Three-dimensional wave dynamics of falling film flows on structured surfaces*

In applications such as falling film absorbers and reactors, unsteady film flows play a crucial role in enhancing heat and mass transfer processes. This study investigates the use of structured surfaces to induce suchtransient film instabilities. To optimize the employed structure dimensions, an in-depth understanding ofthestructure-induced wave evolution is essential, whichis developed through a combined experimental and computational approach. Experimentally, high-speed camera imaging and alight absorption technique are used to reconstruct the spatiotemporal evolution of the interface. The transient film instabilities are observed to evolve from an initially steady base flow, and an optimal structure distance-to-height ratio is identified, at which particularly strong interfacial oscillations are induced in the falling film. Detailed insights into the local flow phenomena under such resonance-like conditions are gained using three-dimensional direct numerical simulations with a hybrid front-tracking/level-set interface capturing algorithm. The simulations show that strong interfacial oscillations are generally associated with the occurrence of unsteady internal recirculation zones. In mass transfer applications, these mixing zones effectivelybroaden the concentration boundary layer, leading to a significant increase in the liquid-side mass transfer coefficient.