condensatio heat transfer m8teacherslides

8/9/2019 Condensatio Heat Transfer M8TeacherSlides

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Condensation and

Boiling

Until now, we have been considering

convection heat transfer in homogeneous

single-phase (HSP) systems Boiling and condensation, however,

provide much higher heat transfer rates

than those possible with the HSP systems


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Condensation occurs when the temperature of a vapor is reduced

below its saturation temperature

Condensation heat transfer

Film condensation

Heat transfer rates in drop wise condensation may be as much as

10 times higher than in film condensation

Condensation

Drop wise condensation

Film

Ts


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Laminar Film condensation on a

vertical wall (VW)

Condensate Film

g

y

y

x

Tsat

(x)

A

T

y

x

A A

y

yyu

l

yyyu

l

ygA)vl

(

A


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Laminar Film condensation on a

vertical wall (cont..)4/1

lwsat

3

lvlfg

4/1

vlfg

lwsatl

)TT(x4

k )(gh

)x(h)(gh

)TT(xk 4

)x(

fg

wsatL

fg

wsatL

4/1

lwsat

3

lvlfg

L

h)TT(Ah

hq m :rateonCondensati

)TT(Ahq :rateferheat transTotallength. platetheisLwhere

)TT(Lk )(gh943.0h coeff.Average


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Example

Laminar film condensation of steamSaturated steam condenses on the outside of a 5 cm-diameter

vertical tube, 50 cm high. If the saturation temperature of the

steam is 302 K, and cooling water maintains the wall temperatureat 299 K, determine: (i) the average heat transfer coefficient, (ii)

the total condensation rate, and (iii) the film thickness at the

bottom of the tube.

Given: Film condensation of saturated steam

Required: (i) Average heat transfer coefficient, (ii) total

condensation rate, (iii) and film thickness

1. Effect of tube curvature negligible

2. Effect of liquid sub cooling negligible

3. Laminar


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Example (contd...)

Condensate Film

g

y

y

x

Tsat

(x)

A

T

4/1

lv)wTsat

T(L

3l

k )vl(g'

fgh

943.0 _ h

: bygiveniscoefficentsferheat trasnAverageThe

3m/kg03.0v kg/J610432.2

fgh

: propertieswaterof TableFrom

Evaluate hfg at the saturation temperature of302 K


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Example (contd...)

kg/s1033.7)10432.2(

)5.0)(05.0()3)(7570(

:israteoncondensatitotalThe(ii)

K W/m7570)1087.0)(3)(5.0(

)611.0(03.0996)81.9)(10432.2(943.0

943.0

/sm100.87

kg/m996

W/mK 611.0

for water Also,

46

2

4/1

6

36

4/13

26-

3

fg fg

lwsat

lvl fh

l

l

l

h

T Ah

h

Qm

T T L

k gh

h

k


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Example (contd...)

m1008.1)81.9)(996(

)1067.4)(103(0.87 Hence,

kg/ms1067.4)05.0)((

)1033.7(

:isfilmof unit width perrateflowmassThe

3

isthicknessfilmTheiii)(

4

3/136-

34

3/1

l

D

m

g lvl


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Boiling occurs when the surface temperature Tw exceedsthe saturation temperature Tsat corresponding to the liquid pressure

re)temperatu(excess TTT where Th)TT(hq :rateferHeat trans

satwe

esatws

Boiling process is characterized by formation of vapor

bubbles, which grow and subsequently detach from the

surface

Bubble growth and dynamics depend on several factors such

as excess temp., nature of surface, thermo physical

properties of fluid (e.g. surface tension, liquid density, vapor

density, etc.). Hence, heat transfer coefficient also depends

on those factors.

Boiling


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Pool Boiling Curve

Pool boiling regimes:

A-B: Pure convection with liquidrising to surface for evaporation

B-C: Nucleate boiling with bubbles

condensing in liquid

C-D: Nucleate boiling with bubbles

rising to surface

D: Peak temperature

D-E: Partial nucleate boiling and

unstable film boiling

E: Film boiling is stabilized

E-F: Radiation becomes a

dominant mechanism for heat

transfer


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Modes of Pool Boiling

Free convection boiling

Nucleate boiling

Transition boiling

Film boiling C T C T C

C T C

C T

e

e

e

e

12012030

305

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