Submerged boiling and jet impingement for cooling high power electronics

• Main Author: Fan, Simiao
• Other Authors: Fei Duan
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Mechanical engineering::Energy conservation
• Online Access: https://hdl.handle.net/10356/148922
• Institution: Nanyang Technological University

The submerged boiling is expected to be a good solution for the ever-increasing cooling demands of electronic devices in the industries including automotive vehicles, aircraft and spacecraft avionics, power inverters, military equipment, mobile devices, supercomputers, and data centers. Although it has been investigated extensively, the development and application of the submerged boiling systems still face some challenging issues: (i) a comprehensive understanding of the heat transfer mechanism is lacked; (ii) results from similar experimental condition are often divergent or even contradictory; (iii) augmentation methods are usually necessary due to poor thermal properties of dielectric fluids; (iv) a universal prediction tool is absent. This thesis contributes to part of the pressing issues mentioned above to further understand the submerged boiling. To distinguish the large-scale component and the short-scale component in the three-dimensional (3D) surface analysis, arithmetic mean heights for the primary surface and the surface roughness component have been defined as 3D roughness parameters. The effects of the 3D roughness parameters on the pool boiling heat transfer are experimentally investigated on eight testing surfaces fabricated through three different preparation methods. Experimental results show that a rougher surface has higher heat transfer coefficient than a smoother one only within the same surface preparation method, if the surface roughness is characterized by the parameter derived from the primary surface. However, a different trend is observed when the parameter from the roughness component is applied to characterize the surface roughness. The boiling curves are found to shift monotonically to the left as the roughness parameter becomes larger, regardless of the surface preparation methods. A correlation is developed to well predict the heat transfer coefficient as a function of the surface roughness. Titania nanotube arrays are anodized on titanium plate in an electrochemical system and the potential application of this type of nanotube in two-phase submerged cooling system is explored. Experimental results show that the titania nanotubes are effective in improving both the cooling performance and the thermal safety margin of a two-phase submerged cooling system for titanium-based devices. However, its applications in devices based on other materials should be cautious unless the potential effect of the thermal interface resistance between the device surface and the nanotube coating can be eliminated. The nanotube structures are recognized to provide faster liquid replenishment hence modify the bubble dynamics and improve the dry spot rewetting capacity. A submerged jet impingement boiling system with a single circular nozzle is proposed to investigate the steady-state heat transfer characteristics at the stagnation point from single phase convection to partial nucleate boiling. The onset of nucleate boiling is highlighted. The impacts of various parameters including jet Reynolds number, liquid subcooling, and nozzle diameter on the heat transfer characteristics of the onset of nucleate boiling are discussed. Empirical correlations are proposed based on the experimental data to predict the local heat transfer coefficient, heat flux, and wall superheat at the onset of nucleate boiling. The future research directions are suggested for the submerged boiling heat transfer. Firstly, the departure characteristics of the single bubble are suggested to be investigated under different experimental conditions to explore its dependence on the control parameters hence help further understand the departure mechanism of nucleation bubbles. The second suggestion is to design the elaborate surface structures to separate the liquid and vapor flow pathways. In particular, the 3D printing method can provide more flexibility in designing the surface structures and it will be very interesting to apply the 3D printing in designing separated liquid-vapor flow pathways. Lastly, models for the nucleation site density, bubble departure diameter, and bubble departure frequency will be developed based on the quantitative analysis of the bubble dynamics study in this thesis. The developed models are expected to improve the accuracy of bubble dynamics models hence enhance the reliability of the Rensselaer-Polytechnic Institute (RPI) boiling model in numerical simulations.