Hernandez Barral, A; (2014) Stratified wavy oil-water flows. Doctoral thesis , UCL (University College London).

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Abstract

The structure of the oil-water interface of stratified flows in a 38 mm ID pipe is investigated in this Thesis with double-wire conductance probes. The fluids used – tap water and Exxsol™ D140 oil (ρo = 830 kgm-3; μo = 0.0055 kgm-1s-1) – are pumped into the facility and brought together in a “Y” inlet section, designed to minimize the mixing between phases (r = 0.6 – 2.4; Umix = 0.5 – 2.5 ms-1). The piping is made of acrylic and the flow was observed with the aid of high-speed imaging. The waves seen on the oil-water interface further downstream the inlet consists of small 3D fluctuations, rather than 2D structures. Conductance probes are used to investigate the oil-water interface in cases where clear wavy structures cannot be followed or analyzed. The signal of interface height in time is found to be stationary and follow a Gaussian distribution when the signal is collected at 256 Hz during 4 min. Based on these properties, a thorough methodology for analysis is presented, which allows estimating time-average parameters of the flow and the power spectrum of the interface. This analysis reveals that, in fully-developed flow conditions, oil and water phases show very little slip and tend to flow both at roughly the mixture velocity regardless of the flow conditions. The power spectrum detects a unique frequency of 19 Hz, but reveals that mechanic vibrations propagating through the facility are a major contribution to the structure of the interface. The 19 Hz frequency corresponds to clearly identifiable waves that develop at the inlet section only if the oil-to-water input ratio is different from 1. The power spectrum at the inlet tends to be dominated by this frequency. This finding is verified with the information of high-speed images. Wave characteristics and their evolution along the inlet are determined from high-speed images collected with a Phantom Miro 4 camera at 1,000 – 1,200 fps. The theoretical analysis of the stability of inlet waves suggests that their origin is a Kelvin-Helmholtz instability and characterizes the waves as dynamic in nature. The two types of oil-water interfaces seen (i.e. that at the inlet with 2D waves and that downstream the pipe with small 3D contributions) are discussed in this Thesis at length and abundant details are given.

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