High-resolution oil-gas two-phase flow measurement with a new capacitance wire-mesh tomograph

The experimental investigation of flow phenomena plays an important role in many fields of research as well as in industrial applications. Wire-mesh tomography allows the study of flows with high spatial and temporal resolution. This tomography modality was introduced about ten years ago and since then it has been employed to investigate a number of single phase and two-phase flow phenomena. In this paper, a new developed wire-mesh tomograph based on capacitance (permittivity) measurements has been employed on a vertical 67 mm diameter pipe up which air and silicone oil mixtures flow under controlled conditions. The wire-mesh sensor consists of two planes of wires forming an angle of 90° and having a small separation between them. An associated electronics measures the local capacitance between the gaps of all crossing points. Thus, the wire-mesh system yields cross-sectional images of the phase distribution. The present system reaches temporal resolution of up to 5 000 frames per second and has a spatial resolution of 2.8 mm. Two visualization techniques were used to display the measured wire-mesh data: (i) virtual sectional side views and (ii) virtual side projections. In the former one the values of phase fraction of the central electrode are displayed along the time. Such an image obtains the character of a side view to the flow on a vertical plane cut through the pipe along the diameter. The second visualization technique applied is the calculation of virtual side projections using a simplified ray-tracing algorithm. Here the three-dimensional phase fraction distribution is assumed to be illuminated by parallel white light and the light intensity departing in the direction of a virtual observer is calculated. Both visualization techniques complement each other in the visualization of flow structures. Different gas and oil flow rates where investigated thus obtaining a variety of flow patterns. All four major accepted flow patterns bubbly, slug, churn and annular flow are reported here. The high-speed capability and the good spatial resolution of the system enable an examination of the flow to be carried out with a high degree of detail. Thus, the spatial resolution is sufficient to detect individual bubbles. Furthermore, quantitative insights of the flow can be obtained by the calculation of axial and radial gas content profiles which are determined by integration of the measured data over appropriate partial volumes.