Thermal Conductivity of Nanostructure-Enhanced Phase Change Materials: Measurements for Solid Eicosane-Based Copper Oxide and Carbon Nanotube Colloids and Numerical Modeling of Anomalous Measurements near Phase Transition
Type of Degreethesis
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In this thesis, thermal conductivity of eicosane-based nanostructure-enhanced phase change materials (NePCM) and modeling of the effectiveness of the widely-used transient hot wire (THW) apparatus near the liquid-solid phase transition temperature are investigated using experimental and numerical approaches. Eicosane (C20H42) with a melting point of 37 °C was selected as the base PCM. Multi-walled carbon nano-tubes (MWCNT) and copper (II) oxide (CuO) nanoparticles stabilized by sodium oleate acid (C18H33O2Na) were selected as nano-scale thermal conductivity enhancers. Three distinct batches of solid eicosane-CuO samples with the same mass fraction of nanoparticles were obtained under ice-water bath, ambient temperature and oven solidification schemes. Thermal conductivity of solid NePCM composites with eight different loadings of CuO nanoparticles and a constant concentration of MWCNTs were measured experimentally at various temperatures using the transient plane source (TPS) technique. Using a controllable temperature bath, measurements were conducted at various temperatures between 10 and 35 °C for the solid samples. Thermal conductivity measurements of the composites were found to be independent of the measurement temperature for a given particle loading regardless of the solidification procedure. The ice-water bath solidification route eicosane-CuO samples consistently exhibited lowest values of thermal conductivity, whereas the samples of oven solidification scheme corresponded to the highest values. Considering eicosane-CuO samples, for mass fractions greater than 2 wt%, a non-monotonic relation between the thermal conductivity and the mass fraction, independent of the temperature range studied, was exhibited. Although no functionalization was performed on the purchased MWCNT powders, the amount of thermal conductivity enhancement for the 0.27 wt% eicosane-MWCNTs solid samples (~25-35%) was much higher than that of the 1 wt% eicosane-CuO solid disks (~1-4%). A 1-D transient heat conduction problem was formulated and solved over a finite cylindrical domain with and without phase transition using ANSYS® FLUENT and MATLAB. The objective was to model the behavior of a THW apparatus near the melting temperature of the medium. The defined FLUENT model was first successfully verified against an ideal mathematical transient hot wire theory for a perfect conductor with a 0.01% error for the monitored temperature values. Another benchmarking was performed against an exact solution for a similar melting problem to verify the adopted enthalpy method and the error for the monitored temperature values was less than 0.5%. Focusing on the FLUENT model, thermal conductivity predictions in both liquid and solid phases were individually performed in the absence of phase change. The difference between the extracted thermal conductivity and the initially-assigned value based on the literature was less than 1% and 0.81% for solid and liquid phases, respectively. The effect of the initial solid state temperature on the predicted thermal conductivity values under the presence of melting was explored. Five different initial temperatures were studied. The aim was to observe if any sharp rise in thermal conductivity values will occur near the melting point of the PCM. It was shown and concluded that there is no abrupt behavior as temperature approaches the melting point of PCM and all the predicted thermal conductivity values are between the assigned values of the liquid and solid states.