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Research Papers

Effects of Vapor Velocity and Pressure on Marangoni Condensation of Steam-Ethanol Mixtures on a Horizontal Tube

[+] Author and Article Information
Hassan Ali

School of Engineering and Materials Science,
Queen Mary University of London,
Mile End Road,
London E1 4NS, UK;
Rachna College of Engineering and Technology,
University of Engineering and Technology,
Lahore 54000, Pakistan

John W. Rose

School of Engineering and Materials Science,
Queen Mary University of London,
Mile End Road, London E1 4NS, UK

1Corresponding author.

Contributed by the Heat Transfer Division of ASME for publication in the Journal of Heat Transfer. Manuscript received March 14, 2012; final manuscript received September 25, 2012; published online February 8, 2013. Assoc. Editor: W. Q. Tao.

J. Heat Transfer 135(3), 031502 (Feb 08, 2013) (10 pages) Paper No: HT-12-1108; doi: 10.1115/1.4007893 History: Received March 14, 2012; Revised September 25, 2012

Careful heat-transfer measurements have been conducted for condensation of steam-ethanol mixtures flowing vertically downward over a horizontal, water-cooled tube at pressures ranging from around atmospheric down to 14 kPa. Care was taken to avoid error due to the presence of air in the vapor. The surface temperature was accurately measured by embedded thermocouples. The maximum vapor velocity obtainable was limited by the maximum electrical power input to the boiler. At atmospheric pressure this was 7.5 m/s while at the lowest pressure a velocity of 15.0 m/s could be achieved. Concentrations of ethanol by mass in the boiler when cold prior to start up were 0.025%, 0.05%, 0.1%, 0.5%, and 1.0%. Tests were conducted for a range of coolant flow rates. Enhancement of the heat-transfer coefficient over pure steam values was found by a factor up to around 5, showing that the decrease in thermal resistance of the condensate due to Marangoni condensation outweighed diffusion resistance in the vapor. The best performing compositions (in the liquid when cold) depended on vapor velocity but were in the range 0.025–0.1% ethanol in all cases. For the atmospheric pressure tests the heat-transfer coefficient for optimum composition, and at a vapor-to-surface temperature difference of around 15 K, increased from around 55 kW/m2 K to around 110 kW/m2 K as the vapor velocity increased from around 0.8 to 7.5 m/s. For a pressure of 14 kPa the heat-transfer coefficient for optimum composition, and at a vapor-to-surface temperature difference of around 9 K, increased from around 70 kW/m2 K to around 90 kW/m2 K as the vapor velocity increased from around 5.0 to 15.0 m/s. Photographs showing the appearance of Marangoni condensation on the tube surface under different conditions are included in the paper.

Copyright © 2013 by ASME
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References

Figures

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Fig. 2

Location of thermocouples in test tube wall

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Fig. 3

Photographs of transition from pseudodropwise to filmwise condensation mode with increasing coolant flow rate (PV = 101 kPa, UV = 0.78 m/s, CiL = 0.05%)

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Fig. 4

Photographs for best enhancement ratio at different pressures and vapor velocity (CiL = 0.025%)

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Fig. 5

Photographs for worst enhancement ratio at different pressures and vapor velocity (CiL = 1.0%)

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Fig. 6

Heat flux versus vapor-to-surface temperature difference for different vapor approach velocities for pure steam—comparison between present data and Eq. (1) of Ref. [26]. (a) PV = 101 kPa, (b) PV = 55 kPa, and (c) PV = 14 kPa.

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Fig. 7

Comparison of present pure steam data with Eq. (1) of Ref. [26]

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Fig. 8

Heat flux versus vapor-to-surface temperature difference for different vapor approach velocities (PV = 101 kPa). Lines show pure steam values from Eq. (1). The data points identified by (a)–(g) in the graph for CiL = 0.05% refer to the photographs in Fig. 3. (a) CiL = 0.025%, (b) CiL = 0.05%, (c) CiL = 0.1%, (d) CiL = 0.5%, and (e) CiL = 1.0%.

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Fig. 9

Heat flux versus vapor-to-surface temperature difference for different vapor approach velocities (PV = 55 kPa). Lines show pure steam values from Eq. (1). (a) CiL = 0.025%, (b) CiL = 0.05%, (c) CiL = 0.1%, (d) CiL = 0.5%, and (e) CiL = 1.0%.

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Fig. 10

Heat flux versus vapor-to-surface temperature difference for different vapor approach velocities (PV = 14 kPa). Lines show pure steam values from Eq. (1). (a) CiL = 0.025%, (b) CiL = 0.05%, (c) CiL = 0.1%, (d) CiL = 0.5%, (e) CiL = 1.0%.

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Fig. 11

Heat-transfer enhancement ratio as compared to Eq. (1) of Ref. [26] (PV = 101 kPa). The data points identified by (a) Fig. 4 and (a) Fig. 5 refer to the corresponding photographs in Figs. 4 and 5. (a) CiL = 0.025%, (b) CiL = 0.05%, (c) CiL = 0.1%, (d) CiL = 0.5%, and (e) CiL = 1.0%.

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Fig. 12

Heat-transfer enhancement ratio as compared to Eq. (1) of Ref. [26] (PV = 55 kPa). The data points identified by (b) Fig. 4 and (b) Fig. 5 refer to the corresponding photographs in Figs. 4 and 5. (a) CiL = 0.025%, (b) CiL = 0.05%, (c) CiL = 0.1%, (d) CiL = 0.5%, and (e) CiL = 1.0%.

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Fig. 13

Heat-transfer enhancement ratio as compared to Eq. (1) of Ref. [26] (PV = 14 kPa). The data points identified by (c) Fig. 4 and (c) Fig. 5 refer to the corresponding photographs in Figs. 4 and 5. (a) CiL = 0.025%, (b) CiL = 0.05%, (c) CiL = 0.1%, (d) CiL = 0.5%, and (e) CiL = 1.0%.

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Fig. 14

Comparison with data of Murase et al. [12,13] for condensation on a horizontal tube

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Fig. 15

Comparison with data of Utaka and Wang [8] for condensation on a 10 mm × 20 mm plane surface with the longer side vertical and Wang et al. [16] for condensation on a vertical tube (10 mm in diameter and 55 mm in height)

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