Nucleate Boiling
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Transcript of Nucleate Boiling
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Presented by – Astha Airan
Chemical Engineering, IIT Bombay
Guide – Prof. Ravi Kumar, IIT Roorkee
BOILING HEAT TANSFER AND
CRITICAL HEAT FLUX
9th INDO-GERMAN WINTER ACADEMY
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OUTLINE
BOILING
POOL BOILING MODES
Nucleate Boiling
Critical Heat Flux
Film Boiling
Bubble Growth & Heat Transfer Models
FORCED CONVECTION BOILING
Burnout & Factors Influencing burnout
Burnout Evaluation Methods
SUMMARY
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MOTIVATION
Nucleate Boiling is an efficient mode of heat transfer. It has
useful application in many areas such as refrigeration, power
generation, chemical processing and nuclear reactors.
Avoiding the Critical Heat Flux is an engineering problem in
heat transfer applications, such as nuclear reactors, where
fuel plates must not be allowed to overheat.
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BOILING
Evaporation occurs at solid liquid interface.
Characterize by formation of bubbles which grow and
subsequently detach from the surface.
POOL BOILING –
Liquid is quiescent.
It’s motion is induced by free convection and mixing induced
by bubble growth and detachment.
CONVECTIVE BOILING –
Fluid motion is induced by external means.
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POOL BOILING MODES
IB
DNB
MFB
Fig : Boiling Curve
Natural convection (Twall < TIB) –
Heat transferred by single phase natural
convection.
Nucleate Boiling (TIB < Twall < TDNB) –
Bubble production commences on surface.
Initially small number of nucleation sites
are active.
At higher flux number of nucleation sites increases
Bubbles coalesce and form irregular columns of vapor leaving the surface .
At D departure from nucleate boiling occurs also known as Critical Heat Flux.
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POOL BOILING MODES
Transition Boiling(TDNB < Twall < TMFB) –
Vapor film begins to form.
Thermal conductivity of vapor is much less than that of
liquid.
Flux decreases.
Film Boiling (TMFB < Twall) –
Heated surface is covered by a continuous film of vapor.
Heat is transferred mainly by radiation.
Flux increases.
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Fig : Pool Boiling Process
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Rohsenow(1952) was the first to develop a model for
nucleate boiling.
From similarity analogy:
Diameter of a bubble on it’s departure from heated surface
can be determined from force balance
Characteristic velocity for agitation of liquid can be found by
the distance the liquid travelled to fill in behind the departing
bubble by the time between bubble departure
NUCLEATE BOILING
Energy it takes to form a
bubble
Rate at which heat is added
over the solid-vapor contact
area
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NUCLEATE BOILING (contd.)
From these equations we get
But this correlation when applied can result in 100% error.
Effect of surface roughness - Increasing surface roughness
provide larger sites for nucleation thus increasing heat flux.
Effect of pressure - As λ increases with increase in pressure the
nucleate boiling heat flux will increase as the liquid is
pressurized.
3
,2
1
Pr,
n
l
elpgl
fs
l
C
TCg
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CRITICAL HEAT FLUX
During nucleate boiling large quantities of vapor are
generated at the heated surface which must be continuously
replenished with liquid.
At the critical condition breakdown of this counterflow
situation occurs owing to the onset of a Helmholtz instability
at the liquid- vapor interface.
Through hydrodynamic stability analysis Zuber calculated
critical heat flux
gl
lglgcrit g
4
12
1
18.0
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FILM BOILING Heat is conducted along a thin vapor film to cause
evaporation at the liquid vapor interface.
Distortion of this interface is increased by gravitational body
forces and opposed by surface tension.
Taylor (1950) showed the interface to be unstable for the
disturbances with wavelength greater than a critical value c.
Berenson's model for horizontal film boiling
2
1
2
gl
cg
4
1
2
1
3
42.0
gl
satg
glg
gT
gkh
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BUBBLE NUCLEATION
HOMOGENEOUS NUCLEATION -For a bubble of radius r
to grow the internal pressure must overcome the collapsing
effect of surface tension
This excess pressure can be converted to the amount of
superheat by Clausius Clapeyron equation
rp
2
lgsat vvTT
P
gsat
SATGLG
vT
TTpp
R
vTTT
gsat
SATG
2
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In Homogeneous Nucleation the rate of nucleation is
The change in Gibbs free energy increases with r for
subcooled liquid. In superheated liquid it first increases with
r till r* then decreases with r
Smaller nucleus than equilibrium size will collapse and a
larger nucleus will grow.
Tk
G
h
TNkn
BP
B exp
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HETEROGENEOUS NUCLEATION –
Occurs at surface cavities.
For subcooled liquid if liquid wets the cavity walls then vapor
pressure will be insufficient to balance surface tension. Nucleation
site become inactive.
If walls of the cavity are poorly wetted curvature of interface
reverses and surface tension resists further penetration .
During heating vapor pressure rises driving the interface towards the
mouth of the cavity. A well-wetted cavity may also be active as a
nucleation site if heating commences before it is completely filled
with liquid.
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BUBBLE GROWTH
For a vapor bubble to grow :
1) Thermal Diffusion Control Growth -The temperature of the bubble
interface must be lower than that of the surrounding liquid so that
heat is supplied to cause evaporation.
2) Inertia Controlled Growth -The pressure inside the bubble must
exceed that some distance away in the liquid, both to do work to
increase the kinetic energy of the surrounding liquid and to
overcome the inter- facial pressure difference caused by surface
tension.
Fig : (a) inertia controlled (b) diffusion controlled
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HEAT TRANSFER MODELSa) Latent-heat transport(Rallis and Jawurek 1964) - Heat is
supplied near the wall to bubbles which then move away into
the bulk liquid. In subcooled boiling there could be
simultaneous evaporation at the base of a bubble and
condensation at its tip.
b) Micro-convection (Bankoff 1961) - Bubble growth and/or
collapse causes random liquid motion very close to the wall.
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c) Vapour- liquid exchange(Engelberg-Forster and Grief 1959)-
A 'Reynolds analogy I model in which bubble growth causes an
exchange of liquid between the wall and bulk regions.
d) Surface quenching(Han and Griffith 1965)- A variation on (c),
assuming transient conduction to the cold liquid contacting
the wall after bubble departure.
e) Wake flow(Tien 1962)-Liquid motion behind a departing,
bubble causes convection from the wall.
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f) Enhanced natural convection (Zuber 1963)-Bubble columns
produce a cellular flow pattern similar to natural convection
above a large horizontal surface.
g) Thermocapillary flow(McGrew, Bamford, and Rehm 1966)-
Small variations in surface tension due to temperature
differences between the base and tip of a bubble cause a jet of
hot liquid to flow away from the wall.
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FORCED CONVECTION BOILING
Forced Convection Boiling Modes
Single phase convection
Subcooled Flow Boiling- Nucleation begins as Twall
becomes Tsat
Saturated Film Boiling –
The thickness of bubble region increases and core of
the liquid reaches saturation and bubbly flow begins.
As the volume fraction of vapor increases
individual bubble coalesces to form slugs of vapor.
The liquid then forms a film which move along the
inner surface in annular flow.
Mist flow till all liquid is converted into vapor.
The vapor is then heated by forced convection.Fig: Flow regimes for forced convection
boiling inside a tube
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BURNOUT IN FORCED CONVECTIVE
BOILING Burnout boiling crises that results in physical damage.
Departure from nucleate boiling (DNB): Under subcooled or
saturated nucleate boiling conditions the crisis is thought to
take the form of the growth of a steam layer on the heated
surface, termed film boiling.
Dryout: In the annular-flow regime the
failure of heat transfer is associated with
the loss of the film of water on the wall
as a result of entrainment and evaporation
leading to a dry-wall regime.
Fig :Two postulated mechanisms of 'burnout': (a)
departure from nucleate boiling (DNB) (b) dryout
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The transition from nucleate
to film boiling is associated
with a large increase in wall
temperature often sufficient to
cause physical damage, while
the transition from annular
flow to the dry-wall regime in
general does not.
Fig :Types of burnout as a function of
quality
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FACTORS INFLUENCING BURNOUT
Fluid Properties
Coolant supply (G) and enthalpy of inlet subcooling (ii)
Distribution of phases
I. Geometry, Length (L) and Diameter (D)
II. Gravity direction
III. Centrifugal forces – due to bends and coils
IV. Complex cross-sections
Heat-flux distribution
Surface condition
Hydraulic stability
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Evaluation of burnout
1. Prediction of the conditions under which burnout will
occur
2. Prediction of the magnitude of the burnout power level
3. Prediction of the effect of operating variables on this power
level.
The method of evaluation depends upon the complexity of
the plant configuration and on the amount of experimental
data already available.
BURNOUT EVALUATION MODEL
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Basic studies of
burnout mechanism
scaling-geometry
effects
Burnout studies using
straight tubes, bends
and coils
Analysis of world
data on burnout
Full-scale testing
using electrically
heated test sections.
Testing with Freon
modeling at reduced
pressure and power
Air-Water
Simulation (adiabatic)
Theoretical models
taking into account
geometry and surface
effects
Correlation of
experimental data
Prediction of
plant
performance
Fig: Burnout Evaluation Models
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BURNOUT IN STRAIGHT TUBES
Experiments were conducted by
changing one of the control parameters
G or ii. In this way a series of results
are obtained of burnout power P vs iifor several values of mass velocity G.
These results can be can be expressed in terms of heat flux ,
quality x, or boiling length Lb in the following ways
4
2
io
iDGPP
xiD
GDLP i 4
2
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correlation in terms of burnout flux and inlet subcooling
correlation in terms of burnout flux and burnout quality x
correlation in terms of burnout quality x and boiling length Lb
where, the fractional boiling length Lb/Lo is given by
o
i
o L
iGD
41
oo x
x
11
1
o
b
o
b
o
L
L
L
L
x
x
1
io
b
ix
x
L
L
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MODELING OF BURNOUT USING FREON
f
w
fi
wi
ii
)()(
fbwb
io
b LLix
xLL )()(,
Empirical relationships exist between high-pressure water and
Freons, which have been used as modeling to investigate the
behavior of high pressure water in straight tubes.
Experiment was conducted with freon and water in 2 tubes.
Make
Make
Then since
Make the ratio of density of liquid and gas phase for water and
freon same.
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Assuming
By reducing the mass velocity scale for the water by a constant
factor the plot for x vs G for water and freon can be
superimposed .
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BURNOUT IN COMPLEX GEOMETRY
Fig : Examples of different geometries
Geometry which
has been
investigated for
boiling water
reactor
Variation of
geometry
involved in
boiler
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BURNOUT EVALUATION OF REACTOR FUEL Information on burnout in reactor fuel is required for optimization of
designs, for setting limiting operating conditions and for safety
studies.Experimental
measurement of
burnout power
characteristic
Measurement of
channel power, flow,
subcooling and
moderator height
Allowance for in-reactor
factors differing from
experiment
Calculation of burnout margin
= (burnout power)/ (operating power)
Allowance for uncertainties due to experimental
accuracy in-reactor instruments, mechanical tolerances,
variations in reactor power, flow, etc.
Satisfactorily low
probability of burnout
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SUMMARY Analytical models are available for the reasonably well-
defined flow patterns of film boiling.
The critical heat flux for transition from nucleate to film boiling can be predicted with sufficient accuracy for design purposes, although further investigation of the processes very close to the surface is desirable to clarify the mechanism of transition.
No satisfactory model has yet been developed for the complicated heat-transfer processes in nucleate boiling. The nucleating characteristics of the surface represent a major difficulty in either theoretical or empirical correlations.
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SUMMARY The evaluation of burnout in industrial plant involves
combination of analysis of existing data, theoretical models, and
detailed experimental investigation using electrical heating.
Progress is being made towards better understanding of the
mechanisms of burnout, and data are being accumulated from
experiments with straight tubes, bends, and coils.
The synthesis of experimental data and theoretical models may
in methods of predicting burnout, even in complex geometries,
but full-scale confirmatory testing of specific designs is must
when high accuracy of prediction is required.
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