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Flow boiling visualization in a vertical circular minichannel at high vapor quality
KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
KTH, School of Industrial Engineering and Management (ITM), Energy Technology.ORCID iD: 0000-0002-9902-2087
KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
2006 (English)In: Experimental Thermal and Fluid Science, ISSN 0894-1777, E-ISSN 1879-2286, Vol. 30, no 8, 755-763 p.Article in journal (Refereed) Published
Abstract [en]

This paper reports on an experimental study of saturated flow boiling of R134a inside a circular vertical quartz tube coated with a transparent heater. The inner diameter of the tube was 1.33 mm and the heated length 235.5 mm. The flow pattern at high vapor qualities and the dryout of the liquid film were studied using a high speed CCD camera at the mass fluxes 47.4 and 124.4 kg/m(2) s in up flow at 6.425 bar. The heat fluxes ranged from 5 to 13.6 kW/m(2) for the lower mass flux and from 20 to 32.4 kW/m(2) for the higher mass flux.

The behavior of the flow close to dryout was found to be different at low and high mass flux. At low mass flux the location of the liquid front fluctuated with waves passing high up in the tube. In between the waves, a thin film was formed, slowly evaporating without breaking up.

At high mass flux the location of the liquid front was more stable. In this case the liquid film was seen to break up into liquid streams and dry zones on the tube wall.

Place, publisher, year, edition, pages
2006. Vol. 30, no 8, 755-763 p.
Keyword [en]
Dryout; Flow boiling; Minichannel; Visualization; Charge coupled devices; Evaporation; Flow visualization; Thin films; Vapors; Dryout; Flow boiling; Mass flux; Minichannels; Pipe flow; Charge coupled devices; Evaporation; Flow visualization; Pipe flow; Thin films; Vapors
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-6867DOI: 10.1016/j.expthermflusci.2006.03.005ISI: 000241181700006Scopus ID: 2-s2.0-33746459316OAI: oai:DiVA.org:kth-6867DiVA: diva2:11697
Note
QC 20100812. Konferens: International Conference on Heat Transfer and Fluid Flow in Microscale, Barga, Italy, Sep 25-30, 2005. ENEA, Inst Thermal Fluid Dynam.Available from: 2007-03-09 Created: 2007-03-09 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Experimental Heat Transfer, pressure drop, and Flow Visualization of R-134a in Vertical Mini/Micro Tubes
Open this publication in new window or tab >>Experimental Heat Transfer, pressure drop, and Flow Visualization of R-134a in Vertical Mini/Micro Tubes
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

For the application of minichannel heat exchangers, it is necessary to have accurate design tools for predicting heat transfer and pressure drop. Until recently, this type of heat exchangers was not well studied, and in the scientific literature there were large discrepancies between results reported by different investigators. The present thesis aims to add to the knowledge of the fundamentals of single- and two-phase flow heat transfer and pressure drop in narrow channels, thereby aiding in the development of this new, interesting technology with the possibility of decreasing the size of electronics through better cooling, and of increasing the energy efficiency of thermal processes and thermodynamic cycles through enhanced heat transfer.

A comprehensive experimental single-phase flow and saturated flow boiling heat transfer and pressure drop study has been carried out on vertical stainless steel tubes with inner diameters of 1.700, 1.224 and 0.826 mm, using R-134a as the test fluid. The heat transfer and pressure drop results were compared both to conventional correlations developed for larger diameter channels and to correlations developed specifically for microscale geometries.

Contrary to many previous investigations, this study has shown that the test data agree well with single-phase heat transfer and friction factor correlations known to be accurate for larger channels, thus expanding their ranges to cover mini/microchannel geometries. The main part of the study concerns saturated flow boiling heat transfer and pressure drop. Tests with the same stainless steel tubes showed that the heat transfer is strongly dependent on heat flux, but only weakly dependent on mass flux and vapor fraction (up to the location of dryout). This behavior is usually taken to indicate a dominant influence of nucleate boiling, and indicates that the boiling mechanism is strongly related to that in nucleate boiling. The test data for boiling heat transfer was compared to several correlations from the literature, both for macro- and mini-channels. A new correlation for saturated flow boiling heat transfer of refrigerant R-134a correlation was obtained based on the present experimental data. This correlation predicts the presented data with a mean absolute deviation of 8%. The frictional pressure drop results were compared to both macro- and mini channel correlations available from the literature. The correlation suggested by Qu and Mudawar (2003) gave the best prediction to the frictional two-phase pressure drop within the studied ranges.

A unique visualization study of saturated flow boiling characteristics in a vertical 1.332 mm inner diameter quartz tube, coated with a transparent heater has also been conducted. The complete evaporation process in a heated circular mini-channel has been studied visually in detail using high speed CCD camera. The study revealed the developments of the flow patterns and the behavior from bubble nucleation to the dry out of the liquid film. The bubble departure frequency, diameter, growth rate, and velocity were determined by analyzing the images. Finally, a flow pattern map for boiling flow in microchannels has been developed based on the test data.

Place, publisher, year, edition, pages
Stockholm: KTH, 2007. viii, 103 p.
Series
Trita-REFR, ISSN 1102-0245 ; 2007:59
Keyword
Minichannel, microchannel, heat transfer, pressure drop, single-phase, two-phase, flow boiling, flow visualization, dry out, bubble behavior, flow pattern.
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-4299 (URN)978-91-7178-594-7 (ISBN)
Public defence
2007-03-26, F3, Lindstedtsvägen 26, KTH, 10:00
Opponent
Supervisors
Note
QC 20100812Available from: 2007-03-09 Created: 2007-03-09 Last updated: 2010-08-12Bibliographically approved

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Palm, Björn

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