Experimental study on solidification process of a phase change material containing TiO2 nanoparticles for thermal energy storage

The solidification process of n-octadecane as a phase change material (PCM) with dispersed titanium dioxide (TiO2) nanoparticles was experimentally studied. Experiments were performed in a rectangular enclosure cooled from one vertical side corresponding to the solid Stefan numbers in the range 0.17-0.239. The Rayleigh numbers at the initial of experiment were in the range 0.92-18.3 × 106. The rheological behavior of liquid PCM/TiO2 samples at higher concentrations tended to Bingham fluids, thus the solidification experiments were conducted for Bingham numbers in the range 0-2.17. The solidification process was characterized by visualizing the progression of solid-liquid interface as well as recording the temperature distribution inside the enclosure. Experimental results showed that heat conduction was the dominant mode of heat transfer during the solidification. Dispersing TiO2 nanoparticles led to enhance in thermal conductance and consequently the increase in solidified volume. An increase of 7%, 9% and 18% in solidified volume fraction was observed at the end of solidification for the mass fractions of 1 wt.%, 2 wt.% and 4 wt.%, respectively. A universal correlation was proposed to predict the solidified volume fraction as a function of Fourier number, Rayleigh number, solid Stefan number, Bingham number and mass fraction of nanoparticles with an error below 11%.