heat transfer enhancement during downward laminar flow condensation of r134a in vertical smooth
Condensation on vertical surface
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Transcript of Condensation on vertical surface
Condensation on
vertical surface
Convective Heat Transfer Seminar
Mostafa Ghadamyari
M. SC. Student
Spring 2014 - Tarbiat Modares university
Outline
Here, We will discuss about:
Condensation definition
Different types of condensation
Film Surface Condensation
Laminar film equation derivation
Laminar film heat transfer coefficient diagram
Turbulent Film heat transfer coefficient diagram
2/12
Condensation definition
A phase is a region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform.
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another one by heat transfer.
Condensation is the change of the physical state of matter from gas phase into liquid phase, and is the reverse of vaporization
3/12
Modes of condensation
Condensation modes:
(a &b). Surface condensation
(a). Film
(b). Dropwise
(c) Homogeneous condensation
(d) Direct Contact condensation
We’ll focus on Film Surface condensation
4/12
Dropwise and Film Surface condensation
(a) Dropwise condensation occurs if the
surface is coated with a substance that inhibits wetting, Silicones, Teflon,
assortment of waxes and fatty acids
(b) Film condensation is generally
characteristic of clean, uncontaminated
surfaces
The condensate provides a resistance to
heat transfer between the vapor and the surface
It’s desirable but difficult to maintain
dropwise condensation, so calculations
are often based on Film condensation5/12
Film condensation
Film condensation has Three distinct regions:
1. Laminar region, near the top, the film is
relatively thin
2. Wavy region, The film becomes thick enough
to show the signs of transition
3. Turbulent region, Ripples appear irregular in
both space and time
Laminar Region complexities:
Flow of liquid interacts with layer of vapor
Tinterface= Saturation temperature of local P
Tw < Saturation temperature < Tvapor
6/12
Laminar Film – Momentum Equation
Momentum equation for simplified laminar film:
Because of slenderness of the film:
Substituting:
Assuming negligible Inertia [Solved by Nusselt]:
2
2(1) l l l
v v dP vu v g
x y dy x
2
2(3) ( )l l l v
Sinking EffectFrictionInertia
v v vu v g
x y x
(2) / vdP dy g
2 21(4) ( , ) ( ) ( )
2l g
l
g x xv x y 7/12
Laminar Film – First thermodynamics law
The first law of thermodynamics:
Substituting conduction & (2):
Substituting (1)
Integrating from y=0
Local mass flow rate:
Vertical enthalpy inflow:
Assume linear temperature distribution:
3
0(1) (y) ( )
3l
l l v
l
gvdx
,0(2) [ ( )]l f P l satH v h c T T dx
''(3) 0 ( ) hg wH H dH d q dy
'
,
3[ ( )]
8(4) ( )
fg
fg P l sat ts
h
a wl
w h c T Tk
T T dy d
3
'
( )(5)
( )l l sat w
fg l v
k v T Tdy d
h g
1/4
'
4 (T T )(6) ( )
( )l l sat w
fg l v
k vy y
h g
1sat
sat w
T T x
T T8/12
Laminar flow – Results
Now we can calculate Heat transfer coefficients:
Similar results can be obtained by Scale Analysis (Similar to laminar boundary layer natural convection)
Rohsenow refined preceding analysis by discarding linear profile assumption and performing an integral analysis.
Rohsenow recommends:
Jacob number = relative measure of subcooling:
To summarize:
1/43 ''' ( )(1)
4 ( )
l fg l vly
sat w l sat w
k h gq kh
T T yv T T
4
(2)3
L y Lh h
1/43 ' ( )(3) 0.943
( T )
fg l vLL
l l l sat w
L h gh LNu
k k v T
',(4) 0.68 ( ) (1 0.68 )
fgfg p l sat w fgh h c T h JaT
, ( )
(5) P l sat w
fg
c T TJa
h
'
(6) ( ) ( )lsat w L
fg
kL T T Nu
h '(7) ( ) (1 0.68 )fgq L h Ja 9/12
Laminar Film - Diagram
In the preceding analysis were derived by Nusselt, based on negligible inertia assumption
The complete momentum equation used by Sparrow & Gregg in similarity solution.
Their solution for Nu falls below Nusselt’ssolution -> Effect of Inertia
Chen abandoned the assumption of zero shear at interface, retaining effect of inertia
His results for Nu are smaller than Sparrow & Gregg’s solution -> Effect of restraining drag of vapor
Better agreement with experimental data 10/12
Turbulent Film - Diagram
Reynolds number of liquid film:
Experimental observations:
Laminar: Re < 30
Wavy: 30 < Re < 1800
Turbulent: Re > 1800
Experiments revealed that heat transfer
rate in wavy and turbulent regions is
considerably larger than laminar section
Following relation developed by Chen for
wavy and turbulent region:
4(1) Re (y)y
l
2
1/3 0.44 6 0.8 1.3 1/2(2) ( ) (Re 5.82 10 Re Pr )L l
L L L
l
h
k g
11/12
Summary
Condensation is phase change from Gas to Liquid
There’re different types of condensation:
Surface (Film, Droplet), Homogeneous, Direct contact
Film surface condensation has three regions:
Laminar, Wavy, Turbulent
Laminar film condensation first solved by Nusselt by neglecting Inertia effect
Complete momentum equation used by Sparrow and Gregg in similarity solution
Chen Solution for laminar film contains vapor drag effect and inertia effect of liquid
Chen reviewed and developed a relation for Wavy and Turbulent regions
12/12
Thank you!
13