|The mechanisms of nanoscale thermal transport within nanoparticle suspensions (nanofluids) and nanostructure-enhanced phase change materials (NePCM) are investigated using molecular dynamics (MD) simulations.
To begin, the terms included in the heat current expression utilized in the Green-Kubo-based equilibrium MD (EMD) simulations are examined by performing simulations on different multi-component systems including model nanofluid and gas, liquid and solid mixtures. The results are compared against those obtained with the application of the heat source and sink and determination of the thermal conductivity using non-equilibrium molecular dynamics (NEMD) and the Fourier’s Law. The results indicate that the proper definition of the heat current in the equilibrium simulations leads to the consistency between the results obtained using the EMD and NEMD simulations. The validated EMD method is utilized to study the role of the Brownian motion-induced micro-convection in the thermal conductivity of well-dispersed nanofluids. An illustrative decomposition of the thermal conductivity into appropriate components shows that while the individual terms in the heat current autocorrelation function associated with the diffusion of nanoparticles achieve significant values, these terms essentially cancel each other if the correctly-defined average enthalpy expressions are subtracted. Otherwise, erroneous thermal conductivity enhancements will be predicted that are attributed to the Brownian motion-induced micro-convection. Consequently, micro-convection does not contribute noticeably to the thermal conductivity and the predicted thermal conductivity enhancements are consistent with the effective medium theory.
Then, the experimentally-observed improving effect of high aspect-ratio carbon-based nano-fillers, e.g. carbon nanotubes (CNT) and graphene sheets, on heat transfer within paraffin-based phase change materials is investigated. Firstly, the thermal conductivity of liquid and solid n-octadecane (as n-paraffin) is determined by using the direct method-based NEMD simulations. Different calculated thermo-physical properties of liquid/solid n-octadecane show good consistency between the numerical results and experimental data. It is observed that through solidification, nano-crystalline domains form in n-octadecane and in agreement with experimental data the value of the thermal conductivity increases. The calculations for a perfect crystal structure of n-octadecane molecules show that the thermal conductivity along the molecular axis is four times higher than the value for the solid case, showing a strong relation between the ordering of paraffin’s molecules and its thermal conductivity. Introducing CNT and graphene sheets in paraffin promotes aligning of paraffin’s molecules in the direction parallel to the CNT axis and graphene surface, respectively and consequently lead to considerable enhancements in its thermal conductivity along those directions. Then, descriptive set of simulations are designed to study the overall thermal conductivity enhancement for the solid/liquid mixtures and also determine the contribution of the proposed mechanism in such enhancement. The results exhibit improvements in the thermal conductivity well beyond the predictions of the effective medium theory and indicate the dominant role of the filler-induced alignment mechanism.
The investigation on the interfacial thermal conductance, which is a key element in heat transfer through multi-component systems, between graphene sheets and paraffin also shed light on the nanoscale thermal transport within such mixtures/composites. The results from the direct method-based NEMD calculations reveal two main statements: (i) for solid phase paraffin the interfacial thermal conductance is higher than the corresponding liquid phase paraffin, more likely due to the more effective filler-induced ordering of paraffin molecules, and (ii) the graphene sheets containing a few number of layers exhibit higher values of the interfacial thermal conductance with respect to the thicker graphene sheets.