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Research Papers: Two-Phase Flow and Heat Transfer

Heat Transfer Performance for a Falling-Film on Horizontal Flat Tubes

[+] Author and Article Information
X. F. Wang

Xi'an Jiaotong University,
Xi'an 710049,
China;
Mechanical Science and Engineering Department,
University of Illinois,
Urbana, IL 61801
e-mail: wangxf@illinois.edu

P. S. Hrnjak

Mechanical Science and Engineering Department,
University of Illinois,
Urbana, IL 61801;
Creative Thermal Solutions,
2209 North Willow Road,
Urbana, IL 61802

S. Elbel

Creative Thermal Solutions,
2209 North Willow Road,
Urbana, IL 61802

A. M. Jacobi

Mechanical Science and Engineering Department,
University of Illinois,
Urbana, IL 61801

M. G. He

Xi'an Jiaotong University,
Xi'an 71009, China

When the sensible heat gain was compared to the electrical power supplied to the heater in 90% of the experiments, the sensible heat gain was within 10% of the electrical power.

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received December 18, 2011; final manuscript received January 21, 2013; published online June 6, 2013. Assoc. Editor: W. Q. Tao.

J. Heat Transfer 135(7), 072901 (Jun 06, 2013) (12 pages) Paper No: HT-11-1579; doi: 10.1115/1.4023689 History: Received December 18, 2011; Revised January 21, 2013

Local and average heat transfer behavior for a falling film on horizontal flat tubes is explored through an experimental approach. Experiments are conducted using water, ethylene glycol, and their mixture (50% by volume) under different heat fluxes and tube spacing, with a range of flow rates that covers all flow modes. It is found that the local heat transfer coefficient decreases with distance from the top of the tube. The distribution of the heat transfer coefficient along the axial direction depends on the flow mode: it is constant for the sheet mode, shows small variations for the jet mode, and has variations as large as 20% for the droplet mode. Heat flux has almost no effect on the average Nusselt number within the experimental range. The average Nusselt number for the flat tube is close to that for round tubes in the droplet flow mode, however, in the jet and sheet modes the flat-tube Nusselt number is much larger than the round-tube Nusselt number. Boundary-layer theory is used to explain the local heat transfer coefficient distribution and the experimental data show good agreement with the boundary-layer theory for most cases. New curve fits for the average heat transfer coefficient for three flow modes at different tube spacing are provided and the maximum deviation of the data from the fit is less than 14%.

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References

Figures

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Fig. 1

Schematic diagram of the experimental setup

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Fig. 2

A sketch of the liquid distributor (see Ref. [29] for more details)

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Fig. 3

Side view of the flat tube (all dimensions in mm)

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Fig. 4

Structure for each piece comprising a test specimen

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Fig. 5

Thermocouple locations on both surfaces of the flat tube. The front surface (thermocouple landings shown as white) is designated as surface 1; the back surface (thermocouple landings shown as black) is designated as surface 2.

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Fig. 6

Temperatures used in the data reduction

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Fig. 7

Local heat transfer coefficient for the sheet mode, mixture at s = 6.5 mm, q″ = 17.59 kW m−2, and Tin = 18 °C: (a) Re = 152, and (b) Re = 121

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Fig. 8

Local heat transfer coefficient, jet mode, mixture at s = 6.5 mm and q″ = 17.29 kW m−2; (a) Re = 91, and (b) Re = 63

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Fig. 9

Local heat transfer coefficient, droplet mode, mixture at s = 6.5 mm and q″ = 17.81 kW m−2; (a) Re = 58, and (b) Re = 39

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Fig. 10

Comparison of hlocal to Eq. (12) with three modes at s = 4.8 mm: (a) water, (b) ethylene glycol and water mixture (50% by volume), and (c) ethylene glycol

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Fig. 11

Nu with Re, ethylene glycol at s = 4.8 mm

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Fig. 12

Nu with Re, mixture at s = 4.8 mm

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Fig. 13

Nu with Re, water at s = 4.8 mm

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Fig. 14

Heat flux effect: (a) ethylene glycol, s = 6.5 mm, Re = 16, hlocal with heat flux, and (b) water, s = 9.6 mm, average Nu with Re

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Fig. 15

Effect of tube spacing for three modes (mixture)

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Fig. 16

Effect of tube spacing with (mixture)

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Fig. 18

Effective gravity in flows with flat tubes and round tubes

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Fig. 17

Comparison with the curve fit for the round tube at s = 9.6 mm, (a) water, and (b) mixture, both at 18 °C

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Fig. 19

Comparison of the curve fit to the experimental data; (a) sheet mode, (b) jet mode, and (c) droplet mode

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