Air-water tests on counter-current flow limitation at lower end of vertical pipes simulating lower part of steam generator U-tube

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, J_G^* and J_L^*, a CCFL correlation for the sharp lower edge was derived. The CCFL constant, which is the value of J_G^*1/2 at J_L^*1/2=0, was C=0.73±0.02. (author)