13. boiling
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13. BOILING HEAT TRANSFER
Many engineering applications involve condensation orboiling phase change
Refrigerator refrigerant boils in the evaporation section
refrigerant condenses in the condenser section Power plant steam production by boiling water in the
boiler steam condensation in the condenser
=p
p kPapp 381,. ==
Evaporation - at the liquid-vapor interface whenthe vapor pressure less than the saturation pressure
at the given temperature.
Water at 20C And relative humidity =60%.Saturation pressure (see Steam tables) kPap 32,=
Evaporation: human body, drying wet clothes, vegetables, fruits, etc.
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Boiling Heat Transfer
Boiling occurs at the solid-liquid interface whenthe liquid is brought into contact with a surface
with temperature Tw above the saturation
temperature Tsat.
Boiling rapid formation of vapor
bubbles.
Bubbles detach from the surface and rise inthe liquid to reach the free surface.
In forced and natural convections physical parameters
involved , , , cp.In boiling new parameters latent heat lv (or l23 or r) and
surface tension
]/[)( 2mWTTTq excesssatwboiling == &Heat flux:
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Boiling Heat Transfer
Pool boiling (var ve velkm objemu) liquid
is quiescent, motion due to free convectionnear the surface, mixing induced by bubble
growth and detachement
Convection boiling (flow boiling) motiondue to external forces and free convection
near the surface, mixing induced by bubble
growth and detachement
Boiling occurs in two basic modes:
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Boiling Heat Transfer
Boiling also occurs as subcooled (liquid temperature below
the saturated temperature bubbles collapse and condense)
and saturated
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Boiling Curve
Notice extremely
high heat flux
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Modes of Pool Boiling
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Boiling Crisis
In case there is no control of
heat flux transfered at the surface
What happens when the heat flux
is increased beyond themax
q&
What happens when the heat flux
is decreased below the minq&
What happens with the vapor film?
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Boiling Heat Transfer
For practical applications the best
is to operate devices in the region
of nucleate boiling highest heat
transfer rates.
Which mechanism is responsible
for such high heat transfer?
Convective motion of bubbles they detach from the surface
and move up new liquid is entrained towards the surface
- up to 98% of heat transferred from the surface
Transfer of latent heat of evaporation
Why increasing the heating (temperature of heated plate i.e.
Texcess) results in an increase of heat transfer rate?
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Boiling Heat Transfer
banTfq =&a = 1.2, b=1/3
n number of nucleation sites
65Tn
3Tfq &
vnuvnit PP
2R
=
Equilibrium bubble radius
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Boiling Heat Transfer
Heat transfer coefficient in the nucleate boiling region for waterat 2.104
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Convective (flow) Boiling Heat Transfer
2
L
V
L
TP
1
+=
Nucleate boiling region
Complication various flow
patterns - slug, annular
0,5
ttLTP X
13,5
=
Annular boiling region
dry-out
burn-out
Roughly, to which point on the boiling curve corresponds
the burn-out point?
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