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

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

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.

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.

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.

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.

Fig : Pool Boiling Process

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

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

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

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

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

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

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.

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

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.

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.

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.

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

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

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

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

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

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

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

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

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.

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 .

BURNOUT IN COMPLEX GEOMETRY

Fig : Examples of different geometries

Geometry which

has been

investigated for

boiling water

reactor

Variation of

geometry

involved in

boiler

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

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.

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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