Turbulent Flame Velocity

The dynamic flame arrester has a cross section which is so small that the flow velocity is always greater than the turbulent flame velocity (combustion velocity) of the flammable mixture. An upstream propagation of the explosion is thus prevented as long as the flow is fast enough. 

Turbulent flame velocity 5t - is a frequently measured parameter for turbulent combustion of premixed gases. It reflects the complex interaction of the combustion chemistry and turbulence. The non-dimensional correlations binding this parameter to the other key characteristics have been derived based on asymptotic analysis. 

A small number of experimental techniques is applicable for measuring a flame velocity in lean hydrogen mixtures. In such mixtures, effects requiring special attention to the arrangement of measurements of both laminar and turbulent flame velocities have been observed. 

Lately, efforts have been made toward the development of techniques to improve laminar and turbulent flame velocity measurements. 

Unlike laminar combustion, turbulent flame velocities depend not only on fuel properties, but, to a greater extent, on the turbulence field characteristics. Therefore, quantification of the turbulent field is a necessary condition for turbulent flame velocity measurement. An informative method of turbulent combustion investigation has been proposed in [1]. A spherical container with four fans installed symmetrically along the container perimeter was used in the experiments. Conditions for w pulsating velocity controllability have been created in the central part of the container ... 

The turbulent flame velocities in the container with fans have been measured by two methods. In the first method, the front boundary displacement of the turbulent flame in the Schlieren photos was observed, similar to the case of the laminar flame the equivalent spherical front radius was found by a mathematical treatment (planimetry, for example). The up-dated Schlieren photography devices have allowed registration in two projections for calculating the flame source volume with better accuracy. 

Turbulent combustion rates obtained by the two aforementioned methods are somewhat different. Therefore, the values obtained by the second method are usually called turbulent flame velocities. A detailed analysis of both methods was recently performed it demonstrates the means of correlation of turbulent flame velocities measured with different methods [7]. 

Parametric investigations of turbulent combustion are based on the definition of a reduced flame radius 7 l (0 that corresponds to the slope of the pressure - t time curve. When the chemical reaction is localized in narrow flamelets (see. Borgi diagram. Fig. 1.4), the spherical volume of radius 7 l (0 is close to the balanced turbulent combustion products volume. For this case, the pressure ratios P = P t) obtained experimentally during the time of burning are used for calculating / l (0-The turbulent flame velocity 5t (often designated as Ut) can be denoted by... 

Figures 3.4 and 3.5 illustrate the measured turbulent flame velocities for H2 + air mixtures at atmospheric pressure and room temperature in the container with fans [8]. The range of stoichiometric and lean mixtures is shown in Fig. 3.4, and the rich mixture range - in Fig. 3.5.

Fig. 3.4 The measured turbulent flame velocities St in H2 + air mixtures at room temperature and atmospheric pressure versus the pulsating speed in the stoichiometric and lean mixtures...

A further conclusion can be drawn on the basis of the measured turbulent flame velocities presented in Figs. 3.4 and 3.5. For this, let us plot the S y/Su ratio relative to the mixture composition at a fixed pulsating speed value. Values 5x at m = 2 m/s have been taken from Figs. 3.4 and 3.5 and supplemented with at m = 5 m/s from [1]. The diagram plotted in Fig. 3.7 shows that in the lean mixtures, at a fixed turbulent intensity value, the turbulent flame velocity exceeds the laminar flame velocity, but this ratio decreases with the increasing H2% content. 

The same conclusion can be made based on the turbulent flame velocities measured by the other method (in a rectangular burner) [11] and illustrated in Fig. 3.8. Let us note the high Sx/S ratio in the lean mixture with less than 13.7% H2 at a turbulent intensity u = 10 m/s. The attempt to summarize the measured results and to verify the possibility of natural turbulence generation by the flame was made in [11]. 

It was revealed in [11] that when the pulsating speed exceeds the laminar flame velocity, the pulsating speed dependence of becomes linear. Therefore, the coordinates were chosen which allowed the use of this empirical dependence and to verify the possibility of a generic expression for the turbulent flame velocity that takes the Karlovitz correction into account ... 

The circle locations around the plotted line in Fig. 3.9 support the Karlovitz hypothesis of additional turbulence generated by the flame and allows the estimation of the turbulent flame velocity in lean mixtures (13.7% H2-27% H2) at atmospheric pressure. 

Fig. 3.10 The turbulent flame velocity versus the turbulence intensity in a H2 + air mixture with 10% H2 at 298 K temperature and three values of the initial pressure...

The results obtained are presented in Fig. 3.10. No pressure effect on the turbulent flame velocity St was observed. The experimental points obtained, at 5 times initial pressure, are chaotically distributed in the area between the two dashed lines. 

Carbon Dioxide Gas Effect on Turbulent Flame Velocity... 

Figures 3.11 and 3.12 present the turbulent flame velocities in the lean H2 + air mixtures diluted with CO2. Figure 3.11 shows the velocities in the mixture with a fuel excess coefficient (f) = 0.39, and Fig. 3.12 - in the mixture with 0 = 0.26 and 0 = 0.21. The experiments were performed at 0.1 MPa and 0.3 MPa pressures and room temperature.

Fig. 3.11 The turbulence intensity effect on the turbulent flame velocity in H2 + air + CO2 at room temperature 298 K 0 = 0.39. The open circles -0.1 MPa pressure, the solid circles -0.3 MPa pressure...

The turbulent flame velocities in the lean H2 + air mixtures diluted with water steam are graphed in Figs. 3.13 and 3.14. 

The general behavior of turbulent flame velocity growth with increasing turbulence intensity is reproduced the water steam additive reduces S - Flame quenching occurs at a turbulence intensity that is close to 4-5 m/s. As can be expected, the leaner the initial mixture, the less water steam is required to extinguish the flame. 

The CO additive effect on turbulent flame velocity has been checked in mixtures where some amount of H2 was replaced by CO. Figure 3.15 shows the effect of such a replacement in a mixture containing 17.4% (H2 + CO). The top curve relates to a mixture with more H2 than CO (CO/H2 = 1/2) and the lower curve relates to a mixture with more CO content than H2 (CO/H2 = 3/2). The maximum turbulent flame velocity values decrease from 6.2 to 2.7 m/s (2.4 times). The laminar flame velocities in the mixtures with CO/H2 = 1/2 and CO/H2 = 3/2 are equal to 47 and 29 cm/s respectively. The laminar flame velocity decreases by 1.6 times in a mixture with higher CO concentration it can be noted that when diluted, the decrease in the turbulent flame velocity is greater than the decrease of the laminar velocity. 

In mixtures with lower fuel concentration, the maximum turbulent flame velocity values decrease as is shown in Fig. 3.16. In a 10% (H2 + CO) binary fuel mixture with H2 concentration twice as high as CO, the St curvature is distinctly seen. For the aforementioned mixtures the initial pressure effect has not been observed. 

According to the turbulent flame velocity diagrams, in mixtures diluted with an incombustible component or in near limiting mixtures, flame extinction is expected at some value of the turbulence intensity. 

Laminar flame velocity in stoichiometric mixture Maximum velocity of laminar flame Turbulent flame velocity (combustion velocity)... 

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