Date of Award

2018

Document Type

Open Access Dissertation

Department

Mechanical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Chen Li

Abstract

Present dissertation has investigated pool and flow boiling and their characteristics via numerical means. A code was developed to investigate and enhance heat transfer performance during different modes of phase change phenomena. Multiphase heat transfer has proven to be one of the most effective means of heat transfer in different industries, therefore, there have been numerous experimental and numerical studies on the subject of phase change phenomena in a wide range of conditions and setups; yet there are complex bubble dynamics and heat transfer characteristics that remain unresolved. To have a more detailed look at and a better understanding of complex characteristics of phase change phenomena, our code focused on the mostly unresolved parts of this phenomena, such as spurious currents, interface diffusion, phase change modeling, micro-layer heat transfer, conjugate heat transfer effects and interfacial heat transfer coefficient.

These complexities arise mostly because of small scale of the phase change phenomena and pace of phase change heat transfer, these scaling issues makes it difficult or in some cases impossible to design a robust and comprehensive experiment, which can study different aspects of phase change heat transfer. On the numerical side, lack of exact solutions and equations to phase change can cause immense problems in modeling and numerical studies. To mention a few of these numerical difficulties one can mention bubble or droplet curvature estimation which does not have an exact mathematical solution that can translate to a viable algorithm, or interfacial temperatures which is said to be most important factor driving phase change.

To address some of these difficulties in numerical simulations we have employed volume of fluid method which benefits from global mass conservation combined with level set method which shows a more promising interface curvature estimation. Combining the two methods has proven to be a challenging task and, in some cases, not so much superior; therefore, a simplified method was employed to capture the best of the two methods. Phase change source terms was simulated based on none equilibrium conditions which states that phase change happens because of deviations of interface temperature from saturation temperature, unlike equilibrium condition which maintains the interface at saturation conditions. Using none equilibrium conditions forces a smaller grid onto simulation, which was cared by introduction of smearing factor. Other numerically challenging phenomena is micro-layer heat transfer which is mostly resolved by simplifying continuity, momentum and energy equations and deriving a set ODEs that are solved outside of main simulation algorithm. This is mostly due the fact micro-layer is mostly sub grid phenomena that cannot be seen by conventional CFD codes. We have employed a method that solves the micro-layer within the main algorithm without the need of solving those simplified set of ODEs and includes the none equilibrium interface conditions in micro-layer simulations.

Present dissertation contains a comprehensive literature review on numerical and experimental studies on the subject of two phase flow temperature driven phase change and heat and mass transfer. Which is followed by a detailed description of mathematical background and code algorithm. Then we have validated our code against numerous experimental studies available in the literature. Eventually, interfacial heat transfer coefficient during sub cooled pool and flow boiling was studied, which has never been studied numerically and there a few experimental studies on it.

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