Effect of segregation and advection governed heterogeneous distribution of nanoparticles on NEPCM discharging behavior

Abstract

A numerical study is performed to investigate the role of heterogeneous distribution of nanoparticles in discharging behavior of nanoparticle-enhanced phase change materials (NEPCM). The numerical model considers solidification, multiphase convection, nanoparticles segregation, sedimentation, drags on nanoparticles and Brownian and thermophoresis diffusion phenomena. The discharging behavior of NEPCM is analyzed for the solidification of n-octadecane (PCM)-Cu-nanoparticles in a rectangular cavity. The discharging stage of NEPCM is also being investigated experimentally. The experiments were equipped with Particle Image Velocimetry (PIV), high-resolution imaging and thermocouples to measure real-time flow field, solidified layer thickness and local temperature. The numerically predicted results are compared with these experimental results to validate the model. Using the numerical model, the transient evolution of solidification interface morphology, nanoparticles concentration, fluid flow, temperature field, and thermophysical properties during solidification of NEPCM is described. Subsequently, the effect of process parameters such as initial nanoparticles concentration and Stefan number on system's thermal performance is delineated. It is noticed that adding nanoparticles up to a certain limit increases the discharging rate. The discharging time for 0 wt%, 2.5 wt%, 4.5 wt% and 6.5 wt% Cu-nanoparticles concentration are found to be 1115, 895, 980 and 1075 min, respectively. Thus, NEPCM with 2.5 wt% Cu-nanoparticles discharges 19.7 % faster than pure PCM. Furthermore, it is found that the heterogeneous distribution of the rejected nanoparticles significantly affects the solid-liquid interface morphology, solidification rate and alters the thermophysical properties locally as well as globally. The effective density and thermal conductivity increase with an increase in the local concentration of nanoparticles, whereas the effective specific heat capacity decreases with the increase in the local concentration of nanoparticles.