**4.1.1 Analytical conditions**

In this section, to evaluate the existing two-phase flow correlation for fluid mixing phenomena, two-phase flow in 2 modeled subchannels for BWRs and FLWRs fuel bundles were performed ("BWR cases" and "FLWR cases"). For this, 16 cases of two-phase flow simulations were performed (see table 6 and 7).


Table 6. Calculation conditions for fluid mixing phenomena in BWRs fuel bundle.

Development of Two-Phase Flow Correlation

(a) Calculated test channel

Fig. 22. Dimensions of calculated test channel and calculation meshes for FLWR cases.

To simulate the operating conditions of the BWR and the FLWR, outlet pressure and inlet temperature of steam and water is set to 7 MPa and saturation temperature at 7 MPa respectively. Steam and water were injected through the steam and water inlet section located at lower part of the modeled test channel. The steam and water inlet sections were optimized to get a developed flow at inlet of test section, and divided into twelve small

Example of the calculated behaviors in the test channel and detail of slug behavior of are shown in Fig.23. As shown in Fig.23, the fluid mixing between Ch.1 and Ch.2 was observed at a gap between the subchannels. Though inlet quality of both subchannels were equivalent in this case (inlet quality ratio (*X*2/*X*1) was equal to 1.0), fluid mixing occurred between 2

The existing two-phase flow correlation for fluid mixing (fluctuating pressure model (Takemoto, 1997) was evaluated using detailed numerical simulation data. The fluctuating

( )

(21)

<sup>2</sup>

2

*K x Bx*

11 1

 

*l T T T l g*

*p w fx p*

conditions for each subchannel.

sections for BWR cases.

subchannels.

**4.1.2 Evaluation of existing correlations** 

pressure model is expressed as follows:

for Fluid Mixing Phenomena in Boiling Water Reactor 307

size was set to 2/3 mm for BWR cases and 0.21mm or 0.16mm for FLWR cases to satisfy the condition that the number of the calculation meshes of gap region must be more than 6. A non-slip wall, constant exit pressure and constant inlet velocity were selected as boundary

Fig. 21. Dimensions of calculated test channel and calculation meshes for BWR cases.

The calculated test channel is shown in Fig.21 (a) and Fig.22 (a). The flow area is divided into two channels by a flat plate (partition plate) as same as the air-water cases. The flow channel is divided into 2 parts, developing section and mixing section. Liquid phase velocity in the BWR cases and the FLWR cases is relatively higher than that in air-water cases. To remove the effects on flow development and fluid mixing, length of the developing section and the mixing section were extended and the outlet section was removed.


Table 7. Calculation conditions for fluid mixing phenomena in FLWRs fuel bundle.

Regular mesh division in the Cartesian system was adopted except for lower part of the developing section. In the lower part of developing section (z=0~350 mm for BWR cases, z=0~200 mm for FLWR cases), to save computational resources, a relatively coarse computational meshes were used (*z*=2mm). The other region, *z* equals to 1mm. Two subchannels and the interconnection were formed by using obstacles. The calculation mesh

Steam

(a) Calculated test channel (b) Calculation mesh

and the mixing section were extended and the outlet section was removed.

Fig. 21. Dimensions of calculated test channel and calculation meshes for BWR cases.

The calculated test channel is shown in Fig.21 (a) and Fig.22 (a). The flow area is divided into two channels by a flat plate (partition plate) as same as the air-water cases. The flow channel is divided into 2 parts, developing section and mixing section. Liquid phase velocity in the BWR cases and the FLWR cases is relatively higher than that in air-water cases. To remove the effects on flow development and fluid mixing, length of the developing section

Inlet liquid mass flux (kg/m2s) Inlet quality (%)

150

600

F3 0.03 F4 0.08

F6 0.03 F7 0.08 F8 0.12

Regular mesh division in the Cartesian system was adopted except for lower part of the developing section. In the lower part of developing section (z=0~350 mm for BWR cases, z=0~200 mm for FLWR cases), to save computational resources, a relatively coarse computational meshes were used (*z*=2mm). The other region, *z* equals to 1mm. Two subchannels and the interconnection were formed by using obstacles. The calculation mesh

1.0 600 600 0.05

Table 7. Calculation conditions for fluid mixing phenomena in FLWRs fuel bundle.

Ch.1 Ch.2 Ch.1 Ch.2

0.05

0.21

0.002

0.05

Developing section

*Slit* Mixing section

Channel 1 Channel 2

200

350

z

Case

F1

F2

F5

x

Gap width (mm)

1.3 600

size was set to 2/3 mm for BWR cases and 0.21mm or 0.16mm for FLWR cases to satisfy the condition that the number of the calculation meshes of gap region must be more than 6. A non-slip wall, constant exit pressure and constant inlet velocity were selected as boundary conditions for each subchannel.

Fig. 22. Dimensions of calculated test channel and calculation meshes for FLWR cases.

To simulate the operating conditions of the BWR and the FLWR, outlet pressure and inlet temperature of steam and water is set to 7 MPa and saturation temperature at 7 MPa respectively. Steam and water were injected through the steam and water inlet section located at lower part of the modeled test channel. The steam and water inlet sections were optimized to get a developed flow at inlet of test section, and divided into twelve small sections for BWR cases.
