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Research Papers: Heat Exchangers

Flow Modes and Mode Transitions for Falling Films on Flat Tubes

[+] Author and Article Information
Xiaofei Wang, P. S. Hrnjak, S. Elbel

 Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China;  Mechanical Science and Engineering, University of Illinois, Urbana, IL 61801 e-mail: wangxf@illinois.edu Mechanical Science and Engineering, University of Illinois, Urbana, IL 61801; Creative Thermal Solutions, 2209 North Willow Road, Urbana, IL 61802 e-mail: pega@illinois.eduCreative Thermal Solutions, 2209 North Willow Road, Urbana, IL 61802 e-mail: stefan.elbel@creativethermalsolutions.com

A. M. Jacobi1

 Mechanical Science and Engineering, University of Illinois, Urbana, IL 61801 e-mail: a-jacobi@illinois.edu Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China e-mail: mghe@mail.xjtu.edu.cn

Maogang He

 Mechanical Science and Engineering, University of Illinois, Urbana, IL 61801 e-mail: a-jacobi@illinois.edu Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China e-mail: mghe@mail.xjtu.edu.cn

1

Corresponding author.

J. Heat Transfer 134(2), 021801 (Dec 13, 2011) (9 pages) doi:10.1115/1.4005095 History: Received October 06, 2010; Revised September 05, 2011; Published December 13, 2011; Online December 13, 2011

Flow mode transitions for a liquid film falling over flat tubes are studied for a wide range of conditions. Hydraulic oil, ethylene glycol, ethylene-glycol-water mixtures (at three different volume ratios), and water are used as working fluids in experiments conducted under adiabatic conditions, without an imposed vapor flow. The flow modes are observed over a range of flow rate and tube spacing. The flow modes are more complex but similar to those manifested on round tubes, and include the sheet, sheet-jet, jet, jet-droplet, and droplet modes. However, the transitions at Ga1/4 ≤ 16 (mainly for hydraulic oil, ethylene glycol) are very different from those on round tubes: there is no sheet-jet mode during transition between the sheet and jet flow modes. Moreover, at all Ga, the Re range over which the jet mode occurs is narrower than that for round tubes, and the hysteresis in mode transitions is not as pronounced as for round tubes. The transitional Re increases slowly with tube spacing, also in contrast to observations of round-tube falling films. The flat-tube falling-film flow regimes are described, and new correlations are provided to relate the transitional Re number to Ga and geometry. By analyzing repeated experiments, it is shown that the standard deviation of the measurements from the correlations is less than 5.2%.

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References

Figures

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Figure 1

A schematic diagram of the experimental apparatus

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Figure 2

A sketch of the liquid distributor (see Ref. [13])

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Figure 3

A typical flat tube used in the experiments

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Figure 4

The observed flat-tube falling-film flow patterns. The liquid is seen as it falls from one flat tube to another. No image processing was used in any images, except for brightness and contrast control. The background is dark, and the liquid appears transparent or bright due to interface curvature and reflection.

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Figure 5

The development of torn sheet (a) Re = 525, (b) Re = 516, (c) Re = 502, (d) Re = 490, and (e) Re = 476 with s = 14.5 mm

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Figure 6

Comparison with results of Hu on round tubes (s = 6.4 mm, s2  = 2 mm)

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Figure 7

Falling-film transition with different tube spacing at s2  = 2 mm

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Figure 8

Comparison with round tube at different tube spacing (curves are calculated per Ref. [8])

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Figure 9

Comparison of two different distributor heights

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Figure 10

Falling-film flow mode transitional map with s*  = 8 (curves are from Eq. 3 and Table 5)

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Figure 11

Comparison of the curve fits to the experimental data (curves are from Eq. 3 and Table 5)

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