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Flow Boiling Heat Transfer Characteristics of a Minichannel up to Dryout Condition
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.ORCID iD: 0000-0002-9902-2087
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
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. 25-34 p.
Keyword [en]
Microchannels, Two-phase, Boiling, Heat Transfer, Dryout
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-27005DOI: 10.1115/MNHMT2009-18224ISI: 000282724400004Scopus ID: 2-s2.0-77954342182ISBN: 978-0-7918-4390-1 (print)OAI: oai:DiVA.org:kth-27005DiVA: diva2:374674
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
In thesis
1. Phase Change Phenomena During Fluid Flow in Microchannels
Open this publication in new window or tab >>Phase Change Phenomena During Fluid Flow in Microchannels
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
Microchannels, Boiling, Heat Transfer
National Category
Energy Engineering
Research subject
SRA - Energy
Identifiers
urn:nbn:se:kth:diva-26796 (URN)978-91-7415-829-8 (ISBN)
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

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