Experiments on falling film evaporation of a water-ethylene glycol mixture on a surface with longitudinal grooves [Elektronische Ressource] / vorgelegt von Miriam Lozano Avilés

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Experiments on falling fllm evaporation of awater-ethylene glycol mixture on a surfacewith longitudinal groovesvorgelegt vonDiplom-IngenieurinMiriam Lozano Avilesaus MadridFakult˜at III-Prozesswissenschaftender Technischen Universitat Berlin˜Institut fur˜ EnergietechnikFachgebiet fur˜ Maschinen- und Energieanlagentechnikzur Erlangung des akademischen GradesDoktor der Ingenieurwissenschaften-Dr.-Ing.-Promotionsausschu…:Vorsitzender: Prof. Dr.-Ing. G. WoznyBerichter: Prof. F. ZieglerBerichter: Prof. Dr.-Ing. H. AuracherTag der wissenschaftlichen Aussprache: 12. Marz˜ 2007Berlin 2007D83 IIIAcknowledgementThis doctoral work was carried out at the Department for Heat, Momentumand Mass Transfer at the Institute for Energy Engineering at the BerlinUniversity of Technology. The project was flnancially supported by theDFG ("Deutsche Forschungsgemeinschaft", German Research Foundation)intheframeoftheGraduiertenkolleg827Transport Phenomena with MovingBoundaries. Supplementaryresourceswerealsoprovidedbythe"Gesellschaftvon Freunden der TU Berlin e.V." (Membership Corporation of Friends ofthe Berlin University of Technology) and the Company Alfa Laval AB, Swe-den. I want to thank all of them for their flnancial contributions.I am very grateful to Prof. Auracher for the technical and scientiflc steeringof the research project. I want to thank also Prof. Ziegler for acceptingthe supervision of the project with particular interest.

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• Publié par: technische_universitat_chemnitz
• Publié le: 01 janvier 2007
• Nombre de lectures: 32
• Langue: English
• Poids de l'ouvrage: 18 Mo

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Experiments on falling film evaporation of a waterethylene glycol mixture on a surface with longitudinal grooves

vorgelegt von Diplom-Ingenieurin Miriam Lozano Aviles aus Madrid

Fakultät III-Prozesswissenschaften der Technischen Universität Berlin Institut für Energietechnik Fachgebiet für Maschinen- und Energieanlagentechnik zur Erlangung des akademischen Grades

Doktor der Ingenieurwissenschaften -Dr.-Ing.-

Promotionsausschuß: Vorsitzender: Prof. Dr.-Ing. G. Wozny Berichter: Prof. Dr.-Ing. F. Ziegler Berichter: Prof. Dr.-Ing. H. Auracher

Tag der wissenschaftlichen Aussprache: 12. März 2007

Berlin 2007 D83

Acknowledgement

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This doctoral work was carried out at the Department for Heat, Momentum and Mass Transfer at the Institute for Energy Engineering at the Berlin University of Technology. The project was financially supported by the DFG (”Deutsche Forschungsgemeinschaft”, German Research Foundation) in the frame of the Graduiertenkolleg 827Transport Phenomena with Moving Boundariesresources were also provided by the ”Gesellschaft. Supplementary von Freunden der TU Berlin e.V.” (Membership Corporation of Friends of the Berlin University of Technology) and the Company Alfa Laval AB, Swe- den. I want to thank all of them for their financial contributions.

I am very grateful to Prof. Auracher for the technical and scientific steering of the research project. I want to thank also Prof. Ziegler for accepting the supervision of the project with particular interest. Thanks also to Prof. Slavtchev and Dr. Zaitsev for their support as falling film experts and the meaningful discussions.

For the very helpful advice and suggestions (and for the very nice moments at the department) I want to thank my colleges Laura Bogdanic, Thorsten Klahm, Martin Buchholz, Heike Heidrich, Olaf Koeppen, Bernhard Wilms and Miroslav Adamov. Special thanks go to Alexander Maun, who involved me in the falling film evaporation and the HF-probes. I also thank my stu- dents for their collaborations, especially Vera Iversen for her patience and outstanding enthusiasm.

The construction of a new test facility in such a short time would not had been able without the help of Walter Frydek, Achim Klein, Klaus Letsch and Manfred Strangalies. Thank you very much for your crucial labor and assistance at solving technical problems.

For the very fruitful team work at the courses of the Graduiertenkolleg I want to thank my colleges, especially Katarzyna Ciunel and Ilja Ausner for their cooperations.

I was very glad to work with changers at Alfa Laval AB in giving me the chance to get a

the team for thermal design of plate heat ex- Lund (Sweden). I thank Matz Andersson for deeper understanding about falling film evap-

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orators in the industry.

I am exceptionally grateful to my parents Jesus Lozano Cabra and Emerita Aviles Martinez, whose efforts enabled the fulfillment of my desire for further education in a foreign country. I thank my brother Alberto and my sister Beatriz for their unconditional support and love. For their help and under- standing I thank all my friends.

And last but not least, I thank my partner Anders Brandstedt for his never- ending patience and support during the difficult moments.

Contents

Zusammenfassung

Abstract

Nomenclature

1

2

3

4

5

V

VIII

Introduction 1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

IX

X

1 1 3

Stateoftheart 4 2.1 Methods to Improve the Heat Transfer . . . . . . . . . . . . . 4 2.2 Structured Surfaces . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2.1 Strategy in the industry . . . . . . . . . . . . . . . . . 5 2.2.2 Fundamental research . . . . . . . . . . . . . . . . . . 6 2.3 Conclusions for Further Studies . . . . . . . . . . . . . . . . . 19

Test Facilities

20

Heat Transfer Coefficient 26 4.1 Definition of the Heat Transfer Coefficient . . . . . . . . . . . 26 4.2 Empirical Correlations . . . . . . . . . . . . . . . . . . . . . . 28 4.3 Determination of the Heat Transfer Coefficient . . . . . . . . . 30

Film Thickness 35 5.1 Available Methods for Film Thickness Measurements . . . . . 35 5.2 High Frequency Needle Probes . . . . . . . . . . . . . . . . . . 40

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6

7

8

5.2.1 5.2.2 5.2.3 5.2.4

Contents

Measuring principle: electromagnetic fundamentals . . Calibration Procedure . . . . . . . . . . . . . . . . . . Conventional statistical data processing . . . . . . . . . Own statistical wave analysis . . . . . . . . . . . . . .

Results 6.1 Preliminary Measurements . . . . . . . . . . . . . . . . . 6.1.1 Summary of Maun’s results . . . . . . . . . . . . 6.1.2 Additional conclusions (new statistical analysis) . 6.2 Final Measurements . . . . . . . . . . . . . . . . . . . . 6.2.1 Heat transfer . . . . . . . . . . . . . . . . . . . . 6.2.2 Wave characteristics . . . . . . . . . . . . . . . . 6.2.3 Statistic data processing after Chu and Dukler . . 6.2.4 Comparison with data from the literature . . . . 6.2.5 New statistical data processing of single waves . . 6.3 Study of the Interaction between Fluid and Substrate . . 6.4 Thermocapillary Film Breakdown on Grooved Surfaces .

. . . . . . . . . . . . . . . . . . . . . .

40 42 44 47

51 . 51 . 51 . 54 . 73 . 73 . 79 . 85 . 90 . 99 . 111 . 117

Summary and Outlook 122 7.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 7.2 Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

Appendices 128 8.1 Correlations for Heat Transfer . . . . . . . . . . . . . . . . . . 128 8.2 Correlations for Transition . . . . . . . . . . . . . . . . . . . . 129 8.3 Flow Regimes for Falling Films . . . . . . . . . . . . . . . . . 130 8.4 Physical Properties . . . . . . . . . . . . . . . . . . . . . . . . 131 8.5 Experimental Uncertainty . . . . . . . . . . . . . . . . . . . . 139

8.6

Contents

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8.5.1 Uncertainty of the heat transfer coefficient . . . . . . . 139 8.5.2 Uncertainty of the Nusselt number . . . . . . . . . . . 142 8.5.3 Uncertainty of the Reynolds number . . . . . . . . . . 143 8.5.4 Uncertainty of the film thickness and wave velocity . . 144 Experimental Parameters and Results . . . . . . . . . . . . . . 145

Bibliography

156

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Zusammenfassung

Fallfilmapparate werden in der Industrie unter anderem zur Aufkonzentration von temperaturempfindlichen fluiden Gemischen eingesetzt. Sie eignen sich hierfür wegen der geringen erforderlichen Wandüberhitzungen. Eine Methode zur Verbesserung des Wärmeübergangs in diesen Apparaten ist der Einsatz von strukturierten Heizflächen, die nicht nur die Austauschfläche vergrössern. Es wird vermutet, dass strukturierte Heizflächen auch die Welligkeit des Fall- filmes beeinflussen und dabei den Wärmeübergang verbessern. Allerdings ist der Mechanismus der Verbesserung noch nicht gründlich verstanden.

In der vorliegenden Arbeit wird der Einfluss von senkrechten berippten und genuteten Heizflächen auf die Hydrodynamik und den Wärmeübergang von verdampfenden Wasser- und Wasser-Ethylenglykol Gemisch Fallfilmen unter- sucht. Der zeitliche Filmdickeverlauf und die Wellengeschwindigkeit werden mit zwei Hochfrequenz-Sonden bei einer Lauflänge von 800mm im Reynolds- bereich von 200 bis 1100 gemessen. Ein neues statistisches Auswertungsver- fahren basierend auf Wahrscheinlichkeitsverteilungen wird entwickelt. Die ergänzende Information der neuen Methode in Bezug auf die Ergebnisse der in der Literatur vorhandenen Methoden wird diskutiert.

Der Wärmeübergangwiderstand in Fallfilmen ist sehr gering aufgrund der niedrigen Filmdicken. Je dünner der Film, desto besser ist der Wärmeüber- gang. Ist der Film jedoch sehr dünn, neigt er zum Aufzureissen und die Leistung des Apparats sinkt schlagartig. Die Stabilität eines Fallfilmes hängt unter anderem von der Benetzbarkeit der Heizfläche ab und damit von Grenz- flächenspannungen. Auch thermokapillare Kräfte beeinflussen das Aufreis- sen des Filmes. In der vorliegenden Arbeit wird daher die Benetzbarkeit der Heizfläche mit Wasser und mit Wasser-Ethylenglykol untersucht. Ferner wird das Aufreissen von Filmen mittels der ”Thin Film Pressure Balance” unter- sucht sowie der Einfluss von Nuten auf das Filmaufreissen von unterkühlten Wasserfilmen aufgrund von thermokapillaren Kräften.

Die Messungen zeigen eine Steigerung des Wärmeübergangs von Fallfilmen auf strukturierten Heizflächen aufgrund deren Wirkung auf die Hydrody- namik des Filmes. Allerdings lässt sich der Verbesserungsmechanismus nicht verallgemeinern, weil er von den physikalischen Eigenschaften der Flüssigkeit abhängt. Darüber hinaus erhöhen längsgenutete Heizflächen die kritische Wärmestromdichte, bei der der Film aufreisst, und unterdrücken die Aus- breitung von trockenen Stellen in Querströmungsrichtung.

Abstract

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Falling film apparatus are used in the industry amongst others to concentrate temperature sensitive liquid mixtures because of their low required wallsu- perheat. A technique to enhance the heat transfer in this kind of apparatus is the use of structured heating surfaces, that not only enlarge the transfer area with respect to a smooth surface. Structured heating surfaces are also believed to modify the waviness of the falling film and thereby to improve the heat transfer. However, the enhance mechanism is not yet understood in-depth.

In the present work, the effect of vertical finned and grooved heating surfaces on the hydrodynamic characteristics and the heat transfer of evaporating water and water-ethylene glycol falling films is studied. Two high frequency probes are used to measure the time variation of the film thickness and the wave velocity at a flow length of 800mm in the Reynolds number range from 200 up to 1100. A new statistical data processing method for the character- ization of wavy falling films based on probability distributions is developed. The additional achieved information with respect to the available methods in the literature is discussed.

The small thickness of falling films results in a low heat transfer resistance. Thus, the thinner the film, the better the heat transfer. However, if the film is very thin, it tends to break and consequently, the performance of the apparatus drops abruptly. The stability of the film is partly determined by the wettability properties of the heating plate and consequently by the inter- facial tensions. Also thermocapillary forces affect the breakdown of falling films. In the present work, the wettability of the test plate by water and a water-ethylene glycol mixture is studied. Furthermore, the breakdown of films by means of the Thin Film Pressure Balance Technique is observed and the influence of the grooves on the thermocapillary breakdown of subcooled water films is investigated.

The analysis of the measurements reveal an improvement of the heat transfer of falling films on structured surfaces due to their impact on the hydrody- namic characteristics of the film. However, the heat transfer enhancement mechanism can not be generalized because it depends strongly on the physi- cal properties of the liquid. Furthermore, longitudinal grooved surfaces have been found to increase the critical heat flux for breakdown and to prevent the spreading of dry patches in transverse direction to the flow.

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Nomenclature

Latin characters

A a b C c P d f g Δh E I k L L l ˙m ˙ M N P q˙ ˙ Q R s t T U w xe ye Z z

area thermal diffusivityλ/(ρc P ) width capacitance specific heat capacity diameter frequency gravity constant latent heat of evaporation current global heat transfer coefficient inductivity heater length length mass flux mass flow rate number of measurements probability heat flux heat flow rate resistance distance between the probes time temperature voltage velocity mass fraction in the liquid mass fraction in the vapour impedance height in AFM

2 m 2 m /s m F J/(kgK) m Hz 2 m /s J/kg A 2 W/(m K) H m m 2 kg/(m s) kg/s - − 2 W/m W Ω m s K V m/s kg/kg kg/kg Ω m

α Γ δ η λ ν ρ σ ϑ ξ

Greek characters

Nomenclature

heat transfer coefficient mass flow rate per unit wall width film thickness dynamic viscosity thermal conductivity kinematic viscosity density surface tension temperature mass fraction

Dimensionlessnumbers

ηw Ca= σ 4 gη Ka= 3 ρσ 3 ∗ ρσ 1 Ka= 4 = gη Ka dσ −q˙ dT Kp= 2/3 λρ(νg) 2 α ν 1/3 N u= ( )( ) λ g ν P r= a ˙ wδ M Re= = ν bη

Abbreviations

ACF AFM CAD CCF CFD HF LDA PHE PIV

2 W/(m K) kg/(ms) m kg/(ms) W/(mK) 2 m /s 3 kg/m N/m ◦ C kg/kg

Capillary number Kapitza number (Alhusseini) Kapitza number (Al-Sibai) dimensionless parameter in film breakdown Nusselt number Prandtl number Reynolds number for a falling film

auto-correlation function atomic force microscopy computer aided design cross-correlation function computational fluid dynamics high frequency laser doppler anemometry plate heat exchanger particle image velocimetry

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