Visualisation of Two-phase Gas-liquid Pipe Flows Using Electrical Capacitance Tomography

Electrical Capacitance Tomography (ECT) can be used to measure concentration and velocity distributions in two-phase flows. ECT is non-intrusive, and the reconstruction of the concentration and velocity distribution can be undertaken in real time and over an arbitrary number of zones in the flow cross-section. The concept of a ‘virtual instrument’ is introduced where zones of the image can be structured for comparison with other measurements. Numerical agreement with gamma-ray density measurements is shown to be excellent in slug and stratified flows. For complex oil/gas slug flows we present a variety of 2-D cross-sectional images, time series velocity and concentration graphs and 3-D contour plots. The good temporal and spatial resolution of ECT throws an extensive new light on these otherwise difficult to measure dynamic flow structures. In particular with bubbly-slug structures known as ‘ghosts’ ECT shows clearly that they are in fact bubbly waves which have extended ‘wings’ up and around the pipe. INTRODUCTION Electrical capacitance tomography is one of the imaging techniques most likely to provide quantitative flow visualization and flowrate information in industrial flows. It has particular advantages in two-phase oil/gas flows typical of those in the petroleum industry [1], [2] and is also widely applicable in gas/solids flows [3], [4], [5]. This paper presents results taken during 2003 using ECT in the multiphase flow loop at SINTEF in Norway (Figure 1). ELECTRICAL CAPACITANCE TOMOGRAPHY ECT measurements are made by placing rings of electrodes around the circumference of the pipe or vessel of interest, measuring the electrical capacitance between each independent pairing of electrodes and using image reconstruction techniques to show the permittivity distribution within the sensor. Figure 2 shows a typical 8-electrode array, while Figure 3 shows a typical cross-sectional image where red is high permittivity material (in this case oil) and blue is low-permittivity (in this case air). In our experiments we used a Tomoflow R100 ECT Flow Analysis System with a twin-plane 8 segment guarded array of electrodes, where the electrode axial length was 30mm. The planes are separated by 100mm axially and the acquisition rate was 200 frames per second on both planes – the fastest rate currently available on a commercial ECT system [6]. The use of capacitance measurement for volume fraction estimation in non-conducting two-phase flows is well known, but the measurements are strongly dependent on sensor geometry and flow regime [7]. ECT offers an important improvement in that it measures the full concentration distribution in any regime or geometry. ECT generates large quantities of information: concentration in two planes (over 800 pixels in each) several