Vortex-induced waves and the mechanism of drop entrainment in transition from stratified to dispersed oil-water pipe flows

This dissertation presents new insights on flow pattern transition from stratified to non-stratified of two-phase oil and water flows in horizontal pipes. A novel approach is implemented to facilitate investigation of drop entrainment which identifies onset of the particular transition, where a cylindrical bluff body is located transverse to flow direction to induce instabilities in the form of vortex-induced interfacial waves in stratified flows. Numerical investigations of two-dimensional single-phase flows performed with CFD code FLUENT shows that vortex shedding frequency increases with decrease in the cylinder diameter while the size of vorticity region expands with increase in cylinder diameter. From the findings, two cylinder diameters, 2 mm and 8 mm are selected for experimental investigation in two-phase flows to generate vortex shedding frequency in the range of 1 to 100 Hz. Findings of high-speed visualization on the flow patterns and interfacial wave characteristics showed that higher instabilities were achieved with increasing cylinder diameter where the transition boundaries were shifted towards lower mixture velocities and waves with higher amplitude were produced. This is attributed to the size of vorticity regions, which are attached to the interface to actuate the vortex-induced waves as demonstrated by the particle image velocimetry (PIV) results. Variations of the vortex shedding behavior achieved by various cylinder diameters were found to be reflected on the resulting vortex-induced waves. The cylindrical bluff body approach is further implemented for the investigations of drop entrainment using a cylinder diameter that corresponds to gap ratio of 0.656 as it provides high instabilities at minimum wall effects. The use of simultaneous PLIF and PIV was introduced to visualize the wave’s evolution with high spatial and temporal resolution while obtaining the velocity field around the waves at the same time. Drop entrainment was identified to occur through detachment of drop from interfacial waves and is formulated into a phenomenological model developed based on force balance. Further analysis of the deformed wave dynamics during drop detachment shows relation to the input flowrate ratio, r