Wall temperature fluctuations: Realization of the criteria for steam generators

qr*

30

20-

Ю-

Рис.11

Рмпац

3 74

f

г

i

'i

_ —"*^

j

к • £rr

\

\

\ /

у _{\ / \ р} А

/ \

7 i $ io ft t'z ft ft is is ft

Течь /У

U-Z

0 I

Рис.12.

2 A 4 S 6 7 S 9 I 0 tS 20 25 30 3$

Развитие тъмпературноео процесса 6 /IP'S.

40

1 1 Т В Р А Т У Р А

X» йяржввнд твхзодогяк и.

250

установке Н&-35О*

Доклад на коввференцих в гЛондолв 17-19 ывв 19вЕИи ий я др«

2» НеКОТОрне результат

янг работ по парогенераторам установки Ш-350» Доклад вв. конфе- ренциж в г.Яондоне П - 1 4 марта 15Г74Г.

Благоволил С М . , Бвхлушиг Р.П», Василенко К»Т«, Грдяичко Е.М., Лукасевич Б.И., СтеколБникаь В.В. % др«

3* Некоторые результата экспериментальных и пуско-наладоч- ннх работ по парогенераторам установки Ш-350.

Доклад на советско-французском семинаре в г.Димигровграде 19-23 марта 1973г.

Лукаоевич Б.И., Баклушин Р.П», Василенко К.Т., Стевшгьни- ков В.В., Федоров B.T.

Наченка Д . С , Грдличко Е.М.,Губанов В.М», Некрасов А.В., Благоволин C.U.

Логвинов С А .

G.3. Wall Temperature Pluctua- P. J. de Munk Netherlands tione - Realization of the

Criteria for Steam Genera- tors

Some aspects in relation to thermal stresses in the tubes of the once-through steam generator.

О iО СЛ iСЛ I

As* a consequence of the once-through system cyclic variations of wall temperature may be expected to occur in the dry-out zone of the steam generating tubes. The inherent thermal stresses in the wall have to be feared even more when the location of

dry-out oscillates in axial direction due to dynamic instabilities.

Both subjects, wall temperature fluctuations in stable dry-out and dynamic instabilities, have been investigated.

1. Wall-temperature fluctuations

Although the phenomenon of fluctuating wall temperatures in the transition region between nucleate boiling and film boiling has been mentioned by many designers and has been recognized as a possible danger for the steam generator due to the corrosion and thermal fatigue, hardly any specific information can be found in the literature on this subject.

Some information can be found dealing with electrically heated systems. Tippets Cll and Bailey and Collier [2] give descriptions of the phenomenon. Fig.l gives schematically the wall tempera- ture of a steam generator tube versus length. Between nucleate boiling and film boiling the wall temperature may oscillate between the extreme values corresponding to a dry and a wet wall. The flow pattern in the transition boiling region is such that the thin water film adherent to the wall is no longer closed and dry spots appear on the surface causing sharp

temperature rises. The surface in this region is intermittently dried and rewetted in. an irregular manner. Going through the transition boiling region all water gradually breaks up from the wall until the stable film boiling region begins where a steam film is adherent to the wall.

In connection with corrosion and thermal fatigue, there is a need for information on the length of the transition boiling region, amplitude and frequency distribution of the fluctuations.

The length of the transition boiling region is particularly important because for thermal stress calculations mechanical designs mostly consider the axial temperature profile.as neg- li'gible compared with the radial temperature profile.

At TNO-Apeldoorn we have made measurements to determine the afore-said quantities. In the so-ealled SWISH-loop, with a

primary sodium circuit and a secondary high-pressure water circuit, we have installed a 10 m long steam generator test section.

Fig.2 gives a scheme of the test section in the uppermost part of which we can observe the wall thermocouples spaced only 5.5 cm apart. This set-up provides for an accurate observation of the phenomena that occur in the transition boiling region.

The measurements have been performed at fully stationary condi-

tions for absolute pressure, primary and secondary mass flows and inlet temperatures. In this situation the locus of dry-out was determined after which the thermocouples present in the transition boiling region were scanned for a while.

The fluctuations measured by the thermocouples at a certain depth in the wall can be extrapolated to fluctuations at the inner surface of the steam generating tube. An example of the last fluctuations is given in Fig.3. The operating conditions

2

during this scan were 180 bar, 1400 kg/m s mass velocity and 68 С inlet subcooling. The ordinate is the temperature difference between the observed maximum temperature and the actual tempera- ture at time t. Of the three thermocouples observed in one scan only one thermocouple registered fluctuations. The other two measured stable wall temperatures at nucleate of film boiling conditions. These observations enable an assessment of the

length of the transition boiling region because the thermocouples are spaced only 5.5 cm apart. This region should therefore be confined to about 10 cm. From a few scans as given in Fig.3 an amplitude distribution has been derived. This distribution is given in Fig..4. A typical amplitude is 15 C. The observed maximum temperature transient is 130 C/s, whereas 75 C/s is a frequently occurring value.

In order to estimate the interest of these measurements for the steam generator design it is necessary to carry out thermal stress and thermal fatigue calculations. Investigation of possible interactions between temperature transients and the magnetite layer are equally useful.

On the other hand the question arises whether there are criteria for admissible temperature fluctuations considered against a certain life time of the component.

2. Stability experiments

Neither theoretical nor experimental works have been reported in the open literature that can predict dynamic instabilities in the geometry and at operating conditions of a sodium heated once- through steam generator.

The dynamic instability in a steam generator with some hundreds of tubes can be considered to be the dynamic instability of one

251

tube that is operated at a constant externally imposed pressure drop between inlet and outlet. It is justified to assume that the instabilities occurring in one tube do not influence the overall pressure drop of the whole component when its initial behaviour is stable.

The experimental set-up for the dynamic stability experiments is based on this idea. The previously mentioned sodium heated test section has been provided with a bypass line of relatively large diameter. The pressure drop in the bypass, and therefore in the test section, will not be influenced by instabilities in the test section. See Fig.5.

The experimental conditions are such that the test section outlet thermocouple measures a slight superheat. In this way instabilities that might occur can be detected very easily. Moreover, the test section flow is continuously recorded.

Up to now the program has been very limited in scope. In the first instance SNR-operating conditions were chosen.

In addition, a few other runs at rather arbitrary conditions were performed as well.

The first measurements had the following conditions:

- absolute pressure 180 bar - inlet subcooling 30, 80°C

- mass velocity 1400, 1120, 840, 560 en 350 kg/m2s - variation of sodium-to-water mass flow in a wide range.

AT.sat

No instabilities have been observed at SNR-operating conditions.

See Fig.6. The outlet superheat is at a constant value of 9 C.

The ripple in the test section outlet temperature is less than However, if lower absolute pressures are permitted, serious purely periodic oscillations occur at pressures as low as

120 bar. See Fig.7. The observed amplitude of the outlet tempera- ture is 25°C, the frequency 0,5 Hz.

In most cases more or less stochastic fluctuations, sometimes of significant amplitude, are predominant, which is probably a compound dynamic instability.

These experiments for search to instability thresholds, that have started on'ly half a year ago, are only of relative importance

since the results are only applicable to the same geometry. In order to obtain a more general validity we intend to construct a second test section of different geometry. The results of both experiments should be fitted in a model that has general validity for the sodium heated once-through evaporator.

References Ш F.E. Tippets

Critical heat fluxes and flow patterns in h.ighu-preasure boiling water flows

ASME Paper 62-WA-162 (1362) [2] N.A. Bailey, J.G. Collier

The estimation of tube wall temperatures in the evaporator region of subcritical once-through sodium heated steal generators.

252

R e p . AEEW-M 1000.

10 m

nucleate boiling

top section

1 6 ew a l l 2 9 H²O 4 6 N^a 1 Ap

other sections 44 temperatures

4Ap

9 m 8 m 7.m 6 m

4 m

2 m length along evaporator

Fig.1 Fig.2

steam out

w

water in Variation of wall superheat in the three Test section for

boiling modes heat transfer and pressure drop measurements

Twall (t) measured by Tss (31) scanning frequency 6.2 Hz

253

Fig.3

Wall temperature fluctuation in the dry-out zone 22

number of fluctuations

t i 2

Г

ОО

6 4 2 0

-

- - -

10 15

Fig. 4 Amplitude

i

20 25

.,-L...

30 35

distribution

1

—*• amplitude of

( K )

to condenser -{outlet headej

др measurement

bypass

sodium in

testsection

sodium out

Fig. 5

test section flow

main flow

20 t(s)18

« 14 12 10 8 6 4 2

12.5"C

= 366.3"C

abs. pressure 180.3 bar p mass velocity I365j<g/m^

| inlet subcooling 75 °C 1- outlet superheat 9.2°C

sodium flow 1022 kg/s sodium inlet ternr> 426 'C

3.78 Hz

T ( C i

iv>_feedwaterf low 156 Hz

p~{turbino flowmeter)

10 20 30 40 50 60 70 80 90 100 Fig.7

operating conditions pressure 120 bar 0

mass velocity 1130 kg/m s inist subcocling 30 С„

sodium inlst temp 473 С sodium flow 0.659 kg/s

/TOT

Set.up for dynamic instability experiments Fig.6 Absence of dynamic instability at SNR-full load Record of outlet temperature during dynamic instabilities