Methane laminar

FIGURE 4.23 Methane laminar flame velocities in various inert gas-oxygen mixtures (after Clingman et al. [27]). 

Let us note the specific character of laminar combustion of a binary fuel H2 + CH4 (Hytane) in air. Assume that the volume fractions of H2 and CH4 are Xh2 and Xch4 so that Xch4 = 1 — Xu2. The dependencies of the laminar flame velocity on this binary fuel composition and the fuel/air ratio are illustrated in Fig. 2.38 based on data obtained in [87, 88]. The solid curve with the solid circles correspond to experimental results, and the dashed line shows the following relations 5hy = > h2 Xn2 + (1 — Xh2) 5 ch4- Here Suy, 5h2 and 5ch4 - hytane, hydrogen and methane laminar flame velocities. 

Van Wingerden and Zeeuwen (1983) demonstrated increases in flame speeds of methane, propane, ethylene, and acetylene by deploying an array of cylindrical obstacles between two plates (Table 4.3). They showed that laminar flame speed can be used as a scaling parameter for reactivity. Van Wingerden (1984) further investigated the effect of pipe-rack obstacle arrays between two plates. Ignition of an ethylene-air mixture at one edge of the apparatus resulted in a flame speed of 420 m/s and a maximum pressure of 0.7 bar. 

It can be seen in Figure 3.1.1 that the total surface area of the propane lean limit flame is much less than that of the methane one. This is because the laminar burning velocity for the limit mixture is much higher for propane than for methane. 

In contrast to the lean propane flame, the burning intensity of the lean limit methane flame increases for the leading point. Preferential diffusion supplies the tip of this flame with an additional amoxmt of the deficient methane. Combustion of leaner mixture leads to some extension of the flammability limits. This is accompanied by reduced laminar burning velocity, increased flame surface area (compare surface of limit methane... 

In the case of flame propagation in the lean limit methane/air mixture, the local laminar burning velocity at... 

This system produces a steady laminar flow with a flat velocity profile at the burner exit for mean flow velocities up to 5m/s. Velocity fluctuations at the burner outlet are reduced to low levels as v /v< 0.01 on the central axis for free jet injection conditions. The burner is fed with a mixture of methane and air. Experiments-described in what follows are carried out at fixed equivalence ratios. Flow perturbations are produced by the loudspeaker driven by an amplifier, which is fed by a sinusoidal signal s)mthesizer. Velocity perturbations measured by laser doppler velocimetry (LDV) on the burner symmetry axis above the nozzle exit plane are also purely sinusoidal and their spectral... 

Stone, R., Clarke, A., and Beckwith, P, Correlations for the laminar-burning velocity of methane/diluent/air mixtures obtained in free-fall experiments. Combust. Flame, 114, 546, 1998. 

Smooke, M.D., Lin, P, Lam, J.K., and Long, M.B., Computational and experimental study of a laminar axi-symmetric methane-air diffusion flame, Proc. Combust. Inst., 23,575,1990. 

Robson, K. and Wilson, M.J.G., The stability of laminar diffusion flames of methane. Combust. Flame, 13, 626, 1969. 

Barlow, R.S., Karpetis, A.N., and Frank, J.H., Scalar profiles and NO formation in laminar opposed-flow partially premixed methane/air flames, Combust. Flame, 127, 2102,2001. 

A number of theoretical (5), (19-23). experimental (24-28) and computational (2), (23), (29-32). studies of premixed flames in a stagnation point flow have appeared recently in the literature. In many of these papers it was found that the Lewis number of the deficient reactant played an important role in the behavior of the flames near extinction. In particular, in the absence of downstream heat loss, it was shown that extinction of strained premixed laminar flames can be accomplished via one of the following two mechanisms. If the Lewis number (the ratio of the thermal diffusivity to the mass diffusivity) of the deficient reactant is greater than a critical value, Lee > 1 then extinction can be achieved by flame stretch alone. In such flames (e.g., rich methane-air and lean propane-air flames) extinction occurs at a finite distance from the plane of symmetry. However, if the Lewis number of the deficient reactant is less than this value (e.g., lean hydrogen-air and lean methane-air flames), then extinction occurs from a combination of flame stretch and incomplete chemical reaction. Based upon these results we anticipate that the Lewis number of hydrogen will play an important role in the extinction process. 

GP 10] [R 18]The best HCN yield of 31% at a p-gauze platinum catalyst (70 ml h methane 70 ml h ammonia 500 ml h air 1 bar 963 °C) is much better than the performance of monoliths (Figure 3.49) having similar laminar flow conditions [2]. A coiled strip and a straight-channel monolith have yields of 4 and 16%, respectively. The micro-reactor performance is not much below the best yield gained in a monolith operated under turbulent-flow conditions (38%). 

It is interesting to note that stratified combustible gas mixtures can exist in tunnel-like conditions. The condition in a coal mine tunnel is an excellent example. The marsh gas (methane) is lighter than air and accumulates at the ceiling. Thus a stratified air-methane mixture exists. Experiments have shown that under the conditions described the flame propagation rate is very much faster than the stoichiometric laminar flame speed. In laboratory experiments simulating the mine-like conditions the actual rates were found to be affected by the laboratory simulated tunnel length and depth. In effect, the expansion of the reaction products of these type laboratory experiments drives the flame front developed. The overall effect is similar in context to the soap bubble type flame experiments discussed in Section C5c. In the soap bubble flame experiment measurements, the ambient condition is about 300 K and the stoichiometric flame temperature of the flame products for most hydrocarbon fuels... 

You want to measure the laminar flame speed at 273 K of a homogeneous gas mixture by the Bunsen burner tube method. If the mixture to be measured is 9% natural gas in air, what size would you make the tube diameter Natural gas is mostly methane. The laminar flame speed of the mixture can be taken as 34cm/s at 298 K. Other required data can be found in standard reference books. 

Unlike premixed flames, which have a very narrow reaction zone, diffusion flames have a wider region over which the composition changes and chemical reactions can take place. Obviously, these changes are principally due to some interdiffusion of reactants and products. Hottel and Hawthorne [5] were the first to make detailed measurements of species distributions in a concentric laminar H2-air diffusion flame. Fig. 6.5 shows the type of results they obtained for a radial distribution at a height corresponding to a cross-section of the overventilated flame depicted in Fig. 6.2. Smyth et al. [2] made very detailed and accurate measurements of temperature and species variation across a Wolfhard-Parker burner in which methane was the fuel. Their results are shown in Figs. 6.6 and 6.7. 

Figures 12.3 and 12.3c show mean velocity (Fig. 12.36) and mean temperature (Fig. 12.3c) fields under bluff-body stabilized combustion of stoichiometric methane-air mixture at inlet velocity 10 m/s, and ABC of Eq. (12.19) at the combustor outlet. Functions Wj, Wij, and W2j in Eq. (12.1) were obtained by solving the problem of laminar flame propagation with the detailed reaction mechanism [31] of Ci-C2-hydrocarbon oxidation (35 species, 280 reactions) including CH4 oxidation chemistry. The PDF of Eq. (12.4) was used in this calculation.

In Fig. 15.4, the measured turbulent flame speeds, normalized with mixture-specific laminar flame velocities obtained recently by Vagelopoulos et al. [14], are compared with experimental and theoretical results obtained in earlier studies. Also shown in the figure are the measurements made by Abdel-Gayed et al. [3] for methane-air mixtures with = 0.9 and = 1 a correlation of measured turbulent flame speeds with intensity obtained by Cheng and Shepherd [1] for rod-stabilized v-flames, tube-stabilized conical flames, and stagnation-flow stabilized flames, Ut/Ul = l + i.2 u /U ) a correlation of measured turbulent 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... 

Westbrook, C. K. 1980. Inhibition of laminar methane-air and methanol-air flames by hydrogen bromide. Combustion Science Technology 23 191-202. 

Gore, J.P., and N. J. Zhan. 1996. NO emission and major species concentrations in partially premixed laminar methane/air co-flow jet flames. Combustion Flame 105 414-27. 

Use laminar premixed free-flame calculations with a detailed reaction mechanism for hydrocarbon oxidation (e.g., GRI-Mech (GRIM30. mec)) to estimate the lean flammability limit for this gas composition in air, assuming that the mixture is flammable if the predicted flame speed is equal to or above 5 cm/s. For comparison, the lean flammability limits for methane and ethane are fuel-air equivalence ratios of 0.46 and 0.50, respectively. 

Discuss how an increased pressure may affect the laminar burning velocity of methane. 

Use GRI-Mech (GRIM30. mec) and a laminar premixed flame code to calculate the flame speed of a methane-air mixture at selected pressures between 0.1 and 10 atm. Evaluate whether the empirical correlation [412] for methane-air flames,... 

The purpose of this exercise is to investigate the effect of an inert (CO2) and a chemically active agent (iron pentacarbonyl, Fe(CO)s) on the flame speed of an atmospheric, stoichiometric methane-air flame. Employ a laminar premixed flame code to determine the flame speed, using GRI-Mech extended with a subset for iron pentacarbonyl chemistry [344] (GRIMFe.mec). 

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... 

In addition to the low-strain limit, which can be used to determine laminar burning velocities, the opposed-flow configuration can also be used to determine high-strain-rate extinction limits. As the inlet velocities increase, the flame is pushed closer to the symmetry plane and the maximum flame temperature decreases. There is a flow rate beyond which a flame can no longer be sustained (i.e., it is extinguished). Figure 17.11 illustrates extinction behavior for premixed methane-air flames of varying stoichiometries. 

M.C. Branch, N. Sullivan, M. Ulsh, and M. Strobel. Surface Modification of Polypropylene Films by Exposure to Laminar, Premixed Methane-Air Flames. Proc. Combust. Inst., 27 2807-2813,1998. 

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. 

M.D. Smooke, P. Lin, J.K. Lam, and M.B. Long. Computational and Experimental Study of a Laminar Axisymmetric Methane-Air Diffusion Flame. Proc. Combust. Inst., 23 575-582,1990. 

N. Sullivan, A. Jensen, P. Glarborg, M.S. Day, J.F. Grcar, J.B. Bell, CJ. Pope, and RJ. Kee. NO Formation and Ammonia Conversion in Laminar Coflowing Non-premixed Methane-Air Flames. Combust. Flame, in press, 2002. 

D.L. Zhu, F.N. Egolfopoulos, and C.K. Law. Experimental and Numerical Determination of Laminar Flame Speeds of Methane/(Ar,N2 CO2)-Air Mixtures as a Function of Stiochiometry, Pressure, and Flame Temperaure. Proc. Combust. Inst., 22 1537-1545,1988. 

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