Edomwonyi-Otu, LC; (2015) Drag Reduction in Oil-water Flows. Doctoral thesis , UCL (University College London).

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Abstract

Liquid-liquid flows occur in many chemical and process industries including the petroleum industry where crude oil and its derivatives are transported over long distances often in mixtures with water. Depending on flow conditions and pipe geometry different flow patterns can appear ranging from fully separated to dispersed ones. The addition of small amounts of some polymeric materials to one of the phases has been found to change the flow patterns and their boundaries and reduce the frictional pressure drop. Understanding these changes and the underlying physical mechanisms is necessary for the design of pipelines for the transport of oil-water mixtures. In this thesis, the effects of a drag reducing polymer (Magnafloc 1011; hydrolysed copolymer of polyacrylamide and sodium acrylate, HPAM, mol. wt. = 10 x 106 g/mol) added in the water phase of an oil-water mixture were studied experimentally in a horizontal 14 mmID acrylic test section. The test fluids were a distillate oil (Exxsol D140: viscosity 5.5 mPas, density 828 kg/m3) and tap water. For some measurements two different molecular weights; 5 x 106 g/mol and 8 x 106 g/mol polyethylene oxide (PEO) polymers were also used. Flow patterns and pressure drop were investigated for a wide range of fluid velocities ranging from 0.052 m/s to 3.620 m/s for single phase water flows while oil and water superficial velocities ranged from 0.008 m/s to 0.580 m/s, and from 0.052 m/s to 0.80 m/s respectively. Both before and after the addition of polymer. Detailed studies of interface height and velocity fields were then carried out in stratified flows. Two types of conductivity probes, a wire probe and a ring probe, were used to measure interface heights in the middle and the wall of the pipe respectively. The velocity profiles and turbulence properties of the water phase were studied with particle image velocimetry (PIV) within the stratified flow regimes of the oil-water flows. The addition of 20 ppm of polymer solution to the water phase resulted in drag reduction of 80 % in single phase water flows and 52 % in oil-water flows. In addition, flow patterns were changed while the region of stratified flows was extended to higher superficial oil and water velocities. In stratified flows with the addition of polymer the in-situ average water velocity, interfacial wave celerity, and wavelength increased while the interface height, amplitude, and power spectrum were decreased. The conductivity probe measurements revealed a curved interface in stratified flows which with the addition of the polymer remained relatively unaffected. A relationship was developed between the two interface heights. The velocity profiles in the water phase became more parabolic compared to the flow without polymer. In addition, the axial component of velocity fluctuations decreased close to the interface and the wall but increased in the middle of the flow, while the Reynolds stresses and radial component of velocity fluctuations reduced throughout the water phase. From the two types of PEO tested, drag reduction was found to increase with polymer molecular weight but also depended on the mechanical degradation of the polymers at high Reynolds numbers and their ionic strength. A two-fluid model was developed that took into account the interface shape and waviness. To calculate its length, the interface was considered circular and the correlation between the two interface heights in the middle and the wall of the pipe was used. The interface waviness was included as roughness in the interfacial friction factor correlation, equal to the average wave amplitude found experimentally. Results showed when both interface curvature and waviness were included; the model predicted better the experimental pressure drop data compared to the two-fluid model with other interfacial shear stress correlations found in the literature. The friction factor correlation in the two-fluid model was also modified to account for drag reduction and it was able to predict the drag reduction in oil-water flows better than correlation available in the literature. The combination of polymer and fibers in single phase water flows resulted in a synergistic effect with drag reduction higher than when either polymer or fibers were used alone.

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