Mixing of confined coaxial flows Valery Zhdanov a, * , Nikolay Kornev a , Egon Hassel a , Andrei Chorny b a Department of Technical Thermodynamics, University of Rostock, Rostock 18059, Germany b Turbulence Laboratory, A.V. Luikov Heat and Mass Transfer Institute, 15, P. Brovka Str., Minsk 220072, Belarus Received 17 April 2006 Available online 7 July 2006 Abstract The paper presents experimental results on the mixing process in a coaxial jet mixer in two mixing regimes. In the first mixing regime, a recirculation zone develops just behind a nozzle near mixer walls, while in the second regime a jet is mixed with the co-flow without developing a recirculation zone. In the both regimes, the mixing process is studied at Re d = 10 000. Behind the nozzle over the range 0.1 < x/D < 9.1, a velocity field in mixer cross-sections is measured by a one-component laser Doppler velocity meter and a scalar field is detected by the laser image fluorescence (LIF) method. A transverse autocorrelation function, integral length scales and probability density functions (PDF) are calculated using instantaneous distributions of a scalar and its fluctuations. It is shown that the scalar field acquires a homogeneous state faster than the velocity one. A quasi-uniform scalar distribution over the mixer cross-section is completed at the distance x/D = 5.1 in the first mixing regime, while this distribution has not been yet attained in the second. Analysis of the tur- bulent statistical moments and the autocorrelation function reveals how unsteady vortex structures exert a dramatic influence on the mixing. When the recirculation zone has developed, long-period antiphase oscillations exist near the mixer walls. Ó 2006 Elsevier Ltd. All rights reserved. 1. Introduction Currently, studies of the mixing in confined coaxial jets have received considerable attention because the jet interac- tion causes a great number of physical phenomena to appear. Among them is the intermittence at the boundaries of mixing layers that originates due to generation, coales- cence, and decay of unsteady vortex structures. In addition, entraining one flow by another can generate a prerequisite to the formation of separated flow regions. These phenomena are the objects of investigation which are of interest from the standpoint of both a deeper insight into transfer pro- cesses and finding of optimal mixing conditions in jet mixers. The elaboration of experimental methods for measure- ment of passive admixture distributions has gained a nota- ble advance in the understanding of the mixing process. The results on a scalar field developed under the action of a velocity one will essentially refine the understanding not only of the velocity evolution, but also of unsteady flow peculiarities that do not always emerge from velocity field experiments. Coaxial jet mixers being rather simple engineering facil- ities find widespread use in different branches of industry and permit realizing the mixing process when the condi- tions for combination of laminar and turbulent flows are regulated through their flowrate ratio. There are a lot of possible mixing regimes. Some of these regimes were already detailed in the investigations of Rehalb [1,2] and Lima [3]. In these investigations, the mixing process was studied at the velocity of the co-flow much larger than that of the jet, in which a laminar flow was realized. At this con- dition, the recirculation zone developed just behind the nozzle along the mixer axis. These investigations were motivated by two following problems that had arisen in the study of chemically reacting flows: the stabilization of the flame front and the filling of the co-flow with sub- stances transported by an internal jet. The mixing with a jet velocity much higher than a co- flow one is less well understood. Under this condition, 0017-9310/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2006.04.022 * Corresponding author. E-mail address: valery.zhdanov@uni-rostock.de (V. Zhdanov). www.elsevier.com/locate/ijhmt International Journal of Heat and Mass Transfer 49 (2006) 3942–3956