Imperial College London
In 2011 alker fibre optics were approached by Imperial College London to design and manufacture a working prototype to be used within the Research department of Fluid Mechanics. alker’s brief was to create a fibre optic sensor assembly which would be used to monitor and measure the flow of bubbles within an oil pipeline.
Min Lu is one of the team of Phd Research students at the Imperial College London and is responsible for the experimental studies of two and three phase slug flows.
alker has extensive experience of working with Universities and Higher Education facilities within the UK. Out other customers within this sector include Oxford, Cambridge, Bristol, Edinburgh, Nottingham, Essex, Glasgow and Brunel Universities.
Multiphase transportation in pipelines is very common in the hydrocarbon industry. The flows can be highly complex therefore it is important to investigate the behaviour of multiphase flow within pipelines to ensure a safe design of the transportation process.
Slug flow is one of the most common flow regimes observed in hydrocarbon transportation, in which the pipe cross section is filled alternatively with liquid and gas. In liquid slug, there are small bubbles entrained and form a bubbly zone. It’s important to investigate the characteristics of the entrained bubbles since they have direct influence on the flow turbulence characteristics.
Optical probes are used to locally detect the presence of the gas bubbles in two liquid-gas phase systems. The measurements are based on different refractive indexes between gas and liquid phase. A light source is transmitted through an optical fibre to the tip. When tip is dipped into a gas phase, the light is mainly reflected, travelling back to the light receiver, when the tip is immersed in a liquid environment, the light is scattered and almost no light is reflected back to the receiver device which leads to a weak electric signal. The time dependant signal consists of a succession of crenel shaped transitions. The duration of each individual crenel represent the residence time of a gas bubble penetrate through the probe tip.
The optical probes in Min Lu’s experiment need to be inserted into a 3 inch horizontal pipe in order to take the measurement. A complete design of the optical probe system was produced with great help from Richard Kershaw, director of alker optical equipment.
Each probe consists of a pair of two fibres. A small part of the plastic silica coating at one end of the fibre was removed then two pieces of the naked cables were inserted into a short stainless steel tube, the tip was carefully welded, then slightly pulled back into the needle tube and mounted in place using epoxy resin.
In order to examine the flow structure at different vertical levels of the flow, 3 optical probes were positioned side by side. To protect the naked optical fibres, each probe was housed by a 2 mm stainless steel tube bent into an elbow. These tubes were tightly clamped by a supporting tube which provides a firm support and prevents excessive vibration in experiments. The probes were housed within a stainless steel tube, the fibres coming out from the tube were assembled into a micro-armoured stainless steel encasement terminated with SMA 905 connectors.
The probe system must be calibrated offline first. Each end of the fibre is connected to an emitter and a receiver, meanwhile an oscilloscope is connected to the receiver in order to give the electrical signal. The first test is to examine the response from the probe when it passes through a gas-liquid interface. This test can be done simply moving the probe tip in and out of the water surface in a glass beaker. Then more complicated tests are performed on bubble penetrations, a digital high-speed video camera is used to enables a comparison to the signal profile from the oscilloscope.