Andrea Düll

Falling film flows over rectangular corrugations can exhibit intense time-oscillatory interfacial motion. This is of considerable interest for heat and mass transfer applications, where structured surfaces play a crucial role in process intensification. Our contribution relies on high-speed imaging and image processing based on an internally referenced light absorption method to obtain a full spatio-temporal characterization of the structure-induced wave evolution. After validating the customized experimental technique, particular emphasis is placed on identifying relationships between the steady and transient characteristics of aqueous falling film flows under operating conditions relevant to, e.g., falling film absorbers for $$\text {CO}_2$$ CO 2 capture applications. The transient film instabilities are found to evolve from an initially steady film flow. In the investigated Reynolds number range, inertia-controlled liquid overshoot in wall-normal direction at the structure element’s upstream edges plays a crucial role in the overall flow destabilization. The developed film flow can be decomposed into a steady and a time-oscillatory flow contribution. The former is characterized by a dominant two-dimensional wave shape with a primary wavelength matching that of the bottom contour, while the latter is more isotropic in shape. Nevertheless, both flow contributions are interconnected, with high oscillation intensities being usually accompanied by a strongly sloped steady base flow. In the context of surface structure optimization, the streamwise length scale of the steady interfacial ridge induced at an isolated structure element may serve as a predictor for identifying structure spacings that exhibit particularly strong transient flow destabilization.