Applications of Advanced Multiscale Simulations in Thermal Performance Improvement of Energy Systems

先進數值模擬在提升能源系統熱性能中的應用

Student thesis: Doctoral Thesis

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Award date17 Nov 2020

Abstract

The thermal performance of systems and devices plays an important role in tremendous energy-related industrial fields and applications, such as electric vehicles and power plants, which is in turn closely linked with our daily life. On the other hand, the giant development of computer technologies has continuously promoted the numerical simulation methods to be indispensable tools for both academic research and industrial applications. In this thesis, the finite volume method (FVM) based computational fluid dynamics (CFD) simulation is used to study the improvement of liquid and PCM (phase change material) based battery thermal management systems, and the molecular dynamics (MD) technique is adopted to investigate the effects of nanostructures on nanoscale boiling heat transfer.

Firstly, a mini-channel liquid thermal management system for high-power prismatic Li-ion batteries are investigated via three-dimensional CFD simulations. The thermal performances of the cooling system using different base fluids (namely, water, ethylene glycol, and engine oil) and their corresponding nanofluids are compared. The thermal conductivities of nanofluids are calculated using a semi-empirical correlation and the dynamic viscosities of nanofluids with different base fluids are obtained by the curve fitting method using experimental data. Due to its high thermal conductivity, water achieves a much better cooling effect than ethylene glycol and engine oil. However, the impact of adding nanoparticles is more remarkable for fluids with lower thermal conductivity. The nanoparticle addition could greatly reduce the cell maximum temperature but has a limited effect on the temperature uniformity. In addition, evident performance enhancement is observed by increasing the suspended nanoparticle volume fractions despite the increased power cost. Different influencing factors including the flow velocity and coolant inlet temperature on the mini-channel cooling performance and the enhancement of adding nanoparticles are also studied.

Subsequently, a hybrid thermal management system that couples PCM/copper foam with helical liquid channels is investigated. The addition of copper foam could improve the overall thermal conductivity of pure PCM while the helical liquid channels help in removing the excess heat, thus promoting the cooling capacity and delaying the complete melting of PCM. A parametric study of the influencing factors on the cooling performance is conducted. Results showed that the helical channel presented better performance than the conventional straight channel and achieved more than 30 K temperature drop than the adiabatic condition. Besides, the reduction of helical pitch and the increments of helical diameter and tube number decreased the battery temperature but raised the pumping power cost. The flow velocity growth could significantly reduce the battery temperature initially, but beyond 0.05 m/s, the temperature became relatively stable. In addition, the foam porosity had an optimal value of 0.92 in the studied range while the battery temperature was reduced with the pore density. It is also suggested that PCM with a lower melting point should be adopted in the hybrid systems.

What follows is the study of the nanoscale boiling process using molecular dynamics simulations. It begins by the investigation of the argon boiling process above solid copper substrates with different surface wettability and surface random roughness. The contact angles of argon droplets on the copper solid substrate with different potential parameters were determined by the circle fitting function. The surface profiles with randomly distributed rough structures were generated using a multivariable Weierstrass-Mandelbrot function. With the application of the thermostat on the solid atoms, bubble nucleated at the solid-liquid interface after experiencing a period of evaporation process. Afterward, the bubble grew into a vapor film and pushed the bulk liquid cluster away from the solid wall. It was found that the rough structures advanced the boiling inception time and increased the evaporation mass flux. The existence of cavities was favorable for bubble nuclei occurrence. Besides, the heat transfer between the solid and fluid atoms became more efficient with the presence of surface roughness. The energy exchange rate was further improved when the surface roughness level was increased.

Afterward, the effects of nanostructure configurations on nanoscale water boiling heat transfer are investigated. The nanostructured substrates showed great superiority over the smooth surface. The bubble nucleation time was accelerated and the heat transfer efficiency was enhanced on structured surfaces. Besides, the heat transfer enhancement differred on surfaces with different nanostructures although with the same nanostructure volume. Through calculation, it was found that the solid-liquid interfacial area varied with the surface structure configuration. To prove the hypothesis that the heat transfer efficiency increased with the interfacial surface area, a solid-liquid-solid system was developed to measure the interfacial thermal resistance between the different solid surfaces and water molecules. The interaction energy per unit area was also calculated to explain the heat transfer variations. Results showed that the increment of surface area which absorbed more water molecules at the interface could increase the interaction energy per unit area, thereby decreasing the interfacial thermal resistance.

Finally, the combined effects of the surface wettability and nanopillar structure on the nanoscale water boiling process are studied. Results showed that the enhancement of surface wettability could shift the bubble nucleation to an earlier time and improve the heat transfer rate between the solid wall and water molecules. The bubble nucleation time was shifted from around 800 ps to 210 ps when the surface interaction strength factor α was increased from 0.4 to 0.8 for the smooth substrate. The addition of nanopillar structure on the substrate further accelerated the boiling reception and increased the energy transport heat flux. It was at approximately 380 ps when the bubble nucleated on the h-20_w-30 nanopillar surface with α = 0.4. Besides, it is shown that the increment of nanopillar height could reduce the required time for bubble nuclei to appear which was due to the improved heat transfer efficiency with the height. Meanwhile, the bubble nucleation time and the heat flux showed no obvious discrepancy between the cases of various nanopillar width. The effects of the nanopillar sizes can be explained by their corresponding interfacial surface areas.