LAUSR

In this study, we investigated the effects of the viscosity ratio (β) and the angle of inclination (θ) on the change in the flow regime of two fluids undergoing exchange flow in a pipe, using a combination of experimental and theoretical methods. In our experiments, we observed that changing either the viscosity ratio or the inclination angle of the pipe causes a change in the fluidity of the two fluids. For the upward-flowing light fluid, we observed four flow regimes: axisymmetric core-annular flow (CAF), eccentric core-annular flow (ECAF), transitional side-by-side flow (eccentric and side-by-side flow at the same time) (TSBS), and side-by-side flow (SBS). In addition, the larger the viscosity ratio, the larger the critical angle at which the flow pattern eventually changes to side-by-side flow. When the tilt angle is larger than 16∘, the flow pattern is a side-by-side flow, regardless of the viscosity ratio, and we find that the viscosity ratio and the inclination angle determine the ratio of the width of the rising fluid to the diameter of the pipe (δa) and the velocity of the increasing fluid (V). We used the velocity model of the fluid in the pipe to compare with our experimentally measured velocities and found some similarities and differences, which we explained. For the downward-flowing heavy fluid, we divided the viscosity ratios into three ranges for our study and found that the changes in flow regimes were different for different ranges of viscosity ratios.