Experimental validation of k–e RANS-CFD on a high-pressure homogenizer valve Andreas H ˚ akansson a,n , Laszlo Fuchs b , Fredrik Innings c , Johan Revstedt b , Christian Tr ¨ ag ˚ ardh a , Bj¨ orn Bergenst ˚ ahl a a Lund University, Department of Food Technology, Engineering and Nutrition, P.O. Box 124, SE-221 00 Lund, Sweden b Lund University, Department of Energy Sciences, P.O. Box 118, SE-221 00 Lund, Sweden c Tetra Pak Processing Systems, Lund, Ruben Rausings Gata, SE-221 86 Lund, Sweden article info Article history: Received 26 September 2011 Received in revised form 21 December 2011 Accepted 22 December 2011 Available online 3 January 2012 Keywords: Emulsion Fluid mechanics Turbulence Computational Fluid Dynamics Homogenisation Hydrodynamics abstract Since the emulsification in the High-Pressure Homogenizer (HPH) is controlled by hydrodynamic forces, the turbulent flow field in the valve region is of significant interest. Computational Fluid Dynamics (CFD) simulations have been used for this in many studies. However, there are reasons to question if the utilized turbulence models, with their inherent assumptions and simplifications, could accurately describe the influential aspects of the flow. This study compares CFD simulations using the methods from previous studies with experimental measurements in a model HPH valve. The results show that the region upstream of the gap can be described accurately regardless of turbulence model and that the gap region can be captured by using one of the more refined k  e models. None of the studied turbulence models were able to describe the details of the highly turbulent region downstream of the gap. The obtained results are also discussed in relation to generalizability and limitations in using CFD simulations for understanding the emulsifica- tion in the HPH. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction The emulsification in a High Pressure Homogenizer (HPH) can be seen as a combination of three simultaneous processes; fragmentation of drops, recoalescence of insufficiently covered droplets and adsorption of emulsifiers (Walstra, 1993; H˚ akansson et al., 2009). All these processes are highly dependent on the flow field in general, and on the turbulence characteristics in particu- lar. Computational Fluid Dynamics (CFD) was used early in order to describe the flow field in the homogenizer, starting with Kleinig and Middelberg (1996, 1997) and Stevenson and Chen (1997). Since then, a large number of studies containing CFD on HPHs of various geometry have been published, for example, Miller et al. (2002), Floury et al. (2004), K¨ ohler et al. (2007), Steiner et al. (2006), Raikar et al. (2009), H˚ akansson et al. (2010) and Casoli et al. (2010). The studies differ in treatment of a large number of factors such as dimensionality (two or three), grid size, wall treatment and turbulence modeling; see Table 1 for an overview of the assumptions made in the aforementioned papers. The CFD results have been used for different purposes, e.g. gaining an overview of the flow (Stevenson and Chen, 1997), investigating possible zones of cavitation (Floury et al., 2004; H˚ akansson et al., 2010), used as basis for predicting drop movement in the valve (Casoli et al., 2010) or predicting strength of fragmenting hydro- dynamic forces (Kleinig and Middelberg, 1997; Steiner et al., 2006; Casoli et al., 2010). Of the studies in Table 1, Kleinig and Middelberg (1996) assume laminar flow since only the inlet chamber, upstream of the gap, is modeled (see Fig. 1). The remaining studies in Table 1 model turbulence with the Reynolds Averaged Navier–Stokes (RANS) equations using turbulence models based on the Boussi- nesq hypothesis; more specifically with different types of k  e turbulence models. RANS-CFD has the clear advantages of being low in computational cost and widely available in commercial implementations. The disadvantage is that the assumptions from the Boussinesq hypothesis and formulations of source terms for the k and (especially) e equations can sometimes give rise to non-physical results. Ideally, the turbulence should therefore be described by more accurate techniques; such as by Large Eddy Simulations (LES) or even Direct Numerical Simulation (DNS). However, these techniques are very computationally demanding and are disregarded in this study, since to our knowledge, no successful CFD using LES or DNS on homogenizer valves has been published. A more constructive approach can therefore be asking Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/ces Chemical Engineering Science 0009-2509/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2011.12.039 n Corresponding author. Tel.: þ46 46 222 9670; fax: þ46 46 222 4620. E-mail address: andreas.hakansson@food.lth.se (A. H ˚ akansson). Chemical Engineering Science 71 (2012) 264–273