Phase-changing flows: numerical methods and fully resolved simulations
Time: Wed 2022-06-15 10.00
Location: Register in advance for this webinar, F3, Lindstedtsvägen 26 & 28
Subject area: Engineering Mechanics
Doctoral student: Nicolo Scapin , Teknisk mekanik, FLOW, Department of Engineering Mechanics
Opponent: Professor Matthias Ihme, Department of Mechanical Engineering, Stanford University
Supervisor: Professor Luca Brandt, Skolan för teknikvetenskap (SCI), FLOW, Department of Engineering Mechanics, KTH, Sweden; Associate Professor Christophe Duwig, Department of Chemical Engineering, KTH, Sweden; Associate Professor Outi Tammisola, FLOW, Department of Engineering Mechanics, KTH, Sweden
Flows with evaporation and boiling are abundant in different contexts, such as geophysics, the biomedical sectors, and industrial applications. Spray combustion, boiling bubble flows, oceanic sprays, formation and evolution of clouds, spreading of infectious diseases are all relevant examples where a deeper understanding of phase-changing flows is of great importance. Fully resolved simulations may assume a central role of investigation as they can overcome the current limitations of the experimental techniques and complement them.
In the first part of this work, we present novel methodologies to perform interface-resolved simulations of phase-changing flows addressing the following three challenges: i) handling abrupt variations of the velocity field across the interface, ii) accurately evaluating the heat and mass interfacial fluxes, iii) incorporate compressibility inevitably present in bounded domains. Both sharp and diffuse interface formulations are considered and the resulting two methods are designed for different classes of multiphase flows. First, we devise a weakly compressible algorithm to describe incompressible evaporating droplets surrounded by a compressible gas medium treated in the low-Mach limit. This approach combines a volume of fluid method and the pressure-splitting techniques of zero-Mach methods to ensure volume conservation of the liquid phase and conservation of the mass of the compressible phase. Next, we develop a fully compressible algorithm for compressible bubbles in boiling flows, where rapid expansions and nonuniformity of the thermodynamic pressure fields make the zero-Mach limit inadequate.
In the second part of the thesis, we discuss how these numerical tools can be utilized to study relevant configurations of evaporating flows. Two flow regimes are considered: i) dispersed droplets, and, ii) a horizontal gas-liquid interface. Droplets are first considered in homogeneous shear turbulence in a dilute condition. Here, we benchmark the semi-empirical correlations for the evaporation rate with the data extracted from DNS of finite-size droplets and study the effect of deformation on the global and local evaporation rate. Thereafter, we move then to a denser regime in a triperiodic domain and study the deviation from the d2-law as a function of initial gas temperature and liquid volume fractions. We confirm that even when evaporation is purely driven by diffusion, deviations from the d2-law cannot be characterized only by the initial volume fraction, but also temperature plays a role: high temperature promotes the departure from the d2-law regime at higher volume fractions, while at ambient temperature, this departure occurs at lower volume fraction. Next, we study the evaporation occurring at a gas-liquid interface in Rayleigh–Bénard convection. For this configuration, we develop an analytical prediction of the interface temperature and the global heat transfer modulation and interface-resolved simulations are employed to assess the validity of the models. The excellent agreement opens the possibility to employ the suggested law for those applications where accurate predictions of interface temperature and heat transfer are sought.