Abstract
The ongoing trend of modern electronic devices towards the device miniaturization in combination with the increase in power dissipation per unit area has made necessary the development of new, more effective cooling methods that will be able to dissipate high heat fluxes on the order of MW m−2 , meeting the demand of such high-performance electronics. Flow boiling within conventional tubes finds application in various engineering fields, and is regarded as one of the most efficient cooling solutions. However, wherever the space is limited, such as in the cooling of electronics chips, conventional tubes cannot be used due to their big size. Therefore, alternative cooling methods have been studied in the past years. The proposed method should offer efficient cooling and at the same time allow room for more advancement in the years to come while maintaining proper functioning. Flow boiling heat transfer within microchannel heat sinks has been already recognised as one of the most efficient thermal management solutions for such high-power density electronic components. The main advantage of the micro/minichannels is that they have relatively large surface area to volume ratios which enable higher heat transfer rates than conventional tubes or conventional channels. Ever since this cooling method was introduced, flow boiling in microchannels has drawn worldwide attention. In more detail, a high number of investigations have been performed to understand the physical mechanisms and influential parameters, aiming to further enhance the heat transfer and flow conditions but also provide answers to fundamental issues concerning flow boiling within microchannels. However, as of now, most of these fundamental open aspects remain unanswered. Numerical simulations can play a significant role in solving some of these fundamental issues and provide useful information that may be difficult to extract from the experimental data. In order to be able to predict such complex phenomena, the development of accurate numerical models and sub-models is required. However, as important characteristics of flow boiling in such micro-scale channels have not been yet clarified, the applicability of the existing methods and correlations used today developed for conventional tubes, in minichannels and microchannels is still under discussion. In this project, various fundamental issues of flow boiling heat transfer are examined numerically (e.g. the effect of surface wettability, aspect ratio, etc. on the first transient stages of flow development), aiming to answer important open aspects in the current state-of-the-art. Additionally, a numerical method which is suitable for more global multiphase Computational Fluid Dynamic methodologies for boiling heat transfer such as the Eulerian-Eulerian two-fluid modelling, will be examined.Particularly, the present PhD thesis is divided into two main parts. In the first part, various controlling parameters of flow boiling within microchannels, that are previously reported in the literature as "unexplored open aspects", are studied qualitatively and quantitatively and their effect in the confined two-phase flow and heat transfer characteristics is identified and quantified. These parameters include: the surface wettability, the aspect ratio, the solid surface thermophysical properties and the hydraulic diameter of the microchannels. Each of these properties have been isolated, by performing parametric analysis and altering only the examined property. This parametric analysis also constitutes one of the main novelties of the present thesis.
In order to conduct this study, high-fidelity simulations were performed, utilising an enhanced customised Volume Of Fluid (VOF)-based solver that has been previously developed within the general framework of the OpenFOAM CFD toolbox. Particularly, the utilised solver enhancements involve an appropriate treatment for spurious velocities dampening (a well-known defect of VOF methods in general), an improved dynamic contact angle treatment, as well as the implementation of a phase-change model in the fluid domain. It is important to note that the utilised solver is also accounting for Conjugate Heat Transfer (CHT) with the solid domain, an important factor that is often not taken into account in flow boiling heat transfer studies.
The results indicate that surface wettability plays a significant role in the flow boiling regime and the associated dominant heat transfer mechanism, with hydrophilic surfaces achieving higher heat transfer rates compared to the hydrophobic surfaces, due to better performance of liquid film evaporation compared to contact line evaporation mechanism. Different channel aspect ratios also result in different flow boiling regimes and therefore in different heat transfer characteristics. It is also evident that as the hydraulic diameter of the microchannels becomes smaller, the difference of the time-averaged heat transfer coefficient of the two-phase flow stage of the simulations with the single-phase stage value progressively increases. Finally, the variation of the solid surface thermophysical properties substantially affects the resulting flow boiling regimes as well as the heat transfer characteristics and enhancement. A modified empirical correlation based on an existing correlation available in the literature is also proposed. This new correlation takes into consideration, for the first time in the literature, the thermal diffusivity of the solid material of the microchannel. It is shown that the improved correlation can better predict the direct numerical simulation VOF results in comparison to the original correlation that does not consider the solid material properties. It should be mentioned that one of the main novelties of the present thesis lies in the fact that the effect of the investigated controlling parameters is quantified on the resulting transient two-phase flow and heat transfer characteristics, focusing on the first transient stages, from the bubble nucleation and growth up to the two-phase flow development.
In the second part of the present PhD thesis, the capabilities of the Eulerian-Eulerian two-fluid model of OpenFOAM are examined initially against experiments of flow boiling within conventional tubes and subsequently against experimental data of flow boiling within microchannels. The purpose of conducting simulations using this numerical model is because it can successfully study complex phenomena such as flow boiling within conventional tubes, at lower computational cost compared to the VOF method and it is therefore applicable to device scale simulations. The results showed that the Eulerian-Eulerian two-fluid model of OpenFOAM is able to adequately predict the radial profiles of the vapour fraction and the liquid temperature, as well as the axial profiles of the heat transfer coefficient and wall temperature of a conventional tube, calibrating in each case the constitutive correlations through tuning of various empirical coefficients, by performing sensitivity analysis. However when the optimised/calibrated numerical simulation set-up was applied to numerically reproduce experiments of flow boiling within microchannels it has been found that the model is not able to predict the heat transfer coefficient trends for all of the examined cases successfully, especially for the case of low to moderate mass fluxes. This is attributed to the different underlying physical phenomena of flow boiling within microchannels compared to conventional tubes, and by the fact that the included sub-models use empirical correlations based on experiments conducted in conventional tubes.
Finally, as it is evident from the overall results of the present thesis, in the future it is necessary to utilise a combination of high fidelity VOF-based numerical simulation results and high-resolution spatiotemporally resolved experimental measurements to develop new closure relationships to be implemented in the Eulerian-Eulerian two-fluid simulation framework to render it capable to accurately predict flow boiling within microchannels.
| Date of Award | Sept 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Anastasios Georgoulas (Supervisor) & Marco Marengo (Supervisor) |