抄録
The fluctuating flow phenomena on a two-phase flow through a vertical sudden expansion pipe system are investigated experimentally and visually. The effect of the volumetric gas flow rate ratio within the range of bubbly flow is investigated. Simple flow control methods are proposed and tested in comparison with the normal expansion case. The first method applies control by mounting a ring shaped obstacle downstream the expansion, and the second by mounting a step-ring just downstream. These two methods are based on a different control concept. The first is based on splitting the vortex region, thus decreasing its intensity, and the second on decreasing the overall generated vortex region length. In single-phase flow, only one dominant frequency is observed. However, when gas is induced, two dominant peaks appear and a tendency of the second peak to shift to lower frequency values when increasing the volumetric gas fraction is observed. When the flow control methods are applied, the fluctuation frequency is not affected, but the fluctuation amplitude decreases. From pressure distribution measurements under several flow conditions, it was confirmed that when the flow control methods are applied, drag reduction is achieved as well.
