BOILING AND CONDENSATION
Boiling: General considerations
• Boiling is associated with transformation of liquid to vapor at a solid/liquid interface due to
convection heat transfer from the solid.
• Agitation of fluid by vapor bubbles provides for large convection coefficients and hence large
heat fluxes at low-to-moderate surface-to-fluid temperature differences.
• Special form of Newton’s law of cooling:
qs” = h(Ts - Tsat) = hATe
where, Tsat is the saturation temperature of the liquid, and ∆T e = (Ts – Tsat) is the excess
temperature.
Special cases
Pool Boiling: Liquid motion is due to natural convection and bubble-induced mixing.
Forced Convection Boiling: Fluid motion is induced by external means, as well as by bubble-
induced mixing.
Saturated Boiling: Liquid temperature is slightly larger than saturation temperature.
Subcooled Boiling: Liquid temperature is less than saturation temperature.
The boiling curve
The boiling curve reveals range of conditions associated with saturated pool boiling on a qs”
vs ∆Te plot.
Water at Atmospheric Pressure
Pool boiling regimes:
A-B: Pure convection with liquid
rising 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.
Free Convection Boiling (∆Te <5oC)
Little vapor formation.
Liquid motion is due principally to single-phase natural convection.
Onset of Nucleate Boiling – ONB (∆Te ≈ 5oC)
Nucleate boiling (5oC < 30oC)
Isolated Vapor Bubbles (5oC < ∆Te < 10oC)
Liquid motion is strongly influenced by nucleation of bubbles at the surface.
h and qs” rise sharply with increasing ∆Te.
Heat transfer is principally due to contact of liquid with the surface (single-phase
convection) and not to vaporization.
Jets and Columns (10oC < ∆Te < 30oC)
Increasing number of nucleation sites causes bubble interactions and coalescence into jets
and slugs.
Liquid/surface contact is impaired. qs”continues to increase with ∆Te while h begins to
decrease.
Critical Heat Flux - CHF, (∆Te ≈ 30oC)
Maximum attainable heat flux in nucleate boiling.
Potential Burnout for Power-Controlled Heating
An increase in qs” beyond qmax.
qs” causes the surface to be blanketed by vapor and its temperature to spontaneously achieve
a value that can exceed its melting point
If the surface survives the temperature shock, conditions are characterized by film boiling.
Film Boiling
Heat transfer is by conduction and radiation across the vapor blanket. A reduction in q s”
follows the cooling the cooling curve continuously to the Leidenfrost point corresponding to
the minimum heat flux min
q
for film boiling.
A reduction in
s
q below
min
q
causes an abrupt reduction in
surface temperature to the nucleate boiling regime
Liquid/surface contact is impaired.
q” continues to increase with A-T while h begins to decrease
Critical Heat Flux - CHF, (ATe *30°C)
> Maximum attainable heat flux in nucleate boiling.
qmax * 1 MW/m2 for water at atmospheric pressure.
Potential Burnout for Power-Controlled Heating
> An increase in q” beyond qmax causes the surface to be
s
blanketed by vapor and its temperature to spontaneously achieve
a value that can exceed its melting point
> If the surface survives the temperature shock, conditions are
characterized by film boiling
Film Boiling
> Heat transfer is by conduction and radiation across the vapor
blanket
> A reduction in q" follows the cooling the cooling
s
curve continuously to the Leidenfrost point corresponding to the
minimum heat flux qmin for film boiling.
> A reduction in q" below qmin causes an abrupt reduction in
s
surface temperature to the nucleate boiling regime
Transition Boiling for Temperature-Controlled Heating
> Characterised by continuous decay of q" (from qmax to qmin)
s
with increasing AT e
Geometry C
Cylinder(Hor.) 0.62
Sphere 0.67 conditions oscillate between nucleate and film
> Surface
boiling, but portion of surface experiencing film boiling
increases with Me
> Also termed unstable or partial film boiling.
8.1 Pool boiling
correlations
Nucleate Boiling
> Rohsenow Correlation, clean surfaces only, ±100% errors
Cs f , n ^
Surface/Fluid
Combination
Critical
heat
flux:
Film Boiling
8.2 Condensation: General considerations
• Condensation occurs when the temperature of a vapour is
reduced below its saturation temperature
• Condensation heat transfer
Film condensation
Dropwise condensation
Ts- TEat
Vapour
Drop
Heat transfer rates in dropwise condensation may be as much as
10 times higher than in film condensation
8.6 Laminar film condensation on a vertical wall
.. 3u |
“fiyjyA hi! A
% J y+Ay
1
(Pj -Py)gAAy
h
fgg(Pl -PV )ki
1/4
Average coeff. hL _ 0.943 L(T
sat - Tw
where L is the plate
length. 9 _ hLA(Taa, - Tw )
Total heat transfer rate : q _ hLA(Tsat - Tw )
yjfc= _
h fg h fg
