Experimental study on flow patterns of air-water two-phase flow in S-shaped flexible riser

Air-water two-phase flow patterns in an S-shaped flexible riser were experimentally studied. The test loop consisted of a horizontal pipeline 114 m in length and 50 mm in i. d., a -2° downward inclined pipe 16 m in length, and an S-shaped riser 15.3 m in vertical height and 24 m in length. The pressure at the bottom of the riser, the riser outlet liquid holdup H_L and the probability density function (PDF) distribution were adopted in the identification of the flow patterns. The experimental results show that there were three types of flow patterns in the flexible riser, i. e., typical severe slugging, transitional severe slugging, and stable flow. For the typical severe slugging, the square shape profile of H_L exhibited a PDF distribution with two peaks at H_L=0 and H_L=1, respectively; the acicular shape profile of H_L for transitional severe slugging also had a bimodal PDF distribution, but the peak at high H_L was much lower than that at low H_L; the PDF of H_L for stable flow displayed a unimodal distribution with a single peak at H_L=~0.27. In addition, the analysis shows that the PDF distributions for typical severe slugging and transitional severe slugging are similar to those for slug flow and churn flow in an upward vertical pipe, respectively, but the flow behavior is totally different from each other.