Developing air-water flow downstream of a vertical 180° return bend

The importance of developing two-phase flow downstream of return bends has been brought about by several authors wishing to quantify irreversible losses in singularities. In a recent work, a method for evaluating such losses in vertical return bends was proposed based on gas holdup and pressure drop measurements in the bend. In the present study, the nature of the developing flow upstream and downstream of a vertical 180° tube bend (curvature ratio of 8.7) is investigated by means of gas holdup and pressure drop measurements at several positions along the 26.4-mm ID tubes connected to the bend. The effect of the bend on the flow characteristics was evaluated for the plug, slug and annular flow regimes in the upward and downward flow directions. The results showed that, although the variations in gas holdup seemed to extend beyond the measuring limits, significant pressure changes were limited to 40 pipe diameters upstream and 60 diameters downstream of the bend.