Heat transfer performance and thermal energy storage in nano-enhanced phase change materials driven by thermocapillarity

Abstract

We study the influence of metallic nanoparticles on heat transfer and thermal energy storage in a phase change material driven by thermocapillarity. Thermocapillary flows break the symmetry of the transport coefficients in the Prandtl number, weighting more strongly the viscosity. The metallic nanoparticles generate competitive effects on the heat transfer rate: (i) higher conductive heat transfer by raising the effective conductivity of the solid and liquid phases of the PCM, and simultaneously (ii) slower convective heat transfer by raising the viscosity of the melted phase and reducing the surface shear stress responsible for the Marangoni effect. We find that metallic nanoparticles with higher diameters support higher convective flows since they exhibit lower decay of the Marangoni number and have lower Prandtl numbers for high concentrations. High aspect ratio geometries with long free surfaces extend the regions governed by thermocapillary flows enhancing the heat transfer performance for most nanoparticle configurations with respect to the base PCM, also leading to reduced thermal energy required for the complete melting of the PCM. We also find that the melting front at the free surface advances following a power-law ∼tα whose exponent grows with the Prandtl number.