Experimental investigation on the non-Newtonian to Newtonian rheology transition of nanoparticles enhanced phase change material during melting

The reliable predictions of the constitutive features of fluids are important for evaluating the thermal performance of phase change materials during melting in the thermal energy storage systems. This paper aims to explore the transition mechanism between non-Newtonian and Newtonian rheological modes of nanoparticles enhanced phase change material (NEPCM) during melting from macro and micro perspectives. Three kinds of NEPCMs containing CuO, Al2O3 and TiO2 nanoparticles in different mass fractions (1–9 wt%) are prepared by the two-step method. Then the measurements of different properties of nanofluids in terms of thermal conductivity, rheological behavior and morphology are performed. The influences of mass fraction and temperature on the viscosity of the nanofluids are analyzed and their relations are evaluated using the fitting function method. The results indicate that the addition of nanoparticles can improve the thermal conductivity of pure paraffin. At low shear rate, all the tested nanofluids behave as shear-thinning fluids at 40 °C, except that the nanofluids with 1 wt% of Al2O3 demonstrate Newtonian behavior. For high shear rate, all the nanofluids display Newtonian behavior. Then, a modified power-law model is constructed for the NEPCMs. In the constructive equation, the coefficient (k) enhances with the increase of mass fraction (c) and it decreases with the increase of temperature (T), while the coupling effects of T and c on power-law index (n) are different for different nanofluids. Finally, it is proved that the phenomenon of de-agglomeration contributes to the transition from non-Newtonian mode to Newtonian mode.