Experimental realization and characterization of unstretched planar one-dimensional diffusion ﬂames Etienne Robert a,b,⇑ , Peter A. Monkewitz b a Department of Mechanics, Kungliga Tekniska Högskolan (KTH), Osquars Backe 18, Stockholm 100 44, Sweden b Laboratory of Fluid Mechanics (LMF), Swiss Federal Institute of Technology (EPFL), Station 9, 1015 Lausanne, Switzerland article info Article history: Received 5 June 2012 Received in revised form 6 August 2012 Accepted 21 November 2012 Available online 22 December 2012 Keywords: Diffusion ﬂame Unstretched One-dimensional abstract A burner conﬁguration that allows the creation of nearly unstretched one-dimensional diffusion ﬂames has recently been introduced. This paper presents the ﬁrst detailed characterization of ﬂames produced in such a burner to assess how well they approach the idealized one-dimensional conﬁguration, which is used extensively for the development of theoretical models for diffusion ﬂame stability. Particular emphasis is directed at quantifying the remaining inhomogeneities in the burner, such as residual ﬂame stretch or variations in the boundary conditions, and identifying possible means to minimize them. Mea- surements made include the velocity ﬁeld, stable species concentration proﬁles and temperature distri- bution throughout the burning chamber. Additionally, the ﬂame position in the burner is determined as a function of mixture strength, reactant transport properties and bulk ﬂow velocity. All the measurements are found to be in good agreement with a simpliﬁed one-dimensional ﬂame model. However, the bound- ary conditions and their physical location have to be determined experimentally on both sides of the ﬂame, which complicates comparison with results from theoretical and numerical models. Despite these limitations, we conclude that this new burner type provides a good experimental approximation of the one-dimensional idealized construct. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. 1. Introduction The fundamental understanding of complex combustion phe- nomena is often acquired through theoretical and numerical mod- els developed in simple conﬁguration. Therefore, the availability of experimental facilities capable of generating good approximations of such conﬁgurations is important to provide validation for these models. This is especially true for the investigation of thermal- diffusive instabilities, which have been studied theoretically for decades in unstretched one-dimensional diffusion ﬂames, long thought to be impossible to realize experimentally. An experimen- tal implementation capable of generating such idealized reaction sheets has recently been introduced [1,2] and applied successfully to the investigation of thermal-diffusive instabilities [2,3] and soot formation in diffusion ﬂames [4]. In this paper, we present the ﬁrst detailed description and characterization of this burner conﬁgura- tion, with an emphasis on the capabilities and limitations of the design, to provide guidance for potential use of this tool in other areas of combustion research where a one-dimensional un- stretched reaction sheet is desirable. This burner was initially developed to provide validation for the- oretical analyses of intrinsic diffusion ﬂame instabilities, which are the subject of an extensive and diverse literature, for instance [5– 9]. This topic also enjoys an increasing interest due to the trend to- wards leaner mixtures in industrial combustors which make them more susceptible to thermal-diffusive instabilities. The latter play a key role in undesired phenomena such as soot formation [10,11] and dynamic extinction and re-ignition processes, for instance. Moreover, the simple 1D conﬁguration is relevant in the context of the ﬂamelet model [12] in which partially premixed turbulent combustion is represented using diffusion ﬂamelets with one- dimensional inner structure. Until recently, however, stability analyses and experimental observations of thermal-diffusive instabilities in diffusion ﬂames have not considered quite the same system. On the one hand, most stability analyses of diffusion ﬂames, e.g., [13–18], consider a variant of the one-dimensional counter- diffusing non-premixed theoretical construct introduced by Kirkby and Schmitz [19] and sketched in Fig. 1a. The combustion chamber is a straight duct open at one end to a fast stream of oxidant and supplied with fuel at the other end through a semi-permeable membrane. All transport processes are supposed one-dimensional with one reactant (the oxidant here) counter-diffusing against the 0010-2180/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.combustﬂame.2012.11.011 ⇑ Corresponding author at: Department of Mechanics, Kungliga Tekniska Hög- skolan (KTH), Osquars Backe 18, Stockholm 100 44, Sweden. Fax: +46 8 796 9850. E-mail addresses: etienne@mech.kth.se (E. Robert), peter.monkewitz@epﬂ.ch (P.A. Monkewitz). Combustion and Flame 160 (2013) 546–556 Contents lists available at SciVerse ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustﬂame