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Dryout Characteristics During Flow Boiling of R134a in Vertical Circular Minichannels
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
2011 (English)In: International Journal of Heat and Mass Transfer, ISSN 0017-9310, 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.

Place, publisher, year, edition, pages
2011. Vol. 54, no 11-12, 2434-2445 p.
Keyword [en]
Microchannels, Critical heat flux, Dryout, Flow boiling, Heat transfer, R134a
National Category
Energy Engineering
URN: urn:nbn:se:kth:diva-27139DOI: 10.1016/j.ijheatmasstransfer.2011.02.018ISI: 000289820100023ScopusID: 2-s2.0-79953058604OAI: diva2:374767
QC 20101206 Uppdaterad från accepted till published (20110518).Available from: 2010-12-06 Created: 2010-12-06 Last updated: 2011-05-18Bibliographically 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.
Trita-REFR, ISSN 1102-0245
Microchannels, Boiling, Heat Transfer
National Category
Energy Engineering
Research subject
SRA - Energy
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)
QC 20101206Available from: 2010-12-06 Created: 2010-11-28 Last updated: 2011-05-18Bibliographically approved

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