| By taking advantage of the large latent heat storage capacity,narrow temperature span during melting,and diverse melting points etc.,phase change materials(PCMs)have been widely studied and applied in areas of large-scale thermal energy storage,industrial waste heat utilization,thermal management for electronic devices and so on.Despite of all those merits owned by commonly utilized PCMs,low thermal conductivity and the resulting poor heat transfer performance seriously restrict their practical applications.With the rapid development of nanotechnology,adding highly-conductive nanostructured materials has been proposed as a novel approach to thermal conductivity enhancement of PCMs.The existing researches mainly focused on preparation and thermal characterization of nano-enhanced PCMs(NePCMs).In recent years,the research interests have gradually moved toward the phase change heat transfer performance assessment of the NePCMs.Restricted by experimental devices and technologies,the present researches on heat transfer of NePCMs mainly rely on numerical simulations.However,remarkable disagreement has been demonstrated between the measured and numerically predicted results of melting of NePCMs,which exposes the invalidness of pure numerical method on prediction of NePCMs’ heat transfer behavior.Moreover,former studies focused more on melting rates rather than heat storage rates,which deviates the goal of store heat by PCM to a certain extent.Based on the above reasons,it is necessary to investigate experimentally the effects and mechanisms of adding nano particles on the heat transfer performances and heat storage rates of PCMs.In the present work,straight-chain saturated fatty alcohols,i.e.,1-dodecanol(C12H26O)andl-tetradecanol(C14H30O),with nominal melting points of 22 0C and 35℃ respectively,were adopted as the base PCMs,while graphene nanoplatelets(GNPs)were used as the nanofillers.The two-step protocol was followed for preparing NePCM samples with loadings of 0.5 wt.%,1 wt.%and 3 wt.%.The size and thickness of the pristine GNPs and their distribution after being dispersed into the base PCM were examined by various microscopes.Rigorous long-term stability tests of the NePCM samples were also performed.In addition,a number of important thermophysical properties of the composite PCM samples were characterized.Two classical problems,i.e.,heating from below on a heated horizontal plate and heating circumferentially in a spherical container,were chose and their constrained melting or unconstrained melting heat ransfer process were experimentally investigated under constant heating temperatures.The results revealed that for constrained melting both on the horizontal plate or in the spherical container,although the introduction of the highly-conductive nanoparticles could pronouncedly enhance the thermal conductivity of pure PCM,an undesirable dramatic growth of the dynamic viscosity was also observed.The competition between the enhanced heat conduction and deteriorated natural convection(due to viscosity increase)had a dominant effect on the melting rate.Meanwhile,due to the decrease of latent heat of fusion upon introduction of the nano-additives,fater melting rates couldn’t ensure higher heat storage rates.Better melting and heat storage rates could only be simultaneously achived under conditions when natural convection can be neglected.In addition,correlations for the instantaneous melt fraction and Nusselt(Nu)number were proposed with uncertainties below ± 20%.For close-contact(unconstrained)melting heat transfer on the horizontal plate,the dramatic growth in viscosity of the concentrated sample might lead to a thickened melt film,causing a higher thermal resistance across the molten layer,which could offset the contribution by the higher thermal conductivity and provide a worse melting and heat storage rate.Correlations with uncertainties below ± 15%were also proposed for providing engineering reference.For unconstrained melting heat transfer in spherical containers,the competition between the increases in thermal conductivity and viscosity of NePCM was shown to determine the melting rate.In addition to the undesirable deterioration of natural convection effect,the increased viscosity also weakened the squeezing effect in the close-contact melting region,thus leading to thicker molten layer that impedes heat conduction through it.Correlations were also proposed to predict the melt fraction and heat transfer for all of the NePCM samples together with an overall uncertainty below±15%.In sum,for the melting heat transfer process of NePCM,as the heat conduction enhancement is always followed by a a series of negative impacts caused by viscosity increase,introducing nanoparticles couldn’t ensure a faster melting rate or heat storage rate.Besides,it can be concluded that as long as the measured thermophysical properties are adopted for data reduction,correlations with acceptable accuracies could be achieved by following the conventional scaling analysis for the classical melting problem with pure PCM,which may provide references for engineering applications. |
