Chapter11-Heat Transfer with Phase Change.pptx

Heat Transfer with Phase Change
Dr. Rashid Ali

Types of Boiling
• Pool boiling: vapor bubbles are formed due to heat
addition to the liquid by a heater surface in contact
with liquid or submerged in the liquid.
• Flow boiling: vapor bubbles are formed in the flowing
liquid.
• Volume or bulk boiling: occurs with in the bulk of the
liquid.
• Nucleate boiling: bubbles of vapor are formed around a
small nucleus of vapor or gas with in the liquid.
• We may have both pool nucleate boiling or volume
nucleate boiling.
•

Types of Boiling
• Film boiling: the vapor blankets the heating surface.
Film boiling is therefore associated with pool
boiling.
• Nucleate and film boiling may coexist in many
situations.

Types of Boiling
• Saturated boiling: where the bulk of the liquid is at
the saturation temperature corresponding to the
system pressure.
• Sub-cooled boiling: where the bulk of the liquid is
below saturation temperature but local boiling
occurs with in the liquid or at solid-liquid interface.
• In both cases, to generate the bubble, the surface
temperature must be higher than the saturation
temperature. i.e. the liquid adjacent to surface is at
saturation temperature but the bulk of the liquid
could be saturated or sub-cooled.

Types of Boiling
• In saturated boiling, the bubbles grow, detach and
rise to the liquid surface.
• In sub-cooled boiling the bubble may collapse
before it reaches the liquid surface.
• Saturated and sub-cooled boiling may be either of
nucleate or film type boiling.
• Volume boiling is most of the times of saturated
type.

Two-Phase flow
• In two-phase flow, both the liquid and the vapor
move together in a channel.
• 2-phase flow may be diabatic or adiabatic.
• 2-phase flow may be classified as single component
flow such as water-steam flow or two component
flow such as water-air flow.
• In all cases the velocities of the two phases are
usually not equal.
• The ratio of the vapor and liquid velocity is called
the slip ratio or simply the slip.

Condensation
• Conversion of vapor to liquid, for example
conversion of steam to liquid.
• Filmwise condensation: continuous flow of liquid
over cooling surface.
• Dropwise condensation: vapor condenses in the
form of drops and cooling surface is not fully
covered with liquid.

Bubble Ebullition Cycle
• The process of bubble formation, growth and
detachment (also called the bubble nucleation
process) during the boiling process.
• Bubbles are formed due to liquid superheat
adjacent to the heating surface.
• Nucleation aids: the nucleation is aided by the
following
• 1-gas or vapor present in the liquid 2-surface
scratches or cavities 3-wetting characteristics of the
liquid-surface.

• 1-gases or vapor present in the liquid: in a nuclear
reactor there are more nucleation sources as
compared to a conventional boiler due to the
presence of ionizing radiations and by oxygen and
hydrogen from the radiolysis of water-cooled
reactors.
• The charged ions acting as nucleation centers
further help bubble motion because of the
electrical repulsive forces of the now-charged
bubbles.

• 2-Scratches or cavities: scratches or cavities
also act as nucleation sites due to entrapped
gas or vapor. More scratches more nucleation
sites and more vigorous boiling on a surface.

• 3- wetting characteristics of heating surface: surface
tension force is important in this phenomenon.
Three types of surface tension forces are acting
• σfg: surface tension acting on the liquid vapor
interface
• σfs: liquid-suraface interface
• σgs: vapor-surface interface
• σgs= σfs+σfg×cosβ
• β is called the contact angle
and is defined between the
vapor bubble and the surface

• If β is larger than 90 degrees, the liquid is not
wetting the solid surface and if less than 90 degrees
it is wetting and 0 degrees means a perfectly
wetting liquid.
• Therefore, the degree of wetting depends upon
how far the contact angle deviates from 90 degrees.

• Bubble Growth: depends upon the contact angle.
• Maximum rate of growth occurs when surface is
unwetted (in this case the superheat required is less).
• From heat transfer standpoint, a poorly wetted surface
is not suitable because vapor bubbles are difficult to
detach from the surface, grow rapidly, coalesce and
form a continuous film barrier to heat flow from the
heating surface to the bulk liquid.
• A desirable boiling surface from wetting point of view is
one that is partially wetted over most of its area, and
rest of the area with tiny unwetted patches which act
as nucleation centers and formed bubbles then overlap
with nearby wet surface where they can detach.

• Another desirable boiling surface would be one that
is also partially wetted over most of its area but
contains scratches or tiny cavities which act as
nucleation sites.
• All the nucleation centers are not equally effective
i.e. at a certain degree of superheat not all the
cavities are active.
• A superheated liquid is one that has higher
temperature than the saturation temperature
corresponding to its pressure.

• Liquid superheat: that is required to initiate bubble
formation depends upon the latent heat of
vaporization of the liquid and the surface tension of
the liquid (surface energy).
• The degree of superheat required to initiate
nucleation in a clean system may be higher in a
clean system.
• Degree of superheat associated with a conical cavity
is given by

• Liquid superheat required by liquid metals is
higher than the nonmetallic liquids due to:
• Low Prandtl numbers which makes temperature
profile much less steep near the heating surface.
• Liquid metals generally have good wetting
characteristics which makes the cavities flooded so
that there is a dearth of active nucleation sites.

• Degree of superheat associated with liquid adjacent
to a free vapor bubble and is given by the force
balance between vapor pressure inside the bubble,
liquid pressure outside the bubble and the surface
tension force, as

• Bubble Detachment: a bubble detaches from the
surface when the buoyancy force overcomes the
capillary force.
• The greater the ratio between the bubble volume
and the contact area, the more easily can the
bubble detach from the surface.
• A wetted surface makes it easier for bubbles to
detach themselves than a non-wetted surface.
• Bubble detachment causes agitation in the liquid
and enhances the heat transfer.

• As the bubble detaches from the surface, the
relatively cooler liquid rushes to fill the space of the
bubble and the process of bubble formation,
growth and detachment continues.
• Two possibilities if heat flux increases during
boiling: 1- bubble detachment rate (frequency)
increases. 2- detached bubbles merge to form larger
vapor plugs.
• In both the cases, the heat transfer is enhanced.
• Note: at low heat fluxes, the bubbles remain
separate i.e. bubbly flow regime

Boiling
• During boiling process, the evaporation occurs at
the liquid vapor interface and the heat is
transferred from solid surface to liquid.
• Heat transfer is given by Newton’s law of cooling
• Ts is the surface temperature and Tsat is the
saturation temperature of the liquid.

Boiling
• Temperature gradient near the surface in a
saturated pool boiling case

Boiling Curve: Pool Boiling
• Nukiyama first identified the different flow regimes
in pool boiling of a Platinum wire in a pool of water.
• The system was a power controlled setup.

Boiling Curve: Pool Boiling
• Heating and cooling curve of Nukiyama’s power
controlled experimental setup is shown. In heating
mode temperature jumps from 30 to more than
1000 at maximum heat flux

Boiling Curve: Heat Transfer Regimes
• O-A: Single phase Free convection
• Nu=0.15 Ra1/3
• Ra=gβ(Tw-Tf)L3/(µfkf/Cpρ2)
• Valid for 107 <Ra< 1010

Boiling Curve: Boiling Regimes
• Free Convection Boiling regime:
• A-B: ∆Te≈5-10
• Surface temperature must be higher than
saturation temperature for boiling to start.
• Point A is called the onset of nucleate boiling
(ONB)
• In this region the heat transfer is from solid to
liquid and not through vapor bubbles.
• In this regime, the h can be calculated as in free
convection process

• Nucleate Boiling regime:
• Nucleate boiling exists between point A and
point C, ∆Te≈5-30
• When ∆Te≈5, there are isolated bubbles
• As ∆Te increases the bubble frequency
increases, bubble merger process is enhanced,
agitation is enhanced and consequently heat
transfer is enhanced, and this happens up to
∆Te≈30,

Boiling Curve: Boiling Regimes Nucleate boiling
• Rohsenow Equation for nucleate boiling on clean
surfaces.
Csf or Csg is a dimensionless constant, its value depends upon the solid-
fluid combination, especially the degree of wettability between the
heating surface and the fluid.
Exponent n in the equation is also dependent on the solid-fluid
combination

Nucleate boiling
• Values of Csf and n for different solid-fluid
combinations used for Rohsenow equation

Nucleate boiling
• Plot for Rohsenow
Equation.
Constant c in the
equation is Dependent
on wetting properties
And is independent of
pressure

Flow Boiling:Nucleate boiling
• Tube flow conditions:
• Tube inner dia: 0.781 mm
• Length: 190 mm
• Fluid: R134a
• Tsat: 30 C and 25 C
• G: 100 to 400 kg/m2 sec

Flow Boiling:Nucleate boiling
• Two-phases have
• Different
• Velocities and
• Shows the slip

Boiling crisis or Burn out or DNB
• Heat flux corresponding to point c on boiling curve.
• Also called critical heat flux (CHF), DNB, Burnout
heat flux, Boiling crisis, hydrodynamic crisis.
• At point c on boiling curve, a further increase in
heat flux results in a sharp or abrupt increase in
temperature of the surface and hence temperature
difference.
• Heat flux corresponding to this temperature is
called burnout heat flux, critical heat flux (CHF),
departure from nucleate boiling (DNB), boiling
crisis.

• Temperature at c’ (for point c’ refer to El Wakil book,
corresponding point in the given figure is E)) is so high
that the safe limits are exceeded and burnout occurs.
• Burnout or CHF in a nuclear fuel element may result in
fuel-cladding rupture which may result in release of
radioactive gases or solids into coolant.
• It is therefore, avoided to operate a boiling system
close to this heat flux or DNB point.
• Therefore, it is of great importance to study and know
the CHF point hence, safe limits of operation.

Flow Boiling: Boiling crisis or Burn out
or DNB
• Flow boiling crisis scenario shown in the figure.
• Curve a:
fluid temperature
Curve b: wall
Temperature at low
heat fluxes.
Curve c: wall
temperature at high
Heat fluxes

Flow Boiling: Boiling crisis or Burn out
or DNB

• Kutateladze and Zuber correlation for CHF
Value of C varies between 0.13 to 0.19 with average value of 0.14

Boiling Curve: Boiling Regimes:
Transition boiling
• C-D: ∆Te≈30-120
• This region is also termed as partial film boiling and
unstable film boiling.
• Conditions oscillate between nucleate boiling an
film boiling.
• Heat transfer coefficient h decreases with increase
in ∆Te.
• This region can not be achieved in heat flux
controlled systems.

Boiling Curve: Boiling Regimes: Film
boiling
• D-E: ∆Te ≥120
• Point D of the boiling curve is also called as
Leidenfrost point or minimum heat flux point.
• In this region the heater surface is completely
covered with vapor blanket and heat transfer is
mainly due to conduction and radiation through the
vapor.
• In this region as the radiative heat transfer
increases therefore heat flux also increase in this
region.

Boiling Curve: Boiling Regimes: Film
boiling
• The Minimum heat flux point, D is of interest
because:
• if the heat flux drops at this point (i.e. D), then the
nucleate boiling is restored.
• If heat flux increases, a stable film boiling can be
achieved beyond this heat flux.
• Zuber’s correlation for q”D=q”min is given as

Parametric effects on boiling and CHF
• Pool Boiling
• Nucleate boiling regime apart from some factors
discussed before (such as number of active
nucleation sites, bubble dynamics etc), it also
depends upon the operating parameters.
• Pressure effect: pressure affects the thermophysical
properties and thereby the nucleate boiling regime.
• Nucleate boiling regime and the CHF point in the
boiling curve shift with pressure.

• There is optimum
• Point for operating
pressure with
Regard to CHF.
It is 1/3 of critical
Pressure after that
CHF decreases
Again. At optimum
Pressure CHF is 4 to 10
Times of that at low pr

• CHF for
Various
Fluids.
It shows
the
Optimum
Pressure
For
High CHF
value

• Effect of Velocity
and Inlet sub-cooling:
Data is for water
Flowing in annuli
Where inner tube is
Heated electrically.
Onset of boiling and
CHF depend upon both
Velocity and
sub-cooling

• CHF is found to increase with certain
additives, particularly those with molecules
much heavier than the boiling fluid, with
ultrasonic and electrostatic fields.
• CHF is found to decrease with the presence of
dissolved gases and surface agents.

• Flow boiling:
• Main parameters
• The affecting parameters are:
• Heat flux
• Mass flux
• System pressure
• Thermo physical properties
• Inlet enthalpy (sub-cooling)
• The size and the shape of the channel

• Less important factors:
• Type of heating such as uniform, non-uniform
• Surface roughness
• Unheated walls near the CHF point
• Dissolved gases and additives

• The exit enthalpy at which CHF is likely to
occur, depends upon both the mass flux and
inlet enthalpy or sub-cooling and to a lesser
extent on pressure.

• Water flow
• In annuli data
• ONB, CHF Both Depend
• On G and Sub-Cooling
• Nucleate Boilng heat
• Flux only Depend on SC

• Data of water in vertical round tubes
• CHF increases with G and decreases with inlet
temperature, inlet quality, tube diameter, and
decreases slightly
with pressure
No effect of flow
Direction (up or down)
on CHF

Chapter11-Heat Transfer with Phase Change.pptx

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Chapter11-Heat Transfer with Phase Change.pptx