Browsing by Author "Somarathna CN"

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    item: Thesis-Abstract
    A Molecular dynamics study on viscous and thermal properties of Nanofluids
    (2023) Somarathna CN; Samaraweera N; Jayasekara S; Perera K
    The aim of this study is to understand the microscopic behavior of heat and momentum transfer in nanofluids. With nanofluids reporting enhanced thermal conductivities (𝜅) and viscosities (𝜂), a microscopic understanding is essential for engineering nanofluids to be practical in heat transfer applications. Therefore, to study the microscopic transport behavior, copper-argon nanofluids simulated by classical molecular dynamics are employed. The Applicability of the Green-Kubo (GK) method in nanofluid 𝜅 evaluation is questioned as the calculated thermal conductivities through the GK method are considerably higher than the direct method in Non-EquilibriumMolecular-Dynamics (NEMD). Green-Kubo calculations are found to be very sensitive to the ill-defined partial enthalpy computation, resulting in an overestimation of the 𝜅. However, the Green-Kubo and the direct method viscosity calculations demonstrate a reasonable agreement. Following the more reliable method, the NEMD direct approach, 𝜅 of the nanofluids consisting of spherical nanoparticles with different diameters are investigated. The computational results are compared with the classical effective medium theories and no anomalous 𝜅 enhancements are observed in the nanofluids having fully dispersed spherical particles. Various microscopic mechanisms such as liquid layering and micro-convection are found to be ineffective for 𝜅 enhancements in nanofluids. However, greatly enhanced 𝜅 are achieved, a maximum of 63% relative to pure argon, in nanofluids consisting of chain-like particle arrangements. This demonstrates the potential origin of anomalous 𝜅 enhancements in experimental measurements and the capability of nanofluids with extended nanostructures to deliver better 𝜅 enhancements. Further investigating the capability of extended nanostructures in nanofluid thermal transport, 𝜅 enhancements of nanofluids consisting of nanowires with different lengths and diameters are evaluated. It is shown that the heat conduction in the parallelly arranged liquid and the nanowires exhibit a coupled thermal behavior owing to the interface thermal resistance (R b ). This contradicts with the predictions of the classical parallel heat conduction model and therefore, a novel model is proposed taking this coupled behavior into account. New heat transfer characteristics at the nanoscale are identified including the R b -driven coupled heat conduction, the reduced 𝜅 of suspended nanowires, and the solid-like liquid layering. Using the new model, the importance of these microscopic thermal characteristics in accurately predicting the effective 𝜅 is shown. The sole contribution from the solid-like liquid layer to the 𝜅 enhancement is found to be in between 20-30% for the nanofluids considered. Extending the investigation of heat transfer phenomena in nanofluids based on spherical nanoparticles, 𝜂 of nanofluids with different nanoparticle sizes, concentrations, and arrangements are evaluated. Both the Green-Kubo and the direct methods are employed and unlike the 𝜅, both methods show a reasonable agreement with one another. Viscosity is observed to decrease as the particle diameter increases in fully dispersed nanofluids. The ratio 𝐶 ⁄ shows a decreasing trend indicating better heat transfer performance in nanofluids with large particles. Nanofluid 𝜂 is 𝜂 𝐶 𝜅 observed to increase rapidly as the concentration increase. This makes 𝐶 ⁄ to increase as well indicating the diminished heat transfer performance in nanofluids with high particle concentrations. As the particles in the nanofluid arrange into chain-like structures, 𝜂 remains unaffected. This makes 𝐶 ⁄ to decrease rapidly indicating the greater heat transfer performance in nanofluids with chain-like nanoparticle arrangements or in general, extended nanostructures.