Air-water tests on counter-current flow limitation (CCFL) were carried out using vertical pipes with the diameter of
D=20 mm and a sharp lower edge which simulated the lower part of U-tubes in a steam generator. Flow patterns were observed to identify the location of CCFL occurrence, and effects of the pipe length
L and the water level in the upper tank
h on CCFL were measured under the conditions of
L=100-700 mm and
h=100-600 mm. As a result, CCFL appeared at the upper end under the conditions of
L/D=5 and
h/D=5, but at the lower end for
L/D≧15 and
h/D=5~30. In the case of the CCFL occurrence at the lower end,
L and
h did not affect CCFL characteristics. Using the Wallis parameters,
JG* and
JL*, a CCFL correlation for the sharp lower edge was derived. The CCFL constant, which is the value of
JG*1/2 at
JL*1/2=0, was
C=0.73±0.02.
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