DIRECT NUMERICAL SIMULATION OF BOILING AT SUPERHEAT LIMIT

Author

Arash Asadollahi, Southern Illinois University CarbondaleFollow

Date of Award

12-1-2021

Degree Name

Doctor of Philosophy

Department

Engineering Science

First Advisor

Esmaeeli, Asghar

Abstract

In this dissertation, we report the results of investigations on the stable and unstable growth of a vapor bubble in superheated liquids using Direct Numerical Simulations (DNS). A front tracking/finite difference scheme is used in conjunction with a Sharp Interface Method (SIM) to solve the governing equations of fluid flow and heat transfer. A spherical vapor seed whose radius is larger than the critical radius is introduced in a pool of superheated liquid and its evolution is followed. The goal of this research is threefold. The first is to identify the criteria that demark the stable growth from the unstable one on the physical parameter space. Secondly, this research seeks to quantify the dynamics of the unstable growth of vapor bubbles of representative fluid systems (water, diethyl ether, and cryogenic data). The third objective is to understand the relative importance and the interaction of various parameters that affect the instability.Regarding the first goal, it is shown that the Jakob number is the most important parameter that demarks stable growth from the unstable one. To this end, we represented a Figure of Merit (FOM) parameter to compare different fluids with different thermophysical properties. Jakob number in collaboration with FOM identify the criteria that would lead to instability. Concerning the second goal, the analyses of the interfacial instability and radius plots indicate an influence region at the onset of instability. It is found that once the radius plots (that are obtained from surface area and volume), separate from each other, can be recognized as a sign of instability in the bubble growth process. At this stage, there is a sudden increase in the growth rate of interfacial wrinkles which is much more than the bubble growth rate. In order to investigate the importance of various parameters on instability, we discussed several examples using analytical and numerical studies. For example, we investigated the impact of the interface temperature on bubble growth. It is shown that the difference between the interface temperature and saturation temperature becomes negligible over time, suggesting a null effect on the bubble growth. The effects of pressure difference and kinetic mobility (resistance to mass transfer across the interface) on interface temperature are investigated. It is indicated that the pressure difference has the highest impact on the interface temperature. Furthermore, the classical theories of interfacial instability of rapid evaporation are provided. The findings of the current research offer important insights such as discovering and understanding the underlying mechanisms in the boiling process at superheat limit.