Modeling of transition boundary from stratified to non-stratified liquid-liquid flows in small diameter pipe

Abstract

Modeling the instability of the interfacial wave in stratified liquid-liquid flows is a popular topic and attracts the interest of scholars. In this study, an experimental study is implemented in horizontal and +5° inclined pipes with an inner diameter of 20 mm. A planar laser-induced fluorescence-60 (PLIF-60) system is utilized to visualize the flow structures at the pipe radial section, such as the curved interface and the droplet entrainment. Image processing is used to correct the PLIF-60 flow images and reconstruct quasi-3D flow structures. Flow pattern maps for horizontal and inclined flows are drawn to indicate the transition boundary between stratified and non-stratified flows. Young-Laplace equation with dynamic contact angle is developed to predict the interface configuration. The predicted interface configuration shows a good agreement with that detected by the PLIF-60 system. In view of the fact that the interface configuration is partly associated with the stratified/non-stratified flow transition, a modified model for the transition boundary is established based on Kelvin-Helmholtz (KH) stability. Comparisons of the proposed transitional criteria with experimental data and available data in liquid-liquid systems show reasonable agreement in small diameter pipes. Additionally, the proposed model with the consideration of interface configuration is basically not dependent on the wave parameters, such as wave amplitude and wavelength, providing a new way for predicting the transition boundary from stratified to non-stratified liquid-liquid flows in small diameter pipe.