Effects of water vapor addition on the laminar burning velocity of oxygen-enriched methane flames A.N. Mazas a,b,c,⇑ , B. Fiorina a,b , D.A. Lacoste a,b , T. Schuller a,b a CNRS, UPR 288 Laboratoire d’Energétique Moléculaire et Macroscopique Combustion (EM2C), Grande Voie des Vignes, 92295 Châtenay-Malabry, France b Ecole Centrale Paris, Grande Voie des Vignes, 92295 Châtenay-Malabry, France c Air Liquide, Centre de Recherche Claude-Delorme, Les Loges en Josas, BP 126, F-78350 Jouy-en-Josas, France article info Article history: Received 25 August 2010 Received in revised form 9 May 2011 Accepted 13 May 2011 Available online 21 June 2011 Keywords: Oxy–fuel combustion Oxygen-enhanced combustion Steam addition Water vapor addition Laminar burning velocity Conical flames abstract The effects of steam addition on the laminar burning velocity of premixed oxygen-enriched methane flames are investigated at atmospheric pressure. Experiments are carried out with an axisymmetric bur- ner on which laminar conical flames are stabilized. A newly devised steam production system is used to dilute the reactants with water vapor. The oxygen-enrichment ratio in the oxidizer, defined as O 2 / (O 2 +N 2 ) (mol.), is varied from 0.21 (air) to 1.0 (pure oxygen). The equivalence ratio ranges from 0.5 to 1.5 and the steam molar fraction in the reactive mixture is varied from 0 to 0.50. For all compositions examined, the reactive mixture is preheated to a temperature T u = 373 K. Laminar flame speeds are deter- mined with the flame area method using a Schlieren apparatus. The deviations induced by stretch effects due to aerodynamic strain and flame curvature are assessed using Particle Imaging Velocimetry measure- ments and flame images, and these data are used to estimate the uncertainty of the flame speed measure- ments. The experiments are completed by numerical simulations conducted with the PREMIX code using different detailed kinetic mechanisms. It is shown that the laminar flame speed of CH 4 /O 2 /N 2 /H 2 O (v) mix- tures features a quasi-linear decrease with increasing steam molar fraction, even at high steam dilution rates. Numerical predictions are in good agreement with experimental data for all compositions explored, except for low dilution rates X H 2 O < 0:10 in methane–oxygen mixtures, where the flame speed is slightly underestimated by the calculations. It is also shown that steam addition has a non-negligible chemical impact on the flame speed for methane–air flames, mainly due to water vapor high chaperon efficiency in third-body reactions. This effect is however strongly attenuated when the oxygen concentration is increased in the reactive mixture. For highly oxygen-enriched flames, steam can be considered as an inert diluent. Ó 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved. 1. Introduction Additives in flames are frequently used for various purposes, including pollutant emissions control, flame stabilization, promo- tion or inhibition of identified chemical reactions. It is well estab- lished that adding a diluent in a reactive mixture leads to significant changes in flame properties. The addition of a diluent can affect the combustion by the three following mechanisms: (a) a dilution effect caused by the reduction in reactants concentra- tion in the reactive mixture; (b) a thermal effect due to the absorp- tion by the additive of part of the heat released by the reaction, leading to a change in flame temperature; (c) a chemical effect ow- ing to the activity of the additive that may alter some reactions paths. Those three mechanisms are concomitant and closely linked. For example, as chemical kinetics strongly depends on flame temperature, it is consequently difficult to analyze these effects separately. Due to the increasing use of flue gas recirculation in combustion processes, water vapor and carbon dioxide are among the most common additives. While the effects of CO 2 dilution have already been thoroughly investigated (see e.g. Refs. [1–5]), the impact of steam addition has been the subject of a relatively limited number of studies, which are summarized below. In this work, the effects of water vapor addition on the laminar burning velocity of premixed oxygen-enriched methane flames are experimentally and numeri- cally examined for various oxygen concentrations in the oxidizer. Varying the oxygen concentration in the oxidizer enables analysis of the sensitivity of water vapor dilution to flame temperature. Furthermore, the high reactivity of oxygen-enriched flames allows to investigate the effects of large steam dilution rates. Finally, studying the impact of steam addition on oxy–fuel flames is of practical interest since oxy–fuel combustion with flue gas 0010-2180/$ - see front matter Ó 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2011.05.014 ⇑ Corresponding author at: Laboratoire EM2C, CNRS UPR 288, Grande Voie des Vignes, F-92295 Châtenay-Malabry Cedex, France. E-mail address: antoine.mazas@airliquide.com (A.N. Mazas). Combustion and Flame 158 (2011) 2428–2440 Contents lists available at ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame