## Abstract

In order to investigate the two-phase flow behavior during countercurrent flow limitation in the hot leg of a pressurized water reactor, two test models were built: one at the Kobe University and the other at the TOPFLOW test facility of Forschungszentrum Dresden-Rossendorf (FZD). Both test facilities are devoted to optical measurement techniques; therefore, a flat hot leg test section design was chosen. Countercurrent flow limitation (CCFL) experiments were performed, simulating the reflux condenser cooling mode appearing in some accident scenarios. The fluids used were air and water, both at room temperature. The pressure conditions were varied from atmospheric at Kobe to 3.0 bars absolute at TOPFLOW. According to the presented review of literature, very few data are available on flooding in channels with a rectangular cross section, and no experiments were performed in the past in such flat models of a hot leg. Commonly, the macroscopic effects of CCFL are represented in a flooding diagram, where the gas flow rate is plotted versus the discharge water flow rate, using the nondimensional superficial velocity (also known as Wallis parameter) as coordinates. However, the classical definition of the Wallis parameter contains the pipe diameter as characteristic length. In order to be able to perform comparisons with pipe experiments and to extrapolate to the power plant scale, the appropriate characteristic length should be determined. A detailed comparison of the test facilities operated at the Kobe University and at FZD is presented. With respect to the CCFL behavior, it is shown that the essential parts of the two hot leg test sections are very similar. This geometrical analogy allows us to perform meaningful comparisons. However, clear differences in the dimensions of the cross section ($H×W=150×10 mm2$ in Kobe, $250×50 mm2$ at FZD) make it possible to point out the right characteristic length for hot leg models with rectangular cross sections. The hydraulic diameter, the channel height, and the Laplace critical wavelength (leading to the Kutateladze number) were tested. A comparison of our own results with similar experimental data and empirical correlations for pipes available in literature shows that the channel height is the characteristic length to be used in the Wallis parameter for channels with rectangular cross sections. However, some limitations were noticed for narrow channels, where CCFL is reached at lower gas fluxes, as already observed in small scale hot legs with pipe cross sections.

1.
Wallis
,
G. B.
, 1969,
One-Dimensional Two-Phase Flow
,
McGraw-Hill
,
New York
.
2.
Henderson
,
F. M.
, 1966. “
Open Channel Flow
,”
The Macmillan
,
New York
.
3.
Richter
,
H. J.
, 1981, “
Flooding in Tubes and Annuli
,”
Int. J. Multiphase Flow
0301-9322,
7
(
6
), pp.
647
658
.
4.
Levy
,
S.
, 1999,
Two-Phase Flow in Complex Systems
,
Wiley
,
New York
.
5.
Richter
,
H. J.
,
Wallis
,
G. B.
,
Carter
,
K. H.
, and
Murphy
,
S. L.
, 1978, “
Deentrainment and Countercurrent Air-Water Flow in a Model PWR Hot Leg
,”
U.S. Nuclear Regulatory Commission
, Report No. NRC-0193-9, Hanover, NH.
6.
Weiss
,
P. A.
, and
Hertlein
,
R. J.
, 1988, “
UPTF Test Results: First Three Separate Effect Tests
,”
Nucl. Eng. Des.
0029-5493,
108
(
1–2
), pp.
249
263
.
7.
Glaeser
,
H.
, 1992, “
Downcomer and Tie Plate Countercurrent Flow in the Upper Plenum Test Facility (UPTF)
,”
Nucl. Eng. Des.
0029-5493,
133
(
2
), pp.
259
283
.
8.
Geffraye
,
G.
,
Bazin
,
P.
,
Pichon
,
P.
, and
Bengaouer
,
A.
, 1995, “
CCFL in Hot Legs and Steam Generators and Its Prediction With the CATHARE Code
,”
Proceedings of the Seventh International Topical Meeting on Nuclear Reactor Thermal-Hydraulics (NURETH-7)
, Saratoga Springs, NY, Sept. 10–15, pp.
815
826
.
9.
Wallis
,
G. B.
, and
Dobson
,
J. E.
, 1973, “
The Onset of Slugging in Horizontal Stratified Air-Water Flow
,”
Int. J. Multiphase Flow
0301-9322,
1
, pp.
173
193
.
10.
Zapke
,
A.
, and
Kröger
,
D. G.
, 2000, “
Countercurrent Gas-Liquid Flow in Inclined and Vertical Ducts—I: Flow Patterns, Pressure Drop Characteristics and Flooding
,”
Int. J. Multiphase Flow
0301-9322,
26
, pp.
1439
1455
.
11.
Vallée
,
C.
,
Seidel
,
T.
,
Lucas
,
D.
,
Beyer
,
M.
,
Prasser
,
H. -M.
,
Pietruske
,
H.
,
Schütz
,
P.
, and
Carl
,
H.
, 2009, “
Influence of the Fluid Properties on Co-Current Two-Phase Flows in a Horizontal Channel Connected to a Riser
,”
Proceedings of the Seventh World Conference on Experimental Heat Transfer, Fluid Mechanics and Thermodynamics (ExHFT-7)
, Krakow, Poland, Jun. 28–Jul. 3, pp.
443
452
.
12.
Minami
,
N.
,
Nishiwaki
,
D.
,
Kataoka
,
H.
,
Tomiyama
,
A.
,
Hosokawa
,
S.
, and
Murase
,
M.
, 2008, “
Experiments on Air-Water Counter-Current Flow in a Rectangular Duct Simulating PWR Hot Leg
,”
Proceedings of the 16th International Conference on Nuclear Engineering
, Orlando, FL, May 11–15, Paper No. ICONE16-48113.
13.
Vallée
,
C.
,
Seidel
,
T.
,
Lucas
,
D.
,
Beyer
,
M.
,
Prasser
,
H. -M.
,
Pietruske
,
H.
,
Schütz
,
P.
, and
Carl
,
H.
, 2009, “
Counter-Current Flow Limitation Experiments in a Model of the Hot Leg of a Pressurised Water Reactor—Comparison Between High Pressure Steam/Water Experiments and Low Pressure Air/Water Experiments
,”
Proceedings of the 13th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-13)
, Kanazawa, Japan, Sept. 27–Oct. 2, Paper No. N13P1107.
14.
Ohnuki
,
A.
, 1986, “
Experimental Study of Counter-Current Two-Phase Flow in Horizontal Tube Connected to Inclined Riser
,”
J. Nucl. Sci. Technol.
0022-3131,
23
(
3
), pp.
219
232
.
15.
Ohnuki
,
A.
,
,
H.
, and
Murao
,
Y.
, 1988, “
Scale Effects on Countercurrent Gas-Liquid Flow in a Horizontal Tube Connected to an Inclined Riser
,”
Nucl. Eng. Des.
0029-5493,
107
(
3
), pp.
283
294
.
16.
Lopez-De-Bertodano
,
M.
, 1994, “
Countercurrent Gas-Liquid Flow in a Pressurized Water Reactor Hot Leg
,”
Nucl. Sci. Eng.
0029-5639,
117
(
2
), pp.
126
133
.
17.
Kim
,
H. Y.
, and
No
,
H. C.
, 2002, “
Assessment of RELAP5/MOD3.2.2γ Against Flooding Database in Horizontal to Inclined Pipes
,”
Ann. Nucl. Energy
0306-4549,
29
, pp.
835
850
.
18.
Navarro
,
M. A.
, 2005, “
Study of Countercurrent Flow Limitation in a Horizontal Pipe Connected to an Inclined One
,”
Nucl. Eng. Des.
0029-5493,
235
(
10–12
), pp.
1139
1148
.
19.
Minami
,
N.
,
Nishiwaki
,
D.
,
Nariai
,
T.
,
Tomiyama
,
A.
, and
Murase
,
M.
, 2010, “
Countercurrent Gas-Liquid Flow in a PWR Hot Leg Under Reflux Cooling—(I) Air-Water Tests for 1/15-Scale Model of a PWR Hot Leg
,”
J. Nucl. Sci. Technol.
0022-3131,
47
(
2
), pp.
142
148
.