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Research Papers: Evaporation, Boiling, and Condensation

Combined Visualization and Heat Transfer Measurements for Steam Flow Condensation in Hydrophilic and Hydrophobic Mini-Gaps

[+] Author and Article Information
Xi Chen

Department of Mechanical and
Nuclear Engineering,
Kansas State University,
Manhattan, KS 66506

Melanie M. Derby

Department of Mechanical and
Nuclear Engineering,
Kansas State University,
Manhattan, KS 66506
e-mail: derbym@ksu.edu

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received February 18, 2016; final manuscript received April 21, 2016; published online May 17, 2016. Editor: Portonovo S. Ayyaswamy.

J. Heat Transfer 138(9), 091503 (May 17, 2016) (11 pages) Paper No: HT-16-1094; doi: 10.1115/1.4033496 History: Received February 18, 2016; Revised April 21, 2016

Condensation enhancement was investigated for flow condensation in mini-channels. Simultaneous flow visualization and heat transfer experiments were conducted in 0.952-mm diameter mini-gaps. An open loop steam apparatus was constructed for a mass flux range of 50–100 kg/m2s and steam quality range of 0.2–0.8, and validated with single-phase experiments. Filmwise condensation was observed in the hydrophilic mini-gap; pressure drop and heat transfer coefficients were compared to the (Kim and Mudawar, 2013, “Universal Approach to Predicting Heat Transfer Coefficient for Condensing Mini/Micro-Channel Flow,” Int. J. Heat Mass Transfer, 56(1–2), pp. 238–250) correlation and prediction was very good; the mean absolute error (MAE) was 20.2%. Dropwise condensation was observed in the hydrophobic mini-gap, and periodic cycles of droplet nucleation, coalescence, and departure were found at all mass fluxes. Snapshots of six typical sweeping cycles were presented, including integrated flow visualization quantitative and qualitative results combined with heat transfer coefficients. With a fixed average steam quality (x¯ = 0.42), increasing mass flux from 50 to 75 to 100 kg/m2s consequently reduced average sweeping periods from 28 to 23 to 17 ms and reduced droplet departure diameters from 13.7 to 12.9 to 10.3 μm, respectively. For these cases, condensation heat transfer coefficients increased from 154,700 to 176,500 to 194,800 W/m2 K at mass fluxes of 50, 75, and 100 kg/m2 s, respectively. Increased mass fluxes and steam quality reduced sweeping periods and droplet departure diameters, thereby reducing liquid thickness and increasing heat transfer coefficients.

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Figures

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Fig. 1

Flow condensation experimental apparatus

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Fig. 2

Front (left) and top (right) views of test section

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Fig. 3

Contact angle of water on bare copper surface (left) and Teflon AF-treated hydrophobic surface (right)

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Fig. 4

Experimental pressure drop (left) and comparison of experimental results and pressure drop model (right)

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Fig. 5

Single-phase validation tests: (a) heat transfer rates, and (b) heat loss through the visualization window

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Fig. 6

Single-phase heat transfer measurement validation tests comparing experimental results to convection in two infinite plates with one side insulated

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Fig. 7

Condensation heat transfer coefficients in hydrophilic gap

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Fig. 8

Experimentally measured filmwise condensation and heat transfer coefficients predicted by Kim and Mudawar [54] in the hydrophilic mini-gap

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Fig. 9

Nucleation, coalescence, and departure states of droplets in a hydrophobic mini-gap

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Fig. 10

Condensation modes in (a) hydrophilic G = 50 kg/m2 s and hydrophobic surface, (b) G = 50 kg/m2 s, (c) G = 75 kg/m2 s, and (d) G = 100 kg/m2 s

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Fig. 11

Nucleation, coalescence, and departure stages with steam quality of 0.42 and mass fluxes of 50, 75, and 100 kg/m2 s

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Fig. 12

Comparison of sweeping periods at average steam quality of 0.42 at mass fluxes of (a) 50, (b) 75, (c) 100 kg/m2 s, and (d) sweeping periods with standard deviations

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Fig. 13

Nucleation, coalescence, and departure stages with G = 50 kg/m2 s and steam qualities of 0.35, 0.42, and 0.55

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Fig. 14

Heat transfer coefficient on hydrophobic surface corresponding to conditions in Figs. 11 and 13

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Fig. 15

Heat transfer coefficient enhancement of hydrophobic mini-gap

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