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 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 for a broad range of operating conditions. 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. The following figure shows a numerical simulation of a fluid (Kaptiza number = 191, Reynolds number = 24) with an inlet disturbance frequency of 30 Hz. The top section displays a snapshot of the entire simulation domain and the bottom part shows how the wave frequency varies with time at three different locations. In addition, the measurement point located at 700 hN (0.45 m) from the inlet is added as a reference. 147