Measurement of vertical gas-liquid two-phase flow by electromagnetic flowmeter and image processing based on the phase-isolation

•An optimized phase-isolation method for vertical flow is proposed.•The behavior of electromagnetic flowmeter worked in phase-isolation is analyzed.•An automatic image processing algorithm is developed.•Electromagnetic flowmeter and image processing are used based on phase-isolation. In this paper, the electromagnetic flowmeter and the image processing are used to measure vertical gas-liquid two-phase flow. In order to reduce the effect of randomly distributed bubbles on the weight function of the electromagnetic flowmeter and the bubbles overlap on the image processing in raw two-phase flow, a special implementation method of phase-isolation for vertical upward flow is developed to achieve a stable and uniform swirling core-annular flow. For this flow pattern, the behavior of the electromagnetic flowmeter is analyzed and an automatic image-processing algorithm is developed. Both the simulated gas column experiment and dynamic air-water two-phase flow experiment were carried out to verify the theoretical equation of liquid flowrate. The results demonstrate that the instrument constant of electromagnetic flowmeter does change in the annular flow cross-section. A cross-section shape coefficient kα is needed in the equation of liquid flowrate, and kα is proved to be only related to the void fraction, independent of the type of electromagnetic flowmeter and the properties of central insulated column. Moreover, the swirl has no effect on the electromagnetic flowmeter. The overall average error of the liquid flow rate is 2.6%, and the relative error is within ±5% in bubble and slug flow condition. The measurement result is immune to the gas velocity.