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Research Papers: Heat Exchangers

Effects of Tube Row on Heat Transfer and Surface Wetting of Microscale Porous-Layer Coated, Horizontal-Tube, Falling-Film Evaporator

[+] Author and Article Information
Chanwoo Park

Assistant Professor
e-mail: chanwoo@unr.edu
Department of Mechanical Engineering,
University of Nevada,
1664 N. Virginia Street,
Reno, NV 89557-0312

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the Journal of Heat Transfer. Manuscript received January 22, 2012; final manuscript received October 23, 2012; published online March 20, 2013. Assoc. Editor: Ali Ebadian.

J. Heat Transfer 135(4), 041802 (Mar 20, 2013) (11 pages) Paper No: HT-12-1023; doi: 10.1115/1.4023014 History: Received January 22, 2012; Revised October 23, 2012

An experimental study was conducted to investigate the effects of tube row on surface wetting and heat transfer of a horizontal-tube, falling-film evaporator. Two types of the evaporator tubes were used for comparison: plain copper and copper coated with a microscale porous-layer. Distilled water was used as solution and heating fluids. A visual observation experiment performed in ambient with no heat input showed that when solution fluid was dripped onto the evaporator tubes from a solution dispenser, the plain tubes were partially wetted, while the porous-layer coated tubes were completely wetted due to capillary liquid spreading, even at low solution flow rates. It was found from the heat transfer experiment performed in a closed chamber under saturated conditions that the porous-layer coated tubes exhibited superior evaporation heat transfer (up to 100% overall improvement over the plain tubes at low solution flow rates) due to complete solution wetting and thin-film evaporation. It was also observed that the surface wetting and heat transfer are greatly influenced by both intertube flow mode of solution fluid and tube wall superheat. The effects of the tube row on the solution wetting and heat transfer were significant, especially for downstream tubes.

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Figures

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Fig. 1

Schematic of the experimental setup for the horizontal-tube, falling-film evaporator

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Fig. 2

Dimensions of the horizontal-tube, falling-film evaporator: (a) side view of the evaporator tube array, (b) dimensions of the solution dispenser, and (c) cross-sectional view of the porous-layer coated evaporator tube

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Fig. 3

Photos of the (a) plain, and (b) porous-layer coated tubes used for the experiment. (c) Isometric view of the porous-layer coated tube. SEM images at (d) × 250 magnification of the porous-layer coating and (e) × 1000 magnifications of the box in Fig. 3(d).

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Fig. 4

(a) Quarter section of the plain tube assembly showing the upstream falling-film flow of the solution fluid, the temperatures used for a thermal analysis and tube dimensions. (b) Thermal resistances and temperature nodes used for the thermal analysis of the plain tube.

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Fig. 7

Comparison of the heat transfer rates and wall superheats of the plain and porous-layer coated tubes for (a) first, (b) second, and (c) third and fourth rows combined in the tube array

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Fig. 8

Comparison of the (a) thermal resistances and (b) external heat transfer coefficients and surface wetting ratio of the plain and porous-layer coated tubes for the third and fourth row combined in the tube array

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Fig. 6

Comparisons of the heat transfer rates and wall superheats for the (a) plain and (b) porous-layer coated tubes

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Fig. 5

Photos of the visualization experiment showing the change of the intertube flow mode with the solution Reynolds number for the (a) porous-layer coated tubes and (c) plain tubes. (b) Variation of the surface wetting ratios with the solution Reynolds number.

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Fig. 9

Comparison of the heat transfer rates of the tube array for the plain and porous-layer coated tubes

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