Particle image velocimetry/planar laser-induced fluorescence applied for the study of hydrodynamic aspects of low-viscosity ratio stratified liquid–liquid flow

Stratified liquid–liquid flow is still an open research subject due to the complex interfacial interactions and its hydrodynamic stability in specific operational conditions. Some challenges are predicting velocity profiles related to the average velocities of the phases and the effect of shape factors, capillary instability, and secondary flow on the flow pattern transition. Understanding the hydrodynamics of horizontal stratified liquid–liquid pipe flows is a fundamental step for properly modeling the stability of stratified liquid–liquid flows. Some studies have focused on measuring hydrodynamic characteristics of stratified liquid–liquid flows in horizontal or slightly inclined pipes by combining two-dimensional Particle Image Velocimetry (2-D PIV) and Planar Laser-Induced Fluorescence (PLIF) techniques. Nevertheless, this study represents the first attempt to study the case of two low Eötvös numbers (2.2. and 9.7) and low-viscosity ratio flows via synchronizing those techniques in the streamwise and spanwise directions. The 2-D PIV technique was used to measure two-phase velocity profiles and turbulence statistics at the flow's diametrical vertical plane for stable and unstable stratified flow conditions. Simultaneously, the mean interface height was measured through the PLIF technique and a homemade scanning algorithm that identifies the liquid–liquid interface. In addition, the interface's cross-section curvature radius was measured at the flow's cross-sectional plane. The axial velocity profiles showed an S-shape. The appearance of radial velocity components near the pipe wall or the liquid–liquid interface revealed momentum transfer between the two phases, suggesting the existence of secondary flow. The cross-section curvature radius data revealed that the higher the in-situ water volumetric fractions, the more concave the cross-section interface and the more unstable the stratified flow.