Numerical modelling of bubbly flows in nanofluids with and without heat transfer
Creators: name: Yuan, Yang
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401210
subject: Microfluidics and nanofluidics
classificationCode: 401706
subject: Numerical modelling and mechanical characterisation
classificationCode: 510703
subject: Particle physics

description: Nanofluids are engineered colloidal dispersions of nano-scale particles (nanoparticles hereafter) in water, or other base liquids. This thesis focuses on the bubbly flows in nanofluids with and without heat transfer. For the former, the nucleate boiling of dilute nanofluids (≤0.01 vol%) in cylindrical containers are investigated numerically. For the latter, the two-phase flows of dilute nanofluids in vertical tubes are numerically studied.<br><br>Dilute nanofluids exhibits largely improved heat transfer performances during both pool and flow boiling, whilst being compared with corresponding pure liquid, and these properties make nanofluid suitable as a heat transfer medium in a stream of equipment dealing with extremely high heat flux and needing high cooling efficiency. Despite the many advantages, the use of nanofluid in industry is still limited. Two major research gaps remain between the preliminary studies and industry applications. One is the difficulty to accurately describe the boiling heat transfer and efficiently predict the relevant heat transfer coefficient (HTC). Because of the inherent complexity, this requires an in-depth understanding of the heated surface characteristics and bubble hydrodynamics in the near-wall region, for both pool and flow boiling. Beyond that, for flow boiling of nanofluids, the heat transfer is closely related to the two-phase flow structures, which needs particular attention. However, to the best of the author’s knowledge, relevant numerical and mechanistic studies are still absent in the open literature. The lack of studies in two-phase flow structures and dynamics is another gap which makes the prospect of nanofluid’s application in industry much gloomier. Recently, with rapid development of computer technology and computational algorithm, Computational Fluid Dynamics (CFD) provides a powerful numerical approach to conduct simulation on gas-nanofluid bubbly flows, and further explore the underlying mechanism behind.<br><br>The main body of this thesis is composed of four parts. In the first part (Chapter 2), a comprehensive literature review, including fundamentals of pool and flow boiling, experimental studies of dilute nanofluids and preliminary numerical modelling of two-phase gas-liquid bubbly flows, was performed to identify the research gaps between previous studies and numerical modelling of dilute nanofluids. In the second part (Chapter 3), a parametric study of the heat flux partitioning (HFP) model for nucleate boiling of nanofluids was conducted with the consideration of the effects of nanoparticle deposition on the heated surface characteristics and bubble behaviours in the near-wall region. Moreover, a new HFP model was proposed, in which a new heat flux component was incorporated to account for the heat transfer by the nanoparticle Brownian motion in microlayer. In the third part (Chapter 4), the flow structures and dynamics of two-phase flows of dilute nanofluids were investigated with the two-fluid model and MUtiple-SIze-Group (MUSIG) model, respectively. In order to identify the individual factors affecting the hydrodynamic behaviours, the heat transfer was not considered. The simulation results showed that both of the above two models need substantial improvement in order to achieve an effective modelling of nanofluids. In the fourth part (Chapter 5), mechanistic studies on the role that nanoparticles have played in affecting the bubble-liquid and bubble-bubble interactions were conducted to clarify the theoretical frame which could be used to develop predictive models for two-phase gas-liquid flows containing nanoparticles.<br><br>In summary, the effects of nanoparticles on boiling heat transfer and flow structures in gas-nanofluid bubbly flows were investigated with and without heat transfer, respectively, and the preliminary heat flux partitioning (HFP) model, two-fluid model as well as the MUSIG model were further developed accordingly. Numerical results were compared with experimental data, which validated the feasibility of new models in simulating nanofluids. <br>