Investigation of upstream and downstream flow conditions in a swirling inline fluid separator – experiments with a wire-mesh sensor and CFD studies

Wire-mesh sensors have been developed to measure gas-liquid distributions in two-phase flows at high speed. In a wire-mesh sensor, a wire electrode grid is formed in the flow cross-section by wires running in two planes of a small axial distance. Its functional principle is based on the measurement of either electrical conductivity (conductivity wire-mesh sensor) or relative permittivity (capacitance wire-mesh sensor) in the electrode crossings. Repeated scanning gives high speed sequences of cross-sectional phase indicator distributions. The sampling frequency can be very high. Thus, for a wire- mesh sensor with 2 × 16 wire electrodes (16 × 16 crossings or image pixel in a square cross-section) 10,000 frames per second can be scanned. The high acquisition speed together with the small wire spacing of typically 3 to 4 mm allows to image gas bubbles in many slices, such that the size and shape of the bubbles and the interfacial area can be resolved, but only in case of very large bubbles. The technical challenge of wire-mesh sensor imaging is the high amount of data, which need to be analyzed by automated computer algorithms.
We used the wire-mesh sensor to study the flow conditions upstream a swirling device, which is used in the oil and gas industry as a separator. Our particular objective was the analysis of the correlation of upstream flow patterns with the shape of the swirling gas cores downstream the swirl. The upstream flow regimes depend on the gas and liquid superficial velocities and can range from bubble flow, via churn flow towards annular flow in vertical pipe orientation. The flow pattern and especially its transition have an impact on the shape and interface stability of the swirling gas core downstream. Hence the wire-mesh data can be used as a predictor for expected hollow vortex variations and can be used for automatic control of the swirling device. To assess the relationship between wire-mesh data and vortex shape we used a high-speed camera to record the downstream interface development at 200 Hz frame rate. As automatic control requires a good process model we intend to use an original computational fluid dynamics (CFD) coupling a Volume-of-Fluid (VoF) method for interface resolution and an Immersed Boundary Method (IBM) for the separator description. For that we performed first CFD studies which give key indications of required model improvements and future numerical research work.