Obstacles, flame

Fuel-pair mixtures, in soap bubbles ranging from 4 to 40 cm diameter and with no internal obstacles, produced flame speeds very close to laminar flame speeds. Cylindrical bubbles of various aspect ratios produced even lower flame speeds. For example, maximum flame speeds for ethylene of 4.2 m/s and 5.5 m/s were found in cylindrical and hemispherical bubbles, respectively (Table 4.1a). This phenomenon is attributed to reduced driving forces due to the top relief of combustion products. 

Obstacles introduced in unconfined cylindrical bubbles resulted only in local flame acceleration. Pressures measured at some distance from the cylindrical bubble were, in general, two to three times the pressure measured in the absence of obstacles. 

The introduction of obstacles within unconfined vapor clouds produced flame acceleration. On a small scale, an array of vertical obstacles mounted on a single plate (60 X 60 cm) resulted in flame accelerations within the array (Van Wingerden and Zeeuwen 1983). Maximum flame speeds of 52 m/s for acetylene-air were found, versus 21 m/s in the absence of obstacles, over 30 cm of flame propagation. 

Harris and Wickens (1989) report large-scale tests in an open-sided 45-m-long apparatus incorporating grids and obstructions. Maximum flame speeds were approximately ten times those found in the absence of obstacles. 

The influence of hemispherical wire mesh screens (obstacles) on the behavior of hemispherical flames was studied by Dorge et al. (1976) on a laboratory scale. The dimensions of the wire mesh screens were varied. Maximum flame speeds for methane, propane, and acetylene are given in Table 4.1b. 

Combustion of a natural gas-air cloud in a highly congested obstacle array leads to flame speeds in excess of 100 m/s (pressure in excess of 200 mbar). 

Interaction of a jet flame and an obstacle array can result in an increase of flame speed and production of pressures in excess of 700 mbar. 

The results in Tables 4. la and 4. lb demonstrate that in the absence of obstacles, the highest flame speed observed was 84 m/s, and it was accompanied by an overpressure of 60 mbar for hydrogen-air in a 10-m radius balloon (Schneider and Pfortner 1981). For all other fuels, flame speeds were below 40 m/s and corresponding overpressures were below 35 mbar. Hence, weak ignition of an unconfined... 

The introduction of obstacles results in some flame acceleration, especially for the more reactive fuels. This effect is especially strong if the flame surface is distorted by the presence of obstacles over its entire surface, such as were present in the experiments of Dorge et al. (1976) and Harrison and Eyre (1986, 1987). The more reactive the fuel, the more effect obstacles seem to have on flame acceleration (Harris and Wickens 1989). 

The presence of horizontal or vertical obstacles (Figure 4.4) in the propane cloud hardly influenced flame propagation. On the other hand, flame propagation was influenced significantly when obstacles were covered by steel plates. Within the partially confined obstacle array, flame speeds up to 66 m/s were observed (Table 4.2) they were clearly higher than flame speeds in unconfined areas. However, at points where flames left areas of partial confinement, flame speeds dropped to their original, low, unconflned levels. 

Flame acceleration was minimal after ignition of dispersed fuel-air clouds under unconhned conditions in the absence of obstacles. 

As previously demonstrated, the introduction of obstacles and partial confinement results in some flame acceleration (Zeeuwen et al. 1983). 

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. 

Urtiew (1981) performed experiments in an open test chamber 30 cm high x 15 cm wide x 90 cm long. Obstacles of several heights were introduced into the test chamber. Possibly because there was top venting, maximum flame speeds were only on the order of 20 m/s for propane-air mixtures. 

Taylor (1987) reports some experiments performed in a horizontal duct (2 m long, 0.05 X 0.05 m cross section). Obstacles were placed in the channel. The top of the duct could be covered by perforated plates with a minimum of 6% open area. Terminal flame speeds of 80 m/s were reported for propane in a channel with a blockage ratio of 50% and a 12% open roof. 

Experiments reported by Harris and Wickens (1989) deserve special attention. They modified the experimental apparatus described in Section 4.1.1—a 45 m long, open-sided apparatus. The first 9 m of the apparatus was modified by the fitting of solid walls to its top and sides in order to produce a confined region. Thus, it was possible to investigate whether a flame already propagating at high speed could be further accelerated in unconfined parts of the apparatus, where obstacles of pipework were installed. The initial flame speed in the unconfined parts of the apparatus could be modified by introduction of obstacles in the confined part. 

Experiments performed with natural gas yielded somewhat different results. Flames emerged from the confined portion of the apparatus at speeds below 500 m/s, then decelerated rapidly in the unconfined portion with obstacles. On the other hand, flames emerging from the confined portion at speeds above 6(K) m/s continued to propagate at speeds of 500-600 m/s in the obstructed, unconfined portion of the... 

The nature of the restrictive boundary conditions for detonation is closely related to the cellular stmcture of a detonation wave (Section 3.2.2). It was systematically investigated in a series of flame propagation experiments in obstacle-filled tubes by Lee et al. (1984). The most important results are summarized below ... 

Literature provides the basis for a user to objectively determine the maximum flame speed that will be achieved with a particular combination of confinement, obstacles, fuel reactivity, and ignition source. ... 

Hjertager, B. H., K. Fuhre, and M. Bjorkhaug. 1988a. Concentration effects on flame acceleration by obstacles in large-scale methane-air and propane-air explosions. Comb. Sci. Tech., 62 239-256. 

Lee, J. H. S., R. Knystautas, and C. K. Chan. 1984. Turbulent flame propagation in obstacle-filled tubes. 20th Symp. (Int.) on Combustion, pp. 1663-1672. The Combustion Institute, Pittsburgh, PA. 

Moen, I. O., M. Donato, R. Knystautas, and J. H. Lee. 1980a. Flame acceleration due to turbulence produced by obstacles. Combust. Flame. 39 21-32. 

Van Wingerden, C. J. M. 1984. Experimental study of the influence of obstacles and partial confinement on flame propagation. Commission of the European Communities for Nuclear Science and Technology, report no. EUR 9541 EN/n. 

Tests were performed in open terrain. Obstacles and partial confinement were also introduced (see also Section 4.1). Under unconfined conditions, flame-front... 

Marx, K. D., J. H. S. Lee, and J. C. Cummings. 1985. Modeling of flame acceleration in tubes with obstacles. Proc. of 11th IMACS World Congress on Simulation and Scientific Computation. 5 13-16. 

Starke, R. and Roth, P, An experimental investigation of flame behavior during explosions in cylindrical enclosures with obstacles. Combustion and Flame, 75,111-121,1989. 

The DDT can be observed in a variety of situations, including flame propagation in smoofh fubes or channels, flame acceleration caused by repealed obstacles, and jet ignition. The processes leading to detonation can be classified into two categories ... 

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