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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?