Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

Air-water two-phase flow tests in a large vertical pipe of 194.1 mm inner diameter are reported. Close to the outlet of a 9 m tall test section two wire-mesh sensors are installed that deliver instantaneous void fraction distributions over the entire cross section with a resolution of 3 mm and 2500 Hz used for fast flow visualization. Void-fraction profiles, gas velocity profiles and bubble size distributions were obtained. A comparison to a small pipe of 52.3 mm inner diameter (DN50) revealed significant scaling effects. Here, the increase of the air flow rate leads to a transition from bubbly via slug to churn turbulent flow. This is accompanied by an appearance of a second peak in the bubble size distribution. A similar behavior was found in the large pipe, though the large bubbles have a significantly larger diameter at identical superficial velocities, the peak is less high but wider. These bubbles move more freely in the large pipe and show more deformations. Shapes of such large bubbles were characterized in three dimensions. They can rather be complicated and far from ideal Taylor bubbles. Also the small bubble fraction tends to bigger sizes in the large pipe.