  Nanoparticle-Enhanced Phase Change Materials (NePCM) have been a subject of
intensive research owing to their potential for enhanced thermo-physical
properties. However, their behavior during phase change processes, such as
melting or solidification, remains inadequately understood\@. This
investigation focuses on the melting process of NePCM in a square cavity,
exploring distinct cases of melting from both the top and bottom sides. The
NePCM comprises copper nanoparticles (2 nm in size) suspended in water. Our
study involves different combinations of constant temperature boundary
conditions and particle volume fractions\@. Utilizing a numerical model based
on the one-fluid mixture approach combined with the single-domain
enthalpy-porosity model, we account for the phase change process and particles'
interaction with the solid-liquid interface. When melting NePCM from the top
side, convection effects are suppressed, resulting in a melting process
primarily governed by conduction. Both NePCM and pure water melt at the same
rate under these conditions. However, melting NePCM from the bottom side
induces convection-dominated melting. For pure water, thermal convection leads
to the formation of convection cells during melting. Contrastingly, melting
NePCM triggers thermosolutal convection due to temperature and particle
concentration gradients. The flow cells formed from thermosolutal convection in
NePCM differ from those in pure water driven by pure thermal convection. Our
simulations reveal that thermosolutal convection contributes to decelerating
the solid-liquid interface, thereby prolonging NePCM melting compared to pure
water. Surprisingly, the viscosity increase in NePCM plays a minimal role in
the deceleration process, contrary to prior literature attributing slow-downs
of the melting process of the NePCM primarily to increased viscosity.

Fluid Dynamics;Soft Condensed Matter