Nucleate boiling on passive and active flexible microstructured surfaces

• Main Author: Nguyen, Thien Binh
• Other Authors: Tran Anh Tuan
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Science::Physics::Heat and thermodynamics; DRNTU::Engineering::Mechanical engineering::Fluid mechanics
• Online Access: https://hdl.handle.net/10356/88657; http://hdl.handle.net/10220/45955
• Institution: Nanyang Technological University

Nucleate boiling is an efficient heat transfer process, which is crucial for numerous industrial applications such as cooling of electronic devices, spray cooling, mixing and cooling in nuclear power plant. Common methods to enhance the heat transfer rate in nucleate boiling, include modification of wettability and surface roughness using structured surfaces. While the physical mechanisms of boiling phenomena on smooth surfaces have been extensively investigated, studies of boiling processes on structured surfaces have been lacking, in particular on surfaces fabricated with passive structures such as micro-structures, nano-structures, and micro-nano hybrid structure, as well as on surfaces fabricated with active structures. The first part of this presentation focuses on enhancement of boiling heat transfer using surfaces fabricated with passive nanopillars. The pillar length and surface area enhancement ratio were varied and demonstrated to correlate with the heat transfer coefficient of the boiling process. Furthermore, a predictive model of critical heat flux and critical temperature for boiling on such surfaces is proposed which shows that the predicted results are consistent with the experimentally measured ones. The second part focuses on the dependence of convective boiling heat transfer on actuation of strip arrays fabricated on a surface. By regulating the frequency of actuation, the heat flux through the surface can be controlled dynamically. Also, a model was derived which related the enhancement of convective heat transfer with the extra mass transfer rate caused by the array actuation. During the derivation, a new coefficient was proposed which represented for the extra mass transfer process, which can be used to further develop the model with different properties of actuators such as shape, stiffness or surface roughness. The last part is an exploratory research on the nucleation, growth and detachment of individual vapour bubbles generated from a heated cavity with the present of a nearby actuated strip. It was proposed to fabricate on the heated surface an actuated strip and a micro-cavity with a precise position and size. The dynamics of the vapour bubble using high-speed imaging coupled with measurement of the local temperature on the heated surface using micro-sensors fabricated on the surface were studied. It was found that the actuation caused the bubble to oscillate around the cavity and to depart faster from the surface with a smaller departure diameter.