VORTEX IDENTIFICATION METHODOLOGY FOR FEED INSERTION GUIDANCE IN FLUID MIXING PROCESSES A. Ducci  and M. Yianneskis Experimental and Computational Laboratory for the Analysis of Turbulence (ECLAT), Division of Engineering, King’s College London, Strand, UK. Abstract: A vortex tracking methodology is developed to determine the precessional frequency and the characteristics of macroinstability (MI) vortices. The methodology is compared with other techniques, frequency analysis and correlation coeffcient, which are most commonly used to assess periodicities in a flow. Two sets of experiments were carried out, 2D PIV and two-point LDA, in two different scaled vessels of diameter 80 mm and 294 mm. The MI characteristics were studied for two different Re (i.e., 32 000 and 3200) by correlating the vortex intensity (vor- ticity and pressure suction) with its radius at different axial positions. This study provides relevant data for the optimisation of mixing time as it indicates ‘preferred’ positions, on the path of the MI vortex, where the insertion of a second phase should take place in order to fully exploit the suction potential of the MI vortex. Keywords: mixing; macro-instability; vortex; l 2 technique; stirred vessel. INTRODUCTION Vortical structures are encountered in most flow processes and their utilization can signifi- cantly enhance mixing of the fluids involved, as has been amply demonstrated for example, by the studies of the mechanism of vortex breakdown (see, e.g., Escudier and Keller, 1985). Mixing operations often involve chemical reactions that are essentially molecular level processes. Consequently only mixing at such a level can affect them directly. However, larger scale (macro-)mixing can have an indirect influence on the course of a process by transporting fluid through environ- ments where turbulence characteristics and/ or chemical composition vary substantially. In particular, vortical motions present in a stir- red reactor can aid substantially macromixing but they are relatively little understood due to lack of sufficient information about vortex strength, location and size; knowledge of these parameters is further complicated by the unsteadiness of such motions in the com- plex, 3D and turbulent flows in the vessels. Vortical structures of potential benefit for mixing practice include the trailing vortices around impeller blades as well as macro- instability vortices that precess around the vessel shaft. Both are however difficult to track in the swirling flows in a mixing vessel. Macro-instabilities (MIs), in particular, have been thoroughly studied and a large number of works can be found in literature mainly concerning the variation of their precessional frequency f, with Re, (i.e., f 0 ¼ f/N ¼ 0.106 for 400  Re  6300 and f 0 ¼ 0.02 for 13 600  Re  54 000), impeller clearence and impeller design (see, e.g., Nikiforaki et al., 2003; Galletti et al., 2004, 2005). Most of these studies have been carried out using LDA and the precessional frequency was obtained through a spectral analysis. More recently Paglianti et al. (2006) have shown that a pressure transducer can also be used to determine the MI frequency. The latter work has motivated the authors to develop a methodology that could offer an insight of MI characteristics such as the MI axis locus, vari- ation of MI size with axial distance (z/T), and MI net vorticity for different axial positions. The relatively recent developments of state- of-the-art anemometers with high spatial and temporal resolution (stereoscopic PIV and multi-channel LDA) and the development of novel methodologies for a proper identifi- cation of a vortex structure, such as the l 2 technique, should enable the study and com- prehensive understanding of the underlying physics of MI vortices. In addition, although it is impractical in most industrial situations to employ PIV and the present approach to identify the presence of MI vortices, use of an inexpensive and easy to install pressure transducer to detect such vortices, as in Paglianti et al. (2006), could be utilised together with the information on vortex radius, size and location offered by the 543 Vol 85 (A5) 543–550  Correspondence to: Dr A. Ducci, Experimental and Computational Laboratory for the Analysis of Turbulence (ECLAT), Division of Engineering, King’s College London, Strand, WC2R 2LS, UK. E-mail: andrea. ducci@kcl.ac.uk DOI: 10.1205/cherd06192 0263–8762/07/ $30.00 þ 0.00 Chemical Engineering Research and Design Trans IChemE, Part A, May 2007 # 2007 Institution of Chemical Engineers