Methane flame velocities

On comparing the two flames, it is evident that the flow structure of the lean limit methane flame fundamentally differs from that of the limit propane one. In the flame coordinate system, the velocity field shows a stagnation zone in the central region of the methane flame bubble, just behind the flame front. In this region, the combustion products move upward with the flame and are not replaced by the new ones produced in the reaction zone. For methane, at the lean limit an accumulation of particle image velocimetry (PIV) seeding particles can be seen within the stagnation core, in... 

It is also interesting to examine the global gas dynamic structure of upward propagating flames. Figure 3.1.6 gives an example of the global velocity field for the lean limit methane flame in the flame coordinates. The velocity distributions for all near limit flames studied share certain features. The central part of the bubble-shaped flame is... 

PIV velocity measurements made it possible to evaluate the flame temperature field [23], following the method demonstrated in Ref. [25]. The calculated thermal structure of lean limit methane flame is shown in Figure 3.1.7. The differences between the structures of lean limit methane and propane flames are fundamental. The most striking phenomenon seen from Figure 3.1.7 is the low temperature in the stagnation zone (the calculated temperatures near the tube axis seem unrealistically low, probably due to very low gas velocities in the stagnation core). 

Thermal structure of lean limit methane flame. Isotherms calculated from the measured gas velocity distribution. 

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... 

Evolution of flow velocity field in flame coordinates during extinction of upward propagating lean limit methane flame. Frames selected from Figure 3.1.12. 

Flame velocity. Hydrogen has a faster flame velocity (1.85 m/s) than other fuels (gasoline vapor—0.42 m/s methane—0.38 m/s). 

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

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 premixed methanol flame [11, 12] does not show the Swan bands of C2, which are prominent in a methane flame [13]. The base of the flame shows strong emission from excited formaldehyde and further up the flame emission from OH and CH occurs. The burning velocity of a stoichiometric methanol—air flame [12] is about 45 cm. sec", and the global activation energy and global order are 43—47 kcal. mole" and unity, respectively [14(a)]. 

Fig. 4.3. Flame velocity sensitivities of stoichiometric, atmospheric methane flame. The sensitivities were calculated with program PREMIX [248] using the Leeds methane oxidation...

FIGURE 1.24 Critical boundary velocity gradient for flashback vs. oxidizer composition for a stoichiometric premixed methane flame through a cylindrical tube. (Adapted from Harris, M. E. et al., in Third Symposium on Combustion, Flame and Explosion Phenomena, Williams Wilkens, Baltimore, 1949, 80.)... 

Following the estimation of predicted output uncertainties, sensitivity studies can then be used to identify the kinetic and thermodynamic data that cause the highest uncertainty in the model simulation result. The contribution of the uncertainty of the parameters can be assessed using Sobol indices as discussed in Sect. 5.5.3. For example, as Fig. 5.22 shows, at stoichiometric equivalence ratio, in a premixed laminar methane-air flame, the uncertainties in the rate coefficients of reactions O2 -1- H = OH -1- O and H -1- CH3 = CH4 cause the highest uncertainty in the calculated laminar flame velocity. Knowing these rate coefficients with lower... 

Fig. 5.23 Local sensitivity coefficients of the laminar flame velocity of a stoichiometric methane-air flame. Grey stripes refer to the local sensitivity coefficients at the nominal parameter set During the Monte Carlo analysis, the local sensitivity coefficients were calculated for each parameter set, which allowed the calculation of the standard deviation of the sensitivity coefficients small bars interconnected with a horizontal line) and the attainable minimum and maximum sensitivity coefficients at any parameter set within the uncertainty limits of parameters (outer larger bars). Adapted with permission from Z or et al. (2005b). Copyright (2005) American Chemical Society...

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. 

It is seen that the obtained dependence is not a linear combination of known values. For 0 < Xh2 < 0.5 H2 added to CH4 has a weak effect on the flame velocity and the methane dominates the flame propagation. For hydrogen content 0.9 < Xh2 < 1 in the mixture, the CH4 additive noticeably slows down the hydrogen combustion. When the binary fuel is diluted with CH4 and in the range 0.5 < X 2 < 0.9 transient variations of hydrogen combustion with moderate difficulties for flame propagation, have been observed. 

Vapor-Phase Mechanisms. Phosphoms flame retardants can also exert vapor-phase flame-retardant action. Trimethyl phosphate [512-56-1] C H O P, retards the velocity of a methane—oxygen flame with about the same molar efficiency as antimony trichloride (30,31). Both physical and chemical vapor-phase mechanisms have been proposed for the flame-retardant action of certain phosphoms compounds. Physical (endothermic) modes of action have been shown to be of dominant importance in the flame-retardant action of a wide range of non-phosphoms-containing volatile compounds (32). 

Typical velocity gradient values for stoichiometric methane—air flames are at flashback about 400 and at blowoff 2000. Thus, if the mixture is... 

The properties of natural gas are dominated by those of methane, notably a low maximum flame speed of 0.33 m/s. This strongly influences burner design, which must ensure that the mixture velocity is sufficiently low to prevent blow-off. Light-back , on the contrary, is very unlikely with such a low flame speed. 

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. 

Flow velocity field determined by PIV. Lean limit flames propagating upward in a standard cylindrical tube in methane/air and propane/ air mixtures, (a) Methane/air—laboratory coordinates, (b) propane/air—laboratory coordinates, (c) methane/air—flame coordinates, and (d) propane/air—flame coordinates. 

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... 

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