Samson, John Ndaa
(2024)
Experimental investigation of oil-water two-phase flow across a large diameter slightly inclined pipe.
PhD thesis, University of Nottingham.
Abstract
The production and transportation of oil and water safely and economically across process equipment and pipeline networks have been a major challenge in the process and petroleum industry. In order to properly design the oil-water separator and transportation system, the hydrodynamic behaviour such as the spatial distribution (flow pattern), holdup and pressure drop of the oil–water two-phase flow needs to be well understood. This study aims to experimentally investigate the flow development and interfacial behaviour of oil-water two-phase flow across a large diameter inclined pipe. The pipe geometry used is a 127 mm ID of length 8.4 m with inclination +1o to +5o to the horizontal. The test fluids are tap water (ρ = 998 kg⁄m^3 , μ = 1 mPa.s) and silicone oil (ρ = 916 kg⁄m^3 , μ = 4.7 mPa.s). Three (3) different configurations of fluid inlet devices were used in order to investigate how the phase distributions are affected by the design of the fluid inlet device. The inlet devices are the T-type (first inlet device), Y-type (second inlet device) and Y-type with a separator plate (third inlet device). Four (4) flushed-mounted conductance probes (CP) located at 9.6D (CP1), 19.2D (CP2), 28.8D (CP3) and 56.6D (CP4) from the fluid inlet device (D is the internal diameter of the test pipe) were used to measure the Oil Volume Fraction (OVF) which gives an indication of the region within the pipe cross-section occupied by the oil phase. A double parallel wire probe (DPWP) was fabricated and mounted on the test-pipe to measure the instantaneous height of the water layer which upon analysis will give an indication of the interfacial wave amplitude for a given flow condition. In addition, Phantom V12.1 high-speed camera was used to capture flow images through a viewing box filled with water to avoid image distortion due to the curvature of the pipe.
Based on experimental observations and inspection of some of the flow images, different flow patterns were identified depending on the fluid inlet device and pipe inclination used. For the T-type fluid inlet device, four (4) distinct flow patterns were observed at +1o pipe inclination and they include stratified wavy with mixing at the interface (SW&MI), dispersed oil in water and water (Do/w&w), dispersed water in oil and oil (Dw/o&o) and dispersed oil in water (Do/w). Upon using the Y-type fluid inlet device, five (5) flow patterns were identified including stratified smooth (ST), stratified smooth with mixing at the interface (ST&MI), dispersed oil in water and water (Do/w&w), dispersed water in oil and oil (Dw/o&o) and dispersed water in oil (Dw/o) and finally, using the Y-type fluid inlet device with a separator plate, only three (3) flow patterns were identified including stratified smooth (ST), stratified wavy (SW) and stratified wavy with mixing at the interface (SW&MI). Increase in pipe inclination from +1o to +5o had a remarkable effect on the spatial distribution of the phases and also from the flow pattern maps across all the three fluid inlet devices, an increase in the region occupied by the water-dominant flow patterns (Do/w&w and Do/w) was observed due to the effect of the axial component of the gravitational force on the denser water phase.
The distribution of OVF in the axial direction was determined and was found to be a function of the water-cut (WC), mixture velocity (Um) and pipe inclination. The slippage characteristics, which indicate the relative speed between two phases, were also determined. It was found that these characteristics are influenced by both the mixture velocity (Um) and the inclination of the pipe.
From the analysis of the instantaneous height of the water layer measured by the Double Parallel Wire Probe (DPWP) and the flow images obtained from the high-speed camera, the interfacial wave characteristics such as the wave amplitude, wavelength and wave speed were determined and their responses to changes in the input flow conditions such as the WC, Um and pipe inclination were examined. The wave aspect ratio which characterises the geometrical structure of the oil-water wavy interface was also determined and was found to be a function of the modified Froude number. An empirical correlation for the wave aspect ratio was proposed and tested on the current experimental data and data from Castro et al., (2012) and Premanadhan et al., (2019) and similar trend was observed across all the data tested.
Stability analysis based on the Inviscid Kelvin-Helmholtz (IKH) stability criteria was used to test the stability of the interface as a result of the competition between the destabilizing effect of the inertial force and the stabilizing effect of the gravitational force and the surface tension force. For all the flow conditions tested, the relative velocities fell below the stability limit line which suggests that the system is stable and by implication, it means any disturbance introduced at the inlet will decay with time as the flow traverses in the axial direction.
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