Hydrodynamics of vertical falling films in a large-scale pilot unit - a combined experimental and numerical study

The hydrodynamics of vertical falling films in a large-scale pilot unit are investigated experimentally and numerically. We study a broad range of operating conditions with Kapitza and Reynolds numbers ranging from Ka = 191-3394 and Re 24-251, respectively. We compare film thickness measurements, conducted by a laser triangulation scanner, with those obtained by directly solving the full Navier-Stokes equations in two dimensions and using the volume of fluid (VOF) numerical framework. We examine the evolution of the liquid film at multiple locations over a vertical distance of 4.5 meters. In both our experiments and simulations we identify a natural wave frequency of the system of approximately 10 Hz. We investigate the formulation of the inlet boundary condition and its effects on wave formation. We show how potentially erroneous conclusions can be made if the simulated domain is shorter than 1000 film thicknesses, by mistaking the forced inlet frequency for the natural wave frequency. We recommend an inlet disturbance consisting of a multitude of frequencies to achieve the natural wave frequency over relatively short streamwise distances.