Visualization of Recirculation Zones Over a Perforated Plate: An Optical Flow Technique for Characterization of Fluid Dynamics in Structured Packing

International audienceThe dynamics of liquid film over the perforated metallic sheets of structured packing highly influences mass transfer and pressure drop characteristics. Regarding the perforations, their role in mass transfer enhancement and liquid redistribution is yet not well understood. On perforated packing, liquid films can be either supported or suspended ('curtain" regime, also known as "twin film"). Twin films account for intensive capillary waves and strong recirculation zones on the free surfaces. Therefore, experiments are required to assess the curtain formation in the perforations and relate it to the dynamics that intensify interfacial mass transfer. Here, a two-dimensional two-component (2D-2C PTV) optical-flow-based technique is applied using defocused fluorescent particles images (to allow discrimination between static and dynamic particles) illuminated by a continuous laser and acquired with a high-speed camera. This provided an opportunity to evaluate the hydrodynamics of the liquid curtain through streamline patterns and to describe key features that promote mass transfer intensification within the suspended liquid curtain. Experiments performed using an in-house test bench allowed the proposed method to be applied satisfactorily to study the behavior of a liquid film flowing over a perforated flat plate. Different recirculation regimes were observed depending on the Reynolds number: horseshoe vortices around perforations-before curtain formation, Dean vortices and kidney-pair vortices-following curtain formation. These eddies appeared chaotic for higher Reynolds numbers. Quantitatively, these recirculation zones were evidenced by negative film velocities, locally detected in the liquid curtain. In structured packing, the recirculation zones in perforations could promote mass transfer through: strong turbulent vortices favoring species transport at the interface; stagnation points with slow local liquid film velocities, increasing the local residence time, which leads to a longer gas-liquid contact time