on the occurrence of burnout downstream of a flow obstacle in boiling two-phase upward flow within a...

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On the Occurrence of Burnout Downstream of a Flow Obstacle

in Boiling Two-phase Upward Flow within a Vertical Annular Channel

Shoji Mori*, Akira Tominaga**, and Tohru Fukano***

* Yokohama National University, Yokohama 240-8501, Japan

E-mail: [email protected]

** Ube National College of Technology, Ube 755-8555, Japan


*** Kurume Institute of University, Fukuoka 830-0052, Japan


Keywords: Multi-Phase Flow, Boiling, Spacer, Burnout, Dryout, Disturbance wave

*Corresponding author. Tel.: +81-453-394010; fax: +81-453-394005

E-mail address: [email protected] (Shoji Mori).

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Abstract

If a flow obstacle such as a spacer is placed in a boiling two-phase flow within a channel, the

temperature on the surface of the heating tube is severely affected by the existence of the spacer.

Under certain conditions, a spacer has a cooling effect, and under other conditions, the spacer

causes dryout of the cooling water film on the heating surface. The burnout mechanism, which

always occurs upstream of a spacer, however, remains unclear.

In a previous paper (Fukano et al. (2002)), we reported that the disturbance wave has a significant

effect on dryout and burnout occurrence and that a spacer greatly affects the behavior of the liquid

film downstream of the spacer.

In the present study, we examined in detail the influences of a spacer on the heat transfer and film

thickness characteristics downstream of the spacer by considering the result in steam-water and air-

water systems. The main results are summarized as follows:

(1) The spacer averages the liquid film in the disturbance wave flow. As a result, dryout tends

not to occur downstream of the spacer. This means that large temperature increases do not

occur there. However, traces of disturbance waves remain, even if the disturbance waves are

averaged by the spacer.

(2) There is a high probability that the location at which burnout occurs is upstream of the

downstream spacer, irrespective of the spacer spacing.

(3) The newly proposed burnout occurrence model can explain the phenomena that burnout

does occur upstream of the downstream spacer, even if the liquid film thickness tFm is

approximately the same before and behind the spacer.

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1. Introduction

Nuclear power stations must be designed to be highly efficient as well as to operate safely. Based

on an experimental result obtained by using a large-scale apparatus, the thermal design of a Boiling

Water Reactor is restricted by heat removal from nuclear rods in close vicinity to cylindrical spacers

that support the nuclear rods (Arai et al. (1992)). However, since this mechanism is not yet fully

understood, clarification of the burnout mechanism near the cylindrical spacers in the Boiling Water

Reactor is necessary. Several studies, including Yokobori et al. (1989), Sekoguchi et al. (1978), and

Feldhaus et al. (2002), have been performed in order to clarify the burnout occurrence mechanism.

Although, generally, the flow pattern is essentially in two-phase flow, most of the above-mentioned

studies did not observe the flow pattern. Few studies have attempted to clarify in detail the burnout

or dryout occurrence mechanisms near the spacer by observing the boiling two-phase flow behavior.

Based on the information described above, Fukano et al. (1996) made a detailed observation of the

behavior of boiling two-phase flow near a flow obstruction in order to clarify the mechanism of dry

patch occurrence by placing a cylindrical flow obstruction in a vertical annular channel. The flow

obstruction was designed to simulate a cylindrical spacer in an actual Boiling Water Reactor.

Furthermore, Fukano et al. (1997) performed an experimental investigation on the effects of the

geometry of the spacer, i.e., a grid spacer or a cylindrical spacer, on dry patch occurrence. They

clarified that dry patches occur more frequently when the grid spacer is used because the wedge-

like gaps formed within the grid spacer hold water near the narrowest region inside the spacer gap

through surface tension. Accordingly, typical drainage occurs just beneath the spacer, when the heat

flux is not so large.

Furthermore, the axial distance between the spacers has a strong effect on the critical heat flux near

the spacer. In an actual nuclear reactor, for example, the distance of 500 mm was adopted. Fukano

(1998) tried to clarify the effect of the existence of an upstream spacer on the dry patch occurrence

on the heating surface around a downstream spacer by observing the flow configuration near both

spacers in detail. Moreover, Fukano et al. (2002) performed a detailed investigation of the wall

temperature fluctuation characteristics near the cylindrical spacer for the case in which repeated

dryout and rewetting of the heating surface occurred. As a result, it was clarified that the

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mechanism of dry patch occurrence was due to the evaporation of a water film that originated

primarily from the drainage of water film in the case of low heat flux, and was due to the

evaporation of the water film (the base film) in the disturbance wave flow in the case of high heat

flux. Fukano et al. (2002) also clarified the influence of the spacer in transient two-phase flow, i.e.,

the influence on the transition of the operating point on parameters such as the heat flux, the mass

flow rate, and the inlet quality of the test section. As a result, even if the flow pattern changes

rapidly by the stepwise change of an operation parameter, the flow transition proceeds safely

provided that the change causes an increase in the vapor velocity, i.e., an increase in the shear force

acting on the water film. On the other hand, if the change causes a decrease in the vapor velocity,

transient burnout may occur, even when the operation condition after the change is less than the

steady burnout condition. Furthermore, Mori and Fukano (2003) performed a detailed observation

of flow phenomena near a spacer using a high-speed video camera for the case in which burnout

occurred in a steady boiling two-phase flow. As a result, it is clarified that the disturbance waves

have a strong effect on burnout occurrence, that is, the interval of the disturbance waves is very

important because the dry patch always occurs at the base film between the neighboring disturbance

waves. In addition, Mori and Fukano (2006) clarified statistically the relationship among the

interval of the disturbance waves, dryout of the thin water film, and burnout of the heating tube for

the case in which a spacer is placed in an annular channel.

The main purpose of the present paper is to clarify in detail the influence of a spacer on the heat

transfer and film thickness characteristics downstream of a spacer. We will propose later herein a

new burnout occurrence model in consideration of the unsteady nature of two-phase flow.

2. Experimental apparatus and procedure

2.1 Experimental apparatus

Figure 1 shows a schematic diagram of the experimental apparatus of the steam-water system. Test

section was placed vertically in a closed forced convection loop. A working fluid, distilled water,

was supplied by a feed pump into the test section after passing through a pre-heater , where the

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temperature of the working fluid at the inlet of the test section, i.e., the degree of inlet subcooling,

was controlled. The two-phase mixture was separated into water and steam in a separator

downstream from the exit of the test section. Both the water and the steam were collected in a

reservoir after being cooled to below saturation temperature in each condenser in order to

prevent cavitation in the feed pump .

As shown in Figure 2, the test section consisted of an annular channel. The inner tube of the test

section, whose dimensions were 1,730 mm in length, 16.0 mm in outer diameter, and 14.0 mm in

inner diameter, was made of stainless steel and was used as a heater by applying direct current

electric power. Both ends of the tube were silver-soldered to copper tubes that were used as power

supply electrodes. The outer tube, the dimensions of which were 1,420 mm in length, 30.0 mm in

outer diameter, and 26.0 mm in inner diameter, was made of Pyrex glass in order to enable

observation of the two-phase flow configuration.

The geometry of a flow obstruction, as shown in Figure 3, was designed to simulate a cylindrical

spacer supporting nuclear fuel rods in an actual nuclear power station. A cylindrical spacer is

hereinafter referred to simply as the spacer. The spacer was constructed of quartz glass in order to

enable observation of the flow configuration on the heating surface inside the spacer. The spacer

was supported by two sets of two stainless steel pipes having a diameter of 1.0 mm, a thickness of

0.2 mm, and a length of 2.0 mm, and a plate spring having a width of 3.0 mm and a length of 30

mm. The pipes corresponded to the spacer pins of an actual Boiling Water Reactor. For the case in

which a spacer was present, a spacer was placed 1,690 mm downstream of the starting point of

heating, and for the case in which two spacers were present, another spacer was placed upstream of

the downstream spacer with a spacing of Ls = 250 mm, as shown in Figure 2. The two cases stated

above are hereinafter referred to as Ls = ∞ and Ls = 250, respectively.

Fourteen thermocouples having diameters of 0.08 mm were placed upstream, inside, and

downstream of an upstream spacer, ranging over an axial distance of 300 mm, as shown in Figure 4.

The thermocouples were pressed against the inner surface of the heating tube wall by expanding a

silicon tube, with a thin mica film of 0.01 mm thickness between them. During the experiments, the

time varying outputs Tin from these thermocouples were simultaneously recorded on a digital

recorder at a sampling frequency of 135 Hz. We calculated the outer surface temperatures of the

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heating tube, Tout, using these data Tin and a heat diffusion equation (Fukano et al. (2002)). As

shown in Figure 4, two strain pressure gages were placed at two locations, 75 mm upstream and 75

mm downstream from the center of an upstream spacer, respectively, to measure the differential-

pressure (DP) fluctuation at this location. The diaphragms of the static pressure gages were fixed

directly to the taps of the Pyrex glass tube in order to measure the pressure fluctuation with

sufficient frequency response. The differential pressure at the upstream spacer was obtained directly

as the difference of the two static pressure gages. Both of the static pressure gages were calibrated

both before and after the experiments, and the differential pressure had an error of not more than

cmH2O. The differential pressure increases rapidly when the disturbance wave passes the

location of the upstream spacer (S. Mori and T. Fukano (2006)).

The flow configuration, i.e., the process of the occurrence of dry patches on the heating tube surface,

was recorded using a high-speed video camera at 400 frames per second simultaneously with the

temperature and pressure fluctuation.

2.2 Definition of burnout occurrence on the heating tube

In the present experiment, we judged burnout of the heating tube to have occurred if Tin exceeded

180ºC for a continuous or rapid increase of Tin, even when the disturbance waves passed over the

dried surface. When burnout occurred, the heating was immediately stopped in order to prevent the

heating tube and the thermocouples from breaking.

2.3 Experimental conditions

The experimental conditions were decided by taking into consideration the operating conditions of a

Boiling Water Reactor, except for pressure, as follows: heat flux, q = (230 to 280) kW/m2;

superficial water velocity, jL = (0.08 to 0.1) m/s; superficial steam velocity at the spacer, jG = (0 to

approximately 48) m/s; quality at the spacer, x = (0 to approximately 0.22); and inlet subcooling,

Tsub = 20 K. The heat flux was gradually increased in small increments (approximately 15 kW/m2)

and was maintained for approximately 5 minutes, and this process was repeated until burnout

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occurred. The glass wall of the test section was not thermally insulated so that the flow inside could

be observed. The heat loss was experimentally verified to be less than approximately 3% of the

added heat. It must be noted that, for the sake of visualization, the data shown here were obtained

under atmospheric pressure, which is far lower than the actual operating pressure of a Boiling Water

Reactor.

2.4 Current burnout occurrence model in a BWR

Later herein, we propose a new burnout occurrence model by taking the unsteady nature of two-

phase flow into consideration. For the sake of comparison with the proposed model, the current

burnout occurrence model is explained in this section. Figure 5 shows a schematic diagram of the

current model. The numbers in the enlarged diagram indicate the following phenomena (Yamamoto

et al. (1997)) as spacer effects.

The flow area is decreased by the presence of the spacer, which causes the acceleration of vapor

flow and increases interfacial shear stress between the vapor and the liquid film. In this region,

the vapor flow promotes the generation of entrainments from the liquid film.

The droplets impinge against the spacer surface and splash onto the liquid film on the fuel rod

surface.

and The other effects promote droplet deposition and entrainment due to increasing turbulence

in the vapor flow at the downstream side of the spacer, as shown and in Figure 5, respectively.

Considering the fact that burnout always occurs upstream of the spacer, the current model considers

that the liquid film flow rate downstream of the spacer becomes larger than that upstream due to the

spacer effects stated above. The liquid film flow rate then gradually becomes smaller with distance

from last transition of the spacer, and burnout occurs at the location where the flow rate is zero, i.e.,

at the location where dryout occurs. The current model is based on time-averaged view point.

However, some researchers have reported results that differ from those obtained by the above

model. For example, it has been reported that the liquid film flow rate in the case of burnout

occurrence is not always zero (Ueda and Isayama (1981)) and that dryout of the liquid film does not

always indicate burnout occurrence (Fukano and Mori (2003)).

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3. Experimental results and discussion

3.1 Influence of the spacer on heat transfer characteristics

Figure 6 shows the cumulative probability distribution of qCHF in the cases of Ls = ∞ and Ls = 250

for Uin = 0.1 m/s. It is clear from this figure that qCHF in the case of Ls = 250 is approximately 5%

larger than Ls = ∞. This indicates that spacer spacing has a significant effect on critical heat flux.

Figures 7 (I) and (II) show typical examples of differential pressure and wall temperature

fluctuations in the case of burnout occurrence for Ls = ∞ and Ls = 250, respectively. The term

“OFF” in the right-hand side of the figures indicates the time at which the heating was stopped in

order to prevent breakage of the heating tube, since Tout exceeds the burnout temperature defined in

Section 2.2. The experimental conditions at TC2 and TC14 for the cases of Ls = ∞ and Ls = 250 are

as follows:

(I) Case of Ls = ∞ : q = 264 kW/m2

Flow conditions at TC14: jG m/s, jL = 0.073 m/s, and x = 0.28


(II) Case of Ls = 250: q = 283 kW/m2



Therefore, flow patterns from TC2 to TC14 in both cases are typical annular flow, and the

experimental conditions for each spacer spacing in the steam-water system are approximately the

same in the remaining figures. The wall temperature at the axial wide region increases rapidly at

almost the same time, as shown in Figures 7 (e.g., at approximately 27 and 26 seconds for Ls = ∞

and Ls = 250, respectively). Based on detailed observation of the behavior of dryout occurrence by

high-speed camera, dryout spreads downstream for a short time (refer to Figure 12). Moreover, wall

temperature decreases rapidly only when disturbance waves pass. For the case of Ls = ∞, wall

temperature increases even at TC3, whereas for the case of Ls = 250, the wall temperature from

Figure 9 Location of probes

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TC1 to TC8 does not increase. That is, dryout tends not to occur just downstream of the spacer for

the case of Ls = 250. This will be considered statistically hereinafter.

Figure 8 shows the frequency distribution of the burnout occurrence and the frequency of large

temperature increase, i.e., increases of more than 10 K due to dryout occurrence, plotted against

location. The location of the highest frequency of burnout occurrence in both cases is the region just

upstream of the downstream spacer, and the frequency of the increase in TOUT from TC2 to TC7 for

the case of Ls = 250 is very small compared with the same location for the case of Ls = ∞, although

the heat flux for the case of Ls = 250 is larger than that for the case of Ls = ∞, which signifies that

the spacer spacing has a great effect on burnout and dryout occurrence, in other words, the presence

of the spacer greatly affects the behavior of the liquid film downstream of the spacer.

In summary, the smaller the spacer spacing, the larger qCHF. Moreover, burnout never occurs and

large temperature increases of more than 10 K rarely occur just downstream of the upstream spacer.

This indicates that the spacer has a strong effect downstream of the spacer.

3.2 Influence of the spacer on film thickness characteristics

In a previous paper (Mori and Fukano 2003), we reported that disturbance waves have a great effect

on burnout occurrence. Therefore, in this section we focused on the change in the shape of the

disturbance wave in front of and behind the spacer. Figure 9 shows a schematic diagram of the

experimental apparatus in the air-water system. The shape of the spacer is cylindrical. The clearance

between spacer and the wall is 0.75 mm, and the spacer thickness is 5 mm. Seven probes for film

thickness measurement ( 5 % accuracy) are situated as shown in the figure. The thickness of the

copper ring electrode is 1.0mm, and the axial distance of one pair in sensor electrodes is 6.0mm.The

shape of the channel, however, is different from that in the steam-water system, as shown in Figure

3. It is considered that the influence of the spacer on the liquid film is qualitatively the same. The

experimental conditions in the air-water system were decided by taking the condition in the steam-

water system into consideration, i.e., jG = 38.8 m/s and jL = 0.1 m/s. The experimental conditions in

the air-water system are approximately the same in the remaining figures. The flow pattern is a fully

developed annular flow.

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Figure 10 shows the minimum, mean, and maximum film thickness in the case of jG = 38.8 m/s and

jL = 0.1 m/s. Cumulative probability densities of 99% and 1% in the probability density distribution

of film thickness fluctuation are defined as minimum film thickness tFmin and maximum film

thickness tFmax, respectively. Mean film thickness tFm is the arithmetic mean value of the film

thickness fluctuation. Note that tFmin and (tFmax - tFmin) are roughly equal to the value of the base film

thickness and height of the disturbance wave, respectively.

As shown in Figure 10, the film thickness increased just in front of the spacer due to the increase in

the static pressure caused by the stagnation of the air flow. The film thickness within the spacer gap

then decreased rapidly because of the acceleration of the steam flow due to the blockage effect of

the spacer. In addition, the cross sectional area at the exit of the spacer was considerably larger for a

gap flow. Therefore, the static pressure and its thickness increased abruptly. These tendencies also

appeared in the mean wall temperature in the vapor-water system (Fukano and Mori, 2003). From

here, we focused on the film thicknesses at P1 and P7. The film thicknesses at P1 and P7 are

approximately equal with respect to tFm and tFmin, but tFmax at P7 is approximately 60% of that at P1.

In general, tFmin can be considered as a criteria indicating whether dryout occurs. However, tFm and

tFmin in front of (P1) and behind (P7) the spacer are approximately the same, as stated above. That is,

the reason why burnout always occurs upstream of the spacer in a BWR cannot be explained from

tFm and tFmin as calculated statistically. Therefore, film thickness fluctuation will be discussed in

detail in the following.

Figure 11 shows the film thickness fluctuation with time at P1 and P7 under the experimental

conditions described in Figure 10. Note that the wave height of the disturbance waves does not

exceed the clearance (0.75 mm) of the spacer. In addition, the disturbance waves pass quickly

(approximately 4 m/s) on the base film. Compared with the change in the shape of the disturbance

waves and the base film upstream and downstream of the spacer, the behavior of the liquid film in

passing the spacer can be characterized as follows.

The wave height of the disturbance waves decreases abruptly after passing the spacer, and the film

thickness between successive disturbance waves becomes thick, while the frequency of passage of

the disturbance waves does not change. Note that the traces of disturbance waves never disappear.

Based on these considerations, we can explain that dryout tends not to occur downstream of the

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spacer even if tFm or tFmin in front of and behind the spacer are approximately equal, because the

duration time of the rather thin liquid film just downstream of the spacer become shorter. This

tendency occurs not only for jL = 0.1 m/s but also for jL = 0.06 m/s to 0.4 m/s in our experiments.

Figure 12 shows the detailed process of the change in shape of disturbance waves near the spacer

from 2.3 s to 2.6 s in Fig. 11. The behavior of disturbance waves is characterized as follows:

Disturbance waves flow upward up to leading edge of the spacer with same shape.

When disturbance waves enter the spacer gap, the wave height of the disturbance waves

becomes smaller because of the increase in interfacial shear force due to the blockage effect.

The wave height of the disturbance wave becomes larger at the exit of the spacer due to the

decrease in interfacial shear force.

The wave height of the disturbance waves becomes small again because disturbance waves are

re-averaged by the vapor flow in the main flow. Note that the traces of disturbance waves never

disappear.

Figure 13 shows the change of the flow configuration in the case of dryout occurrence downstream

of the upstream spacer in the steam-water system. The white parts in the figure indicate dry patches.

As shown in the figure, incipient dry patches occurred at 10 mm downstream from the last

transition of the spacer, and spread downstream momentary. These dry patches then disappeared

instantaneously when disturbance waves passed. Burnout, however, never occurred just

downstream of the spacer, although dryout occurred occasionally for a short time, even downstream

from the spacer. This fact signifies that dryout just downstream of the spacer does not cause the

burnout because dryout is rewetted easily by the passing of the disturbance waves even when dryout

does occur.

Figure 14 shows the value of superheat in the case of Ls = 250 normalized by the case of Ls = ∞.

In consideration of the equivalent heat conduction layer, as shown in the following equation, the

ratio of superheat is approximately equal to the ratio of thickness in an equivalent heat conduction

layer. That is, a value that is smaller than 1 means that the film thickness is reduced by the

placement of a spacer.

)( satoutL TT

q

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The values of TC2 and TC3 are less than 1, and the values of TC4 to TC14 are roughly equal to 1,

indicating that the spacer makes the liquid film at TC2 and TC3 thinner. Note that TC3 corresponds

to the incipient location of dryout occurrence in Fig. 13.

3.3 Proposed burnout occurrence model

The time averaged current burnout occurrence model as explained in Section 2.4 cannot explain

why burnout always occurs upstream, rather than downstream, of the spacer, because tFm and tFmin

before(P1) and behind (P7) the spacer are approximately the same, as shown in Figure 10. And

according to the detailed observation by high-speed camera, a lot of droplets cannot be identified in

the vapor core. Moreover, dryout occurrence and temperature fluctuation was often synchronized

with the passing of the disturbance wave. Therefore, we consider that not the liquid droplets but

disturbance waves play an important role in the dryout and rewetting phenomena. Taking these

results into consideration, we proposed the new burnout occurrence model in consideration of the

unsteady nature of two-phase flow.

Figure 15 shows a schematic diagram of the proposed burnout occurrence model. The behaviors

indicated by the numbers in the figure are as follows:

Disturbance waves flow upward on the base film up to the leading edge of the spacer

without the influence of the spacer.

When disturbance waves pass the spacer, the wave height of the disturbance waves become

small, and the film thickness between the disturbance waves becomes large. As a result,

dryout tends not to occur downstream of the spacer. However, traces of disturbance waves

remain even if the disturbance waves are averaged by the spacer.

The film thickness between the disturbance waves decreases because the traces of the

disturbance waves flow upward and again develop into typical disturbance waves. Therefore,

the film thickness between disturbance waves becomes thin. As a result, dryout occurs at the

base film at a location far downstream from the last transition of the spacer.

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Incipient dryout spreads downstream instantaneously, and then the dry patches disappear with

the passing of the following disturbance wave. Dryout cannot be rewetted by the disturbance

wave when the amount of water of the disturbance wave is too small. As a result, the wall

temperature increases rapidly, provided that the wall surface is not covered with liquid film,

and burnout occurs without exception if the wall temperature reaches the Leidenfrost

temperature. Burnout then occurs upstream of the downstream spacer because the dryout time

is long at this location.

The proposed burnout occurrence model can explain phenomena whereby burnout occurs

upstream of the downstream spacer, even if tFm or tFmin in front of the spacer and downstream of

the wake region of the spacer are approximately the same

.

4. Conclusion

In the present study, we examined in detail the influences of a spacer on the heat transfer and film

thickness characteristics downstream of the spacer by considering the result in steam-water and air-

water systems. Based on our results, we clarified the following:

(4) The smaller the spacer spacing, the larger qCHF.

(5) The spacer averages the liquid film in the disturbance wave flow. That is, when the

disturbance wave passes the spacer, the wave height of the disturbance waves becomes

smaller and the film thickness between the disturbance waves becomes larger. As a result,

dryout tends not to occur downstream of the spacer. This means that large temperature

increases do not occur there. However, traces of disturbance waves remain, even if the

disturbance waves are averaged by the spacer.

(6) There is a high probability that the location at which burnout occurs is upstream of the

downstream spacer, irrespective of the spacer spacing.

(7) The newly proposed burnout occurrence model can explain the phenomena that burnout

does occur upstream of the downstream spacer, even if the liquid film thickness tFm is

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approximately the same in front of the spacer and downstream of the wake region of the

spacer.

5. Refernces

1. Arai,K., Tsunoyama,S., Yokobori,S., and Yoshimura.K., 1992. Critical power characteristics in

a rod bundle with narrow gap. Dynamics of Two-Phase Flow, CRC Press, Boca Raton, FL, 739-

752

2. Fukano, T., Goto, A., Tsurusaki, Y., and Morooka, S., 1996. Dryout of water film on heated

tube surface caused by an obstruction in a boiling two-phase vertical upward flow. Chem. Eng.

Comm., Vols. 141-142, 191-206.

3. Fukano, T., Egashira, R., and Naitoh, K., 1997. Dryout of water film on a heated tube surface

caused by an obstruction in a boiling two-phase vertical upward flow in an annular channel

(effect of the geometry of the spacer). Proc. of the Physics of Heat Transfer in Boiling and

Condensation, Moscow, Russia, 257-262.

4. Fukano,T., 1998. Dryout of water film on a heated tube surface caused by an obstraction in a

boiling two-phase vertical upward flow in an annular channel (effects of an upstream spacer).

Proc. 11th IHTC., Vol.2, 249-254.

5. Fukano,T. , Mori, S., Akamatsu, S., and Baba, A., 2002. Relation between temperature

fluctuation of a heating surface and generation of drypatch caused by a cylindrical spacer in a

vertical boiling two-phase upward flow in a narrow annular channel. Nuclear Engineering and

Design , vol.217, 81-90.

6. Fukano,T., Kawakami,Y., Shimizu,H., and Sekoguchi,K., 1980. Film thickness in gas-liquid

two-phase flow (4th report, film thinning mechanism during the drainage). Bulletin of the JAME,

vol. 23, No.178, 553-560

7. Fukano,T. , Tanaka,T., Kutaragi,K., and Kanamori,M., 1992. The effect of a flat-plate-type

obstacle on a thin liquid film flow. Dynamic of two-phase flows, CRC Press, Boca Raton, FL,

pp.161-183.

15

8. Fukano, T., Ishida, K., Morikawa, K., Nomura, H., Takamatsu, Y., Sekoguchi, K.,1979. Liquid

films flowing concurrently with air in horizontal duct-1st report, flow pattern. Bull. JSME 22-

172,1374-1382.

9. Fukano T., Mori S. and Nakagawa T., 2003, Fluctuation characteristics of heating surface

temperature near an obstacle in transient boiling two-phase flow in a vertical annular channel,

2003, Nuclear Engineering and Design, Volume 219, Issue 1, 47-60

10. G. Feldhaus, B. J. Azzopardi and W. Zeggel, 2002, Annular flow experiments in rod bundles

with spacers, Nuclear Engineering and Design, Volume 213, Issues 2-3, 199-207

11. Mori S. and Fukano T., Influence of a flow obstacle on the occurrence of burnout in boiling

two-phase upward flow within a vertical annular channel, 2003, Nuclear Engineering and

Design, Volume 225, Issue 1, 49-63

12. Mori S. and Fukano T., Relation between the Occurrence of Burnout and Differential-pressure

Fluctuation Characteristics caused by the Disturbance Waves Passing by a Flow Obstacle in a

Vertical Boiling Two-phase upward flow in a narrow annular channel, 2006, Nuclear

Engineering and Design, in press

13. Sekoguchi, K., Tanaka, O., Furukawa, T., Esaki, S., and Fukano, T., 1978. Experimental

investigation of the alternate dry and rewet (ADR) on heated tubes with and without an

obstruction in flow boiling. Proc. 6th IHTC., Vol. 4, 355-360.

14. Shiralker，B.S.， and Lahey，R.T.，Jr.，1973. The effect of obstacles on a liquid flow，

Trans ． ASME， J． Heat Transfer， 95-4， 528-533．

15. Yamamoto, Y., Hoshide, A., Mitsutake, T., Morooka, S., 1997, Analytical study on effects of

BWR fuel spacer on droplet deposition, Nuclear Engineering and Design, Volume175, 119–129

16. Yokobori, S., Ohta, M., Terasaka. H., and Morooka, S., 1989, A phenomenological study on the

dryout mechanism in fuel rod, Fourth International Topical Meeting on Nuclear Reactor

Thermal-Hydraulics. Nureth-4, 1054-1061.

17. Ueda, T., and Isayama, Y., 1981, Critical heat flux and exit film flow rate in a flow boiling

system, International Journal of Heat and Mass Transfer, Vol. 24-7, 1267-1276

16

Fig.1 Schematic diagram of the experimental apparatus

Shoji Mori, Akira Tominaga and Tohru Fukano

① Test Section② Separator③ Safety Valve④ Deaeration Chamber⑤ Condenser⑥ Reservoir A⑦ Pump⑧ Flow Meter⑨ Mixing Chamber⑩ Pre-heater⑪ Valve A⑫ Valve B⑬ Valve C⑭ Valve D⑮ Reservoir B

(TC):Thermocouple

P :Pressure Transduser

①

②

③

④

⑤

⑥

⑦

⑧⑨

⑩

⑪

⑫

⑬

⑭

⑮

(TC)

P

P

Cooling Water

Cooling Water

⑤

17

Fig.2 Test section and location of spacers


Flow

Hea

ting

reg

ion

1730

Flow

Vie

win

g se

ctio

n14

20

250

1690

Flow

280

Spacer

Spacer

30

Glass Spacer

φ26

φ16

18

Fig.3 Schematic diagram of the glass spacer


30 1Glass spacer

Heating tube

Stainless steel tube(φ1.0×φ0.7×2.0)

Glass tube2

Plate spring

Heating tube

26

16

Glass spacer

19

Fig.4 Location of thermocouples and static pressure gages


20

Fig.5 Current burnout occurrence model in BWR


Dryout=Burnout

Hea

ting

tube

Flo

w

Water film

Spacer

Droplet

③

④

①

②

Hea

ting

tube

Spacer

Spacer

21

Fig. 6 Cumulative probability distribution of qCHF


220 240 260 2800

20

40

60

80

100Ls=∞(Data points=31)Ls=250(Data points=28)

qCHF , kW/m2

Cum

ulat

ive

prob

abili

ty ,

%

Uin=0.10 m/s

22

Fig. 7 Temperature and differential pressure fluctuation

in the case of burnout occurrence

q = 264 kW/m2, jG = 43 m/s,

jL = 0.073 m/s, x = 0.28

q = 283 kW/m2, jG = 48 m/s,

jL = 0.070 m/s, x = 0.30


25 26 27 28 29 30

100

150

200

0

100

200

t , s

Tou

t , ℃

TC7TC6TC5TC4TC3TC2TC1

(b)off

DP

, c

mH

2O

DP

(Ⅰ) Case of Ls=∞

25 26 27 28 29 30

100

150

200

0

100

200

t , s

Tou

t , ℃

DP

, c

mH

2O

TC14TC13

(a) off

TC12TC11TC10TC9TC8

DP

25 26 27 28 29 30

100

150

200

0

100

200

t , s

Tou

t , ℃

TC7TC6TC5TC4TC3TC2TC1

(b)

off DP

, c

mH

2O

DP

(Ⅱ) Case of Ls=250mm

25 26 27 28 29 30

100

150

200

0

100

200

t , s

Tou

t , ℃

DP

, c

mH

2O

TC16TC15TC14TC13

(a)

off

TC12TC11TC10TC9TC8

DP

23

0 20 40 60 80 1000123456789

101112131415

Loc

atio

n of

TC

Frequency , %

Flo

w

(a) Frequency distribution of locations of burnout occurrence

Ls= ∞Ls= 250 mm

0 1 2 30123456789

101112131415

Spa

cer

Frequency , number of occurrences/min

Ls=250 mm

q=283 kW/m 2

Flo

w

(b)Frequency distribution of locations in case Tout increases beyond 10 K

Spa

cer

Spa

cer

Ls= ∞Ls= 250 mm

Ls=∞

q=264 kW/m 2

jG=43 m/sjL=0.073 m/sx=0.28

jG=35 m/sjL=0.078 m/sx=0.23

jG=48 m/sjL=0.070 m/sx=0.30

jG=38 m/sjL=0.076 m/sx=0.25

Fig.8 Frequency distribution of locations of the burnout

and the dryout occurrence (Uin = 0.1 m/s)


24

Fig.9 Location of probes for film thickness measurement


13.5

10

ƒÓƒÓƒÓƒÓƒÓ

P2

P5

P6

P7

P4

P3

φ14.5

0.75

φ26

2525

2522

.57.

5

Spacer

Flow P1

66

66

66

6

25

Fig.10 Film thickness distribution tFmin,, tFm, and tFmax along the spacer

( jL = 0.10 m/s, jG =38.8 m/s)


0 0.1 0.20

1

2

3

4

5

6

7

8

Loc

atio

n of

pro

be

tFmin , mm

Flo

w

Spacer

0.13 mm

0.12 mm

P

P

P

P

P

P

P

0 0.1 0.2 0.3 0.4 0.50

1

2

3

4

5

6

7

8

tFm , mm

Flo

wSpacer

0.22 mm

0.23 mm

P

P

P

P

P

P

P

0 0.5 1 1.50

1

2

3

4

5

6

7

8

tFmax , mm

Flow

Spacer

0.74 mm

0.47mm

Clearance:0.75 mm

P

P

P

P

P

P

P

26

Fig. 11 Film thickness fluctuation at P1 and at P7

(jL = 0.10 m/s, jG = 38.8 m/s)


2 2.2 2.4 2.6 2.8 30

0.2

0.4

0.6

0.8

1P1

t , s

t F ,　

mm

▽▽▽P7

Cle

aran

ce :

0.75

mm

27

Fig. 12 Film thickness fluctuation near the spacer

(jL = 0.10 m/s, jG = 38.8 m/s)


2.3 2.4 2.5 2.60

0.2

0.4

0.6

0.8

1

P5

t , s

t F　

, m

m

P1P2P3 ①

②

③

④

P4○○○

△△△

□□□

▽▽▽

P6

P7

Cle

aran

ce :

0.75

mm

28

①25.950 s ②25.975 s ③26.000 s ④26.025 s Spa

cer

Fig.13 Change of the flow configuration in the case of dryout

occurrence downstream of the upstream spacer (Ls = 250)

q = 283 kW/m2

jG = 38 m/s

jL = 0.076 m/s

x = 0.248


29

Fig. 14 Influence of Ls on the degree of superheat ΔT

(Uin = 0.1 m/s)


10 20 30 400

1

2

jG , m/s (at the upstream spacer)

ΔT

Ls=

250

/ Δ

TL

s=∞

TC1TC2TC3TC4

TC9TC10TC11TC12

TC5TC6TC7TC8

TC13TC14

30

Fig. 15 Proposed burnout occurrence model near the spacer


Hea

ting

tub

e

Spacer

Spacer

Flo

w

DW

Vap

or f

low

Vap

or f

low

①

②

③Dryout

DW

DW

DW

DW

DW

Base film

on the occurrence of burnout downstream of a flow obstacle in boiling two-phase upward flow within a...

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