Abstract
Given that phase transition involves the absorption or release of significant latent heat, boiling heat transfer plays an integral role in contemporary energy sectors, such as nuclear power plants, high-performance servers, and desalination plants. Therefore, improving the thermal efficiency of phase-change processes can address the pressing requirements of diverse emerging applications. However, with the development of micro-machining and the miniaturization of electronic devices, boiling heat transfer has now involved micro or even nanoscale behavior, which is difficult to observe with macroscale experimental measurement. On the other hand, newly developed techniques have helped broaden the application of many traditional technologies, boiling heat transfer being one of the technologies with a long history should not miss such opportunities. Hence, with the attempt to upgrade the application potential of boiling heat transfer, this thesis carries out Molecular Dynamics (MD) simulations to unravel the enhancing mechanisms behind the nanoscale boiling heat transfer. Moreover, an experiment is conducted to explore boiling heat transfer’s possibility of incorporating Triboelectric Nanogenerator (TENG).Firstly, nanoscale boiling on two types of plane surfaces is studied by MD, i.e. hydrophilic/hydrophobic patterned surfaces and wettability thermos-responsive surfaces. During the dynamic process of boiling, the hydrophilic/hydrophobic patterned surfaces create a lateral temperature and density gradient near the solid-liquid interface, which lowers the inception temperature, increases the evaporation rate, and enhances heat transfer. Meanwhile, the potential, thermal resistance, and the arrangement of the liquid near the wettability thermo-responsive surfaces are in a dynamic variation when the wettability changes with the surface temperature, which alters the difficulty level of heat transfer and leads to a better performance than the wettability uniform surfaces.
Then, the wicking and boiling performance of two porous structures, foam, and mesh, are compared via MD. During the wicking process, it is found that foam and mesh respond to wettability differently. When the wettability of solid atoms is set as hydrophilic, the mesh structure is the fastest to absorb liquid atoms compared to the foam structure. Yet it becomes non-wettable to liquid when the wettability switches to neutral or hydrophobic whereas the mesh structure is still able to be wetted by the liquid under neutral wettability. In terms of the boiling process, both porous structures perform better than the plain surface. More importantly, surfaces with foam structures can keep a higher heat transfer rate in a wider range because their random pore structures retain more liquid atoms than mesh’s uniform pore structures.
Furthermore, a nanoscale heat pipe with hydrophilic/hydrophobic pattern applied to its hot and cold end is studied by MD. Our observations indicate that the hydrophilic/hydrophobic pattern improves the hot and cold end in different ways, the pattern enhances the evaporation rate at the hot end because of the weak solid-liquid attraction force introduced by the hydrophobic area within the pattern. Meanwhile, the hydrophobic area boosts the mobility of condensed liquid atoms so that the condensation capacity of the cold end is strengthened. As a result, the liquid circulation from the hot end to the cold end is augmented leading to a better overall performance.
Lastly, an experiment is conducted to explore boiling heat transfer’s possibilities of incorporating TENG. To avoid the negative influence of high temperature and humidity, a set of magnet gear is adopted realizing remote force transmission so that the TENG can be placed independently from the boiling chamber. Then, the application demonstrations of driving low-power electronic devices and monitoring the heat transfer have proven the potential of boiling heat transfer.
| Date of Award | 12 Dec 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Jiyun ZHAO (Supervisor) |