Acetylene, flame speed

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. 

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. 

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. 

Several methods have been adopted for the measurement of flame speeds. If the flames are sufficiently actinic to affect a photographic plate, permanent records may be obtained on revolving drums bearing the films. Mallard and Le Chatelier 2 employed this method m their researches on mixtures of carbon disulphide with nitric oxide or oxygen, the flames of which are well known to be highly actinic, whilst Mason and Wheeler 3 were able to apply the method with conspicuous success to mixtures of acetylene and air. The actual flame speed is obtained by comparison with the waves made simultaneously on the photographic drum by means of a tuning-fork of known frequency. 

The flame speed is the rate of propagation of a flame front through a flammable mixture, with respect to a fixed observer. Materials such as hydrogen and acetylene that have high flame speeds are more prone to detonation. 

The high burning speed and explosive potential of oxy-acetylene flames made it difficult to produce a usable premix oxy-acetylene burner. The total consumption burner, on the other hand, while capable of burning such a fuel combination, produced such a brilliant and turbulent flame that it was difficult to obtain analytical results with it. Various eflForts to modify the total consumption burner met with only moderate success. 

Several commercial atomic absorption instruments contain monochromators with similar dispersion but are seldom equipped with slits giving less than 0.1 nm bandpass. Even then the emission mode is more sensitive than absorption, at least for the alkali metals, using the air-acetylene flame. However, their scan speeds, when provided, are too fast for more general use in emission. 

A detonation occurs when the flame velocity reaches supersonic speeds above 600 m/s and generally in the 2000-2500 m/s range. Peak overpressures can be 20—100 times the initial pressure, with typical values of 20 bar. Detonation can be initiated either by use of a high explosive charge or from a deflagration wave that accelerates due to congestion and confinement. Certain chemicals are more prone to create detonations than normal hydrocarbons. These include ethylene, acetylene, and hydrogen. 

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