Methane flat flame

The experimental setup for diode-laser sensing of combustion gases using extractive sampling techniques is shown in Fig. 24.8. The measurements were performed in the post-flame region of laminar methane-air flames at atmospheric conditions. A premixed, water-cooled, ducted flat-flame burner with a 6-centimeter diameter served as the combustion test-bed. Methane and air flows were metered with calibrated rotameters, premixed, and injected into the burner. The stoichiometry was varied between equivalence ratios of = 0.67 to... 

Fig. 13.11 Low-pressure, premixed, methane-air, flat flame with a molecular-beam mass-spectrometry probe positioned above the flame [187]. Photograph is courtesy of Dr. A. Mcllroy, Sandia National Laboratiories.

Fig. 16.8 Illustration of a premixed flat-flame burner. Fuel and oxidizer are first premixed, and then flow through a porous burner face. A steady, one-dimensional flat flame is stabilized by heat transfer to the cooled burner face. The solutions shown here are for a methane-air flame, in which the air contains water vapor at 100% relative humidity. By plotting the temperature and selected species profiles, one can observe some of the complexities of flame structure.

At the relatively low inlet velocity of U =30 cm/s, the flame is stabilized by heat transfer to the inlet manifold. This is essentially the situation in the typical flat-flame burner that is found in many combustion laboratories (e.g., Fig. 16.8). The laminar burning velocity (flame speed) of a freely propagating atmospheric-pressure, stoichiometric, methane-air flame is approximately 38 cm/s. Therefore, since inlet velocity is less than the flame speed, the flame tends to work its way back upstream toward the burner. As it does, however, a... 

W.A. Hahn and J.O. L Wendt. NOx Formation in Flat, Laminar, Opposed Jet Methane Diffusion Flames. Proc. Combust. Inst., 18 121-128,1981. 

Figure 15. CARS spectra in the region of the CO Q-branch from a rich methane-air flat flame. Above is shown the background-free CARS spectrum, below the conventional CARS spectrum (15,).

Using the frozen excitation model to analyze the data shown in Fig. 3, and calibrating the system via Rayleigh scattering (8J, a total OH number density of 4 x 1C>16 cm 3 was calculated for an assumed flame temperature of 2000 K in the methane-air torch. Nt was not compared directly with the results of absorption studies future flat flame burner studies will involve direct comparison of absorption and fluorescence. 

Two flat flame burners have been employed, a 4 cm 10 cm burner with a ceramic-lined chimney for NO measurements (4) and a 2.6 cm x 8.6 cm open-faced burner with a nitrogen shroud flow for CO measurements. Both burners operate at atmospheric pressure with laminar, premixed methane-air mixtures. These burners work satisfactorily over a broad range of fuel-air equivalence ratios, but both have cold boundary regions which cause non-uniform conditions along the optical axis that can be important in the data analysis (4). 

Until recently it was further assumed that the hydrocarbon oxidation reactions have equilibrated prior to the onset of NO formation because the NO reactions are relatively much slower (5) at temperatures of stoichiometric hydrocarbon-air combustion and because they take place over an extensive portion of the mixing region. Fenimore (6) and Harris et al. (7) have conducted recent experimental studies of NO formation in atmospheric flat flames their data support this simplified picture for posf-combustion-zone formation. However, Fenimore (6) noted a substantial amount of NO was formed very rapidly in the flame front of methane-air and ethylene-air flames but not in CO-air or H2-air flames. Figure 1 shows Fenimores data on NO formation in four ethylene-air flames as a function of reaction time from the burner surface to the probe tip. The positive intercepts are indicative of flame zone or prompt NO. Fenimore subsequently postulated that reactions such as... 

Chander, S., and Ray, A. "Heat Transfer Characteristics of Three Interacting Methane/ Air Flame Jets Impinging on a Flat Surface." International Journal of Heat and Mass Transfer 50 (2007) 640-53. 

In a steady-state methane-air flame at approximately atmospheric pressure, the temperature is raised from 70 to 3200°F. The incoming air-gas mixture and the products of combustion may both be considered ideal gases with a molecular weight of 28 g/mol. The flame is a thin, flat region perpendicular to the gas flow. If the flow comes into the flame at a velocity of 2 ft/s, what is the pressure difference from one side of the flame to the other This problem and its consequencesj are discussed elsewhere [9]. 

The burning velocities of propane-air and methane-air flames were measured on a flat burner using the heat flux method (De Goey et al., 1993 Van Maaren et al.,1994). This technique allows burning velocities to be measured with much higher accuracy (3% for stoichiometric flames and 5-10% for lean and rich flames) over a wide range of equivalence ratios. 

Figure 2. Flow diagram for the oxidation of CH4 in stoichiometric methane-air flames at F = 1 bar, = 298 K. The calculations were done with a one-dimensional laminar flat flame model using a mechanism described by Warnatz et al (1982). The thickness of the arrows is proportional to the reaction rates integrated over the whole flame front.

Fig. 6.20 Experimental particle paths in an opposed stagnation flow. A mixture of 25% methane and 75% nitrogen issues upward from the bottom porous-plate manifold and a mixture of 50% oxygen and 50% nitrogen issues downward from the top porous-plate manifold. The inlet velocity of both streams is 5.4 cm/s. Both streams are seeded with small titania particles that are illuminated to visualize the flow patterns. The upper panel shows cold nonreacting flow that is, the flame is not burning. In the lower panel, a nonpremixed flame is established between the two streams. Thermal phoresis forces the particles away from the flame zone. The fact that the flame region is flat (i.e., independent of radius) illustrates the similarity of the flow. Photographs courtesy of Prof. Tadao Takeno, Meijo University, Nagoya, Japan, and Prof. Makihito Nishioka, Tsukuba University, Tsukuba, Japan.

The photograph is included to make two points. First, the particle paths show qualitatively that the flow follows the anticipated streamlines. Even for the relatively small dimensions, the edge effects that could interrupt similarity behavior at the outflow appear to be minor. Second, and more striking, is the fact that the flame zone is extremely flat. Here is a situation that includes a considerable amount of chemistry (methane combustion) and complex heat and mass transfer. The fact that the flame zone shows no radial dependence is is convincing evidence that the fluid mechanical similarity is indeed valid. 

Four flat, disc-shaped laminar flow flames were probed and analyzed using standard microprobing techniques. The flames were composed primarily of CO, H2, 02> and Ar with small amounts of CH4 or natural gas added to simulate intermediate Btu gas mixtures. Gas compositions used in the probings are presented in Table 1. Flames A and B contained excess air, air/fuel equivalence ratio = 1.13 Flames C and D were slightly fuel rich, air/fuel equivalence ratio = 0.93. Each of the mixtures had a CO/H2/X (X = methane or natural gas) mole ratio of 1/1/0.22. 

Schuller, Durox, and Candel [115] studied the noise produced by acoustically excited air/methane flames impinging on a water-cooled flat plate. 

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