Combustion and Flame 155 (2008) 181–195 www.elsevier.com/locate/combustflame Simultaneous Rayleigh temperature, OH- and CH 2 O-LIF imaging of methane jets in a vitiated coflow Robert L. Gordon a,∗ , Assaad R. Masri a , Epaminondas Mastorakos b a School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW 2006, Australia b Department of Engineering, University of Cambridge, Cambridge, UK Received 7 November 2007; received in revised form 6 March 2008; accepted 2 July 2008 Abstract This paper details a quantitative joint temperature, OH, and CH 2 O imaging experiment designed to investigate the stabilization of lifted turbulent methane flames issuing into a high temperature vitiated coflow. Temperature is determined through Rayleigh imaging, and the data are used to quantify OH-LIF excited at 283.011 nm, and to enable to semi-quantification of CH 2 O-LIF excited at 355 nm. A fuel with Rayleigh cross-section equal to that of the vitiated coflow was used to improve accuracy in the processing of the Rayleigh temperature. Results of the experiment have been presented, and compared to simulations of laminar transient autoignition flamelets. The images were classified in three main categories: (i) CH 2 O only, (ii) ignition kernels, and (iii) liftoff flames. Images of type (i) and (ii) were dominant in the early part of the jet, while images of type (iii) were dominant after the mean stabilization height. By examining OH and CH 2 O conditional on the size of the kernel, it was found that the sequence of conditional data was analogous to the evolution of autoignition, following the key stages of (1) build- up of a precursor pool, (2) initiation of reaction, and (3) formation of a steady flame. Viewed in such a sequence, CH 2 O peaks prior to the autoignition and then decays after ignition, and OH is found to peak at ignition and these peaks are maintained into the established steady flames. This is in qualitative agreement with the laminar transient flamelet calculations. The data are consistent with the view that autoignition is the main stabilization mechanism in this lifted flame.  2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Autoignition; Turbulent lifted flames; Rayleigh temperature; OH-LIF; CH 2 O-LIF 1. Introduction Autoignition is a phenomenon of great practical interest to combustion engineers in the design of gas * Corresponding author. Address for correspondence: FG Energie- und Kraftwerkstechnik, Petersenstr. 30, 64287 Darmstadt, Germany. Fax: +49 (6151) 16 6555. E-mail address: gordon@ekt.tu-darmstadt.de (R.L. Gordon). turbines, dual-fuel diesel engines, supersonic com- bustion ramjets, and HCCI engines. The processes are transient, and usually occur at lower temperatures than those found in standard flames. The ignition en- vironment also some times contains hot combustion products. While some of these features are common to those found in flameless combustion [1,2], the time- dependent nature of autoignition raises special issues about the initiation of reaction and heat release. Ideally, investigations of autoignition should be capable of resolving the complexities of this process, 0010-2180/$ – see front matter  2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2008.07.001