Combustion and Flame 146 (2006) 200–214 www.elsevier.com/locate/combustflame Time-varying behaviour of turbulent swirling nonpremixed flames Yasir M. Al-Abdeli a,∗ , Assaad R. Masri b , Gabriel R. Marquez c , Sten H. Starner b a School of Engineering, University of Tasmania, TAS 7001, Australia b School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW 2006, Australia c Cordukes Ltd, Industrial Project Division, Silverwater, NSW 2128, Australia Received 29 June 2005; received in revised form 21 February 2006; accepted 21 March 2006 Available online 11 May 2006 Abstract This paper presents the results of experimental investigations into the instabilities of turbulent swirling non- premixed flames. The unconfined flames considered here possess features similar to those found in practical combustors. Extensive time-mean (space-resolved) flow field and composition data already exist in these flames. Results presented herein seek to complement those data with time-resolved measurements acquired using high- speed imaging of laser Mie scattering and shadowgraphs. Mie scattering measurements are designed to identify instabilities specific to the fuel jet that is geometrically centred on the bluff body. Conversely, shadowgraphs reveal the behaviour of the fuel jet as well as a recirculation zone that stagnates on the face of the bluff body. Results from these two imaging techniques are augmented with observations from the frequency spectra of velocity mea- surements. Overall, the results indicate that two modes of instability exist in these flames: (i) a precession mode in the centre (fuel) jet and (ii) a puffing mode characterised by an expansion and collapse of the toroidal recirculation zone which forms on the face of the bluff body.  2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Swirl; Turbulent; Flame; Unsteady; Mie scattering; Shadowgraph; LDV 1. Introduction Swirling flames are complex in nature but attrac- tive to use in many engineering applications [1,2] due to favourable mixing characteristics and low emis- sion of pollutants [3–7]. The complexity arises not * Corresponding author. School of Engineering, Private Bag 65, University of Tasmania, TAS 7001, Australia. Fax: +61 3 6226 7863. E-mail address: yasir.alabdeli@utas.edu.au (Y.M. Al-Abdeli). only due to the existence of recirculation zones but also because of flow instabilities inherent to both re- acting and isothermal swirling flows. Understanding the origins and nature of these instabilities remains a challenge despite numerous excellent reviews [8–10] as well as many thorough experimental and numerical studies in the area [11–16]. However, the fact remains that the time-varying nature of these flows renders the experimental investigations difficult and the numeri- cal simulations time consuming. For a classical swirl burner configuration consist- ing of a pipe supplying fuel in a coflowing or coun- 0010-2180/$ – see front matter  2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2006.03.009