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Phase Change Phenomena During Fluid Flow in Microchannels
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Phase change phenomena of a fluid flowing in a micro channel may be exploited to make the heat exchangers more compact and energy efficient. Compact heat exchangers offer several advantages such as light weight, low cost, energy efficiency, capability of removing high heat fluxes and charge reduction are a few to mention. Phase change phenomena in macro or conventional channels have been investigated since long but in case of micro channels, fewer studies of phase change have been conducted and underlying phenomena during two-phase flow in micro channels are not yet fully understood. It is clear from the literature that the two-phase flow models developed for conventional channels do not perform well when extrapolated to micro scale.

In the current thesis, the experimental flow boiling results for micro channels are reported. Experiments were conducted in circular, stainless steel and quartz tubes in both horizontal and vertical orientations. The internal diameters of steel tubes tested were 1.70 mm, 1.224 mm and the diameter of quartz tube tested was 0.781 mm. The quartz tube was coated with a thin, electrically conductive, transparent layer of Indium-Tin-Oxide (ITO) making simultaneous heating and visualization possible. Test tubes were heated electrically using DC power supply. Two refrigerants R134a and R245fa were used as working fluids during the tests. Experiments were conducted at a wide variety of operating conditions.

Flow visualization results obtained with quartz tube clearly showed the presence of confinement effects and consequently an early transition to annular flow for micro channels. Several flow pattern images were captured during flow boiling of R134a in quartz tube. Flow patterns recorded during the experiments were presented in the form of Reynolds number versus vapour quality and superficial liquid velocity versus superficial gas velocity plots. Experimental flow pattern maps so obtained were also compared with the other flow pattern maps available in the literature showing a poor agreement. Flow boiling heat transfer results for quartz and steel tubes indicate that the heat transfer coefficient increases with heat flux and system pressure but is independent on mass flux and vapour quality. Experimental flow boiling heat transfer coefficient results were compared with those obtained using different correlations from the literature. Heat transfer experiments with steel tubes were continued up to dryout condition and it was observed that dryout conditions always started close to the exit of the tube. The dryout heat flux increased with mass flux and decreased with exit vapour quality. The dryout data were compared with some well known CHF correlations available in the literature. Two-phase frictional pressure drop for the quartz tube was also obtained under different operating conditions. As expected, two-phase frictional pressure drop increased with mass flux and exit vapour quality.

Place, publisher, year, edition, pages
Stockholm: KTH , 2010. , xii, 141 p.
Series
Trita-REFR, ISSN 1102-0245
Keyword [en]
Microchannels, Boiling, Heat Transfer
National Category
Energy Engineering
Research subject
SRA - Energy
Identifiers
URN: urn:nbn:se:kth:diva-26796ISBN: 978-91-7415-829-8 (print)OAI: oai:DiVA.org:kth-26796DiVA: diva2:372785
Public defence
2010-12-17, M3, Brinellvägen 64, KTH, Stockholm, 20:35 (English)
Opponent
Supervisors
Funder
StandUp
Note
QC 20101206Available from: 2010-12-06 Created: 2010-11-28 Last updated: 2011-05-18Bibliographically approved
List of papers
1. A Visualization Study During Flow Boiling of R134a In A Horizontal Microchannel
Open this publication in new window or tab >>A Visualization Study During Flow Boiling of R134a In A Horizontal Microchannel
2010 (English)In: ASME 2010 8th International Conference on Nanochannels, Microchannels, and Minichannels Collocated with 3rd Joint US-European Fluids Engineering Summer Meeting, ICNMM2010, 2010, 85-94 p.Conference paper, Published paper (Refereed)
Abstract [en]

In this paper, the experimental flow boiling visualization results of a microchannel are presented and discussed. A series of visualization experiments have been conducted in a horizontal, circular, uniformly heated microchannel, to record the two-phase flow patterns evolved during the boiling process and to study the ebullition process. A high speed camera (REDLAKE HG50LE) with a maximum of 100000 fps together with tungsten lights was used to capture the images along the test section. Microchannel was made of circular fused silica tube having an internal diameter of 0.781 mm and a uniformly heated length of 191 mm. Outside of the test tube was coated with a thin, electrically conductive layer of Indium Tin Oxide (ITO) for direct heating of the test section. Refrigerant R134a was used as working fluid and experiments were performed at two different system pressures corresponding to saturation temperatures of 25 degrees C and 30 degrees C. Mass flux was varied from 100 kg/m(2)s to 400 kg/m(2)s and heat flux ranged from 5 kW/m(2) to 45 kW/m(2). Visualization results show that the bubble growth is restricted by the tube diameter which results in very short existence of isolated bubbly flow regime except essentially restricted to a very short length of test tube. Flow patterns observed along the length were: Isolated bubble, elongated bubble, slug flow, semi annular and annular flow. Rigorous boiling and increased coalescence rates were observed with increase in heat flux. Bubble frequency was observed to increase with both heat and mass flux. A comparison with our previous flow boiling visualization studies, carried out for a test tube of 1.33 mm internal diameter, shows that the number of active nucleation sites is less while the bubble frequency is higher for the current study. Mean bubble length and bubble velocity during elongated bubble flow pattern have also been calculated from the images obtained during the tests.

Keyword
microchannels, two-phase, flow patterns, heat transfer, visualization
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-27037 (URN)10.1115/FEDSM-ICNMM2010-30218 (DOI)000290556300011 ()2-s2.0-84855815291 (Scopus ID)978-079185450-1 (ISBN)
Conference
ASME 2010 8th International Conference on Nanochannels, Microchannels, and Minichannels, ICNMM2010 Collocated with 3rd Joint US-European Fluids Engineering Summer Meeting; Montreal, QC; Canada; 1 August 2010 through 5 August 2010
Note

QC 20101206

Available from: 2010-12-06 Created: 2010-12-06 Last updated: 2014-09-02Bibliographically approved
2. Flow Patterns and Flow Pattern Maps for Microchannels
Open this publication in new window or tab >>Flow Patterns and Flow Pattern Maps for Microchannels
2010 (English)In: 2010 3rd International Conference on Thermal Issues in Emerging Technologies, Theory and Applications - Proceedings, ThETA3 2010, 2010, 33-42 p.Conference paper, Published paper (Refereed)
Abstract [en]

Dense packaging of electronic components generates very high heat fluxes and therefore results in challenges for proper thermal management of such components. Microchannel based evaporators with phase changing liquids are regarded as a promising solution for such high heat flux cooling applications. Due to confinement of flow and differences in the relative importance of governing phenomena, the two-phase flow and heat transfer characteristics of microchannels have been shown to be different from those of conventional sized channels. The fact that microchannel is an attractive cooling option but at the same time there is a clear lack of understanding of related hydrodynamic and thermal transport phenomena which provides an impetus for microchannel research. This paper presents the flow patterns and flow pattern maps obtained for an experimental study of R134a during flow boiling in a horizontal microchannel. The microchannel was a fused silica tube, the outer surface of which was coated with thin, transparent and electrically conductive layer of Indium-Tin-Oxide (ITO). The microchannel was 781 m in internal diameter and 191 mm in heated length. Operating parameters during the experiments were: mass flux 100-400 kg/m2 s, heat flux 5-45 kW/m2, saturation temperature 25 and 30 °C. A High speed camera was used with a close up lens to capture the flow patterns evolved along the channel. Flow pattern maps are presented in terms of superficial gas and liquid velocity and in terms of Reynolds number and vapor quality plots. The results are compared with some flow pattern maps for conventional and micro scale channels available in literature.

Keyword
Bubble columns, Fused silica, Heat flux, Microchannels, Multiphase flow, Reynolds number, Thermonuclear reactions, Tin
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-27132 (URN)10.1109/THETA.2010.5766377 (DOI)2-s2.0-79957992041 (Scopus ID)978-161284266-0 (ISBN)
Conference
2010 3rd International Conference on Thermal Issues in Emerging Technologies, Theory and Applications, ThETA3 2010; Cairo; Egypt; 19 December 2010 through 22 December 2010
Note

QC 20101206

Available from: 2010-12-06 Created: 2010-12-06 Last updated: 2014-09-02Bibliographically approved
3. Flow Boiling Heat Transfer Characteristics of a Minichannel up to Dryout Condition
Open this publication in new window or tab >>Flow Boiling Heat Transfer Characteristics of a Minichannel up to Dryout Condition
2010 (English)In: MNHMT2009, VOL 2, New York: AMER SOC MECHANICAL ENGINEERS , 2010, 25-34 p.Conference paper, Published paper (Refereed)
Abstract [en]

In this paper the experimental flow boiling heat transfer results of a minichannel are presented. A series of experiments was conducted to measure the heat transfer coefficients in a minichannel made of stainless steel (AISI 316) having an internal diameter of 1.7mm and a uniformly heated length of 220mm. R134a was used as working fluid and experiments were performed at two different system pressures corresponding to saturation temperatures of 27 degrees C and 32 degrees C. Mass flux was varied from 50 kg/m(2) s to 600 kg/m(2) s and heat flux ranged from 2kW/m(2) to 156 kW/m(2). The test section was heated directly using a DC power supply. The direct heating of the channel ensured uniform heating and heating was continued until dry out was reached. The experimental results show that the heat transfer coefficient increases with imposed wall heat flux while mass flux and vapour quality have no considerable effect. Increasing the system pressure slightly enhances the heat transfer coefficient. The heat transfer coefficient is reduced as dryout is reached. It is observed that dryout phenomenon is accompanied with fluctuations and a larger standard deviation in outer wall temperatures.

Place, publisher, year, edition, pages
New York: AMER SOC MECHANICAL ENGINEERS, 2010
Keyword
Microchannels, Two-phase, Boiling, Heat Transfer, Dryout
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-27005 (URN)10.1115/MNHMT2009-18224 (DOI)000282724400004 ()2-s2.0-77954342182 (Scopus ID)978-0-7918-4390-1 (ISBN)
Conference
ASME Micro/Nanoscale Heat and Mass Transfer International Conference, Shanghai, PEOPLES R CHINA, DEC 18-21, 2009
Note
QC 20101206Available from: 2010-12-06 Created: 2010-12-02 Last updated: 2011-11-03Bibliographically approved
4. Flow Boiling Heat Transfer of Refrigerants R134a and R245fa in a Horizontal Microchannel
Open this publication in new window or tab >>Flow Boiling Heat Transfer of Refrigerants R134a and R245fa in a Horizontal Microchannel
2010 (English)In: Proceedings of 2nd European Conference on Microfluidics, 2010Conference paper, Published paper (Refereed)
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-27138 (URN)
Conference
2nd European Conference on Microfluidics, December 8-10, Toulouse, France
Note
QC 20101206Available from: 2010-12-06 Created: 2010-12-06 Last updated: 2011-11-03Bibliographically approved
5. Dryout Characteristics During Flow Boiling of R134a in Vertical Circular Minichannels
Open this publication in new window or tab >>Dryout Characteristics During Flow Boiling of R134a in Vertical Circular Minichannels
2011 (English)In: International Journal of Heat and Mass Transfer, ISSN 0017-9310, E-ISSN 1879-2189, Vol. 54, no 11-12, 2434-2445 p.Article in journal (Refereed) Published
Abstract [en]

In this paper, the experimental results of dryout during flow boiling in minichannels are reported and analysed. Experiments were carried out in vertical circular minichannels with internal diameters of 1.22 mm and 1.70 mm and a fixed heated length of 220 mm. R134a was used as working fluid. Mass flux was varied from 50 kg/m(2) s to 600 kg/m(2) s and experiments were performed at two different system pressures corresponding to saturation temperatures of 27 degrees C and 32 degrees C. Experimental results show that the dryout heat flux increases with mass flux and decreases with tube diameter while system pressure has no clear effect for the range of experimental conditions covered. Finally, the prediction capabilities of the well known critical heat flux (CHF) correlations are also tested.

Keyword
Microchannels, Critical heat flux, Dryout, Flow boiling, Heat transfer, R134a
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-27139 (URN)10.1016/j.ijheatmasstransfer.2011.02.018 (DOI)000289820100023 ()2-s2.0-79953058604 (Scopus ID)
Note
QC 20101206 Uppdaterad från accepted till published (20110518).Available from: 2010-12-06 Created: 2010-12-06 Last updated: 2017-12-11Bibliographically approved
6. Experimental Investigation of Two-phase Pressure Drop in a Microchannel
Open this publication in new window or tab >>Experimental Investigation of Two-phase Pressure Drop in a Microchannel
2011 (English)In: Heat Transfer Engineering, ISSN 0145-7632, E-ISSN 1521-0537, Vol. 32, no 13/14, 1126-1138 p.Article in journal (Refereed) Published
Abstract [en]

Experimental results of two-phase pressure drop in a horizontal circular microchannel are reported in this paper. A test tube was made of fused silica having an internal diameter of 781 mu m with a total length of 261 mm and a heated length of 191 mm. The outer surface of the test tube was coated with an electrically conductive thin layer of ITO (indium tin oxide) for direct heating of the test section. Refrigerants R134a and R245fa were used as the working fluids, and mass flux during the experiments was varied between 100 and 650 kg/m(2)-s. Experiments were performed at two different system pressures corresponding to saturation temperatures of 25 degrees C and 30 degrees C for R134a and at three different system pressures corresponding to saturation temperatures of 30 degrees C, 35 degrees C, and 40 degrees C for R245fa. Two-phase frictional pressure drop characteristics with variation of mass flux, vapor fraction, saturation temperature, and heat flux were explored in detail. Finally, the prediction capability of some well-known correlations available in the literature, some developed for macrochannels and others especially developed for microchannels, was assessed.

National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-27140 (URN)10.1080/01457632.2011.562463 (DOI)000299957500005 ()2-s2.0-79960485611 (Scopus ID)
Note
Updated from accepted to published. QC 20120412Available from: 2010-12-06 Created: 2010-12-06 Last updated: 2017-12-11Bibliographically approved

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