Effects of 90° vertical elbows on two-phase flow parameters

This study investigates two-phase flow from vertical-upward to horizontal and horizontal to vertical-downward direction through 90° vertical elbows. Specifically, effects of 90° vertical elbows on two-phase pressure drop, development of bubble distribution, and one-dimensional transport of bubble velocity are studied. Pressure measurements are obtained along the test section over a wide range of flow conditions. The two-phase pressure drops across the elbows are modeled using the modified Lockhart-Martinelli correlation which accounts for the minor loss effect. It is demonstrated that the modified Lockhart-Martinelli correlation can predict the pressure loss across elbows with different orientations and angles by varying the minor loss coefficient. A database is established by measuring two-phase flow parameters across 90° vertical elbows using a four-sensor conductivity probe. It is found that immediately downstream of the 90° vertical-upward elbow, the bubbles have a bimodal distribution along the horizontal radius of the pipe cross-section causing a dual-peak in the profiles of local void fraction and interfacial area concentration. As the flow develops, the bimodal bubble distribution dissipates and develops to a single-peaked bubble distribution near the top of the pipe. Immediately downstream of the 90° vertical-downward elbow, a bimodal bubble distribution is also observed near the inner curvature of the elbow. As the flow develops, the bimodal bubble distribution migrates to the pipe center and finally develops to a characteristic center-peaked bubble distribution. For the area averaged void-weighted bubble velocity, it varies significantly in the elbow regions due to the changes in local velocity profile and bubble distribution caused by the elbow effects.