Critical heat flux in horizontal annular channels with variable eccentricity

0735-1933/91 $3.00 + .00 Printed in the United States

INT. COMM. I/EAT MASS TRANSFER VoL 18, pp. 659-667, 1991 ©Pergamon Press pie

CRITICAL HEAT FLUX IN HORIZONTALANNULAR CHANNELS ~TH VARIABLE ECCENTRICITY

J. L. Ballflo and J. Convertl Comlsl6n Naclonal de Energia At6mlca Centro At6mico Barlloche - Dlv. Termohldr&ulica 8400 - Bariloche, Rio Negro Argentina

(Communicated by J.P. Hartnett and WJ. Minkowycz)

ABSTRACT Measurements of critical heat flux (CHF) were performed for a horizontal annular channel (diameters 11.2 and 54 mm, approx. Im heated length) in which the eccentricity in the vertical direction of the inner heater could be varied. Measurements for low mass fluxes (up to 1600 kg/mZ/s) using freon-12 at a pressure of approx. 10.3 bar and saturation inlet conditions, show a strong influence of eccentricity on CHF; this influence is discussed on a basis of the flow pattern under which CHF occurs.

Introduction

The magnitude

of heat transfer rates which can be achieved

boiling systems

is limited by the boiling

can occur when,

due to a hydrodynamic

heater surface,

causing an abrupt drop in the local heat transfer coefficient

and a high wall temperature Among

the

huge

amount

crisis

phenomenon.

in two-phase

instability,

This situation

a dry patch appears

on a

increase if power is the controlled variable. of

data,

very

few

correspond

to

horizontal

channels. Differences

can be expected

of the transversal to

stratify

channel;

the

on the

between

gravity force present flow,

other

with hand,

the

lighter

turbulent

distribution.

659

horizontal

and vertical

in horizontal phase

mixing

flow.

(vapor)

tends

at

CHF because

This force tends the

to homogenize

top the

of

the

phase

660

J.L. Balifio a n d J. C o n v e r t i

Due

to the buoyancy

force,

different

flow

Vol. 18, No. 5

patterns

(namely

bubble elongated and horizontal slug) appear in horizontal or

liquid

superficial

force decreases, The

that

of

above

a

certain

for

flow

flux,

cooled

the

stratified, When the gas

importance

of

buoyancy

and vertical flow patterns are similar. patterns

(ref.

by water

found that CHF was lower in horizontal

appears

and vertical

horizontal

Merilo et al.

tubes

enough,

in horizontal

mass

identical.

kg/m2/s

is hlgh

different

for differences

essentially 4000

and horizontal

occurrence

explanation

velocity

flow.

and

to

be

CHF.

a

reasonable

It can be stated

vertical

CHF

values

are

I) found this value to be approx.

or freon-12;

below

this

value,

they

flow than in vertical.

Apparatus and Procedure

The test

section

consisted

of

an a~.nular

channel;

the

tube

diameter

was

5 4 mm. The heater wall

w a s a SS 3 0 4

thickness

of

The heater

at

the wall

was

mm.

with

The heated

instrumented

inner side,

temperatures axial

0.25

L tube

an

length

with

ten

outer

was approx.

at the upper

and

lower points

it was of the

after this,

cement to prevent possible deformations The heater had an electrical

of

11.2

mm a n d

a

1 m.

chromel-alumel

in such a way that

locations uniformly spaced;

dieuneter

thermocouples

possible heater

placed

to monitor

in five

the

different

the inner space was filled with

under pressure.

resistance of approx.

83

m~ and was heated

with a 50 kW d.c. power supply. By means of two positloners,

It was possible

to change

the eccentricity

of the heater. A

schematlc

locatlons A

dlaEl-am

is presented

simplified

experiments

of

the

in fig.

schematic

is shown

in fig.

test

dlaEram 2 (ref.

pump was heated up in the preheaters; regulating pressure

the was

power

az[justed

released by

section

showing

the

thermocouple

i.

by

of 2).

freon-12

Subcooled

loop

freon-12

the Inlet temperature

the

controlling

the

preheaters. pressure

In

The a

used

in

the

coming from the

was controlled

test

section

pressurizer.

by

inlet

The

test

the voltage

drop

section flow rate was adjusted by means of a throttlln E valve. Power

in the

test

section

was determined

by measuring

Vol. 18, No. 5 across

CRITICAL HEAT FLUX IN ANNULAR CHANNELS

the

source.

test

The

section

section

inlet

flow rate

and

across

pressure

was

was monitored

a

shunt

measured

resistance

wlth

wlth a turbine

a

built

DP-cell,

661

in

the

while

flowmeter.

The

power

the

test

thermocouple

outputs were monitored wlth digital thermometers.

(C I J k D P r O x , 1000 (heated

length)

I

I m

-

-

"

II FLOW

I

DI RECTION

7/////////////////////////////////~///////~. • l~rmoccxJpt, locations

I C'

ECCENTRICI~ECTION

C°C '

o

from

FIG.

I.-

Schematic diagram (out of scale) of the test section (dimensions in mm).

CHF

detection

was

performed

a

resistance

difference

bridge

of electrical

temperature changes, The pressure, the

sharp

were

bridge

unbalance

3).

resistances

inlet temperature

preset value,

measuring The

the

signal

voltage was

unbalance

proportional

of the two halves of the heater,

signal to

the

due to

as the power was increased.

measurements

resistance

(ref.

by

signal.

was

performed

by

maintaining

and flow constant; manually

When

the

balanced absolute

test

section

inlet

a slow power ramp was set and until

value

power was automatically cut off.

the

CHF of

caused

thls

a

signal

sudden

and

exceeded

a

662

J.L. Balifio a n d J. C o n v e r t i

In many occasions, thermocouple

signals

Vol. 18, No. 5

the power cutoff signal and a sudden increase in some

were

detected

which the dry patch appeared first

simultaneously,

but

there

were

not measured. The ra_nEe of variables explored are given in table 1.

TABLE 1 . -

R a n g e of variables e x p l o r e d .

Inlet pressure:

approx.

Mass flux:

240 - 1600 k E / m 2 / s

Inlet conditions:

saturation

Tube diameter:

54.0

10.3 bar

Heater diameter:

1 1 . 2 mm approx. -lS

Eccentricity:

(T

mm

Hested length:

to

IN

= 41.8

C)

1 m 15 mm f r o m t h e

concentric

position.

PRES,.~JRIZER

~ F-I

~RE~TER I1

.

TEST SECTION (HORIZONTAL)

FIG. 2.-

cases

in

in regions where the wall temperature was

Schematic dia&oeam of the freon-12 loop.

Vol. 18, No. 5

CR1TICAL HEAT FLUX IN ANNULAR CHANNELS

Results a n d Discussion

Experimental

The experimental in

terms

position.

of

the

results length

Positive

are shown

the

663

in fig.

heater

is

eccentricities

3.

Eccentricity

displaced

mean

from

is measured

its

upwards

concentric

displacements.

350

I'n

300

[] D

F]

[] [3 [] o

D 250

0

0

0 '~ 200

0

0

150

8 I O0

8

50

l | i , l l , l , , l , l , , i t , , i l , l i , l ¢ l , , l l , I l l , * l , i

-20

-15

-10

-5

0

5

10

15

20

EccgNTmcrr~ ( ~ )

FIG. 3.-

CHF as a function of eccentricity

( p = (10.3

CHF concentric

~ 0.4)

bar,

o

G =

( 2 4 3 ~ 3) k g / m 2 / s

A

G =

(788 ! 5) kE/m2/s

u

G = (1576 ~ 8) kg/m2/s

decreases

sharply

position,

as

the

and remains

heater

constant

T

IN

is

=

(41.8 ~ 0.7)

displaced

°C)

upwards

(or even decreases

from

slightly)

its

as It

moves d o w n w a r d s . The i n f l u e n c e

o f mass f l u x

o n t h e maximum CHF v a l u e

is only moderate,

and

664

the

J.L. Balifio a n d J. Converti

main effect

with the role

is

flattening

of turbulence

the

CHF v s .

Vol. 18, No. 5

eccentricity

profile.

This

agrees

as a mixing agent.

For each condition CHF was measured twice;

it can be seen that CHF data

showed good repeatability. An

explanation

superficial

for

velocities

the for

data the

trend

liquid

can and

be gas

obtained phase,

by

based

calculating on

a

the

mass

and

energy balance with saturation inlet conditions

PL VSL + PC, VSG = G PL hL VSL + p c h G V SG -

Q

(1) + G h

A

(2)

L

From (I) and (2) we easily obtain

VSL

°[, PL

(3)

A G h LG

Vs

-

I n fig. horizontal

4 the calculated

flow-pattern

map

superficial

(ref.

(4)

q A hLe

Pc;

4).

velocities

are shown

It can be seen

that

i n a typical

the data

points

fall in the intermittent flow region (bubble elongated or horizontal slug). Assuming that CHF occurs under an intermittent flow pattern, of the heater will (fig.

5).

the position

limit the height of the liquid film under the gas pockets

Therefore,

pockets in horizontal

CHF

should

depend

intermittent flows.

strongly

on

the

dynamics

of

gas

This assumption is supported by the

following experimental facts: i)

Except for cases

of the test section,

in which the heater was placed close to the bottom

CHF was detected first by the thermocouples

the top of the heater.

This suggest

located at

"dryout" as the mechanism responsible of

CHF.

ii)

In many cases,

when the heater was placed close

test section and for low mass fluxes, the heater.

to the top of the

CHF occurred at the upstream portion of

There were also cases in which measurements were repeated and CHF

was detected at the downstream as well as the upstream portion of the heater, with no significant differences phenomenon of unsteady nature.

in the CHF values obtained.

This indicates a

Vol. 18, No. 5

CRITICAL HEAT FLUX IN ANNULAR CHANNELS

665

10 DISPERSED / INTERMITTENT

I

I

FLOW

I

I

I

m,:f,

I I

I

A

I

SLUG

Z~l~

BUBBLE

ELONGATED

/

\ /

\

O(3D

I \

0.1

I

/

\

T,

l

-

/

\

ANNULAR

I

\ \ WAVE

STRATIFI ED

\

\

\

\

\ \

0.01

i

, J , ,,i,I

0.01

i

,

, , = ,,i,l

0.1

,

\

I I

, , ,~,,I

,

1

,

, , ,,,,

10

100

w= (,'-I,) FIG.

4.-

Typical

horizontal

£1ow-p&ttern

showln8

the superflclal

( p = (10.3 ~ 0.4) bar, o

G =

map,

velocities. T

IN

= (41.8 ~ O. 7) °C)

(243 ~ 3) kg/mZ/s

A

G

=

(788

~

5)

kE/m2/s

u

G

=

(1676

:

8)

kE/m2/s

Conclusions

For concentric

low

mass

position

The calculation other

experimental

intermittent

fluxes,

the

displacement

causes a stron 8 decrease of superflclal evldences

flow pattern.

of

velocltles

lndlcate

the

heater

upwards

of CHF in horlzontal

that

at

the crltlcal

C}IF occurs

under

from

its

flow. condltlon a

klnd

and of

666

J.L. Balifio and J. Converti

POCKET

GAS

Vol. 18, No. 5

IC

.//////i,//////////i#//////4/////////// I

I 0

0

0

o

• 0

o

o

¢~ ¢~

o ~ o

HEATER

~

i 0

o"

o

0

0

~>

"

Y

FLOW I DIRECTION

f//////////////////////~, ////////// I

SECTION C-C'

IC' FIG 5.-

Idealization of the horizontal intermittent flow in the test section.

Nomenclature

A G h Q: T V

: cross sectional : mass flux. : enthalpy. heat power. : temperature. : velocity.

flow area.

G r e e k letters. p : density. Subscripts. G : gas: IN : inlet. L : liquid. SL : superficial liquid. SG : superficial gas. LG : liquld-gas phase change.

AcknowledKements

The financial

support for this work was provided by the C o m i s i 6 n Nacional

de E n e r g i a A t 6 m l c a and the International The c o n t r i b u t i o n test s e c t i o n

of Daniel

Mateos

is d e e p l y acknowledged.

Atomic E n e r g y Agency. and M a r t i n E n e v o l d s e n

in building

the

Vol. 18, No. 5

CRITICAL HEAT FLUX IN ANNULAR CHANNELS

667

References

I.

M.

Merllo,

S.Y.

Ahma~t, "Experimental

study

horizontal tubes cooled by freon-12". pp. 2.

J.L. en

463-478

Converti,

Laboratorio

de

T & c n l c o CNEA-CAB 8 9 / 0 4 5 3.

J.L.

Balifio,

desbalance

CHF

in

vertlcal

Int. J. Multlphase Flow,

and

vol. 5,

(1979).

Ballfio,J. el

of

N.H.

Rulval,"Descripci6n

Termohidr&ullca

del

CAB"

del circutto (in

de f r e 6 n

spanish).

12

Informe

(1989).

H. N e n d l e t a ,

"Detector

de reslstencla~"

de fluJo

(in spanish).

cal6rico

critlco

basado en el

I n f o r m e T 6 c n l c o CNEA/CAB 8 9 / 0 4 3

(1989). 4.

J.M.

Nandhane, G.A. Gregory, K. Azlz,

flows in h o r l z o n t a l pipes". (1974).

Int.

"A Flow p a t t e r n map For g a s - l i q u i d

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1, pp. $37-553