An experimental realization of an unstrained, planar diffusion flame D. Lo Jacono a , P. Papas a, * , M. Matalon b , P.A. Monkewitz a a Fluid Mechanics Laboratory, Swiss Federal Institute of Technology (EPFL), CH-1015 Lausanne, Switzerland b McCormick School of Engineering and Applied Science, Northwestern University, Evanston, IL 60208, USA Abstract A unique burner was constructed to experimentally realize a one-dimensional unstrained planar non- premixed flame, previously considered only in idealized theoretical models. One reactant, the fuel mixture in the current experiments, is supplied through a porous plug at the bottom of the combustion chamber and flows vertically up towards the horizontal flame. The crux of the design is the introduction of the oxi- dizer from above in such a way that its diffusion against the upward product flow is essentially one-dimen- sional, i.e., uniform over the burner cross-section. This feature was implemented by introducing the oxidizer into the burner chamber from the top through an array of 625 closely spaced hypodermic needles, and allowing the hot products to escape vertically up through the space between the needles. Due to the injection of oxidizer through discrete tubes, a three-dimensional ‘‘injection layer’’ exists below the exit plane of the oxidizer supply tubes. Experimental evidence suggests that this layer is thin and that oxidizer is supplied to the flame by 1-D counterdiffusion, producing a nearly unstrained flame. To characterize the burner, flame position measurements were conducted for different compositions and flowrates of H 2 –CO 2 and O 2 –CO 2 mixtures. The measured flame locations are compared to an idealized one-dimensional model in which only diffusion of oxidizer against the product flow is considered. The potential of the new burner is demonstrated by a study of cellular structures forming near the extinction limit. Consistent with previous investigations, cellular instabilities are shown to become more prevalent as the initial mixture strength and/ or the Damko ¨ hler number are decreased. As the extinction limit is approached, the number of cells was observed to decrease progressively. Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Counterdiffusion burner; Unstrained planar diffusion flame; Cellular instabilities 1. Introduction The unstrained one-dimensional diffusion flame has served for over six decades, starting with Zeldovich [1] and Spalding [2,3], as an idealized construct for theoretical modelling. The idealized burner in these studies is supplied from the bot- tom with one reactant, the fuel in this study, through a ‘‘semi-permeable plate’’ (impermeable in the downward direction). The other reactant diffuses from the top of the chamber to the flame, against the upward flow of products. Conditions at the boundary where the oxidizer originates are maintained constant by a fast-flowing oxidizer 1540-7489/$ - see front matter Ó 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.proci.2004.07.011 * Corresponding author. Present address: Division of Engineering, Colorado School of Mines, Golden, CO 80401, USA. Fax: +1 303 273 3602. E-mail address: ppapas@mines.edu (P. Papas). Proceedings of the Combustion Institute 30 (2005) 501–509 www.elsevier.com/locate/proci Proceedings of the Combustion Institute