity addition to fluids.