PROOF COPY [FE-03-1127] 022506JFG PROOF COPY [FE-03-1127] 022506JFG Thierry Lemenand Pascal Dupont Dominique Della Valle Hassan Peerhossaini 1 e-mail: hassan.peerhossaini@polytech.univ-nantes.fr Phone: +33-2-40-68-31-42 Fax: +33-2-40-68-31-41 Thermoﬂuids & Complex Flows Research Group, Laboratoire de Thermocinétique de Nantes, CNRS UMR 6607, Rue Christian Pauc, BP 50609, F-44306 Nantes, France Turbulent Mixing of Two Immiscible Fluids The emulsiﬁcation process in a static mixer HEV (high-efﬁciency vortex) in turbulent ﬂow is investigated. This new type of mixer generates coherent large-scale structures, enhanc- ing momentum transfer in the bulk ﬂow and hence providing favorable conditions for phase dispersion. We present a study of the single-phase ﬂow that details the ﬂow struc- ture, based on LDV measurements, giving access on the scales of turbulence. In addition, we discuss the liquid-liquid dispersion of oil in water obtained at the exit of the mixer/ emulsiﬁer. The generation of the dispersion is characterized by the Sauter diameter and described via a size-distribution function. We are interested in a local turbulence analy- sis, particularly the spatial structure of the turbulence and the turbulence spectra, which give information about the turbulent dissipation rate. Finally, we discuss the emulsiﬁer efﬁciency and compare the HEV performance with existing devices. DOI: 10.1115/1.2073247 Keywords: Liquid-Liquid Dispersion, Turbulent Spectra, Static Mixer, Energetic Efﬁciency, Energy Dissipation Rate, High Efﬁciency Vortex (HEV), Longitudinal Vortices 1 Introduction The global trend in chemical and manufacturing industries is towards improved energy efﬁciency, cleaner synthesis, reduced environmental impact, and smaller, safer, multifunctional process plants. Such concerns are the driving force for the intensiﬁcation of batch processes, which are being replaced with continuous high-intensity in-line mass- and heat-transfer equipment. In this context the process-intensiﬁcation PI approach, in which the ﬂuid dynamics of the process is matched to the reaction in order to improve selectivity and minimize the by-products, takes on par- ticular importance. It is estimated that in a typical large chemical plant, between 5 and 45 million Euros are wasted every year through inefﬁciencies. In some cases it is estimated that optimizing reactor performance for maximum yield and selectivity would save between 0.4 and 0.8 million Euros per product per year. Overall, reactor-related problems are believed to account for between 0.5% and 3.0% of total turnover, which for European Union chemical industries amounts to 1.9 to 11.4 billion Euros per year 1. Systems involving more than one component or phase contain interfaces between the components. Our ability to predict the per- formance of such systems is extremely limited. In addition, many multiphase processes are carried out in stirred-tank reactors. Poor ﬂow patterns and low inhomogeneous mixing are characteristic of stirred-tank reactors and typically afford energy dissipation rates in the range 1–10 W kg -1 . High selectivity requires high rates of micromixing, which need turbulent energy dissipations higher than 100 W kg -1 . Therefore, fast exothermic reactions when carried out in stirred tanks start before mixing is complete, leading to slow apparent rates of reac- tion and formation of by-products that must be separated further downline. The high-efﬁciency vortex HEV heat exchanger- reactor is selected for its capacity to generate large-scale vortex motions and enhance turbulent energy dissipation in the ﬂow. A typical potential application of this device in manufacturing processes is the “mixhead” of resin-injection-molding RIM ma- chines. Mixing, often called the heart of RIM, is what differenti- ates it from other reaction-molding processes such as thermoset injection molding or sheet molding. Most mixhead designs were developed by trial and error. Even today, newly designed mix- heads are mounted on a machine and a typical reaction is tried on them 2; mixing quality is then judged by the visual appearance of the product. Therefore, understanding the basic physical phe- nomena underlying mixing in ﬂows in manufacturing processes is fundamental to a predictive approach to these processes. In this paper we give a global characterization of a special static mixer designed for use as a reactor-heat exchanger for process intensiﬁcation in liquid-liquid mixing and reaction. Its design is based on curved bafﬂes ﬁxed on the tube walls that generate lon- gitudinal vortices, substantially increasing transport phenomena over the simple pipe and even over some static mixers known for their high efﬁciency. The emulsiﬁcation performance of this sys- tem, indicative of its mixing abilities, is presented in this study. Oil-in-water emulsions obtained with the static mixer are charac- terized by the granulometric distributions. The mean size, size distribution, and power consumption of the mixer are compared with those in some existing devices. The turbulent characteristics of the ﬂow in the mixer are studied extensively and the physical phenomena underlying the high efﬁ- ciency of the mixer are addressed. 2 Experimental Setup and Methods The Perspex HEV test section designed and constructed for this work Fig. 1 is a straight tube of inner diameter 20 mm along which seven tab arrays are ﬁxed. Each of the seven arrays consists of four trapezoidal tabs positioned at 90° to one another and ﬁxed on the tube walls. The tabs are turbulence promoters and generate longitudinal vortical structures. The test section is 180 mm long and the distance between two successive tab arrays is 20 mm one tube diameter. The test section is connected to a preconditioner and postcon- ditioner, which are 300 mm straight transparent tubes of circular inner cross section and 20 mm inner diameter. The preconditioner is used to provide a fully developed ﬂow at the inlet of the test section, and the postconditioner to observe the effect of the test section on mixing quality. A schematic diagram of the experimental setup is shown in Fig. 2. It consists of feed loops for oil and water. The ﬂow rates are controlled by valves and measured with two ﬂowmeters with overlapping ranges. 1 Corresponding author. Contributed by the Fluids Engineering Division for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received by the Fluids Engineering Division November 17, 2003; ﬁnal manuscript received: June 10, 2005. Associate Editor: Steven Ceccio. Journal of Fluids Engineering NOVEMBER 2005, Vol. 127 / 1 Copyright © 2005 by ASME PROOF COPY [FE-03-1127] 022506JFG