ARTICLE pubs.acs.org/EF
Effects of Dilution on Laminar Burning Velocity of Premixed Methane/Air Flames B. Galmiche,† F. Halter,†,* F. Foucher,† and P. Dagaut‡ † ‡
Institut PRISME, Universite d’Orleans, Polytech Vinci - 45072 Orleans cedex, France CNRS, 1C, Ave de la Recherche Scientifique - 45071 Orleans cedex 2, France ABSTRACT: Effects of dilution on premixed methane/air combustion are investigated through experiments and numerical simulations on laminar burning velocities. Experiments are carried out at atmospheric pressure and 393 K; computed results are obtained using the GRI mechanism and the Premix code of the Chemkin package. Laminar burning velocities are determined for several diluents (nitrogen, carbon dioxide, vapor water, a mixture of the three previous gases representative of exhaust gases, helium, and argon) and for different dilution percentages. The equivalence ratio is kept constant equal to stoichiometric. Excellent agreements between experimental data and computed results are obtained. Because heat capacity seems to possess the predominant effect on the laminar flame, an explicit correlation between the heat capacity of the diluent and the laminar burning velocity is proposed.
’ INTRODUCTION In many combustion systems, such as internal combustion engines, one way to reduce NOx emissions consists of recirculating hot exhaust gases in the fresh gases. Indeed, exhaust gas recirculation (EGR) induces a diminution of the combustion temperature and therefore leads to substantial reductions in NOx formation. On the other hand, the dilution of fresh gases affects the flame reactivity, which can induce stability problems. Therefore, it is of paramount importance to know precisely the dilution impact on the combustion process. Depending on the diluent characteristics, dilution leads to modifications in the thermophysical properties (changes in mixture heat capacity) and could also affect the elementary chemical reactions. Few prior studies have examined the effect of nitrogen and carbon dioxide on premixed methane-air flame properties.1-3 It was shown that a larger decrease of the laminar burning velocity is obtained for carbon dioxide dilution. The heat capacity effect and the carbon dioxide dissociation explain this observation.1 The influence of water vapor addition on the laminar burning velocity has also been investigated previously for methane oxygen-enhanced flames.4 Experimental data were compared to compute results using the GRI 3.0 mechanism. It was shown that the laminar burning velocity decreases linearly as the water steam molar fraction increases, and this evolution was found to be independent of the equivalence ratio. The present study aims at characterizing the dilution effect on stoichiometric methane/air mixtures for several different diluents: nitrogen, carbon dioxide, water vapor, a mixture of these three previous gases, helium, and argon. New experimental data are presented for a large set of diluents and dilution levels. The response of the GRI mechanism to dilution then is tested in terms of laminar burning velocity for all the diluents tested. The main goal of the present paper is to provide a global correlation allowing the prediction of the laminar burning velocity, regardless of the diluent and the dilution level.
(diameter of 0.8 mm), linked to a conventional capacitive discharge ignition system, are used to form the spark gap (1 mm) at the center of this chamber. Before filling the chamber with the different gases, a vacuum is created. The volumes of methane, air, and diluents (nitrogen, carbon dioxide, helium, and argon) were then introduced with thermal mass-flow meters. For water vapor, a Corriolis mass flow meter was used. To obtain a perfectly homogeneous premixed mixture, the mass-flow meters are simultaneously opened and a fan is installed into the chamber to mix gases. A piezoelectric pressure transducer is used to check the pressure level. Measurements were performed for an initial pressure of 0.1 MPa ((0.03 MPa) and an initial temperature of 393 K ((3 K). This temperature has been chosen to be higher than the water boiling temperature, to avoid any condensation problems. Two opposite and transparent windows (diameter of 10.5 cm) provide optical access into the chamber. An argon-ion laser is used for the lighting (457.9-514.5 nm). A parallel light is created by using two plano-convex lenses (diameters of 15 and 70 mm and focal lengths of 25 and 1000 mm, respectively). After passing through both lenses and the combustion chamber, the beam is displayed on a screen. A more-detailed description of the entire system was presented earlier.5 The visualization of the flame is obtained using a classical shadowgraphy method. Instantaneous images are recorded using a high-speed video camera (6000 pictures per second). An analysis of the different errors linked to the experimental procedure in a previous study5 has shown that the maximum uncertainty in the determination of the laminar burning velocity was
