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Heat and mass transfer characteristics of a new combined absorber-evaporator exchanger operating near the triple point of water

Wärme- und Stoffübergang bei einem, in der Nähe des Tripelpunktes von Wasser betriebenen, neuen, kombinierten Absorber/Verdampfer-Austauscher

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Abstract

The present experimental study investigates the controlling mechanism involved in a new combined vertical film-type absorber-evaporator exchanger operating near the condition of the triple point of water. This peculiar exchanger plays the most important role in the VFVPE process that can be utilized in many industrial applications, water pollution prevention, desalination, and purification of chemicals, for example. The method of analogy of the heat and mass transfer near the film surface is used to calculate the interfacial concentration and temperature, and thus determining the heat and mass transfer coefficients. It is shown that the working temperature level has the negligible effect on the characteristics of the mass transfer. The mass transfer coefficients are higher than those obtained in the case of isothermal absorption due to the convective effect arisen from vapor absorption in the falling solution film. The water flow rate in the evaporator side has a minor effect on the performance of this combined exchanger. The overall mean heat transfer coefficient remains nearly constant in the lower range of the solution flow rate of the absorber; however, it would increase with increasing solution flow rate in the higher range. The correlating equations for both the heat and mass transfer coefficients are suggested.

Zusammenfassung

Der Wärme- und Stoffübergang bei einem, in der Nähe des Tripelpunktes von Wasser betriebenen, neuen, kombinierten Absorber/Verdampfer-Austauscher wurde experimentell untersucht. Dieser Austauscher spielt eine entscheidende Rolle im VFVPE-Prozeß, der vielfache industrielle Anwendung findet, wie z.B. bei der Süßwassergewinnung, der Meerwasserentsalzung oder der Reinigung von Chemikalien. Die Phasengrenz-temperatur und-Konzentration, welche zur Berechnung von Wärme- und Stoffübergangskoeffizienten benötigt werden, lassen sich über eine Nu/Sh-Analogie in der Nähe der Filmoberfläche ermitteln. Die experimentell bestimmten Sherwood-Zahlen bei der Absorption von Dämpfen in Flüssigkeiten liegen höher als die entsprechenden Meßwerte bei der isothermen Rieselfilm-Absorption. Es wird gezeigt, daß der Kühlwasser-Volumenstrom auf der Verdampferseite nur unbedeutenden Einfluß auf das Betriebsverhalten des kombinierten Austauschers hat. Der Gesamt-Wärmeübergangskoeffizient bleibt im Bereich niedriger Volumenströme des Lösung nahezu konstant; er würde jedoch im Bereich höhrer Volumenströme mit diesen ansteigen. Für Wärme-und Stoffübergangs-koeffizienten werden auf den Messwerten basierende Berechnungsgleichungen angegeben.

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Abbreviations

a :

thermal diffusivity (m2/s)

C :

brine concentration by weight (%)

C p :

specific heat capacity (KJ/Kg K)

D :

mass diffusivity (m2/s)

E T :

Ackermann's correction factor

g :

gravitational acceleration constant (m/s2)

h :

enthalpy (KJ/Kg)

k :

thermal conductivity (KW/mK)

L :

total film length (m)

Le :

Lewis number,Sc/Pr

m a :

local mass absorption rate (Kg/sm2)

M T :

total mass absorption rate per unit width (Kg/sm)

Nu :

Nusselt number

P :

pressure (torr)

Pr :

Prandtl number,v/a

q a :

local heat flux (KW/m2)

Q T :

total heat transfer rate per unit width (KW/m)

Re :

film Reynolds number, 4Γ/μ

Sc :

Schmidt number,v/D

Sh :

Sherwood number

T :

temperature (K)

x :

longitudinal coordinate (m)

U :

overall mean heat transfer coefficient (KW/m2K)

α :

heat transfer coefficient (KW/m2K)

β :

mass transfer coefficient (m/s)

Γ:

flow rate per unit width (Kg/sm)

δ:

film thickness (m)

ΔC 1m :

logarithmic mean concentration difference

Δh a :

heat of absorption (KJ/Kg)

ΔT 1m :

logarithmic mean temperature difference

μ :

viscosity (Kg/sm)

ν :

kinematic viscosity (m2/s)

ρ :

liquid density (Kg/m3)

a :

absorption

b :

brine

B :

bulk condition

c, d :

constant

e :

evaporation

i :

inlet

n :

constant

o :

outlet

p :

plate

ph :

film interface

v :

water vapor

w :

water film

⋆:

at thermodynamic equilibrium

-:

average

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The authors acknowledge the discussion with Prof. Chen-Yen Cheng. The authors are also grateful to Prof. Neima Brauner for his kind offer of the physical properties of CaCl2 solution. This work is financially supported by the National Science Council, ROC under contract NSC 84-2212-E-002-011

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Chen, Y.M., Sun, C.Y. Heat and mass transfer characteristics of a new combined absorber-evaporator exchanger operating near the triple point of water. Heat and Mass Transfer 31, 291–299 (1996). https://doi.org/10.1007/BF02184041

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