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Register a new userNanolayer and nano-convection based enhanced thermal conductivity of Copper-CO2 nanofluid: A molecular dynamics approach
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The use of CO2 as a natural refrigerant in data center cooling, oil recovery and in CO2 capture and storage which is gaining traction in recent years involves heat transfer between CO2 and the base fluid. A need arises to improve the thermal conductivity of CO2 to increase the process efficiency and reduce cost. One way to improve the thermal conductivity is through nanoparticle addition in the base fluid. The nanofluid in this study consists of copper (Cu) nanoparticle and CO2 as a base fluid. No experimental data are available on the thermal conductivity of CO2 based nanofluid. In this study, the effect of the formation of a nanolayer (or molecular layering) at the gas-solid interface on thermal conductivity is investigated using equilibrium molecular dynamics (EMD) simulations. This study also investigates the diameter effect of nanoparticle on the nanolayer, thermal conductivity and self-diffusion coefficient. In addition to this, diffusion coefficients are calculated for base fluid and nanofluid. The thickness of the dense semi-solid layer formed at the nanoparticle-gas interface is studied through radial distribution function (RDF) and density distribution around the nanoparticle. This thickness is found to increase with nanoparticle diameter. Enhancement in thermal conductivity and diffusion coefficient with nanoparticle diameter are strongly correlated, indicating that the dominant modes of heat and mass transfer are the same. The output of the current work demonstrates the enhancement in thermal conductivity due to nanoparticles addition which may improve data center cooling efficiency and CO2 capture and storing.
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Nanofluids are known to have significantly different thermal properties relative to the corresponding conventional fluids. Heat transfer at the solid-fluid interface affects the thermal properties of nanofluids. The current work helps in understanding the role of two nanoscale phenomena, namely ordering of fluid layer around the nanoparticle (nanolayer) and thermal resistance at the interface of solid-fluid in the enhancement of thermal conductivity of Al2O3 - CO2 nanofluid. In this study, molecular dynamics (MD) simulations have been used to study the thermal interfacial resistance by transient non-equilibrium heat technique and nanolayer formed between Al2O3 nanoparticle (np) and surrounded CO2 molecules in the gaseous and supercritical phase. The nanoparticle diameter (dNP) is varied between 2 and 5 nm to investigate the size effect on thermal interfacial resistance (TIR) and thermal conductivity of nanofluid and the results indicate that the TIR for larger diameters is relatively high in both the phases. The study of the effect of surface wettability and temperature on TIR reveals that the resistance decreases with increase in interaction strength and temperature, but is entirely independent at higher temperatures, in both gaseous and supercritical nanofluid. A density distribution study of the nanolayer and the monolayer around the nanoparticle revealed that the latter is more ordered in smaller diameter with less thermal resistance. However, nanolayer study reveals that the nanoparticle with bigger diameters are more suitable for the cooling/heating purpose, as the system with larger diameters has higher thermal conductivity. Results show that the nanolayer plays a significant role in determining the effective thermal conductivity of the nanofluid, while the influence of TIR appears negligible compared to the nanolayer.
CO2 cooling systems are the wave of the future for industrial refrigeration. CO2 refrigeration systems are gaining traction in recent years which involves heat transfer between CO2 and the base fluid. The high viscosity of CO2 is of interest to the oil and gas industry in enhanced oil recovery and well-fracturing applications. A need arises to improve the thermal conductivity and viscosity of CO2 to increase the efficiency of these significant applications. Aggregation of nanoparticles is one of the crucial mechanisms to improve the thermal conductivity and viscosity of nanofluids. Since the aggregation morphology of nanoparticles is unclear so far, we have evaluated the stable configurations of the aggregation of nanoparticles by determining the potential energy of the different configurations system. In this paper, Green-Kubo formalism is used to calculate the mentioned thermo-physical properties of the different aggregated nanofluids. The nanofluid in this study consists of alumina (Al2O3) nanoparticles and CO2 as a base fluid. Results indicate that the enhancement in the thermal conductivity and viscosity of nanofluid is inversely proportional to the potential energy of the system. The results also mark that various morphologies of the aggregated nanoparticles have different enhancements of thermo-physical properties of the nanofluid. This study is conducive for the researchers to perceive the importance and influence of aggregation morphology of nanoparticles and their stability on the thermal conductivity and viscosity of nanofluid.
Advanced manufacturing (AM) technologies, such as nanoscale additive manufacturing process, enable the fabrication of nanoscale architected materials which has received great attention due to their prominent properties. However, few studies delve into the functional gradient cellular architecture on nanoscale. This work studied the gradient nano-Gyroid architected material made of copper (Cu) by molecular dynamic (MD) simulations. The result reveals that, unlike homogeneous architecture, gradient Gyroid not only shows novel layer-by-layer deformation behavior, but also processes significantly better energy absorption ability. Moreover, this deformation behavior and energy absorption are predictable and designable, which demonstrates its highly programmable potential.
Solid-state lithium-ion batteries (SSLIBs) are considered to be the new generation of devices for energy storage due to better performance and safety. Poly (ethylene oxide) (PEO) based material becomes one of the best candidate of solid electrolytes, while its thermal conductivity is crucial to heat dissipation inside batteries. In this work, we study the thermal conductivity of PEO by molecular dynamics simulation. By enhancing the structure order, thermal conductivity of aligned crystalline PEO is obtained as high as 60 W/m-K at room temperature, which is two orders higher than the value (0.37 W/m-K) of amorphous structure. Interestingly, thermal conductivity of ordered structure shows a significant stepwise negative temperature dependence, which is attributed to the temperature-induced morphology change. Our study offers useful insights into the fundamental mechanisms that govern the thermal conductivity of PEO but not hinder the ionic transport, which can be used for the thermal management and further optimization of high-performance SSLIBs.
Nanograined bulk alloys based on bismuth telluride (Bi2Te3) are the dominant materials for room-temperature thermoelectric applications. In numerous studies, existing bulk phonon mean free path (MFP) spectra predicted by atomistic simulations suggest sub-100 nm grain sizes are necessary to reduce the lattice thermal conductivity by decreasing phonon MFPs. This is in contrast with available experimental data, where a remarkable thermal conductivity reduction is observed even for micro-grained Bi2Te3 samples. In this work, first-principles phonon MFPs along both the in-plane and cross-plane directions are re-computed for bulk Bi2Te3. These phonon MFPs can explain new and existing experimental data on flake-like Bi2Te3 nanostructures with various thicknesses. For polycrystalline Bi2Te3-based materials, a better explanation of the experimental data requires further consideration of the grain-boundary thermal resistance that can largely suppress the transport of high-frequency optical phonons.
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