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Research Papers: Evaporation, Boiling, and Condensation

Study of Flow Boiling Characteristics of a Microchannel Using High Speed Visualization

[+] Author and Article Information
Björn Palm

e-mail: bjorn.palm@energy.kth.se
Department of Energy Technology,
Royal Institute of Technology,
Brinellvägen 68,
SE 100 44 Stockholm, Sweden

Claudi Martin-Callizo

Sapa Heat Transfer AB,
612 81 Finspång, Sweden

Mohammad H. Maqbool

Department of Energy Technology,
Royal Institute of Technology,
Brinellvägen 68,
SE 100 44 Stockholm, Sweden

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received September 3, 2013; final manuscript received February 14, 2013; published online June 27, 2013. Assoc. Editor: Srinivas Garimella.

J. Heat Transfer 135(8), 081501 (Jun 27, 2013) (8 pages) Paper No: HT-11-1430; doi: 10.1115/1.4023879 History: Received September 03, 2011; Revised February 14, 2013

This paper presents the visualization results obtained for an experimental study of R134a during flow boiling in a horizontal microchannel. The microchannel used was a fused silica tube having an internal diameter of 781 μm, a heated length of 191 mm, and was coated with a thin, transparent, and electrically conductive layer of indium-tin-oxide (ITO) on the outer surface. The operating parameters during the experiments were: mass flux 100–400 kg/m2 s, heat flux 5–45 kW/m2, saturation temperatures 25 and 30 °C, corresponding to saturation pressures of 6.65 bar and 7.70 bar and reduced pressures of 0.163 and 0.189, respectively. A high speed camera with a close up lens was used to capture the flow patterns that evolved along the channel. Flow pattern maps are presented in terms of the superficial gas and liquid velocity and in terms of the Reynolds number and vapor quality plots. The results are compared with some flow pattern maps for conventional and micro scale channels available in the literature. Rigorous boiling and increased coalescence rates were observed with an increase in the heat flux.

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Figures

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Fig. 1

(a) Details of the test section. (b) Schematic diagram of the experimental set up.

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Fig. 2

Flow patterns observed during flow boiling in the test section of D = 0.781 mm, (1) G = 400 kg/m2 s, q″ = 10.25 kW/m2, (2) G = 185 kg/m2 s, q″ = 13.40 kW/m2, (3) G = 185 kg/m2 s, q″ = 10.97 kW/m2, (4) G = 185 kg/m2 s, q″ = 15.04 kW/m2, (5) G = 400 kg/m2 s, q″ = 19.41 kW/m2, (6) G = 100 kg/m2 s, q″ = 16.50 kW/m2, and (7) G = 400 kg/m2 s, q″ = 47.81 kW/m2

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Fig. 3

Experimental flow pattern map for D = 0.781 mm, Tsat = 25 °C, and inlet sub cooling of 2 °C: B = bubbly, S = slug, SA = semi annular, A = annular, and M = mist

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Fig. 4

Effect of the system pressure on the experimental flow pattern transition lines for D = 0.781 mm: solid lines are for Tsat = 30 °C and dotted lines are for Tsat = 25 °C

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Fig. 5

Effect of the tube diameter on the flow pattern transition lines, Tsat = 30 °C

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Fig. 6

Comparison of the experimental transition lines for the intermittent and nonintermittent flow pattern with the transition line prediction correlation of Garimella et al., Tsat = 30 °C

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Fig. 7

Comparison of the current experimental data points at Tsat = 25 °C with the experimental transition lines of Triplett et al. for the air-water two-phase flow in a circular test section of 1.097 mm

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Fig. 8

Comparison of the current experimental data points at Tsat = 25 °C with the experimental transition lines of Chen et al. for R134a in a circular test section of 1.10 mm at system pressures of 6 bar and 10 bar

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Fig. 9

Comparison of the current experimental data points at Tsat = 30 °C with the experimental transition lines of Revellin and Thome for R134a in a circular test section of 0.5 mm at Tsat = 30 °C and ΔT = 3 °C

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Fig. 10

Number of active nucleation sites versus heat flux for different mass fluxes and two system pressures

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Fig. 11

Bubble growth after detachment with time, Tsat = 25 °C

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Fig. 12

Bubble frequency versus heat flux for different mass fluxes, Tsat = 25 °C

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Fig. 13

Mean velocity of elongated bubbles against vapor quality for different mass fluxes, Tsat = 30 °C

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