Modelling Subcooled Nucleate Boiling using Multiphase CFD : A Development for Industrial Free-Hardening Applications

Sammanfattning

Quenching is a critical heat treatment process used in manufacturing industries to rapidly cool metallic components in order to achieve desired mechanical properties. The thermal behaviour during quenching is strongly influenced by boiling phenomena involving complex interactions between heat transfer, turbulence, phase change, and multiphase flow. Numerical modelling of such processes therefore remains highly challenging, particularly when attempting to simulate subcooled boiling conditions relevant to industrial quenching applications. The objective of this thesis was to investigate the feasibility of modelling boiling flows in COMSOL Multiphysics for future application toward quenching process simulations. The work initially originated from an industrial quench bath used in heat treatment operations. However, due to limited availability of essential input data, including vapour-phase properties of quench oil and fan performance curves, the work was redirected toward the development of a generic modelling framework using water as the working fluid and the ivf SmartQuench experimental setup as a reference case. A literature study revealed that the Eulerian wall boiling approach using the RPI heat flux partitioning model is one of the most widely used methods for boiling simulations. An attempt was therefore made to implement the Euler–Euler multiphase framework together with the extended RPI wall boiling model and the Lee evaporation-condensation model in COMSOL 6.4. Since COMSOL lacks built-in boiling functionality and does not provide capability for modelling heat transfer within its Euler–Euler implementation, several custom developments were required, including manual implementation of thermal wall functions, phase change source terms, and interfacial boiling models. However, the Euler–Euler implementation experienced significant convergence and numerical stability difficulties, limiting its practical applicability within the scope of this work. As a result, increasing focus was directed toward the Mixture Model formulation, which provided a more numerically stable and computationally manageable approach for handling boiling-related multiphase behaviour within COMSOL’s finite element framework. In parallel, a simplified boiling approach based on the Rohsenow correlation was also investigated in order to evaluate the possibility of approximating nucleate boiling behaviour using a single-phase formulation at lower computational cost. Thus, the work demonstrates both the possibilities and limitations of implementing boiling flow models in COMSOL Multiphysics. Overall, the thesis establishes an initial modelling framework for boiling and quenching simulations in COMSOL and provides insights into the suitability, limitations, and implementation challenges of different multiphase modelling approaches for industrial heat treatment applications.