Experimental investigation of two-phase countercurrent flow limitation in a bend between horizontal and inclined pipes

Abstract

The countercurrent flow limitation (CCFL) or onset of flooding determines the maximum rate at which one phase can flow countercurrently to another phase. In the present study, the experimental data of the countercurrent flow limitation for air and water in a bend between a horizontal pipe and a pipe inclined to the horizontal are investigated. Water is introduced into the upper leg and flows downward while the air injected into the horizontal leg flows countercurrently. The flow patterns are visualized. The different mechanisms that lead to flooding and that are dependent on the water flow rate are observed. For low and intermediate water flow rates, the onset of flooding appears simultaneously with the slugging of unstable waves that are formed at the crest of the hydraulic jump. At low water flow rates, slugging appears close to the bend; at higher water flow rates, it appears far away from the bend in the horizontal section. For high water flow rates, no hydraulic jump is observed, and flooding occurs as a result of slug formation at the water flow outlet close to the end of the horizontal pipe. The influence of the inclination angle of the bends, the water inlet conditions, and the length of the horizontal pipes is of significance for the onset of flooding.