Magnetic resonance velocimetry of thin falling films

Falling film microreactors are a type of liquid–gas contactor based on a gravity-driven film flow. These devices achieve high interfacial areas per unit volume compared to other contactors. Nevertheless, they are limited by the liquid-side mass transfer. To overcome this issue, the operating conditions must be chosen in a way to enhance gas absorption and ensure efficient transfer within the film. The present study looks into the use of magnetic resonance velocimetry to study the velocity field inside of falling films at the microscale. Two parameters were considered, the channel geometry, and the volumetric flow rate. The aim was not only to determine the conditions that optimize the interfacial area, but also to showcase the capabilities and advantages of magnetic resonance velocimetry for this type of flow configuration. In addition, a straightforward mathematical model, based on the Navier–Stokes equations, is used to predict the thickness of the film, since previous approaches, mainly those of Kapitza and Nusselt, are not valid for flows at the microscale, where surface tension phenomena predominate. It was shown that the round channel yields by far the highest surface-to-volume ratio, twice as high as for the rectangular channel for the same volumetric flow rate. In addition, film breakup was not observed in the round channel even for volumetric flow rates of down to 0 . 27 m L m i n − 1 . • Magnetic resonance is used to measure the velocity of liquid films at least 100 μ m thin. • The surface-to-volume ratio in a round channel is greater than 12,000 m 2 /m 3 . • Round channels produce stable films with a relatively uniform thickness.