Hysteresis in flow patterns in annular swirling jets M. Vanierschot * ,1 , E. Van den Bulck 2 Katholieke Universiteit Leuven, Department of Mechanical Engineering, Division of Applied Mechanics and Energy Conversion, Celestijnenlaan 300A, B-3001 Heverlee, Belgium Received 17 January 2006; received in revised form 31 May 2006; accepted 2 June 2006 Abstract This study investigates the influence of swirl on the mean cold flowfield of an annular jet with a stepped-conical expansion. Both the axial and azimuthal velocity components are measured using a two component Laser Doppler Anemometry system in forward scattering mode. A detailed description of the radial profiles of both mean axial and azimuthal velocity as well as three components of the Reynolds stress are given. Four different jets are identified as a function of the swirl number: ‘Closed Jet Flow’, ‘Open Jet Flow Low Swirl’, ‘Open Jet Flow High Swirl’ and ‘Coanda Jet Flow’. These flow patterns change with varying swirl number and there exists hysteresis when increasing and subsequently decreasing the swirl. Also a method for jet identification based upon pressure measurements is presented to replace the time consuming LDA measurements. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Annular swirling jet; Vortex breakdown; Hysteresis; Stepped-conical 1. Introduction Application of swirl to jets is of great importance in a variety of engineering applications, e.g. aeronautics, com- bustion, cyclone separators, Ranque-Hilsch tubes, etc. In combustion systems, swirl leads to better flame stability and lower pollution; and it is used in gasoline and diesel engines, gas turbines, industrial furnaces and many other applications [1]. The large-scale effects of swirl are well known: an increase in jet growth and entrainment and a faster decay than non-swirling jets. These effects increase with increasing swirl [2,3]. Escudier et al. [4] studied the influence of swirl on a con- fined annular jet with a funnelling cone and an expansion (geometry to the left in Fig. 1). At low swirl, the central cyl- inder acts as a bluff body to the flow and a reverse flow zone is created behind it. Increasing the swirl leads to no significant change in the axial-radial flow field. At a certain critical swirl level, an isolated recirculation zone appears downstream. This phenomenon is called vortex breakdown. The recirculation zone moves upstream with increasing swirl and at a certain level of swirl the wake behind the central cylinder and the isolated recirculation zone are joined [6]. Vortex breakdown can be divided in six different types, but only two are observed for high Reynolds num- bers: axisymmetric (bubble) breakdown and spiral break- down. Over the past 45 years many studies on this phenomenon have been made [7–9]. However, up to now, no consensus has been reached on the basic mechanisms that cause the breakdown. Sheen et al. [6] studied the different recirculation zones in confined and unconfined annular swirling jets with a straight sudden expansion (geometry in the middle of Fig. 1). Seven different flow patterns were identified behind the central rod based on the Reynolds number and the swirl number, and the reported patterns for high Reynolds numbers are in close agreement with the ones presented here. In this paper, a study is made of the mean cold flow field of an annular swirling jet. The stepped-conical expansion 0894-1777/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2006.06.001 * Corresponding author. E-mail addresses: Maarten.Vanierschot@mech.kuleuven.be (M. Vani- erschot), Eric.VandenBulck@mech.kuleuven.be (E. Van den Bulck). 1 Ir., Research associate applied flow and combustion section. 2 Prof. dr. ir., Head of the applied flow and combustion section. www.elsevier.com/locate/etfs Experimental Thermal and Fluid Science 31 (2007) 513–524