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Enhanced Boiling Heat Transfer on a Dendritic and Micro-Porous Copper Structure
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A novel surface structure comprising dendritically ordered nano-particles of copper was developed during the duration of this thesis research project. A high current density electrodeposition process, where hydrogen bubbles functioned as a dynamic mask for the materials deposition, was used as a basic fabrication method. A post processing annealing treatment was further developed to stabilize and enhance the mechanical stability of the structure.

The structure was studied quite extensively in various pool boiling experiments in refrigerants; R134a and FC-72. Different parameters were investigated, such as; thickness of the porous layer, presence of vapor escape channels, annealed or non-annealed structure. Some of the tests were filmed with a high speed camera, from which visual observation were made as well as quantitative bubble data extracted. The overall heat transfer coefficient in R134a was enhanced by about an order of magnitude compared to a plain reference surface and bubble image data suggests that both single- and two-phase heat transfer mechanisms were important to the enhancement.

A quantitative and semi-empirical boiling model was presented where the main two-phase heat transfer mechanism inside the porous structure was assumed to be; micro-layer evaporation formed by an oscillating vapor-liquid meniscus front with low resistance vapor transport through escape channels. Laminar liquid motion induced by the oscillating vapor front was suggested as the primary single-phase heat transfer mechanism.

The structure was applied to a standard plate heat exchanger evaporator with varying hydraulic diameter in the refrigerant channel. Again, a 10 times improved heat transfer coefficient in the refrigerant channel was recorded, resulting in an improvement of the overall heat transfer coefficient with over 100%. A superposition model was used to evaluate the results and it was found that for the enhanced boiling structure, variations of the hydraulic diameter caused a change in the nucleate boiling mechanism, which accounted for the largest effect on the heat transfer performance. For the standard heat exchanger, it was mostly the convective boiling mechanism that was affected by the change in hydraulic diameter.

The structure was also applied to the evaporator surface in a two-phase thermosyphon with R134a as working fluid. The nucleate boiling mechanism was found to be enhanced with about 4 times and high speed videos of the enhanced evaporator reveal an isolated bubble flow regime, similar to that of smooth channels with larger hydraulic diameters. The number and frequency of the produced bubbles were significantly higher for the enhanced surface compared to that of the plain evaporator. This enhanced turbulence and continuous boiling on the porous structure resulted in decreased oscillations in the thermosyphon for the entire range of heat fluxes.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , 75 p.
Series
Trita-REFR, ISSN 1102-0245 ; 11:02
Keyword [en]
enhanced boiling; R134a; FC-72; flow boiling; heat transfer; high speed visualization; instability; micro-channels; micro-structured; nano- and micro-technology; nano- and micro-porous structured surfaces; plate heat exchanger; pool boiling; porous media; thermosyphon; two-phase heat transfer
National Category
Energy Engineering
Research subject
SRA - Energy
Identifiers
URN: urn:nbn:se:kth:diva-47538ISBN: 978-91-7501-163-9 (print)OAI: oai:DiVA.org:kth-47538DiVA: diva2:455631
Public defence
2011-11-25, E1, Lindstedtsvägen 3, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
StandUp
Note
QC 20111111Available from: 2011-11-11 Created: 2011-11-10 Last updated: 2011-11-11Bibliographically approved
List of papers
1. Dendritically ordered nano-particles in a micro-porous structure for enhanced boiling
Open this publication in new window or tab >>Dendritically ordered nano-particles in a micro-porous structure for enhanced boiling
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2006 (English)In: Proceedings of 13th International Heat Transfer Conference, NAN-07, 2006, Vol. NAN-07Conference paper, Published paper (Refereed)
Abstract [en]

Presented research is an experimental study of the pool boiling performance of copper surfaces enhanced with a newly developed structure. The enhanced surfaces were fabricated with an electrodeposition method where metallic nano-particles are formed and dendritically connected into an ordered micro-porous structure. To further alter the grain size of the dendritic branches, some surfaces underwent an annealing treatment. The tests were conducted with the test objects horizontally oriented and submerged in a refrigerant: R134A, at saturated conditions and at an absolute pressure of 4 bar. The heat flux varied between 0.1 and 10 W/cm2. The boiling performance of the enhanced surfaces was found to be dependent on controllable surface characteristics such as thickness of the structure and the interconnectivity of the grains in the dendritic branches. Temperature differences less than 0.3 °C and 1.5 °C at heat fluxes of 1 and 10 W/cm2 respectively have been recorded, corresponding to heat transfer coefficients up to 7.6 Wcm-2K-1. The micro-porous structure has been shown to facilitate high performance boiling, which is attributed to its high porosity (∼94%), a dendritically formed and exceptionally large surface area, and to a high density of well suited vapor escape channels (50 – 470 per mm2).

National Category
Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-8691 (URN)1-56700-225-0 (ISBN)
Note
QC 20100924Available from: 2008-06-04 Created: 2008-06-04 Last updated: 2011-11-11Bibliographically approved
2. Nature-inspired boiling enhancement by novel nanostructured macroporous surfaces
Open this publication in new window or tab >>Nature-inspired boiling enhancement by novel nanostructured macroporous surfaces
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2008 (English)In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 18, no 15, 2215-2220 p.Article in journal (Refereed) Published
Abstract [en]

World energy crisis has triggered more attention to energy saving and energy conversion systems. Enhanced surfaces for boiling are among the applications of great interest since they can improve the energy efficiency of heat pumping equipment (i.e., air conditioners, heat pumps, refrigeration machines). Methods that are used to make the state-of-the-art enhanced Surfaces are often based on complicated mechanical machine tools, are quite material-consuming and give limited enhancement of the boiling heat transfer. Here, we present a new approach to fabricate enhanced surfaces by using a simple electrodeposition method with in-situ grown dynamic gas bubble templates. As a result, a well-ordered 3D macro-porous metallic surface layer with nanostructured porosity is obtained. Since the structure is built based on the dynamic bubbles, it is perfect for the bubble generation applications Such as nucleate boiling. At heat flux of 1W cm(-2), the heat transfer coefficient is enhanced over 17 times compared to a plain reference Surface. It's estimated that such ail effective boiling surface Would improve the energy efficiency of many heat Pumping machines with 10-30%. The extraordinary boiling performance is explained based on the structure characteristics.

Keyword
heat-transfer, saturated fc-72, silicon, walls
National Category
Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-17788 (URN)10.1002/adfm.200701405 (DOI)000258795500009 ()2-s2.0-50249158588 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
3. Boiling heat transfer on a dendritic and micro-porous surface in R134a and FC-72
Open this publication in new window or tab >>Boiling heat transfer on a dendritic and micro-porous surface in R134a and FC-72
2011 (English)In: Applied Thermal Engineering, ISSN 1359-4311, E-ISSN 1873-5606, Vol. 31, no 16, 3595-3603 p.Article in journal (Refereed) Published
Abstract [en]

A visualization study was conducted with the aim of deepening the understanding of the boiling mechanism in a dendritic and micro-porous copper structure for enhanced boiling heat transfer. The unique structure has earlier been shown to enhance heat transfer in pool boiling applications as well as in convective boiling in both small and large channels. Pool boiling tests were conducted in R134a and in the dielectric fluid FC-72 and were visualized with a high speed imaging system. Data on bubble size, bubble frequency density, heat transfer coefficient and the latent and sensible heat flux contributions were collected and calculated at heat flux varying between 2 and 15 W/cm(2). The enhanced surface produces smaller bubbles and sustains a high bubble frequency density in both fluids, even at low heat flux. An enhanced latent heat transfer mechanism of up to 10 times, compared to that of a plain reference surface, is the main reason for the improved boiling heat transfer performance on the enhanced surface. The data also suggests that the high nucleation bubble frequency density leads to increased bubble pumping action and thus enhancing single-phase convection of up to 6 times. The results in this study highlight the importance of both two and single-phase heat transfer within the porous structure.

Keyword
Enhanced boiling, Pool boiling: porous media, Boiling mechanism, R134a, FC-72
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-45581 (URN)10.1016/j.applthermaleng.2011.07.027 (DOI)000295115100056 ()2-s2.0-80052025181 (Scopus ID)
Funder
Formas
Note
QC 20111107Available from: 2011-11-07 Created: 2011-10-31 Last updated: 2017-12-08Bibliographically approved
4. The Use of a Nano- and Microporous Surface Layer to Enhance Boiling in a Plate Heat Exchanger
Open this publication in new window or tab >>The Use of a Nano- and Microporous Surface Layer to Enhance Boiling in a Plate Heat Exchanger
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2009 (English)In: Journal of heat transfer, ISSN 0022-1481, E-ISSN 1528-8943, Vol. 131, no 10Article in journal (Refereed) Published
Abstract [en]

Presented research is an experimental study of the performance of a standard plate heat exchanger evaporator, both with and without a novel nano- and microporous copper structure, used to enhance the boiling heat transfer mechanism in the refrigerant channel. Various distance frames in the refrigerant channel were also employed to study the influence of the refrigerant mass flux on two-phase flow heat transfer. The tests were conducted at heat fluxes ranging between 4.5 kW/m(2) and 17 kW/m(2) with 134a as refrigerant. Pool boiling tests of the enhancement structure, under similar conditions and at various surface inclination angles, were also performed for reasons of comparison. The plate heat exchanger with the enhancement structure displayed up to ten times enhanced heat transfer coefficient in the refrigerant channel, resulting in an improvement in the overall heat transfer coefficient with over 100%. This significant boiling enhancement is in agreement with previous pool boiling experiments and confirms that the enhancement structure may be used to enhance the performance of plate heat exchangers. A simple superposition model was used to evaluate the results, and it was found that, primarily, the convective boiling mechanism was affected by the distance frames in the standard heat exchanger. On the other hand, with the enhanced boiling structure, variations in hydraulic diameter in the refrigerant channel caused a significant change in the nucleate boiling mechanism, which accounted for the largest effect on the heat transfer performance.

Keyword
boiling, convection, evaporation, flow through porous media, heat, exchangers, microchannel flow, nanofluidics, nucleation, refrigerants, two-phase flow, small-diameter channels, refrigerant vaporization, flow
Identifiers
urn:nbn:se:kth:diva-18668 (URN)10.1115/1.3180702 (DOI)000268749900011 ()2-s2.0-77955289061 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
5. Experimental investigation of an evaporator enhanced with a micro-porous structure in a two-phase thermosyphon loop
Open this publication in new window or tab >>Experimental investigation of an evaporator enhanced with a micro-porous structure in a two-phase thermosyphon loop
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2009 (English)In: HT2008: PROCEEDINGS OF THE ASME SUMMER HEAT TRANSFER CONFERENCE - 2008, VOL 2, NEW YORK: AMER SOC MECHANICAL ENGINEERS , 2009, 327-334 p.Conference paper, Published paper (Refereed)
Abstract [en]

Following is an experimental study of six different evaporators in a closed two-phase thermosyphon loop system, where the influence of various evaporator dimensions and surfaces was investigated. The evaporators featured a 30 mm long rectangular channel with hydraulic diameters ranging from 1.2-2.7 mm. The heat transfer surface of one of the tested evaporators was enhanced with copper nano-particles, dendritically connected into an ordered micro-porous three dimensional network structure. To facilitate high speed video visualization of the two-phase flow in the evaporator channel, a transparent polycarbonate window was attached to the front of the evaporators. Refrigerant 134A was used as a working fluid and the tests were conducted at 6.5 bar. The tests showed that increasing channel diameters generally performed better. The three largest evaporator channels exhibited comparable performance, with a maximum heat transfer coefficient of about 2.2 W/(cm(2)K) at a heat flux of 30-35 W/cm(2) and a critical heat flux of around 50 W/cm(2). Isolated bubbles characterized the flow regime at peak performance for the large diameter channels, while confined bubbles and chaotic churn flow typified the evaporators with small diameters. In line with previous pool boiling experiments, the nucleate boiling mechanism was significantly enhanced, tip to 4 times, by the nano- and micro-porous enhancement structure.

Place, publisher, year, edition, pages
NEW YORK: AMER SOC MECHANICAL ENGINEERS, 2009
Keyword
Thermosyphon, electronics cooling, enhanced boiling, nano- and micro-structures, porous networks, two-phase heat transfer, micro-channels, R134A, high speed visualization
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-30867 (URN)10.1115/HT2008-56471 (DOI)000265637100037 ()2-s2.0-70349160840 (Scopus ID)978-0-7918-4848-7 (ISBN)
Conference
ASME Heat Transfer Summer Conference Jacksonville, FL, AUG 10-14, 2008
Note
QC 20110304Available from: 2011-03-04 Created: 2011-03-04 Last updated: 2012-03-21Bibliographically approved
6. Heat transfer, flow regime and instability of a nano- and micro-porous structure evaporator in a two-phase thermosyphon loop
Open this publication in new window or tab >>Heat transfer, flow regime and instability of a nano- and micro-porous structure evaporator in a two-phase thermosyphon loop
2010 (English)In: International journal of thermal sciences, ISSN 1290-0729, E-ISSN 1778-4166, Vol. 49, no 7, 1183-1192 p.Article in journal (Refereed) Published
Abstract [en]

Two-phase flow instabilities which may occur at low and high heat loads were studied for a thermosyphon loop with R134a as refrigerant. The heat transfer surface of the evaporator was enhanced with a copper nano- and micro-porous structure. The heat transfer of the enhanced evaporator was compared to a smooth surface evaporator. Finally, the influence of the liquid level and the inside diameter of the riser on the instability of the system have been investigated. It was found that the enhanced structure surface decreased the oscillations at the entire range of heat fluxes and enhanced the heat transfer coefficient. Three flow regimes were observed: Bubbly flow with nucleate boiling heat transfer mechanism, confined bubbly/churn flow with backflow and finally churn flow at high heat fluxes.

Keyword
Instability, Thermosyphon loop, Natural circulation, Heat transfer, Electronic cooling, Nano- and micro-porous structure surfaces
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-27545 (URN)10.1016/j.ijthermalsci.2010.01.016 (DOI)000278233600013 ()
Note
QC 20101216Available from: 2010-12-16 Created: 2010-12-13 Last updated: 2017-12-11Bibliographically approved

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