Investigations on the transition between stratified and non-stratified horizontal oil-water flows
The work presented in this thesis aims to investigate experimentally and theoretically the transition from stratified to non-stratified horizontal oil-water flows and to improve the understanding of the dual continuous pattern, where both phases retain their continuity at the top and bottom of the pipe respectively but there is dispersion of one phase into the other. Two experimental facilities were used in this study; a 38 mm ID stainless steel test section in a pilot scale flow facility and a 14 mm ID acrylic test section in a small flow facility running with water and oil (5.5 mPa s viscosity and 828kg/m3 density) as test fluids. A high speed video camera was employed to examine wave characteristics and flow development, capture mechanism of drop formation and determine the onset conditions of drop entrainment and the dual continuous pattern in both facilities. In the 38 mm ID test section, a conductivity probe was also used to investigate wave structures before and at the onset of entrainment. A high frequency impedance probe was used to find the phase distribution of the oil-water flow while the local drop velocity and chord length distributions were measured using a dual impedance probe. In the 14 mm ID test section, the influence of adding polymer in horizontal oil-water flows was also investigated. While the flow pattern map developed by Lovick and Angeli (2004a) was used for the 38 mm ID test section, a new flow pattern map was constructed for the 14 mm ID pipe. Dual continuous flow was found to occur for a smaller range of superficial velocities in the small test section compared to the large one. Visual observations from the two test sections revealed that no drops are formed when interfacial waves are absent. In addition annular flow with oil flowing at the core was observed in both pipes for low oil velocities and relatively high water velocities. The results from the high speed pictures and the conductivity probe showed that the amplitudes of the waves are increased as the superficial velocities of the two phases increase and as a result the required superficial water velocity,Usw,for the onset of entrainment decreases as the superficial oil velocity, Uso, increases. The model suggested by Trallero (1995) for the transition from stratified to non-stratified flow failed to predict the experimental results. Moreover, the high speed video images and the conductivity probe results showed that the amplitudes of the waves found at 2m from the inlet are smaller than those observed at 7 m. When drops and the onset of entrainment were observed at 7m from the inlet, these were not observed at 2m from the inlet, which means that all drops forming downstream the pipe resulted from the waves. In the large pipe, the presence of a bend after the inlet section (T junction) resulted in larger drops than when no bend was present (Y junction). The high speed images also revealed that drops formed as a result of the relative movement between the oil and water phases. The faster phase will undercut the other one until a drop is detached from the wave crest. The entrained fractions during dual continuous flow, or the fraction of one phase dispersed into the continuum of the other were calculated from the phase distribution data obtained with both inlet configurations (T junction and bend and Y -junction). The entrained fraction of water in oil (Ew/o) increased as the input water flow rates increased at constant superficial oil velocity. Similarly, the entrained fraction of oil in water (Eo/w) increased as the oil flow rates increased at a constant water superficial velocity. Moreover, the entrained fractions when the bend was used were higher than those obtained without it. From the chord length measurements in dual continuous flow, chord length and drop concentration were found to decrease with increasing distance from the interface while the number density of large drops decreased as Usw increased at each Uso. Also, oil drops were in general larger than water drops. Drop velocity measurements also revealed that water drops were faster than the velocity of the upper layer while oil drops could be either slower or faster than the velocity of the lower layer. The results showed that average chord length L32 was almost constant for the oil drops while it tended to decrease for the water drops as the respective layer velocity increased. In the 14 mm ID pipe, the addition of a polymer in the oil-water flow had a significant effect on the flow patterns and pressure drop. The transition from stratified to nonstratified patterns was clearly delayed and the pressure drop was found to decrease after adding the polymer. The wavy interface in the stratified, dual continuous and annular flows was damped when polymer was present. The interfacial and water wall shear stress were also found to decrease after the addition of the polymer. Theoretically a model was developed based on Kelvin-Helmholtz (KH) instability to predict whether waves in stratified wavy flow with certain amplitudes and lengths are stable or not. The model compared well with the Viscous KH correlation developed by Trallero (1995) and with some experimental results. The model was extended to predict the onset velocities of entrainment by including an empirical wave amplitude and length. The prediction agreed well with the experimental onset velocities from a number of studies. Based on a balance between drag force and surface tension on the crests of the waves, another equation was developed to predict the critical wave amplitude and length required for drop formation. This equation was used together with the stability equation to define three regions in a wave amplitude against length graph. These are; stable wave region; unstable wave region, where waves are unstable but drops may not form because waves need to grow more before drops can detach; drop entrainment region. The model agreed well with the experimental results. Finally, an entrainment model to predict the fraction of one phase entrained into the other during dual continuous flow, that was based on a balance between rate of drop entrainment and rate of drop deposition. The model was modified with experimental data from the current study and was then validated against data from literature. The comparison was reasonable in many cases.