Abstract
The linear stability theory is used to study stability characteristics of laminar gravity-induced condensate film flow down an arbitrarily inclined wall. The coupled equations describing the velocity and temperature disturbances are solved numerically. The results show that laminar condensate films are unstable in all practical situations. Several stabilizing effects are acting on the film flow; these are: the angle of inclination, the surface tension at large wave numbers, the condensation rate at small Reynolds numbers, and to a certain extent the Prandtl number. For a vertical plate, the expected wavelengths of the disturbances are presented as functions of the Reynolds numbers of the condensate flow.
Zusammenfassung
Mit Hilfe der linearen StabilitÄtstheorie werden die StabilitÄtseigenschaften laminarer Kondensatfilme an ebenen WÄnden untersucht. Die Gleichungssysteme, die Temperatur- und Geschwindigkeitsstörungen beschreiben, werden numerisch gelöst. Es zeigt sich, da\ Kondensatfilme in jedem praktischen Fall ein unstabiles Verhalten aufweisen. Der stabilisierende Einflu\ von OberflÄchenspannung, Schwerkraft und Stoffübertragung durch Kondensation werden diskutiert. Für eine senkrechte Wand werden die zu erwartenden WellenlÄngen der Störungen als Funktion der Reynoldszahlen des Kondensatfilms angegeben.
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Abbreviations
- C*=C *r + iC *i :
-
dimensional complex wave velocity
- C=C*/u0 :
-
dimensionless wave velocity
- cp :
-
specific heat at constant pressure
- g:
-
gravitational acceleration
- hn :
-
defined by Eq. (16)
- hfg :
-
latent heat
- k:
-
thermal conductivity
- Pe=PrRe:
-
Peclet number
- Pr:
-
Prandtl number
- Py :
-
defined by Eq. (15)
- q:
-
iaPe
- Re=u0 δΝ:
-
Reynolds number
- S:
-
temperature disturbance amplitude
- t* :
-
dimensional time
- t=t* u0/δ:
-
dimensionless time
- T:
-
dimensional temperature
- Ts :
-
saturation temperature
- Tw :
-
wall temperature
- δT =Ts−Tw :
-
temperature drop across liquid film
- u*, v* :
-
dimensional velocity component
- v=v*/u0 :
-
dimensionless velocity components
- u0 :
-
dimensional surface velocity of undisturbed film flow
- x*, y* :
-
dimensional coordinates
- x=x*/δ:
-
dimensionless coordmates
- Yn :
-
functional vector defined by Eq. (20)
- α:
-
dimensionless wave number
- Β :
-
roots of Eq. (20)
- γn:
-
defined by Eq. (21)
- δ :
-
local thickness of undisturbed condensate film
- λ * :
-
wavelength, dimensional
- \(\lambda = \frac{{\lambda *}}{\delta }\) :
-
wavelength, dimensionless
- \(\theta = \frac{{T - T_s }}{{T_w - T_s }}\) :
-
temperature variable
- Ν :
-
kinematic viscosity of liquid
- ρ :
-
liquid density
- ρ g :
-
vapor density
- σ :
-
surface tension
- Φ :
-
stream function disturbance amplitude
- ψ :
-
stream function
- Ω :
-
angle of inclination
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Lee, C.Y., Marschall, E. Laminar stability analysis of condensate film flow. Wärme- und Stoffübertragung 7, 14–21 (1974). https://doi.org/10.1007/BF01438316
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DOI: https://doi.org/10.1007/BF01438316