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Film Dynamics and Deposit Formation in Evaporating Multi-Component Liquids

Bender, Achim (2020)
Film Dynamics and Deposit Formation in Evaporating Multi-Component Liquids.
Technische Universität Darmstadt
doi: 10.25534/tuprints-00011435
Ph.D. Thesis, Primary publication

Abstract

Deposit formation from evaporating liquid films and drops is an important phenomenon in many industrial applications. In internal combustion engines, deposits form from fuel films on ports, cylinder walls, and pistons. In the exhaust gas treatment, deposits form from the urea-water solution, which is injected into the exhaust pipe to reduce nitrogen oxide emissions. In both cases, the deposit formation has a negative influence on the process efficiency and increases the emission of pollutants.

The influence of physical parameters on the evaporation and deposition process is not understood. While some individual aspects of the evaporation and deposit formation process have been addressed numerically in the literature, the influence of some key phenomena remain unknown. Furthermore, a single multiscale model, taking all relevant physical processes and their interactions into account, is not available. Such a model would be important in order to gain a basic understanding of the process and to deduce strategies to avoid deposit formation in the future. This thesis is a first step in the development of such a model. Based on the analysis of previous investigations, it is concluded that the process can be separated into two stages.

In the first stage, a thin liquid film, which is influenced by evaporation, turbulent shear stress, and chemical reactions, is present on a structured wall. This liquid film ruptures at some time during the process and then continues to evaporate in the second stage in which the deposits form.

Long-wave theory is used to derive various models to investigate the evolution and stability of thin liquid films. These models consider a film on a heated or cooled structured wall evaporating into a pure vapor atmosphere or into an ambient gas, a liquid film with a time-dependent chemical reaction subject to a laminar shear flow, and a liquid film sheared by a turbulent shear stress from the gas flow. The resulting evolution equations are solved with a finite difference solver developed in this work. The linear stability of the solution is addressed and parametric studies are conducted. It is shown that the investigated physical phenomena have a big influence on the film stability and evolution and that there are strong interactions between the individual phenomena. This makes a full numerical simulation of the film development necessary.

Evaporation and deposit formation from sessile binary drops are investigated with an arbitrary Lagrangian-Eulerian method. The mesh is deformed to follow the shape of the evaporating drop and the deposit shape. The developed model is validated against a correlation and experimental data. The results show that ring shaped deposits occur in the vicinity of the three-phase contact line in the absence of Marangoni flow. A parametric study for urea-water drops is conducted. The temperature of the wall, initial composition of the drop, and drop size influence the evaporation of the drop, the time of first deposit formation, and the deposit growth rate, as well as the resulting deposit shape. The deposit shape changes from a ring-shaped to a cap-shaped pattern with increasing importance of thermocapillarity.

Item Type: Ph.D. Thesis
Erschienen: 2020
Creators: Bender, Achim
Type of entry: Primary publication
Title: Film Dynamics and Deposit Formation in Evaporating Multi-Component Liquids
Language: English
Referees: Gambaryan-Roisman, Prof. Dr. Tatiana ; Valluri, Dr. Prashant ; Stephan, Prof. Dr. Peter
Date: 2020
Place of Publication: Darmstadt
Refereed: 5 February 2020
DOI: 10.25534/tuprints-00011435
URL / URN: https://tuprints.ulb.tu-darmstadt.de/11435
Abstract:

Deposit formation from evaporating liquid films and drops is an important phenomenon in many industrial applications. In internal combustion engines, deposits form from fuel films on ports, cylinder walls, and pistons. In the exhaust gas treatment, deposits form from the urea-water solution, which is injected into the exhaust pipe to reduce nitrogen oxide emissions. In both cases, the deposit formation has a negative influence on the process efficiency and increases the emission of pollutants.

The influence of physical parameters on the evaporation and deposition process is not understood. While some individual aspects of the evaporation and deposit formation process have been addressed numerically in the literature, the influence of some key phenomena remain unknown. Furthermore, a single multiscale model, taking all relevant physical processes and their interactions into account, is not available. Such a model would be important in order to gain a basic understanding of the process and to deduce strategies to avoid deposit formation in the future. This thesis is a first step in the development of such a model. Based on the analysis of previous investigations, it is concluded that the process can be separated into two stages.

In the first stage, a thin liquid film, which is influenced by evaporation, turbulent shear stress, and chemical reactions, is present on a structured wall. This liquid film ruptures at some time during the process and then continues to evaporate in the second stage in which the deposits form.

Long-wave theory is used to derive various models to investigate the evolution and stability of thin liquid films. These models consider a film on a heated or cooled structured wall evaporating into a pure vapor atmosphere or into an ambient gas, a liquid film with a time-dependent chemical reaction subject to a laminar shear flow, and a liquid film sheared by a turbulent shear stress from the gas flow. The resulting evolution equations are solved with a finite difference solver developed in this work. The linear stability of the solution is addressed and parametric studies are conducted. It is shown that the investigated physical phenomena have a big influence on the film stability and evolution and that there are strong interactions between the individual phenomena. This makes a full numerical simulation of the film development necessary.

Evaporation and deposit formation from sessile binary drops are investigated with an arbitrary Lagrangian-Eulerian method. The mesh is deformed to follow the shape of the evaporating drop and the deposit shape. The developed model is validated against a correlation and experimental data. The results show that ring shaped deposits occur in the vicinity of the three-phase contact line in the absence of Marangoni flow. A parametric study for urea-water drops is conducted. The temperature of the wall, initial composition of the drop, and drop size influence the evaporation of the drop, the time of first deposit formation, and the deposit growth rate, as well as the resulting deposit shape. The deposit shape changes from a ring-shaped to a cap-shaped pattern with increasing importance of thermocapillarity.

Alternative Abstract:
Alternative abstract Language

Ablagerungsbildung von verdunstenden Flüssigkeitsfilmen und Tropfen ist ein wichtiges Phänomen in vielen industriellen Anwendungen. In Verbrennungsmotoren bilden sich Ablagerungen aus Kraftstofffilmen auf Ventilen, Zylinderwänden und Kolben. Bei der Abgasnachbehandlung bilden sich Ablagerungen von Harnstoff-Wasser-Lösungen, die in den Abgasstrang eingespritzt werden, um Stickoxidemissionen zu reduzieren. In beiden Fällen haben die Ablagerungen einen negativen Einfluss auf die Effizienz des Prozesses und sie erhöhen den Schadstoffausstoß.

Der Einfluss von physikalischen Parametern auf den Verdunstungs- und Ablagerungsprozess ist nicht ausreichend verstanden. Obwohl mittels numerischer Simulation einzelne Aspekte des Verdunstungsprozesses und der Ablagerungsbildung in der Literatur behandelt wurden, bleibt die Rolle einiger bedeutender Phänomene bisher unklar. Weiterhin gibt es bislang kein einzelnes Multiskalen-Modell, welches alle relevanten physikalischen Phänomene und deren Interaktionen berücksichtigt. Ein solches Modell wäre wichtig, um ein grundlegendes Verständnis für den Prozess zu entwickeln und daraus Strategien zur Vermeidung von Ablagerungen abzuleiten. Diese Arbeit ist ein erster Schritt in der Entwicklung eines solchen Modells. Aus der Analyse von früheren Untersuchungen kann abgeleitet werden, dass sich der Prozess in zwei Stufen aufteilen lässt.

In der ersten Stufe existiert ein dünner Flüssigkeitsfilm auf einer strukturierten Wand, der von Verdunstung, der turbulenten Scherströmung und chemischen Reaktionen beeinflusst wird. Dieser Flüssigkeitsfilm reißt im weiteren Prozessverlauf auf und tritt in die zweite Stufe ein, in der er weiter verdunstet und Ablagerungen bildet.

Die Long-Wave Theory wird verwendet, um mehrere Modelle herzuleiten, mit denen die Entwicklung und Stabilität von dünnen Flüssigkeitsfilmen untersucht wird. Diese Modelle berücksichtigen verdampfende oder verdunstende Filme auf beheizten oder gekühlten strukturierten Wänden, einen Flüssigkeitsfilm mit einer zeitlich veränderlichen chemischen Reaktion, geschert von einer laminaren Schubspannung, und einen Flüssigkeitsfilm, der von einer turbulenten Schubspannung geschert wird. Die resultierenden Evolutionsgleichungen werden von einem Finite-Differenzen-Löser gelöst, der in dieser Arbeit entwickelt wurde. Die lineare Stabilität der Lösung wird betrachtet und Parameterstudien werden durchgeführt. Es wird gezeigt, dass die untersuchten physikalischen Phänomene einen großen Einfluss auf die Stabilität und Entwicklung des Films haben und dass es starke Wechselwirkungen zwischen den einzelnen Phänomenen gibt. Dies macht eine volle numerische Simulation der Filmentwicklung notwendig.

Die Verdunstung und Ablagerungsbildung aus aufgesetzten binären Tropfen wird mithilfe einer Arbitrary Lagrangian-Eulerian Methode untersucht. Das Gitter wird verformt, um der Form des verdunstenden Tropfens und der Form der Ablagerung zu folgen. Das entwickelte Modell wird mit Hilfe von einer Korrelation und experimentellen Daten validiert. Die Ergebnisse zeigen, dass sich ringförmige Ablagerungen in der Nähe der Dreiphasenkontaktlinie bilden, wenn der Marangoni-Effekt vernachlässigt werden kann. Eine Parameterstudie für Harnstoff-Wasser Tropfen wird durchgeführt. Die Wandtemperatur, die initiale Zusammensetzung des Tropfens und die Tropfengröße beeinflussen die Tropfenverdunstung, die Zeit, zu der sich die ersten Ablagerungen bilden, die Wachstumsrate der Ablagerungen sowie deren resultierende Form. Die Form der Ablagerungen ändert sich von ringförmig zu kappenförmig, wenn Thermokapillarität berücksichtigt werden muss.

German
URN: urn:nbn:de:tuda-tuprints-114351
Classification DDC: 600 Technology, medicine, applied sciences > 620 Engineering and machine engineering
Divisions: 16 Department of Mechanical Engineering
16 Department of Mechanical Engineering > Institute for Technical Thermodynamics (TTD)
Date Deposited: 29 Mar 2020 19:55
Last Modified: 29 Mar 2020 19:55
PPN:
Referees: Gambaryan-Roisman, Prof. Dr. Tatiana ; Valluri, Dr. Prashant ; Stephan, Prof. Dr. Peter
Refereed / Verteidigung / mdl. Prüfung: 5 February 2020
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