Interface-Resolving Simulations of Gas-Liquid Two-Phase Flows in Solid Structures of Different Wettability

This PhD study is devoted to numerical investigations of two-phase flows on and through elementary and complex solid structures of varying wettability. The phase-field method is developed and implemented in OpenFOAM®. The numerical method/code is verified by a series of test cases of two-phase flows, and then applied to investigate: (1) droplet wetting on solid surfaces; (2) air bubble rising and interacting with cellular structures and (3) gas-liquid interfacial flows in foam structures

Numerical Simulation of Droplets with Dynamic Contact Angles

The numerical simulation of droplet impact is of interest for a vast variety of industrialprocesses, where practical experiments are costly and time-consuming. In these simulations, the dynamic contact angle is a key parameter, but the modeling of its behavior is poorly understood so far. One of the few models, which considers the overall physical context of the involved 'moving contact line problem' is Shikhmurzaev’s interface formation model. In addition to keeping the problem well-posed, all surface and bulk parameters, such as the contact angle, are determined as part of the solution rather than being prescribed functions of contact line speed. In this thesis, we couple an asymptotic version of the interface formation model with our three-dimensional incompressible two-phase Navier-Stokes solver NaSt3DGPF developed at the Institute for Numerical Simulation, Bonn University. With this sophisticated model, the droplet shapes, heights and diameters compare very well with those from a range of practical experiments

Lubricant transport towards tribocontact in capillary surface structures

To counter lubricant shortage at a frictional contact (starvation), lubrication liquids, e.g. oils, are actively transported from a distant location towards the undersupplied tribocontact. This is done via small channels or generally via structures cut into a flat surface. In this way one can use capillary force as a cheap and reliable driver of the lubricant flow. Numerical modeling and experiments show that this method can be considered a promising new option to enhance tribocontact operation

Development of a two-phase flow model for the investigation of collisions between heavy gasoil droplets and catalytic particles in Fluid Catalytic Cracking Reactors

The goal of this work is to study computationally the flow induced by the collision between a single gasoil droplet and a spherical catalytic particle under realistic Fluid Catalytic Cracking (FCC) conditions. FCC reactors are found in the fossil fuel refineries and are used to upgrade heavy fuel (gas oil) to lighter products (gasoline or LPG), which are industrially more important. Gasoil is injected in the reactor and atomizes; the produced droplets vaporize intensely and come in contact with the hot fluidized solid catalysts. The “cracking” reactions accommodated at the particle porous surface (ex. zeolite) result in the decomposition of gasoil to lighter products. The two-phase flow model developed solves the incompressible Navier-Stokes equations for mass and momentum, along with the energy conservation equation. The VOF methodology is used to track the liquid-gas interface, while a dynamic local grid refinement technique is adopted, so that high accuracy is achieved with a relative low computational cost. A local evaporation model coupled with the additional solution of the species transport equation is utilized to consider phase change. Cracking surface reactions are taken into account via a simplified 2-lump scheme. The model is successfully validated in fundamental droplet dynamics flow conditions, such as droplet acceleration, droplet impingement onto flat and solid surfaces under isothermal conditions and droplet evaporation. Insights into these phenomena provide important information that are missing from experimental measurements. The numerical novelties of the current work include the implementation of a new Wetting Force Model to simulate drop-solid interaction, as well as the proposition of a sharpening scheme for the volume fraction field, to suppress diffusion. Concerning FCC collisions, the numerical model is able to reproduce both the hydrodynamics (drop deformation, spreading, breakup), as well as the chemical products (gasoil converted to gasoline). It is found that droplets of similar size to the catalytic particles tend to be levitated more easily by hot catalysts, thus resulting in higher cracking reaction rates/cracking product yield, and limited possibility for liquid pore blocking. For larger sized droplets, solid-liquid contact increases. The main ambition of the current Thesis, which is to combine the droplet hydrodynamics with the chemical reactions acts as a novel step towards the understanding of such micro-scale physical phenomena that are difficult to capture/measure in experimental apparatus. This fundamental numerical tool can provide insight to the spray system strategy of an FCC reactor for a wide range of operating conditions

Modelling multiple-material flows on adaptive unstructured meshes.

The ability to distinguish between regions with different material properties is essential when numerically modelling many physical systems. Using a dual control volume mesh that avoids the problem of corner coupling, the HyperC face value scheme is extended to multiple dimensions and applied as a device for material advection on unstructured simplex meshes. The new scheme performs well at maintaining sharp interfaces between materials and is shown to produce small advection errors, comparable to those of standard material advection methods on structured meshes. To further minimise numerical diffusion of material interfaces a total variation bounded flux limiter, UltraC, is defined using a normalised variable diagram. Combining the material tracking scheme with dynamically adapting meshes, the use of a minimally dissipative bounded projection algorithm for interpolating fields from the old mesh to the new, optimised mesh is demonstrated that conserves the mass of each material. More generally, material conservation during the advection process is ensured through the coupling of the material tracking scheme to the momentum and mass equations. A new element pair for the discretisation of velocity and pressure is proposed that maintains the stability of the system while conserving the mass of materials. When modelling multiple materials the use of independent advection algorithms for each material can lead to the problem of non-physical material overlap. A novel coupled flux limiter is developed to overcome this problem. This automatically generalises to arbitrary numbers of materials. Using the fully coupled (and rigorously verified) multi-material model, several geophysically relevant simulations are presented examining the generation of waves by landslides. The model is validated by demonstrating close agreement between model predictions and experimental results of wave generation, propagation and run-up. The simulations also showcase the powerful capabilities of an unstructured, adaptive multi-material model in real world scenarios