Methane-air mixtures

Experiments on a small scale with stoichiometric methane-air mixtures were carried out by Chan et al. (1980). Comparisons of results of these experiments with those performed by Moen et al. (1982) revealed that simple scaling is not possible for the results of explosions with very high flame speeds, in other words, flame speeds resulting from very intense turbulence. 

Chan et al. (1983) studied flame propagation in an obstructed channel whose degree of confinement could be varied by adjustment of exposure of the perforations in its top. Its dimensions were 1.22 m long and 127 x 203 mm in cross section. Results showed that reducing top confinement greatly reduced flame acceleration. When the channel s top confinement was reduced to 10%, the maximum flame speed produced for methane-air mixtures dropped from 120 m/s to 30 m/s. 

Figure 7-46 illustrates a typical relationship of limits of flammability and ignitibility for a methane air mixture. Note that energy required to ignite a flammable mixture (within its LET and UEL) varies with the composition, and that a 0.2 millijoule (mj) spark is inadequate to ignite even a stoichiometric mixture at atmospheric pressure at 26°C, while 1-mj spark can ignite any... 

Figure 7-46. Ignitibiiity curve and limits of flammability for methane-air mixtures at atmospheric pressure and 26°C. By permission, U.S. Bureau of Mines, Bulletin 627 [43].

The effects of turbulence must be taken into account when sizing a relief area. For example, the explosion violence of turbulent methane-air mixture is comparable to that of zero turbulence of hydrogen-air mi.xtures. From the investigations [54], the nomograms from Figure 7-63 can be applied for turbulent gas mixtures under the following conditions [54] ... 

Global velocity distribution behind flame front. Upward propagation in 5.15% methane/air mixture, (a) vector map, (b) and (c) scalar maps of axial and radial velocity components, respectively. Spots are caused by condensation of water vapor on the glass walls. 

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

Temperature profiles for methane-air mixture with = 0.50 in 50 mm tube diameter (a) neglecting and (b) considering radiation heat transfer. 

History of upward flame propagation and extinction in lean limit methane/air mixture. Square 5 cm x 5 cm vertical tube. Green color frames indicate PIV flow images. Red color represents direct photography of propagating flame. Extinction starts just after frame c. Framing rate... 

The mixture used in the present simulation is stoichiometric methane-air. Table 3.2.1 shows the chemical reaction schemes for a methane-air mixture, which has 27 species, including 5 ion molecules such as CH% CHO% F130+, CH3+, and C2IT3O and electron and 81 elementary reactions with ion-molecule reactions [9-11]. The reaction rate constants for elementary reaction with ion molecules have been reported in Refs. [10,11]. 

Relationship between minimum ignition energy and equivalence ratio for hydrogen-air and methane-air mixtures. 

Yuasa, T., Kadota, S., Tsue, M., Kono, M., Nomuta, H., and Ujiie, Y, Effects of energy deposition schedule on minimum ignition energy in spark ignition of methane-air mixtures, Proc. Combust. Inst., 29, 743, 2002. 

Kravchik, T. and Sher, E., Numerical modeling of spark ignition and flame initiation in a quiescent methane-air mixture. Combust. Flame, 99, 635,1994. 

Kadota, S., et al.. Numerical analysis of spark ignition process in a quiescent methane-air mixture with ion-molecule reactions. The 2nd Asia-Pacific Conference on Combustion, p. 617, 1999. 

Further measurements on the flame speed have been obtained with the use of a rotating tube [11] and vortex ring combustion [12]. Figure 4.2.4 shows the flame speed in vortex rings [12]. The values of slope in the V( -plane is nearly equal to unity for the near stoichiometric methane/air mixtures. Thus, this value is much lower than the predictions of JPi/P, and flPn/P >. 

Schlieren image of a very clear tulip or two-lip flame formed in a moderate length, square cross section tube (300mm long x 38.1mm on the side) from a stoichiometric methane/air mixture, with a line igniter across the left wall. 

Variation of the normalized remaining percentage of CH4 fuel (c/Cj) after a run, measured by the gas chromatography, plotted over a very wide range of normalized turbulent intensities (u /Sl 10 100), where the subscript "i" refers to the initial condition. Both very rich (0 = 1.45 Cj = 13.2%) and very lean = 0.6 q = 5.92%) pure methane/air mixtures are investigated, showing critical values of Ka for the transition across which global quench occurs. 

For the adiabatic condition in which RHL is suppressed, the flame response exhibits the conventional upper and middle branches of the characteristic ignition-extinction curve, with the upper branch representing the physically realistic solutions. It can be noted that the effective Le of this lean methane/air mixture is sub-unity. It can be seen from Figure 6.3.1 that, with increasing stretch rate, first increases owing to the nonequidiffusion effects (S > 0), and then decreases as the extinction state is approached, owing to incomplete reaction. Furthermore, is also expected to degenerate to the adiabatic flame temperature, when v = 0. 

Figure 6.3.6 further compares and of lean to stoichiometric methane/air mixtures for all five cases— plug flow, potential flow, asymmetric plug flow, radiative plug-flow, and radiative asymmetric plug-flow. The... 

Variations of extinction stretch rate and extinction temperature of methane/air mixtures with equivalence ratio for the five different cases as in Figure 6.3.4. 

Subsequently, the problem was investigated by Karpov and Severin [6]. They used closed vessels with a diameter of 10cm and 10, 5, and 2.5cm width, initially at atmospheric pressure. The vessels were filled with different lean hydrogen and methane/air mixtures and rotational speeds in the range of 130-4201/s were employed. They also included data from the study of Babkin et al. [3] in their analysis. Unfortunately, they did not observe the flame itself and measured only the pressure rise in the vessel, which was compared with pressure development in the vessel without rotahon, to draw a conclusion with respect to flame speeds and quenching. 

Flame speeds as a function of time for an 8.45% methane/air mixture and different rotation rates. Vessel vented on the axis of rotation. 

Axial (uj left-half and radial ( ,) right-half velocify components at 15.0ms, CO = 314s for 0 = 0.879, methane/air mixture, dosed vessel [17]. 

E.S. Oran, J.P Boris, T. Young, M. Flanigan, T. Burks, and M. Picone, Numerical simulations of detonations in hydrogen-air and methane-air mixtures. Proceedings 18th Symposium (Int.) on Combustion, The Combustion Institute, Pittsburgh, PA, pp. 1641-1649,1981. 

Fig. 5.3.8 Photograph of the detection region of the NMR probe with radiofrequency coil. A methane—air mixture was ignited above the zeolite pellets. The mixture also contained xenon for NMR detection. Hp-129Xe NMR spectra with 30% xenon (from high-density xenon optical pumping) in 70% methane is depicted. (1) The spectrum in the absence of combustion and (2) the spectrum during combustion. Adapted from Ref. [2],...

The mechanisms by which explosives can cause ignition of methane/air mixtures are the following ... 

By ignition of the methane/air mixture on mixing with the hot gaseous products of the explosion. 

By hot reacting particles of explosive escaping into the methane/air mixture. 

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