Experimental apparatus and flow instrumentation for the investigation of a quasi-real slug flows in vertical ducts

Abstract

This paper presents the description of the experimental apparatus, instrumentation, and processing techniques developed for the study of quasi-real slug flow using Particle Image Velocimetry, High-Speed Camera and Laser Diode Photocell techniques. Most experimental studies of the flow fields around Taylor bubbles, aiming a deeper insight into slug flow pattern, analyze the situation of isolated Taylor bubbles rising in stagnant or co-current liquid flow. Furthermore, most of these studies, consider highly viscous fluids (or, in general, low values of the inverse viscosity number, Nf=ρ2gD3/μ2), to get “well behaved” Taylor bubbles, where flow fields are more easily measured. However, in most industrial situations slug flows are characterized by the presence of small dispersed bubbles in the liquid stream flowing together with large Taylor bubbles, and high values of the inverse viscosity number. On the other hand, the chaotic nature of a real slug flow would not allow an adequate characterization of the flow around single Taylor bubbles, mainly, due to the lack of the repeatability of the flow field measurement, making difficult the calculation of the averaged fields. Thus, in this work, a specific experimental apparatus and instrumentation were developed, which allows the study of the flow around Taylor bubbles with and without dispersed small bubbles in the liquid stream under controlled conditions. This allows the PIV measurements, around Taylor bubbles in flows with high Nf numbers, be done under repeatable conditions and thus, the determination of the averaged flow fields. The laser diode photocell technique is used to synchronize the PIV system with the passage of Taylor bubbles at the test section as well as to measure the terminal velocity and length of Taylor bubbles. A dynamic masking procedure was developed to mask out the Taylor bubbles noses and tails in the PIV images, since, for high values of the inverse viscosity number (Nf≈13,275) the interface fluctuations are strong and, therefore, no fixed masking can be used.