International Journal of Multiphase Flow 95 (2017) 188–198 Contents lists available at ScienceDirect International Journal of Multiphase Flow journal homepage: www.elsevier.com/locate/ijmulfow Hydrodynamics of vertical falling films in a large-scale pilot unit –a combined experimental and numerical study Anders ˚ Akesjö a,∗ , Mathias Gourdon a,b , Lennart Vamling a , Fredrik Innings c , Srdjan Sasic d a Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-41296 Gothenburg, Sweden b Valmet AB, Regnbågsgatan 6, P.O. Box 8734, SE-40275 Gothenburg, Sweden c Tetra Pak, Ruben Rausings Gata SE-22186 Lund, Sweden d Department of Applied Mechanics, Chalmers University of Technology, SE-41296 Gothenburg, Sweden a r t i c l e i n f o Article history: Received 20 December 2016 Revised 4 May 2017 Accepted 5 June 2017 Keywords: Falling film Hydrodynamics VOF Large scale unit a b s t r a c t 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 m. 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 poten- tially 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 distur- bance consisting of a multitude of frequencies to achieve the natural wave frequency over relatively short streamwise distances. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Falling liquid film is a technique where a thin liquid film is flowing down an inclined or vertical wall in the presence of a gas layer. Instabilities in the liquid film grow as they travel downwards and evolve into various flow regimes depending on the operating conditions. Advantages, such as a large contact area and high heat transfer at low flow rates give this technique excellent heat and mass transport characteristics (Kalliadasis et al., 2012). As a con- sequence, falling films are utilized in a wide range of engineering and technological applications, such as chemical reactor columns or heat exchangers. Regardless of the field of application, it can be assumed that hydrodynamics governs other physical phenomena (e.g. heat and mass transfer), as well as the overall performance of a reactor (Dukler, 1976). For that reason, there is great interest to study hydrodynamics of falling films. The hydrodynamics of falling films have been extensively stud- ied in the literature (Alekseenko et al., 1994; Kalliadasis et al., 2012) and a number of papers exist using analytical, experimental ∗ Corresponding author. E-mail addresses: anders.akesjo@chalmers.se (A. ˚ Akesjö), mathias.gourdon@chalmers.se (M. Gourdon), lennart.vamling@chalmers.se (L. Vamling), fredrik.Innings@tetrapak.com (F. Innings), srdjan@chalmers.se (S. Sasic). or numerical methods. Kapitza and Kapitza (1965) were one of the pioneers in this field and used shadow photography of the liquid in order to study the wave development. Since then, several authors have carried out similar studies e.g. Alekseenko et al. (1985); Ishi- gai et al. (1972); Oyakawa (1994); Patnaik and Perez-Blanco (1996); Plerson and Whitaker (1977); Wasden and Dukler (1989). Density, viscosity, gravity, surface tension and the flow rate were identified as the important factors for the hydrodynamics. These factors are typically expressed as functions of non-dimensional numbers, such as Reynolds (Re) and Kapitza (Ka) numbers, in order to distinguish between influences from flowrates and material properties and to classify different flow regimes. The average film thickness has been extensively measured and mapped for different running con- ditions (Brötz, 1954); (Lukach et al., 1972). In addition, as measure- ment techniques have become increasingly sophisticated, it has be- come possible to study the flow structure beneath the liquid inter- face and measure the velocity components with techniques such as Laser Doppler Velocimetry (LDV) (Dietze et al., 2009), Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) (Zadrazil et al., 2014). When it comes to modelling of dynamics of falling films, analytical modelling has been a subject of research for a long time e.g. Brauner (1989). Such studies have often predicted a http://dx.doi.org/10.1016/j.ijmultiphaseflow.2017.06.003 0301-9322/© 2017 Elsevier Ltd. All rights reserved.