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An Experimental Study on Micro-Hydrodynamics of Evaporating/Boiling Liquid Film
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety. (Nuclear power safety)
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Study of liquid film dynamics is of significant importance to the understanding and control of various industrial processes that involve spray cooling (condensation), heating (boiling), coating, cleaning and lubrication. For instance, the critical heat flux (CHF) of boiling heat transfer is one of the key parameters ensuring the efficiency and safety of nuclear power plants under both operational and accident conditions, which occurs as the liquid layers (microlayer and macrolayer) near the heater wall lose their integrity. However, an experimental quantification of thin liquid film dynamics is not straightforward, since the measurement at micro-scale is a challenge, and further complicated by the chaotic nature of boiling process.

The object of present study is to develop experimental methods for the diagnosis of liquid film dynamics, and to obtain data for the film instability under various conditions. A dedicated test facility was designed and constructed where micro conductive probes and confocal optical sensors were used to measure the thickness and dynamic characteristics of a thin liquid film on various heater surfaces, while a high speed camera was used to get visual observation. Extensive tests were performed to calibrate and verify the two thickness measuring systems. The micro conductive measuring system was proven to have a high reliability and repeatability with maximum system error less than 5µm, while the optical measuring system is capable of recording the film dynamics with spatial resolution of less than 1 mm. The simultaneous measurement on the same liquid film shows that the two techniques are in a good agreement with respect to accuracy, but the optical sensors have a much higher acquisition rate up to 30 kHz, which are more suitable for rapid process.

The confocal optical sensors were therefore employed to measure the dynamic thickness of liquid films (ethanol, hexane and water) evaporating on various horizontal heater surfaces (aluminum, copper, silicon, stainless steel and titanium) to investigate the influences of heat flux, the surface and liquid properties on the film instability and the critical thickness. The critical thickness of water film evaporating on various surfaces was measured in the range of 60-150 mm, increasing with the increased contact angle or increased heat flux (evaporating rate) and also with the decreased thermal conductivity of the heater material. The data suggest the conjugate heat transfer nature of the evaporating liquid film dynamics at higher heat fluxes of interest to boiling and burnout. In the case of hexane on the aged titanium surface with contact angle of ~3o, the liquid film is found resilient to rupture, with film oscillations at relatively large amplitude ensuing as the averaged film thickness decreases below 15 µm.

To interpret our experimental findings on liquid film evolution and its critical thickness at rupture, a theoretical analysis is also performed to analyze the dynamics of liquid films evaporating on heater surfaces. While the influences of liquid properties, heat flux, and thermal conductivity of heater surface are captured by the simulation of the lubrication theory, influence of the wettability is considered via a minimum free energy criterion. The thinning processes of the liquid films are generally captured by the simulation of the lubrication theory. For the case with ideally uniform heat flux over the heater surface, the instability of the liquid film occurs at the thickness level of tens micro meters, while for the case of non-uniform heating, the critical thicknesses for the film rupture are closer to  the experimental data but still underestimated by the lubrication theory simulation. By introducing the minimum free energy criterion to considering the influence of surface wettability, the obtained critical thicknesses have a good agreement with the experimental ones for both titanium and copper surfaces, with a maximum deviation less than ±10%. The simulations also explain why the critical thickness on a copper surface is thinner than that on a titanium surface. It is because the good thermal conductivity of copper surface leads to uniform temperature distribution on the heat surface, which is responsible for the resilience of the liquid film to rupture.

A silicon wafer with an artificial cavity fabricated by Micro Electronic Mechanical System (MEMS) technology was used as a heater to investigate the dynamics of a single bubble in both a thick and thin liquid layer under low heat flux (<60 kW/m2). The maximum departure diameter of an isolated bubble in a thick liquid film was measured to be 3.2 mm which is well predicted by the Fritz equation. However, in a thin liquid layer with its thickness less than the bubble departure diameter, the bubble was stuck on the heater surface with a dry spot beneath. A threshold thickness of the liquid film which enables the dry spot rewettable was obtained, and its value linearly increases with increasing heat flux.

In addition, another test section was designed to achieve a constant liquid film flow on a titanium nano-heater surface which helps to successfully carry boiling in the liquid film from low heat flux until CHF. Again, the confocal optical sensor was employed to measure the dynamics of the liquid film on the heater surface under varied heat flux conditions.  A statistical analysis of the measured thickness signals that emerge in a certain period indicates three distinct liquid film thickness ranges: 0~50 µm as microlayer, 50~500 µm as macrolayer, 500~2500 µm as bulk layer. With increasing heat flux, the bulk layer disappears, and then the macrolayer gradually decreases to ~105 µm, beyond which instability of the liquid film may lose its integrity and CHF occurs. In addition, the high-speed camera was applied to directly visualize and record the bubbles dynamics and liquid film evolution. Dry spots were observed under some bubbles occasionally from 313 kW/m2 until CHF with the maximum occupation fraction within 5%.  A dry spot was rewetted either by liquid receding after the rupture of a bubble or by the liquid spreading from bubbles’ growth in the vicinity. This implies that the bubbles’ behavior (growth and rupture) and their interactions in particular are of paramount importance to the integrity of liquid film under nucleate boiling regime.   

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. , xv, 103 p.
Series
Trita-FYS, ISSN 0280-316X ; 2011:52
Keyword [en]
liquid film, critical thickness, boiling, critical heat flux, rupture, dry spot, confocal optical sensor
National Category
Engineering and Technology
Research subject
SRA - Energy
Identifiers
URN: urn:nbn:se:kth:diva-50216ISBN: 978-91-7501-165-3 (print)OAI: oai:DiVA.org:kth-50216DiVA: diva2:461325
Public defence
2011-12-16, Sal FR4, Roslagstullsbacken 21, AlbaNova, Stockholm, 09:30 (English)
Opponent
Supervisors
Projects
VR-2005-5729, MSWI
Funder
StandUp
Note
QC 20111205Available from: 2011-12-05 Created: 2011-12-02 Last updated: 2012-03-28Bibliographically approved
List of papers
1. Diagnostic techniques for the dynamics of a thin liquid film under forced flow and evaporating conditions
Open this publication in new window or tab >>Diagnostic techniques for the dynamics of a thin liquid film under forced flow and evaporating conditions
2010 (English)In: MICROFLUID NANOFLUID, ISSN 1613-4982, Vol. 9, no 6, 1077-1089 p.Article in journal (Refereed) Published
Abstract [en]

Motivated by quantification of micro-hydrodynamics of a thin liquid film which is present in industrial processes, such as spray cooling, heating (e.g., boiling with the macrolayer and the microlayer), coating, cleaning, and lubrication, we use micro-conductive probes and confocal optical sensors to measure the thickness and dynamic characteristics of a liquid film on a silicon wafer surface with or without heating. The simultaneous measurement on the same liquid film shows that the two techniques are in a good agreement with respect to accuracy, but the optical sensors have a much higher acquisition rate up to 30 kHz which is more suitable for rapid process. The optical sensors are therefore used to measure the instantaneous film thickness in an isothermal flow over a silicon wafer, obtaining the film thickness profile and the interfacial wave. The dynamic thickness of an evaporating film on a horizontal silicon wafer surface is also recorded by the optical sensor for the first time. The results indicate that the critical thickness initiating film instability on the silicon wafer is around 84 mu m at heat flux of similar to 56 kW/m(2). In general, the tests performed show that the confocal optical sensor is capable of measuring liquid film dynamics at various conditions, while the micro-conductive probe can be used to calibrate the optical sensor by simultaneous measurement of a film under quasi-steady state. The micro-experimental methods provide the solid platform for further investigation of the liquid film dynamics affected by physicochemical properties of the liquid and surfaces as well as thermal-hydraulic conditions.

Keyword
Thin liquid film, Film dynamics, Diagnostic technique, Confocal optical sensor, Micro-conductive probe
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-27358 (URN)10.1007/s10404-010-0626-z (DOI)000284335800006 ()2-s2.0-78649754159 (Scopus ID)
Funder
Swedish Research Council, VR-2005-5729
Note
QC 20101214Available from: 2010-12-14 Created: 2010-12-13 Last updated: 2011-12-26Bibliographically approved
2. An experimental study of rupture dynamics of evaporating liquid films on different heater surfaces
Open this publication in new window or tab >>An experimental study of rupture dynamics of evaporating liquid films on different heater surfaces
2011 (English)In: International Journal of Heat and Mass Transfer, ISSN 0017-9310, Vol. 54, no 7-8, 1538-1547 p.Article in journal (Refereed) Published
Abstract [en]

Experimental data were obtained to reveal the complex dynamics of thin liquid films evaporating on heated horizontal surfaces, including formation and expansion of dry spots that occur after the liquid films decreased below critical thicknesses. The critical thickness of water film evaporating on various material surfaces is measured in the range of 60-150 mu m, increasing with contact angle and heat flux while decreasing with thermal conductivity of the heater material. In the case of hexane evaporating on a titanium surface, the liquid film is found resilient to rupture, but starts oscillating as the averaged film thickness decreases below 15 mu m.

Keyword
Liquid film, Film evaporation, Film dynamics, Film rupture, Film thickness measurement
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-31339 (URN)10.1016/j.ijheatmasstransfer.2010.11.036 (DOI)000287279600025 ()2-s2.0-78751643541 (Scopus ID)
Funder
Swedish Research Council, VR-2005-5729
Note
QC 20110317Available from: 2011-03-17 Created: 2011-03-14 Last updated: 2011-12-05Bibliographically approved
3. An experimental study on boiling phenomena in a liquid layer
Open this publication in new window or tab >>An experimental study on boiling phenomena in a liquid layer
(English)In: International journal of thermal sciences, ISSN 1290-0729, E-ISSN 1778-4166Article in journal (Other academic) Submitted
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-50404 (URN)
Note
QS 2011 QS 20120326Available from: 2011-12-05 Created: 2011-12-05 Last updated: 2017-12-08Bibliographically approved

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