Thesis--Amsterdam. / In Periodical Room.

Thesis (doctoral)--University of Lund. / Description based on print version record.

The subject of this thesis is the development of an accurate, general and robust numerical method capable of predicting the flow behavior of two-phase immiscible fluids, separated by a well defined interface. In the quest of an accurate and robust numerical method for the modeling of two-phase flows, one has to keep in mind the intrinsic properties and difficulties associated with the problem: (i) those flows are mostly three-dimensional, (ii) some flows are steady, others unsteady, (iii) the interface might encounter a lot of topology changes (like merger or break-up), (iv) large jumps of density and viscosity might exist across the interface (e.g. ratio of density of 1/1000 for water and air), (v) surface tension forces may play a very important role in the interface dynamics. Hence, the influence of this force should be accurately evaluated and incorporated into the model, (vi) mass conservation is of primary importance. All these issues are addressed in this thesis, and special techniques are proposed for their treatment, which enables to construct the desired computational method. The chosen computational method combines a pressure stabilized finite element method for the Navier Stokes equations with a discontinuous Galerkin (DG) method for the level set equation. Such a combination of those two numerical methods results in a simple and effective algorithm that allows to simulate diverse flow regimes presenting large density and viscosity ratios (ratio up to 1/1000).

In the present study a parallel three dimensional Volume of Fluid (VOF) method is developed to simulate Marangoni force in immiscible fluids with variable surface tension. Conservation equations are solved based on cell-averaged one-field volume tracking scheme. Evaluating the convective term in the energy equation along the boundary between the fluids highly depends on the position and orientation of the interface; hence, using average cell values simply ignores the interface shape and leads to computational uncertainty. As a remedy to this issue, the original idea behind the volume tracking method is used not only to advect mass and momentum but also energy across cells. To verify the proposed algorithm, results are compared against theoretically predicted thermocapillary migration velocity of a droplet at the limit of zero Marangoni number. However, at relatively high Marangoni numbers, thermal boundary layers are very thin and challenging to resolve. To demonstrate the capabilities of the heat transfer module, simulations of a Fluorinert droplet moving in silicon oil under applied temperature gradient in microgravity are compared against the available experimental results and the migration velocity of the droplet are reported.

In the present study a parallel three dimensional Volume of Fluid (VOF) method is developed to simulate Marangoni force in immiscible fluids with variable surface tension. Conservation equations are solved based on cell-averaged one-field volume tracking scheme. Evaluating the convective term in the energy equation along the boundary between the fluids highly depends on the position and orientation of the interface; hence, using average cell values simply ignores the interface shape and leads to computational uncertainty. As a remedy to this issue, the original idea behind the volume tracking method is used not only to advect mass and momentum but also energy across cells. To verify the proposed algorithm, results are compared against theoretically predicted thermocapillary migration velocity of a droplet at the limit of zero Marangoni number. However, at relatively high Marangoni numbers, thermal boundary layers are very thin and challenging to resolve. To demonstrate the capabilities of the heat transfer module, simulations of a Fluorinert droplet moving in silicon oil under applied temperature gradient in microgravity are compared against the available experimental results and the migration velocity of the droplet are reported.

This thesis assesses the extent to which jet precession can be used to control the mean and instantaneous particle distributions in particle-laden jet flows. Investigations were conducted, providing quantitative, planar measurements of instantaneous particle distributions in the first 10 nozzle diameters of a particle-laden co-annular nozzle with centrally located Precessing Jet (PJ). Equipment was specifically designed to conduct the investigations, a laser diagnostic technique developed and a methodology to quantify particle clusters was devised. The experimental facilities are scaled to simulate the near burner region of a typical rotary cement kiln. The laser diagnostic technique, called planar nephelometry, enables non-intrusive, quantitative, instantaneous, planar measurements of particle distributions without the need to identify individual particles. The methodology to quantify particle clusters is designed to enable statistical comparison of clusters without ambiguity. Measurements of the influence of particle mass loading and jet precession on the distribution of particles emerging from an particle-laden co-annular nozzle, with a centrally located PJ nozzle, are presented. These data include mean and standard deviation of the particle distributions and statistics on particle cluster characteristics. The results indicate that small amounts of momentum through the PJ nozzle causes an elongation of the jet, but larger amounts of momentum through the PJ nozzle will result in a wider mean particle distribution and greater mean centreline decay rate. An increase in jet precession also results in an increase in the fluctuations in the particle distributions. The transition is determined by the interplay of momentum of the particle-laden and precessing streams. The physical characteristics of identified particle clusters in the instantaneous planar flow field are also influenced by jet precession. An initial increase in the amount of jet precession results in an overall decrease in the average number of both small- and large-clusters. The size of small-clusters generally reduces with increasing jet precession, whereas large-clusters reach maximum sizes for an intermediate relative momentum of jet precession. Analogous to the influence of jet precession on the mean distribution of particles, increasing jet precession also results in a greater spread of small- and large-clusters. Results also indicate that increasing the mass flow rate of particles results in an elongation of the jet. However, these variations correspond to an increase in annular jet momentum, rather than an addition of secondary phase. The particle mass flow rate has a minor influence on the general characteristics of particle clusters. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1370427 / Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2009

<div>Incompressible single-phase and two-phase flows are widely encountered in and underlie many engineering applications. In this thesis, we aim to develop efficient methods and algorithms for numerical simulations of these classes of problems. Specically, we present two schemes: (1) a modied consistent splitting scheme for incompressible single-phase flows with open/out flow boundaries; (2) a three-dimensional hybrid spectral element-Fourier spectral method for wall-bounded two-phase flows.</div><div><br></div><div><div>In the first part of this thesis, we present a modied consistent splitting type scheme together with a family of energy stable outflow boundary conditions for incompressible single-phase outflow simulations. The key distinction of this scheme lies</div><div>in the algorithmic reformulation of the viscous term, which enables the simulation of outflow problems on severely-truncated domains at moderate to high Reynolds numbers. In contrast, the standard consistent splitting scheme is observed to exhibit a numerical instability even at relatively low Reynolds numbers, and this numerical instability is in addition to the backflow instability commonly known to be associated with strong vortices or backflows at the outflow boundary. Extensive numerical experiments are presented for a range of Reynolds numbers to demonstrate the effectiveness and accuracy of the proposed algorithm for this class of flows.</div></div><div><br></div><div><div>In the second part of this thesis, we present a numerical algorithm within the phase-field framework for simulating three-dimensional (3D) incompressible two-phase flows in flow domains with one homogeneous direction. In this numerical method, we represent the flow variables using Fourier spectral expansions along the homogeneous direction and C0 spectral element expansions in the other directions. This is followed by using fast Fourier transforms so that the solution to the 3D problem is obtained by solving a set of decoupled equations about the Fourier modes for each flow variable. The computations for solving these decoupled equations are performed in parallel to effciently simulate the 3D two-phase</div><div>ows. Extensive numerical experiments are presented to demonstrate the performance and the capabilities of the scheme in simulating this class of flows.</div></div>

A distribuição do tamanho de bolhas é uma importante característica em sistemas bifásicos. A área interfacial de bolhas está fortemente relacionada às taxas de reações químicas e de transferência de massa em aplicações industriais como colunas de destilação e fermentação, sistemas biológicos, oxidação, hidrogenação, tratamento de água, e em processos naturais, como dinâmicas de aerossóis e transferência de gás oceano-atmosfera. Pontas de provas intrusivas são largamente usadas para determinar a distribuição do tamanho de bolhas em colunas de borbulhamento bifásicas. Embora o tamanho de bolhas não possa ser obtido diretamente pelo uso de uma simples ponta de prova porque estas medem somente comprimentos perfurados em bolhas, é possível relacionar distribuições de cordas à distribuição de tamanho de bolhas usando análises estatísticas. Este trabalho apresenta a implementação de um sistema capaz de medir distribuição de tamanho de bolhas em uma coluna de borbulhamento composta por água e ar através de uma ponta de prova condutiva. Imagens obtidas por uma câmera de vídeo CCD (Charge-Coupled Devices) monocromática são usadas para validar e calibrar o sistema. / The distribution of bubble sizes is a critical feature in two–phase systems. The interfacial area of bubbles is strongly related to chemical reaction and mass transfer rates in industrial applications such as distillation and fermentation columns, biological systems, oxidation, hydrogenation, waste water treatment and in natural processes such as aerosol dynamics and air-sea gas transfer. Intrusive probes are widely used to determine bubble size distribution in two-phase bubble columns. Although bubble size cannot be obtained by a simple probe because it measures only the pierced length of the bubbles, it is possible to relate chord distributions to bubble size distribution by the use of statistical analysis. This work proposes a system implementation which is able to determine bubble size distribution in a water-air bubble column using an intrusive conductance probe. Images obtained by a monochromatic video camera CCD (Charge-Coupled Devices) are used to validate and calibrate the system.

Aquesta tesi centra els seus esforços en l'àmbit de la microfluídica, un camp relativament recent dins de la Mecànica de Fluids, amb un futur prometedor i amb un ritme d'investigació intens en les seves diferents especialitzacions. En aquest sentit, la tesi presenta dos aportacions científiques principals. Primer, aporta una eina numèrica d'elaboració pròpia per realitzar simulacions de fluxos reactius en microcanals. Eina que s'aplica satisfactòriament a la identificació dels principals processos de transport involucrats en la oxidació parcial del metà per a produir gas de síntesi, i a l'estudi de l'efecte que tenen alguns paràmetres d'operació en aquest procés reactiu. Segon, estén el coneixement dels fluxos multifàsics en microunions en T, estudiant experimentalment fluxos de dues fases amb fluids principalment miscibles i en condicions supercrítiques, que son portats al seu equilibri vapor-líquid. Durant aquest estudi, a més, reporta un succés inesperat que presenta futurs reptes en l'aplicació d'aquest tipus de fluxos multifàsics. / The present thesis focuses on microfluidics, a relatively recent field of Fluid Mechanics with promising expectations and with an intense scientific interest on its different areas. In this regard, the thesis aims to provide two main scientific contributions. First, it presents an in-house numerical tool to carry out simulations of reactive flows within microchannels. The tool is successfully applied to the identification of the main transport phenomena involved on the partial oxidation of methane to produce synthesis gas, and to the analysis of the effect of several operating parameters on this reactive process. Second, it extends the knowledge on multiphase flows in microfluidic T-junctions with an experimental study of two-phase flows of mixtures of potentially miscible fluids, in supercritical conditions and in vapour-liquid equilibrium. In this study it is also reported an unexpected phenomenon, which brings new challenges to the application of these kind of multiphase flows.

The normal injection of a liquid jet into a gaseous crossflow has many engineering applications. In this thesis, detailed numerical simulations based on the level set method are employed to understand the physical mechanism underlying the jet ``surface breakup''. The numerical observations reveal the existence of hydrodynamic instabilities on the jet periphery. The temporal growth of such azimuthal instabilities leads to the formation of interface corrugations, which are eventually sheared off of the jet surface as sheet-like structures. The sheets finally undergo disintegration into ligaments and drops during the surface breakup process. Temporal linear stability analyses are employed to understand the nature of these instabilities. To facilitate the analysis, analytical solutions for the flow fields of the jet and the crossflow are derived. We identify the ``shear instability'' as the primary destabilization mechanism in the flow. This inherently inviscid mechanism opposes the previously suggested mechanism of surface breakup (known as ``boundary layer stripping''), which is based on a viscous interpretation. The influence of the jet-to-crossflow density ratio on the flow stability are also studied. The findings show that a higher density jet leads to higher wavenumber instabilities on the jet surface and thereby subsequent smaller drops and ligaments. The stability characteristics of the most amplified modes (i.e., the wavenumber and corresponding growth rate) obtained from stability analyses and numerical simulations are in good agreement. The stability results of the jet also show that the density may have a non-monotonic stabilizing/destabilizing effect on the flow stability. To investigate such effect, the concept of wave resonance are employed to physically interpret the inviscid instability mechanism in two-phase flows with sharp interfaces and linear velocity profiles. We demonstrate that neutrally stable waves are formed due to the density jump in the flow, in addition to the well-known vorticity (Rayleigh) waves. Under certain conditions, such neutral waves are capable of resonating and generating unstable modes. The resonance of different pairs of neutral waves, therefore, results in either stabilizing or destabilizing effect of density variation. We predict similar reasoning behind the density behavior in the jet in crossflow configuration with smoothly varying velocity and density profiles.