Experimental study on gas carry-under in liquid down flow from a two-phase mixture

The gas carry-under characteristics in liquid down flow from a two-phase mixture flow have been studied for various flow parameters, based on experiments with a small scale air-water system simulating the concept of a natural circulation BWR with no separators. For high void fraction in the riser, as the liquid superficial velocity jf increased to 0.17 m/s, the void fraction in the lower part of the downcomer αd increased sharply due to the descent of comparatively large bubbles (diameter: about 4∼6 mm). In the region of jf > 0.17 m/s, on increasing jf, the void fraction αd increased until it reached a maximum value at jf = 0.3. For liquid descending velocities higher than 0.3 m/s, αd became almost constant and the level of the mixture above the riser had little effect on the void fraction αd due to the phase separation of the large bubbles formed by bubble coalescence in the upper part of the downcomer. The void fraction αd increased as the void fraction αd increased until bubble coalescence occurred in the upper part of the downcomer, and αd became constant and independent of αd after the occurrence of bubble coalescence. Under the conditions of high void fractions in the riser, 0.4 < αr < 0.64 (upper limit of the tests), and high liquid descending velocities in the downcomer, 0.3 m/s < jf < 0.4 m/s (upper limit of the tests), the void fraction αd was represented by a dimensionless number (G = η4g/σ3ρf)and by the upper limit of void fractions in bubbly flow, αd =0.3 (author)