BOILING

AND

CONDENSATION

Isotherms of real gas

Evaporation

Boiling

Boiling

and

evaporation

- the liquid-to-vapour

phase change

processes that occur at a solid-

liquid interface when the surface is heated
above the

saturation temperature T

sat

of the

liquid →

→

→

→ convection heat transfer.

Evaporation

occurs when the vapour

pressure

is less than the saturation

pressure

of the liquid at a given

temperature, and it involves no
bubble formation or bubble motion.

Boiling

occurs when a liquid is

brought into contact with a surface
maintained at a temperature T

S

sufficiently above the saturation
temperature T

sat

of the liquid.

Refrigerators
Steam power plants
Electronic system
cooling

Water
20

0

C

Boiling →

→

→

→ convection heat transfer →

→

→

→ the boiling heat flux

from a solid surface to the fluid – Newton`s law of cooling:

excess

sat

S

boiling

T

h

T

T

h

q

∆

=

−

=

•

)

(

)

/

(

2

m

W

where ∆

∆

∆

∆T

excess

= T

S

– T

sat

is called the excess temperature,

which represents the excess of the surface above the
saturation temperature of the fluid

Two phases (liquid and vapour) involves:
- two sets of thermophysical properties (density

ρ

ρ

ρ

ρ

, dynamic

viscosity

µ

µ

µ

µ

, conductivity

Λ

Λ

Λ

Λ

, and the specific heat

C

p

)

- the latent heat of vaporisation

h

fg

- the surface tension

σ

σ

σ

σ

Parameter

h

fg

represents the energy absorbed as a unit mass of

liquid vaporizes at a specified temperature or pressure.

Surface tension

→

→

→

→ bubbles →

→

→

→ thermodynamic non-

equilibrium

conditions →

→

→

→ different temperature in the

bubble than in liquid.

The pressure difference between the liquid and the
vapour is balanced by the surface tension at the
interface →

→

→

→ the driving force for heat transfer between

two phases.

When the liquid is at a higher T than the bubble, heat will
be transferred from the liquid to the bubble →

→

→

→ the bubbles

grow and rise to the top under influence of buoyancy.

Surface tension

Surface tension in liquids →

→

→

→ in an elastic membrane (2D

effect) →

→

→

→ analogy to tension in an elastic spring (1D effect)

∆

∆

∆

∆l

∆

∆

∆

∆x

T

r

∆

T

r

∆

−

Any line element of the surface of
the „membrane” is in equilibrium
due to equal and opposite forces
exerted perpendicular to ∆

∆

∆

∆l by the

parts of the „membrane” on either
side.

The surface tension σ

σ

σ

σ →

→

→

→ the magnitude of the tensile force

per unit length:

l

|

T

|

limit

0

l

∆

∆

=

→

∆

r

σ

)

/

(

m

N

Note

: The surface tension in a liquid does not change as the

liquid surface is „stretched”.

Effects related to surface tension: liquid drop formation
(surface energy minimum condition) or soup bubbles.

Natural convection boiling

regime

- the fluid motion is governed by
natural convection currents, and
heat transfer from the heating
surface to the fluid is by natural
convection.

Nucleate boiling

regime - bubbles

form at various preferential sites
on the heating surface, and rise
to the top.

Transition boiling

regime - part

of the surface is covered by a
vapor film.

Film boiling

regime - the heater

surface is completely covered by
a continuous stable vapor film,
and heat transfer is by combined
convection and radiation.

Mechanisms of boiling

→

→

→

→ pool boiling

Pioneering work by
S. Nukiyama (1934)

∆

∆

∆

∆T

excess

=T

S

-T

sat

,

0

C

The boiling curve

water

The burnout point C

Boiling regimes (methanol)
on a horizontal 1 cm-diameter
steam-heated copper tube

(b) Transition boiling

(a) Nucleate boiling

(c) Film boiling

The actual boiling curve
obtained with platinum
wire in water as the heat
flux is increased and then
decreased.

The boiling curve

Condensation

Temperature of a vapour - reduced below T

sat

Larger values of
heat transfer rate
→

→

→

→ preferred mode
of condensation

Condensation on a plate

• Influence of gravity.

• T

s

must be below T

sat

of the

vapour for condensation to occur.

• Temperature of the condensate is
T

sat

at the interface and decreases

gradually to T

s

at the wall.

• The velocity of the condensate at
the wall is zero because of the „no-
slip” condition.

• Velocity maximum at the liquid –
vapour interface.

v

v(y)

3

/

1

2

3

/

1

Re

47

.

1













Λ

≅

−

l

l

vert

v

g

h

Heat transfer coefficient:

l

v

ρ

ρ

〈〈

〈

〈

30

Re

0

Film condensation on an inclined plate

4

/

1

)

(cos

θ

vert

inclined

h

h

=

for laminar flow of condensate

Dropwise condensation of steam on a vertical surface

One of the most effective mechanisms of heat transfer →

→

→

→

extremely large heat transfer coefficients (more than 10 times
larger than in case of film condensation) →

→

→

→ preferred mode of

condensation (efficient condensers) →

→

→

→ adding a promoting

chemical into the vapour, treating the surface with a promoter
chemical or coating the surface with a polymer (teflon) or a
noble metal (gold, platinum, silver)

Heat exchangers