High resolution experimental measurement of turbulent flow field in a high pressure homogenizer model and its implications on turbulent drop fragmentation 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 25 October 2010 Received in revised form 5 January 2011 Accepted 11 January 2011 Available online 19 January 2011 Keywords: Homogenization Turbulence Hydrodynamics Emulsion Particle image velocimetry Fragmentation abstract Particle image velocimetry is performed on a model of a high pressure homogenizer, scaled for qualitative similarity of the one phase turbulent flow field in a production scale homogenizer. Flow fields in gap entrance, gap and gap outlet chamber are obtained with high resolution. The measure- ments show gap flow development and formation of a turbulent wall adherent jet when exiting into the outlet chamber. Turbulent kinetic energy spectra show how the turbulent energy available for fragmentation is transported over distance along the jet centre axis. The high resolution images are also used together with a Kolmogorov–Hinze theory framework for discussing drop fragmentation together with a direct evaluation of disruptive stresses from measure- ments. For the turbulent inertial mechanism large drops experience high fragmenting force close to eight gap heights downstream of the gap exit whereas this occurs closer to 20 gap heights for smaller drops. The turbulent viscous mechanism is most efficient at a downstream distance of eight gap heights into the outlet chamber for all drops sizes. & 2011 Elsevier Ltd. All rights reserved. Introduction The high pressure homogenizer (HPH) is used in a wide range of applications for creating emulsions. In the HPH, large pre- emulsion drops are accelerated in a narrowing inlet chamber, forced through a narrow gap (  10–100 mm) under high pressure (  10–100 MPa) and released into a larger outlet chamber. This process results in drop size reduction. The nature of the fragmen- tation is somewhat different between geometries, but for produc- tion scale and large pilot scale machines it is usually described as being caused mainly by turbulence (Phipps, 1985; Walstra and Smulders, 1998). There are also some studies showing that cavitation takes place in the homogenizer and could participate in fragmentation (Floury et al., 2004; H ˚ akansson et al., 2010; Kurzhals, 1977). The position of fragmentation has previously been much debated, however there is now a strong experimental basis showing fragmentation taking place in the outlet chamber downstream of the gap exit (Budde et al., 2002; Innings and Tr¨ ag ˚ ardh, 2005; Innings et al., 2011) and has recently been discussed by different authors in relation to modelling (e.g. H˚ akansson et al., 2009; Steiner et al., 2006). Turbulent drop fragmentation has been described by the Kolmogorov–Hinze model (Hinze, 1955; Kolmogorov, 1949). Two different mechanisms have been proposed: turbulent inertial (TI) and turbulent viscous (TV) fragmentation. One distinguishes between the two in that for the TI mechanism, drops are fragmented by pressure fluctuations induced by small eddies and for the TV mechanism by shearing of larger eddies. (Hinze, 1955; Walstra and Smulders, 1998). In order to use the Kolmogorov–Hinze theory for predicting drop fragmentation, information on the velocity fluctuations and spatial gradients are needed. However, direct measurements of these quantities in the turbulent region of the high pressure homogenizer are hindered by geometrical reasons and because of the large velocity gradients in comparison to the small geometrical length scales in the turbulent region. As an alternative to experimental measure- ments, in a large number of studies, computational fluid dynamics (CFD) has been used in order to calculate the flow field (Casoli et al., 2010; Floury et al., 2004; H ˚ akansson et al., 2010; Kleinig and Middelberg, 1997; Raikar et al., 2009; Steiner et al., 2006; Stevenson and Chen, 1997). However, all these simulations are based on the Boussinesq assumption (turbulent-viscosity Contents lists available at 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.2011.01.026 n Corresponding author. Tel.: + 46 46 222 9670; fax: + 46 46 222 4620. E-mail address: andreas.hakansson@food.lth.se (A. H ˚ akansson). Chemical Engineering Science 66 (2011) 1790–1801