Abstract
Even though its application has a long history, boiling is still an extremely efficient way of heat transfer today. It has extensive applications in fields such as energy, chemical engineering, aerospace, and thermal management. Over the past century, the academic community has conducted extensive research on boiling, including its enhancement methods and physical mechanisms. However, due to its inherent chaotic nature, its mechanism is still not fully understood. Many boiling enhancement methods have been proposed, but many of these new methods are constrained by robustness and economy, making it difficult to be widely adopted in industry. Therefore, this thesis aims to develop more robust and cost-effective methods for boiling enhancement by novel aqueous solutions, and to explore the physical mechanism by utilizing advanced measurement tools and improved bubble modeling.Firstly, in order to enhance the boiling heat transfer coefficient, the feasibility of surfactants aqueous solutions for the boiling heat transfer coefficient enhancement was investigated. For saturated pool boiling, the HTC can increase by up to 130%. Experiments on pool boiling and flow boiling had both been carried out. Surfactants cause the boiling bubbles to exhibit unique dynamic characteristics, namely that bubbles are difficult to merge, forming bubble swarms, wet foams and periodic bubble clusters in the flow boiling. It was surprisingly found that when the surfactant concentration was very low, the change in the surface tension of the fluid was negligible, but the boiling performance changed significantly, which was inconsistent with the traditional mechanistic explanations. Therefore, a new theoretical explanation based on the non-coalescence of bubbles was proposed to account for this heat transfer coefficient enhancement. Furthermore, the critical heat flux was also affected by surfactants, and a mechanism explanation based on bubble dynamics had been proposed.
Then, in order to increase the critical heat flux, the method of enhancing the critical heat flux by using a dilute glycerin aqueous solution has been innovatively proposed and verified. In saturated pool boiling at atmosphere, the critical heat flux of 2% wt glycerin aqueous solution can enhance 42% compared with water. The critical heat flux enhancement of this glycerin aqueous solution does not conform to the traditional mixture boiling theory that considers the fluid properties changing playing the dominant effect on the boiling performance. The microlayer has been identified as the breakthrough point for the analysis, and qualitative and quantitative analyses of the behavior of microlayer were both conducted. The dilute glycerin solution concentrates during evaporation of the microlayer. The high glycerin concentration prevents the microlayer from being completely dry-out, and its high viscosity retarded the receding of contact line and delayed the expansion of dry spots. Based on these qualitative analyses, a microlayer evaporation and movement model and a critical heat flux model based on the percolation theory were established, thereby explaining this novel critical heat flux enhancement phenomenon.
Furthermore, a mechanistic model that can simulate the full bubble lifecycle from nucleation on the wall to collapse in the subcooled mainstream was proposed. Even though the process is quite complex, the number of fitting parameters used has been reduced to just one, namely the static contact angle. The evaporation of the microlayer and the near-wall superheated layer adopts mechanistic models that are more in line with physical principles, rather than the commonly used empirical correlations. Experimental data from different researchers were used for validation, proving that this model has a satisfactory ability to predict the evolution of bubbles during subcooled flow boiling. The separate effects of contact angle, mass flux, wall superheat degree and mainstream subcooling degree on the full bubble lifecycle evolution were also studied.
Finally, an efficient method for processing infrared temperature data of boiling experiments has been proposed to accelerate the boiling research. A series of pool nucleate boiling experiments using thin titanium foil heaters were carried out, and the temperature and heat flux distributions are analyzed. Because the high-speed infrared camera can obtain a large amount of raw temperature data, and through processing, it can obtain valuable bubble parameters that cannot be obtained through traditional thermometry. However, processing these data is very challenging. A self-adaptive method based on statistics was proposed to identify different heat transfer modes and nucleation sites which can be executed by computer automatically and rapidly. This method can identify different heat transfer modes (evaporation, convection and transient conduction), and then nucleation sites and bubble dynamics parameters. The experimental data and other open-access data from literature are used for validation, and relatively satisfactory performance is achieved.
The two methods proposed in this thesis for enhancing the boiling heat transfer coefficient and critical heat flux by configuring simple solutions possess both effectiveness, robustness and economic efficiency, and have application potential in the industry. For exploring the boiling mechanism, the proposed full bubble lifecycle model and the infrared temperature data processing method can also provide powerful tools for future boiling researchers. The primary conclusions, limitations and future research directions of this thesis have also been thoroughly discussed in the end.
| Date of Award | 22 Dec 2025 |
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| Original language | English |
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| Supervisor | Jiyun ZHAO (Supervisor) |