Boiling Heat Transfer
Source:
• Vishwas V. Wadekar, HTFS, Aspen Technology
• J.P. Holman
Boiling Heat Transfer
• Definitions/Terminology
– T surface > Tsat of liquid
 boiling may occur and
heat flux depends on T
– Pool boiling process
 heated surface is
submerged below a free
surface of liquid.
– Subcooled or local boiling
 Tof liquid < Tsat
– Saturated or bulk boiling
 Tof liquid = Tsat
Two Modes of Heating
Region I = Single phase
• No bubbles, wall
superheat too low
• Motion of fluid near
surface = free convection
currents
• Liquid near heated
surface = superheated
slightly, when it rises to
liquid surface, it
evaporates.
• Calculation uses free
convection relations.
Region II
• Bubbles begin to form
on surface of a wire
and dissipated in
liquid after breaking
away from surface.
• This region indicates
the beginning of
nucleation boiling
Nucleate
boiling
Coefficient
increases with
Temp excess
Region III
Nucleate
boiling
• Tx increases,
bubbles form more
rapidly and rise to
surface of liquid and
dissipated.
Coefficient
increases with
Temp excess
Region IV
• Bubbles form so rapidly
and they blanket the
heating surface and
prevent the inflow and of
fresh liquid from taking
their place.
• Bubbles coalesce and
form vapor film (cover
the surface)
• Film cause thermal
resistance due to
reduction in heat flux.
transition
boiling
Film boiling region: this region is
transition region (from nucleate to
film boiling)
The film is unstable.
Region V
• Vapor film at wall
• Stable film boiling
• Surface temperature
is high to maintain
stable film boiling.
film boiling
Region VI
• Heat loss from surface
is the result of
thermal radiation.
•
•
•
Point a  wire is unstable, small increase in T  Critical heat flux
Point b  this temp. is higher than melting Temp. of wire (cause of burnout
results)
If maintain at point a  partial nucleate boiling and unstable film region
p = pv-pl
pl , Tl
Bubbles
pv , Tv
• If Tv = Tsat and Tl < Tsat  heat
conducted out of bubble and vapor
condense  bubble collapse
• If Tl > Tv  a metasatable condition
 bubble growth after leaving the
surface
Copper rod heated and immersed in isopropanol
free convection boiling  nucleate boiling  film boiling
• Boiling of methanol on a
horizontal steam-heated copper
tube
Nucleate boiling
q/a = 242.5 kW/m2
Temp excess = 37C
Transition boiling
q/a = 217.6 kW/m2
Temp excess = 62C
Film boiling
q/a = 40.9 kW/m2
Temp excess = 82C
Calculation of boiling heat transfer
• Nucleate pool boiling : Rohsenow
• This eqn. can use for geometries other than
horizontal wire.
• Geometry is not a strong factor in determining heat
flux for pool boiling.
Vapor-liquid surface tension for water
Heat flux data for
water boiling on a
platinum wire
(numbers in
parentheses are
pressure in
MN/m2)
Example
• A heated brass plate is submerged in a
container of water at atmospheric pressure.
The plate temperature is 242F. Calculate the
heat transfer per unit area of plate.
Forces convection boiling
occurred when surface Temp > Tsat of liquid
This equation is applicable to forced convection where
the bulk liquid temp. is subcooled (local forced
convection boiling)
For fully developed nucleate boiling
independent of flow velocity or forced convection effects
For low pressure boiling water
For high pressure boiling water
Peak heat flux for nucleate pool boiling
• Zuber equation:
Simplified relations for boiling heat
transfer with water
For forced convection local boiling
inside vertical tubes:
Valid for 5-170 atm
p is pressure in Mpa
Example:
Water at 5 atm flows inside a tube of 2.54 cm diameter
under local boiling conditions where the tube wall
temperature is 10C above the saturation temperature.
Estimate the heat transfer in a 1.0 m length of tube.