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Heat transfer and evaporation during single drop impingement onto a superheated wall

Batzdorf, Stefan (2015)
Heat transfer and evaporation during single drop impingement onto a superheated wall.
Technische Universität Darmstadt
Ph.D. Thesis, Primary publication

Abstract

This thesis is aiming for the numerical simulation of the impingement process of a single droplet onto a wall which is superheated against the fluid's saturation temperature corresponding to the bulk pressure. The heat transfer during drop impingement is of particular importance in spray cooling which is a promising technology for the removal of high heat fluxes at a small temperature difference.

While the hydrodynamics of an impinging droplet have been studied extensively in the past, the heat transfer to the droplet during the impingement process in the non-isothermal case is not yet fully understood, in particular if evaporation comes into play. Moreover, many studies on pool boiling heat transfer have demonstrated that the evaporation at the 3-phase contact line, where the solid, liquid, and gas phase meet, might contribute significantly to the overall heat transfer. Hence, it can be expected that a proper knowledge of the physical processes at the contact line might be crucial for the understanding of the entire process. However, up to now no attempt has been made to model the heat transfer of an impinging droplet just above the boiling point taking into account the microscale thermodynamic effects at the contact line.

To shed light on the individual heat transfer processes involved in the overall process and to quantify their individual importance, a numerical simulation of the drop impingement is conducted within this thesis. Numerical simulations provide data on small length and time scales which cannot be resolved with available measurement techniques. The numerical model is based on the Volume of Fluid method to track the evolution of the droplet shape. Evaporation is accounted for at the surface of the droplet. Special attention is payed to the modeling of the evaporative heat transfer in the vicinity of the moving 3-phase contact line.

The developed numerical model is validated with the help of highly resolved experimental data on single drop impingement. A good agreement of the model predictions to the measurements is achieved. At the same time the detailed information provided by the simulation are employed to identify the dominant phenomena governing the heat transfer during the entire impingement process. Moreover, the model is utilized to quantify the impact of the governing influence parameters. Thereby it is made use of the great advantage of numerical simulations that any parameter can be controlled individually without any additional effort. Even though the focus of this thesis is on single droplets, also the interaction of individual droplets during their impingement is addressed briefly.

Item Type: Ph.D. Thesis
Erschienen: 2015
Creators: Batzdorf, Stefan
Type of entry: Primary publication
Title: Heat transfer and evaporation during single drop impingement onto a superheated wall
Language: English
Referees: Stephan, Prof. Dr. Peter ; Weigand, Prof. Dr. Bernhard ; Gambaryan-Roisman, Prof. Dr. Tatiana
Date: 2015
Place of Publication: Darmstadt
Refereed: 21 April 2015
URL / URN: http://tuprints.ulb.tu-darmstadt.de/4542
Abstract:

This thesis is aiming for the numerical simulation of the impingement process of a single droplet onto a wall which is superheated against the fluid's saturation temperature corresponding to the bulk pressure. The heat transfer during drop impingement is of particular importance in spray cooling which is a promising technology for the removal of high heat fluxes at a small temperature difference.

While the hydrodynamics of an impinging droplet have been studied extensively in the past, the heat transfer to the droplet during the impingement process in the non-isothermal case is not yet fully understood, in particular if evaporation comes into play. Moreover, many studies on pool boiling heat transfer have demonstrated that the evaporation at the 3-phase contact line, where the solid, liquid, and gas phase meet, might contribute significantly to the overall heat transfer. Hence, it can be expected that a proper knowledge of the physical processes at the contact line might be crucial for the understanding of the entire process. However, up to now no attempt has been made to model the heat transfer of an impinging droplet just above the boiling point taking into account the microscale thermodynamic effects at the contact line.

To shed light on the individual heat transfer processes involved in the overall process and to quantify their individual importance, a numerical simulation of the drop impingement is conducted within this thesis. Numerical simulations provide data on small length and time scales which cannot be resolved with available measurement techniques. The numerical model is based on the Volume of Fluid method to track the evolution of the droplet shape. Evaporation is accounted for at the surface of the droplet. Special attention is payed to the modeling of the evaporative heat transfer in the vicinity of the moving 3-phase contact line.

The developed numerical model is validated with the help of highly resolved experimental data on single drop impingement. A good agreement of the model predictions to the measurements is achieved. At the same time the detailed information provided by the simulation are employed to identify the dominant phenomena governing the heat transfer during the entire impingement process. Moreover, the model is utilized to quantify the impact of the governing influence parameters. Thereby it is made use of the great advantage of numerical simulations that any parameter can be controlled individually without any additional effort. Even though the focus of this thesis is on single droplets, also the interaction of individual droplets during their impingement is addressed briefly.

Alternative Abstract:
Alternative abstract Language

Die vorliegende Dissertation zielt auf die numerische Simulation des Tropfenaufpralls auf eine Wand ab, die gegenüber der Sättigungstemperatur des Fluids überhitzt ist. Der Wärmeübergang während des Tropfenaufpralls ist von besonderer Bedeutung im Rahmen der Sprühkühlung, die eine hervorragend geeingnete Technologie zur Übertragung großer Wärmemengen bei kleinen Temperaturdifferenzen darstellt.

Während die Hydrodynamik eines aufprallenden Tropfens in der Vergangenheit intensiv untersucht wurde, ist der Wärmeübergang während des Aufpralls im nicht isothermen Fall noch nicht vollständig verstanden, insbesondere wenn Verdampfungsprozesse relevant sind. Außerdem haben zahlreiche Forschungsarbeiten zu Siedevorgängen in der Vergangenheit gezeigt, dass die Verdampfung nahe der 3-Phasen Kontaktlinie, an der die flüssige, feste und gasförmige Phase aufeinander treffen, einen signifikanten Teil zum global Wärmeübergang beitragen kann. Daher ist zu erwarten, dass für ein tieferes Verständnis der globalen Vorgänge die genaue Kenntnis der physikalischen Vorgänge an der Kontaktlinie unumgänglich ist. Bislang ist jedoch noch kein Versuch unternommen worden, den Wärmeübergang beim Tropfenaufprall unter Berücksichtigung der Mikrothermodynamik an der Kontaktlinie zu modellieren.

Zur Erlangung eines tieferen Verständnisses der grundlegenden Wärmetransportmechanismen während des Tropfenaufpralls und um die Bedeutung der individuellen Mechanismen zu quantifizieren, wird im Rahmen dieser Dissertation eine numerische Simulation des Tropfenaufpralls durchgeführt. Numerische Simulationen bieten einen Einblick in Vorgänge auf Längen- und Zeitskalen, die jenseits der Auflösung etablierter Messtechniken liegen. Das numerische Modell basiert auf der Volume of Fluid Methode zur Verfolgung der Tropfenform. Die Verdampfung an der Phasengrenze wird im Rahmen des Modells berücksichtigt. Besondere Aufmerksamkeit wird der Modellierung des Wärmeübergangs durch Verdampfung im Bereich der bewegten 3-Phasen Kontaktlinie gewidmet.

Das entwickelte numerische Model wird anhand hochaufgelöster experimenteller Daten zum Einzeltropfenaufprall validiert. Die numerischen Vorhersagen geben die Messdaten dabei in guter Übereinstimmung wieder. Gleichzeitig geben die detaillierten Berechnungsergebnisse Aufschluss über die dominanten Wärmetransportvorgänge während des Aufpralls. Darüber hinaus wird das Modell eingesetzt, um den Einfluss der wichtigsten Einflussgrößen zu untersuchen. Dabei wird der große Vorteil numerischer Simulationen ausgenutzt, dass alle Prozessparameter unabhängig kontrollierbar sind. Auch wenn der Schwerpunkt der vorliegenden Arbeit auf der Untersuchung von Einzeltropfen liegt, wird auch die Wechselwirkung von zwei simultan aufprallenden Tropfen qualitativ bewertet.

German
Uncontrolled Keywords: Drop impact, 3-phase contact line, phase change, dynamic contact angle, VOF, numerical simulation
Alternative keywords:
Alternative keywordsLanguage
Topfenaufprall, Kontaktlinie, Phasenwechsel, Kontaktwinkel, VOF, Numerische SimulationGerman
URN: urn:nbn:de:tuda-tuprints-45424
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: 10 May 2015 19:55
Last Modified: 07 Aug 2019 13:55
PPN:
Referees: Stephan, Prof. Dr. Peter ; Weigand, Prof. Dr. Bernhard ; Gambaryan-Roisman, Prof. Dr. Tatiana
Refereed / Verteidigung / mdl. Prüfung: 21 April 2015
Alternative keywords:
Alternative keywordsLanguage
Topfenaufprall, Kontaktlinie, Phasenwechsel, Kontaktwinkel, VOF, Numerische SimulationGerman
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