Measurements on interfacial characteristics and droplet entrainment in horizontal liquid-liquid flows: A Comparison of PLIF Versus WMS

The accurate detection of horizontal liquid–liquid two-phase flow structures is all-important for understanding the flow dynamic behaviors and uncovering the flow pattern transition mechanism. In this article, a 10 $\times $ 10 conductance wire-mesh sensor (WMS) is designed to detect the interface structures and the entrained organic droplets. The liquid–liquid flow structures at the pipe radial section are visualized by the WMS. Due to the limitation of the WMS spatial resolution, adhesions between the droplets and the stratified interface are always encountered in the 2D flow visualizations. To overcome the effect of the adhesions, watershed algorithm and axial gradient correction algorithm are used to separate the adherent droplets and the interface. Thus, the dispersed droplets and the liquid–liquid interface can be extracted to reconstruct the 3D flow visualizations. To validate the performance of the conductance WMS, we design a novel planar laser-induced fluorescence (PLIF) system with a high temporal-spatial resolution. The incident direction of the laser sheet is along the pipe radial section, and the angle between the laser sheet and the high-speed camera is 60°, i.e., PLIF-60. The phase distribution at the pipe radial section can be accurately detected by the PLIF-60 system. By comparison, the conductance WMS performs well in detecting unstable interface structure, and the maximum relative errors of the average interfacial height and length measured by WMS are both less than 10%. The WMS measurement results allow indicating the evolution characteristics of the liquid–liquid interface and the droplet entrainment. The PLIF method is precise in measuring the organic droplets over the WMS method, especially for the droplets smaller than 1 mm. The detection capability of the WMS for the droplet distribution is investigated. The limitation of the WMS in detecting small droplets is quantitatively demonstrated by the probability density function (PDF) of the droplet diameters and the droplet entrainment rate.