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Boiling heat transfer performance of plain and porous tubes in falling film flow of refrigerant R114

Wärmeübergang beim Sieden in vertikaler Rieselfilm-Strömung für glatte und poröse Rohre mit Kältemittel R114

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Abstract

The falling film flow characteristic and heat transfer performance of four types of commercially available porous tubes (High flux tube by Union Carbide, Gewa-T, Wieland, Thermoexcel-E and -EC by Hitachi) were compared with an ordinary sand-blasted tube and a plain copper tube. The test evaporator consisted of a single 2 m vertical tube. The evaporating liquid was refrigerant R 114 flowing down the outside of the test tube. Flow observations were made by high-speed photography. Nucleate boiling was dominant in all the tests with porous surfaces. No boiling hysteresis was observed. All structured surfaces showed an excellent performance compared with plain tube.

Thermoexcel-E had the highest heat transfer coefficient, 12 times better than smooth tube. Ordinary sand-blasted tube could not compete with porous surfaces. Its performance during surface evaporation was nevertheless 1.4 to 1.8 times and during nucleation 1.7 times higher.

Zusammenfassung

Die Charakteristik und der Wärmeübergang in einer Riesel-Film-Strömung für vier verschiedene kommerzielle Rohre mit porösen Oberflächen (High flux-Rohr von Union Carbide, Gewa-T von Wieland, Thermoexcel-E und -EC von Hitachi) wurden mit einem gewöhnlichen ebenen und einem sandbestrahlten Rohr verglichen. Der Versuchsverdampfer war ein einzelnes vertikales Rohr, 2 m lang. Die siedende Flüssigkeit war das Kältemittel R114 und strömte abwärts entlang der Außenfläche. Strömungsbeobachtungen wurden mit der Hochgeschwindigkeits-Kinematographie gemacht. Blasensieden war vorherrschend in allen Versuchen mit porösen Oberflächen. Es wurde keine Hysterese beobachtet. Alle strukturierten Flächen zeigten einen hervorragenden Wärmeübergang im Vergleich zur glatten Fläche. Thermoexcel-E hatte den besten Wärmeübergangskoeffizienten, 12mal besser als die glatte Fläche. Sandgestrahltes Rohr konnte nicht mit porösen Rohren konkurrieren. Jedoch war die Leistung bei konvektiver Verdampfung 1,4-bis l,8mal und bei Blasensieden l,7mal höher als für die glatte Oberfläche.

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Abbreviations

C :

constant in Eq. (1)

d :

diameter, m

h :

heat transfer coefficient, W/m2 K

L :

evaporation length, m

m :

mass flowrate, kg/s

n :

exponent in Eq. (1)

Pr :

Prandtl number

q :

heat flux density, W/m2

Re :

Reynolds number=4m/dη

T :

temperature, K

Δ T :

temperature difference, K

λ :

thermal conductivity, W/Km

ν :

kinematic viscosity. m2/s

g :

gravity acceleration, 9.81 m/s2

h* :

(ν 2/g λ 3)1/3 h nondimensional heat transfer coefficient

η :

dynamic viscosity, kg/ms

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Authors and Affiliations

  1. Department of Mechanical Engineering, Helsinki University of Technology, Otakaari 1, 02150, Espoo, Finland

    N. -E. Fagerholm (Doctor of Technology), A. -R. Ghazanfari Ph.D., K. Kivioja (Master of Science in Technology) & E. Järvinen (Master of Science in Technology)

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  1. N. -E. Fagerholm
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  2. A. -R. Ghazanfari Ph.D.
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  3. K. Kivioja
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  4. E. Järvinen
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Fagerholm, N.E., Ghazanfari, A.R., Kivioja, K. et al. Boiling heat transfer performance of plain and porous tubes in falling film flow of refrigerant R114. Wärme- und Stoffübertragung 21, 343–353 (1987). https://doi.org/10.1007/BF01376289

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