Geometrical parametric effects on CCFL in a nuclear reactor hot leg

An experimental study of countercurrent flow limitation (CCFL) has been performed for air-water countercurrent flow in a horizontal pipe connected to a upwardly inclined pipe through a bend. The CCFL determines the maximum rate at which one phase can flow counter currently to another phase. A total of 85 onset of CCFL and 13 zero penetration limits were obtained. The parametric effects of the pipe diameter, horizontal pipe length, and inclined pipe length on CCFL were also examined. Three different mechanisms that lead to CCFL were observed in accordance with the inlet water flow rate. For low inlet water flow rate, the CCFL occurred simultaneously with the formation of the large unstable waves at the crest of the hydraulic jump in the horizontal pipe. The hydraulic jump appeared far away from the bend as the inlet water flow rate increased; at intermediate inlet water flow rate, no hydraulic jump was observed, and CCFL occurred as a result of large wave formation at the end of the horizontal pipe; at high inlet water flow rate, no hydraulic jump and no large wave formation at the end of the horizontal pipe were observed, but the entrainment of the water droplet into the water tank that is connected to the inlet of the test section induced CCFL. The effects of the horizontal pipe length-to-diameter ratio, L/D, and inclined pipe length on CCFL were significant in the present experimental range. Based on the present experimental data, an empirical correlation for CCFL was developed in terms of square roots of dimensionless superficial velocities. The present correlation predicted the large pipe diameter data more closely than the existing empirical correlations. The discrepancy of the two correlations are mainly due to the fact that the experimental range of L/D and inclined pipe length were quite different