Experimental investigations on the rewetting of hot horizontal annular channels

INT. ~ . HEAT MASS TRANSFER 0735-1933/83/040299-13503.00/0 Vol. 10, pp. 299-311, 1983 ©Pergamon Press Ltd. Printed in the United States

.EX~PE~RIM,.~T?\L Iq,~/ESTIGATIONS ON T.~TE RE~TETT!NG OF HOT ~ORTZO~"FAL AI,~IU[AR C~%AkqIELS V. Venkat Raj Reactor ~nglneer!ng Division Dhabha Atomic Research Centre Trombay, ~ombay - 400085 (Cc~municated by J.H. Whitelaw)

A 9STP~\CT This paper deals with exper~nental investigations on the rewetting of hot horizontal annular channels. Experiments have been carried out, at atmospheric pressure, on stainless steel test section. The temperature of the inner surface of the annular channel v~s varied from 200 to 500°C and the flow rate from about 0.7 kg/mln, to 9.5 kg/mln. These experiments indicate that stratification effects are significant, more so at lower flows. The rewetting front is incl/ned. The rewettL~g velocity increases as the flow rate is increased and decreases as the initial wall temperature is increased. IntroductiQn The Pressurised Heavy Water Reactors tal core config,~ratlon. zontal channels

(PI~s)

have a horizon-

The phenomenon of reverting of hot hori-

is of direct relevance to heat removal during the

~mergency Core Coollng

(ECC) phase of Loss of Coolant Accident

(LOC~%) in these reactors.

Frcm a review of published

literature

it is seen that there is a pauc'Ity of data on rewettlng of hot horizontal

surfaces,

whereas an enormous ~mount of infomnation on

rewetting of hot vertical

sttrfaces is available.

tlons on rewetting of hot horizontal

The few publlca-

channels are of very recent

orlgin. Experimental

investigations

on the rewettlng o f

hot horizon-

tal tubes have been carried out by Lee et.al.[l]and Chan and BanerJee [2].

The rewetting velocity was found to decrease with

increasing initial wall t~nperature and power input, but increase with increasing coolant flow rate. 299

Lee et.al,

found that increasing

300

V.V. Raj

Vol. i0, No. 4

subcooling of coolant at inlet caused the rewetting velocity to increase.

It varied approximately proportional to the 2/3 power

of inverse wall thickness.

The average rewetting velocities for

horizontal tubes were about 20 to 30 percent smaller than those for vertical tubes.

Chan and BanerJee identified three regions

in the rewetting channel, viz., (i) the totally quenched region; (ii) the partially quenched region where the quench front is inclined; and (iil) film boiling region.

Because of the strati-

fied nature of flow the cha~nel is quenched bottom first, midside next and top last.

Average rewettlng velocities over the length

of the channel estimated independently for the top, midside and bottom of the channel were found to be close to one another, but not equal, the deviation being more at high flows.

For a given

flow, the inverse re~.~tting velocity was found to vary linearly with initial wall t~,perature.

The effect of inlet subcooling

was found to be dependent on the initial ~ i i

temperature.

As part of a phased programme of investigations on the rewetting of hot horizcntal channels planned by this author, experiments on the rewettlng of hot horizontal exposed surface and rectangular ducts were carried out [ 3 ] .

These studies

indicated that the rea.;etting behaviour of a hot horizontal surface is affected by the initial surface temperature, water flow rate and whether the surface is exposed or not.

The sputter-

ing temperature was measured to be approximately 200°C at atmospheric pressure.

A transition in the rewetting behaviour was

found to occur at initial wall temperature of about 350°C (estimated Leidenfrost temperature).

Subsequent to these studies the

rewetting behavi~Ir of a hot horizontal annular channel has been investigated experimentally.

This is described and discussed in

this paper. ExDer imenta I SetuD A schematic of the experimental setup along with the test section is shown in figure i. a source of water

The setup consists essentially of

supply, a source of power supply, test section,

valves for isolation and control purposes and instrumentation for measuring and recording flow, pressure differential and temperatures of interest.

Demlneralised water was used in the experiments.

Vol. i0, No. 4

~ I N G

OF HORIZONTAL ANNULAR CHANNELS

301

_.C)

TIE]'IPERATURE ELEMENT

[ ~.~

FLOW ELEMENT

DIFFEREHTIAL PRESSURETRANSMITTER DEMINERALISEO WATERSUPPLY

DRAIN

THERMOCOUPLE.J LEADS I

I

| ~

I

I

]~ LOW VOLTAGEPOWERSU PL "

I

SWITCH I

MULTI CHANNEL OSCILLOGRAPHICRECORDER

TEMPERATURE IHOICATOR

1

EXPERIMEHTAL SET-UP

THERMOCOUPLES

t

/~,,.

" s.s.poRTiO.

INNER TUBE OF TEST SECTION

Details

FIG. 1 of the E x p e r i m e n t a l Set-Up

and Test Section

302

v.v. Raj

Vol. i0, No. 4

The inner tube of the test section was heated by a high current, variable power source.

The flow was measured using an orifice.

Electronic differential pressure transmitters were used for recording the flow and pressure differential across a portion of the test section.

The outputs of these along w~th those of the

thermocouples monitoring fed to a multichannel conditioners. temperature

temperatures

at different

locations were

fast recorder through appropriate

signal

The thermocouple outputs were also taken to a

indicator through a selector switch for monitoring

the temperature of the test section while it was being heated. The test section was insulated by winding asbestos rope around the outer tube. Test S e.ct.!on. The test section is of annular geometry with the inner tube heated by electrical resistance heating.

Even though the

experiments were carried out at essentially atmospheric pressure, the design and fabrication of the test section were complicated because of the following reasons. (i)

Provision must be made for accommodating

the thermal

expansion and sudden contraction of the inner tube. (ii) Since the sharp circumferential

temperature gradients

encountered during rewetting would cause the inner tube to bow, the test section design should facilitate easy dismantling and assembly,

so that the inner tube can be removed and straightened

when required. (iii) The thermocouples

should be fixed to the inner tube

and the leads taken in such a way that there are no projections on the outer surface of the inner tube which could affect the rewetting process;

also the thermocouples

should not detach frem

the surface when subjected to alternate heating and quenching cycles. The test section design was finally arrived at to meet the fore-mentioned requirements. inside an outer stainless forming an annulus.

It consists of an inner tube housed

steel

(s.s.) tube of 2.66 an. I.D. thus

The inner tube consists of a central s.s.

portion 1.2 m. long and 15 nln. O.D.

which forms the heated

Vol. i0, No. 4 length. end.

REWETrING OF HORIZONTAL ANNULAR CHANNELS

303

Copper tubes were brazed on to the s.s. tube at either

On assembly these protrude out of the test section and the

bus bars are clamped on to them as shown in figure I. able flange with an

A detach-

'o' ring seal provided at one end ensures

leak tightness whale providing for the thermal expansion and contraction of the inner tube.

As shown in figure 1 thermocouples

were fixed to the s.s. portion of the inner tube, at locations named A,B,C,D and E starting from the inlet end and 250 mm apart. When the test section was assembled the thermocouples, designated A T , B T and so on (where T stands for top and A,B etc. location), were at the top. were f ~ e d

for the

In addition, three more thermocouples

at cross section C so that one of these

(CB) was at

the bottom and the other two (CM) on the mldside when the inner tube ~,~s fixed. Experimental ~rocedure and Ranqe of Parameters A f t e r installing the test section a check was made to ensure that it was horizontal. the valves ~ r e

For each experimental run, to begin with,

adjusted to get the desired flow.

Subsequently

the test section was isolated and the water in it was completely drained.

The test section power supply was switched on and the

power was increased gradually.

When the test section temperature,

which was nearly uniform all along the length, reached the desired value the recorder drive was started, the test section power was switched off and water flow was initiated.

The temperature

transients and the flow and pressure drop were recorded. Subsequently the flow was readjusted to a new value and the experiment was repeated.

It was noted that for initial ~ i i

temperature values of 300°C and more the rewetting transient caused considerable bowing of the inner tube.

Hence, after every

run the test section was dismantled and the inner tube was straightened.

The experiments were carried out over the following

r~nge of parameters. Flow

--

0.7 to 9.5 kg/min.

Initial surface t e m p e r a t u r e - 200 to 500°C. The experimental observations and data are presented in the following paragraphs.

304

v.v. Raj

Vol. i0, No. 4

zxperlm.,enta.l Observat,lqns, During the experimental runs, as the flow was initiated, flow leaving the test section exit was visually observed. noted that the behaviour of f l ~

the

It was

issuing out is very much depend-

ent on the initial surface temperature and flow rate.

At lower

flow rates both steam and water start issuing out of the test section right from the beginning in a steady stream, thus indlcat. ing the highly stratified nature of the flow in the ar~ulus.

At

higher flows considerable chugging occurs, ~Tater and steam coming out in alternate spurts initially. runs, also revealed oscillations.

The flow trace,

for these

At higher wall temperatures

this chugging was seen to occur at lower flow rates. The transient traces obtained during rewettlng,

for initial

temperature of 500"C and flow of 6.86 ~g/min, are shown in figure 2. progresses,

It may be seen that as the rewettlng transient the temperatures

at different axial locations along

the top of the inner tube fall in spurts with small sharp drops interspersed between periods of nearly constant temperature or very gradual temperature drops. tures at different

This continues till the tempera-

locations drop sharply to less than 200"C

(i.e. sputtering tem!>~rature) , when reverting occurs. (CB) and midslde

At bottom

(CM) locations rewetting occurs much earlier and

aljo the number of prior intermittent This indicates that the progress

sharp drops are much less.

of the wet front becomes more

halting as x./e l>roceed from the bottom to the top locations in the horizontal channel.

The pressure drop trace indicates continuous

oscillation with sudden intermittent peaking in pressure drop. The flow trace also shows corresponding

oscillations.

oscillations are more at higher flows.

These oscillations which

continue till the channel

is completely rewetted are in llne with

the visual observatlohs wherein considerable noted.

The

flow chugging was

The observations made here are typical for higher flows

(greater than about 5

kg/min. ) at this temperature.

initial wall temperature

As the

is made less and less, the intermittent

sharp drop~s in temperature prior to rewetting also reduce, ez
thus indicating a smoother progress

The pressure drop oscillations are also less.

in

of the wet

Vol. i0, No. 4

REWEITING OF HORIZONTAL ANNULAR CHANNELS

305

THERMOCOUPL E _ET

DT

PRESSURE, DIFFERENTIAL

I

/

II~

I

'

'

'. . . .

J

4

Rewetting

l

8 TIME

Transients

12 IN SECONDS

16

24

20

FIG. 2 (Initial Temperature

- 500°C)

ET DT, CB

~t

m Z ,<

CM

LU

CT

i

|

0

|,

|

~

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a,

$

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TIME

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Transients

L .

12

t.,

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16 IN

l

.

20

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24

SECONDS - - - - m -

FIG. 3 (Initial Temperature

- 400°C)

I

26

306

V.V. ~aj

Vol. 10, No. 4

At flow rates of about 4 kg/min. tJng behaviour changes. very few intermittent

(medium flows), the r e w e t -

As the rewetting transient ~roceeds,

sharp, drops

in te~nl~erat~ire are observed.

At the bottom and mldside locations,

the surface temperatLtre

which rez~ains more or less steady falls sharply to less than the sputtering temperature as rewettino occurs. the t~nreratures

end when rewett!ng reduced,

At the top locations,

fall gradually followed by a sharp drop at the occurs.

As the initial wall temperature

the period of gradual fall also reduces.

is

The flow and

pressure drop oscillations are also much less. %t very low flow rates it was noted that the sharp drop in tem~eratllre as a conseqJ ence of rewetting occurs only at the bott~n locations.

At the top and even mldside locations no sharp

drop was obse~¢ed but the temperatur~ was seen to fall gradually, the slope of the temperature transient being more at midside locations.

This indicates that the flow is highly stratified.

It is most likely that ovcr the entire length of the channel, incl~iding the rewetted portion,

water flows only at the lower

portion of the cross section, while the rest of the sl-~ce is covered by steam.

This conclusion

is supported by the visual

observations discussed earlier. Under certain conditions of flow and temperature

location E

was ~ound to rewet earlier than D, thus indicating the formation of an auxiliary wet front at the exit end, as observed earlier experiments

on rectangular ducts and exposed surface

The order in which the various thermocouple examined.

[3]

Q

locations rewet was

It was noted that CB which is located on the bottom of

the inner tube invariably,

in the

(do~tream

of BT) r e ~ t t e d

earlier than BT

and in some cases even earlier than AT.

cates that the wet front is inclined with the tlpe

This indiof the wet

front at the bottom and top of the inner tuJ~e being separated by approximately less.

250 to 500 mm.

At higher flows this distance is

The observations also indicate that the refilling

moves much ahead of the r ~ e t t l n g

ing do~mstr~%m of the rewettlng front. effects this film boiling portion of the channel,

front

front, %.:ith film boiling occur~-ecause of stratification

is likely to occur only over the lower

especially at lower flows.

Vol. I0, No. 4

REWETTING OF HORIZONTAL ANNULAR CHANNELS

307

Experiments with Inner Tube Bowed During the experiments it was found that at initial wall temperatures of 300"C and above considerable bowing of the inner tube occurred on rewetting.

Some rewetting runs were carried out

with the inner tube in bowed condition.

Under this condition the

inner tube did not heat up to uniform temperattire along its length.

The temperature was found to peak up in the middle.

~,~en water was admitted it was found that at those downstream locations ~4here the initial surface temperature was less than that at the upstream locations rewettlng occurred earlier, i.e. before the main w~t front progressing from the inlet end reached these locations.

This has significance in the context of fewer-

ring of nuclear reactor fuel cladding with a peaked temperature distribution.

The locations downstream of peak temperature are

likely to rewet earlier thus resulting in a reduction in the total time required for quenching the entire fuel channel. Criterion for Diterminin q Occurrence of Rewettinq Normally surface re',~ttlng is considered to have occurred when the surface temperature falls sharply. experiments,

In the present

as already explained, depending on the initial

conditions the surface temperature falls in spurts or gradually till the sharp drop to less than sputtering temperature occurs. Hence rewetting is considered to have occurred when the sharp drop to less than sputtering temperature occurs.

At very low

flow rates no sharp drop is observed at the top locations at all, but only a sudden increase in the rate of fall of temperature. This is indicative of a sudden improvement in heat removal and hence in such cases rewettlng is considered to have occurred when a sharp change in the slope of the temperature trace occurs. Rewettln~ Velocity AS ~xplalned earlier, at a given axial location the bottom of the inner tube rewets first, the midside next and the top last.

In the experiments discussed in this paper only the rewet-

ring velocity along the top of the inner tube could be measured. ~ne rev~tthqg velocity along the midside and bottom locations

308

V.V. Raj

Vol. I0, No. 4

could not be measured since thermocouples ~ r e axial location only.

Chan and Banerjee[2] p

provided at one

in their ex~neriments

on rewettlng of horizontal tubes, found no significant difference between the rewetting velocities at these locations.

However,

experiments presently being carried out by this author indicate that, depending on the initial conditions, there can be considerable differences between the rewetting veloc~ies along the bottom, mldside and top locations of the inner tube of t~e annulus. Constancy of Rewettlnq velocity alonq the le/%qth of the channel From the transient traces of the thermocouples located axially along the top of the inner tube the time for wet front travel along the top of the tube was determined for each run. This time was plotted against the axial distance travelled by the wet front.

The data for higher values of initial wall

temperature were found to fall nearly on a straight llne. However, at lower values of initial wall temperature the deviation from linearity was found to be more.

This indicates that

a t higher values of initial wall temperature, say greater than

400"C, the rewetting velocity can be considered to be reasonably constant along the length of the test section, for a given set of initial conditions. this is not so.

However at temperatures less than 400"C

In their experiments on rewetting of a hot

horizontal tube Chan and BanerJee [2] also found that, in general, the local rewetting velocities tend t o be more uniform along the length of the channel for hSgh initial wall temperatures and/or high inlet flows. EffeGt o~ /low Rate. on Rewett!nq Velocity The rewetting velocities, along the top of the inner tube, estimated from the various experimental runs are plotted against the flow rate, in figure 4, for different values of initial surface temperature.

It can be seen that, at all temperatures,

the rewetting velocity increases as the flow rate increases.

At

lower values of initial wall temperature the effect of flow is much more predominant as can be seen from the larger slopes of the curves for 200"C and 300"C.

The slopes of curves for 400°C

-~

200

~ ,~

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,~oo° c

S00 C

- - 3oo"c --,o,c

--

--

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2

Flowrate

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vs.

i~~°°[i ~

"

o

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4oo~-

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t

FIG. 4 Rewetting

a

Velocity

4 6 FLOWRATE KgJ M,'..

A

LE6END :I SYMBOL-- INITIA~ SURFACE TEMPERATURE

t B

/ .

I 10

.

300

400

!

8 Kg/M~,

FLOWRATE

INITIAL SURFACE TEMPERATURE--°C

200

$

/

FIG. 5 Initial Surface T e m p e r a t u r e vs. R e w e t t i n g V e l o c i t y

100

loo

,so

5¢

o,

~

-"

200

250

300

!

500

~O O ~D

Ca

H

o ~

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o <

310

v.v. Raj

and 500°C are much less and are nearly same.

Vol. i0, No. 4 These observations

are in line with those made earlier on horizontal exposed surface and rectangular ducts [33 Effect of InitialSurface Temperature on R ewettlnq Velocity ~n figure 5 the re%~tting velocity is plotted against the initial surface temperature for different flow rates.

It may be

noted that as initial surface temperature is decreased, below a certain value there is a sudden sharp increase in rewetting velocity.

This sharp increase is much more at higher flow rates.

The temperature at which thls occurs is also dependent on the flow rate.

While at higher flow rates this temperature is about

400°C, at lower flow rates it ks about 300°C. indicate

a change

These observations

in rewetting behavlour in the initial surface

temperature range of 300"C to 400°C and are in line with the earlier data [33 on horizontal exposed surface and rectangular ducts. Conclusions (i)

The refilllng and rewettlng of a hot horizontal channel

is affected by stratification effects which become more and more significant at lower flow rates. (ii)

The refilling front moves ahead of the rewetting front

with film boiling occuring ahead of the wet front. (iii)

The rewetting front is inclined with the top and bottom

edges separated by as much as 250 to 500 n~. (iv)

At t~nperatures above 400"C the rewetting velocity can be

considered to be reasonably constant along the length of the test section, for given initial conditions. (v)

With peaked temperature distribution,

do~nstream locations

at lower temperature rewet earlier than upstream locations at higher temperature. (vi)

The rewetting velocity increases as the flow rate is

increased, the effect being more predominant at lower values of initial surface temperature.

Vol. I0, NO. 4 (vii)

REWEITING OF HORIZONTAL ANNULARCHANNELS

311

AS the initial surface temperature is reduced the rewetting

velocity increases, the increase being more sharD below a certain

temperature. Acknowledqeme~ts The author wishes to thank Professor S.P.Sukhatme (Tndian Institute of Technology, ~ombay) and :Ir. S.k'.Mehta (IIead, Reactor Engineering Division, B.~.R.C., ~ombay) ~or their comments and useful suggestions.

Thanks are due to the staff of workshop and

Engineering Laboratory of ~eactor Vngineering Division for their help in fabricating the experimental setup and car~llng out the experiments. References I.

Y.Lee, W.J.Chan and D.C.Groeneveld, Proc. 6th Int. Heat

Transfer Conference, 5, 95, 1978. 2.

A.M.C.Chan and S.BanerJee, Trans. ASME, J. Heat Transfer,

103, 281, 1981. 3.

v.Venkat RaJ, S.P.Sukhatme and S.K.Mehta, Droc. 7th Tnt.

Heat Transfer Conference, 1982.