Obstacles flame velocity

Flame velocity versus fuel concentration for H2/air mixtures in the 10 m long tubes of 5, 15, and 30 cm internal diameter with obstacles (orifice plates) BR = 1 - d /D - blockage ratio, where d is the orifice diameter and D is the tube diameter. (From Lee, J.H., Advances in Chemical Reaction Dynamics, Rentzepis, P.M. and CapeUos, C., Eds., 246,1986.)... 

Compared to Fig. 7 the measured flame velocities are essentially higher, the maximum values are here between 75 m/s and 135 m/s for the same fuel-air-mixtures. Hence, the acceleration effect of the obstacle structure depends strongly on the shape parameters of the grid, i.e., the induced turbulence intensities and macro length scales obviously play an important role in the propagation process. 

A tube with obstacles can model some industrial explosions. A simple analysis indicates that there are two contributing factors involved in flame acceleration along a tube with obstacles. They are the character of the motion of the gas in front of the flame and the turbulence induced by the interaction of the motion of the gas in front of the flame with the boundary condition in the tube with obstacles. To study in detail the mechanism of flame acceleration by means of obstacles, explosion tests and measurements of the parameters of mean flow velocity and root mean squared (RMS) turbulent velocity were performed. The characteristics of methane-air and comstarch-air flame acceleration were investigated in a closed tube 0.19 m in diameter and 1,86 -m long filled with obstacles. The nonuniformity in the mean flow velocity and the RMS turbulent velocity across the tube with and without obstacles were measured in a substitutional tube in which the air free flow velocity ranged from 9 m/s to 177 m/s. Experimental results demonstrated that in the environment with obstacles flame acceleration caused by the nonuniformity of flow velocity is more efficient than that caused by the RMS turbulent velocity. 

For the case of quasi-stationary combustion wave propagation velocity in a tube with obstacles, the velocity that is obtained due to the flame acceleration, depends on the mixture composition, the tube dimensions and the obstacle geometry. Figure 8.3 [16] presents such a dependence for HAM in 5-30 mm diameter tubes. The blockage ratio in the experiments was BR = 0.39-0.43. 

Fig. 8.3 Change in final flame velocity versus H2 vol.% in H2 + air mixture in tubes with obstacles i - CJ velocity ...

The high-speed flames propagate in a tube with repeat-able obstacles with the steady-state velocity, which is maintained for the rest of their passage over the obstacles. In some cases, the steady-state flame propagation velocity of the combustion products may approach the... 

The types of obstacles used in stabilization of flames in high-speed flows could be rods, vee gutters, toroids, disks, strips, etc. But in choosing the bluff-body stabilizer, the designer must consider not only the maximum blowoff velocity... 

A second rapid increase in the flame speed occurs at H2 25% for both tubes. This corresponds to a transition to the detonation regime. The detonation velocity in the obstacle field is typically about 1500 m/sec and is practically independent of the H2 concentration up to H2 - 45%. For higher H2 concentrations, the detonation velocity abruptly drops back to the values for deflagration speeds of the order of 800 m/sec. The severe pressure Cor momentum) losses due to the presence of the obstacles accounts for the sub-Chapman-Jouguet detonation velocities observed. The normal velocities are about 2000 m/sec as observed in smooth tubes for H2 concentrations in the range (i.e., 25% H2 45%). 

When a dependence of the local burning velocity on the turbulence field is induced in the computations, a much more rapid burning will occur in the shear layers. The computer generated movie shown in Figure 7 corresponds very closely to the actual high speed schlieren photographs of flame acceleration through a baffle obstacle (25). 

The aims of the present paper are, first, to study the characteristics of cornstarch-air flame acceleration in environments with obstacles by means of a comparative method and, second, to measure the nonuniformity in the mean flow velocity and the rms turbulent velocity across the tube under the conditions with and without obstacles along the tube when the air flow velocities are in the range of 9 - 177 m/s. Finally, an examination of the role played by the mixture motion and turbulence in flame acceleration is carried out by means of a simple analysis. It demonstrates that in the obstacle environment the flame acceleration caused by the nonuniformity of mean flow velocity is more efficient than... 

Flame acceleration and the rate of pressure rise are significantly affected by obstacles. The maximum rate of pressure rise under the environments with and without obstacles is shown in Fig. 6(a) for methane-air mixtures and in Fig. 6(b) for comstarch-air mixtures. The mean propagation velocities in the tube with and without obstacles is... 

The experimental measurements of the nonuniform flow velocity and the RMS turbulent velocity profiles in the tube environment with and without obstacles demonstrate that the values of two factors [(Um - U)/U and cu /U] and their summary (K) under the tube enviroment with obstacles are, at least, one order larger than those under the tube environment without obstacles (see Table 2). This indicates that obstacle environments create conditions more favorable for the flame acceleration. 

Point-symmetry, also referred to as spherical or unconfined geometry, has the lowest degree of flame confinement. The flame is free to expand spherically from a point ignition source. The overall flame surface increases with the square of the distance from the point ignition source. The flame-induced flow field can decay freely in three directions. Therefore, flow velocities are low, and the flow field disturbances by obstacles are small. 

When the expansion level is large, the flame accelerates to high velocities approximately equal to the sound speed in the combustion products before DDT conditions are reached. This means that the flame propagates through obstacles, in obstructed channels or in a chain of connected volumes and gas flows are generated which are required to increase the flame surface area. 

Let us focus on flame acceleration investigations in tubes with obstacles [16]. Some variations of this process (flame acceleration after ignition having passed several obstacles, the flame either dies out or reaches a steady constant velocity) have been recorded. The self-quenching mode is observed in lean near-limit mixtures (hydrogen + acetylene mixtures are an exception), which is explained by the fast turbulent mixing of hot combustion products with cold reagents [20]. 

When the flame leaves the obstructed part of the tube and gets to the smooth part, various modes of flame propagation are possible (Fig. 8.4) [16]. In the case of the sound deflagration mode in a tube with obstacles, the flame propagation either sharply slows down to a comparatively slow constant velocity or the front reaccelerates to close to detonation velocity values. This is observed ahead of the... 

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