Flame propagation detonations

The pressure developed by decomposition of acetylene in a closed container depends not only on the initial pressure (or more precisely, density), but also on whether the flame propagates as a deflagration or a detonation, and on the length of the container. For acetylene at room temperature and pressure, the calculated explosion pressure ratio, / initial > deflagration and ca 20 for detonation (at the Chapman-Jouguet plane). At 800 kPa (7.93... 

Detonation arresters are typically used in conjunction with other measures to decrease the risk of flame propagation. For example, in vapor control systems, the vapor is often enriched, diluted, or inerted, with appropriate instrumentation and control (see Effluent Disposal Systems, 1993). In cases where ignition sources are present or pre-dic table (such as most vapor destruct systems), the detonation arrester is used as a last-resort method anticipating possible failure of vapor composition control. Where vent collec tion systems have several vapor/oxidant sources, stream compositions can be highly variable and... 

A deflagration or detonation flame arrester fails hy definition if any flame propagates from the unprotected to the protected side. Failures can result for a numher of reasons, some of which are listed helow ... 

Fourth, the blast effects produced by vapor cloud explosions can vary greatly and are determined by the speed of flame propagation. In most cases, the mode of flame propagation is deflagration. Under extraordinary conditions, a detonation might occur. 

Such hot spots react instantaneously as localized, constant volume sub-explosions (Urtiew and Oppenheim 1966 Lee and Moen 1980). If the mixture around such a sub-explosion is preconditioned sufficiently to ignite on shock compression, a detonation wave will engulf the entire process of flame propagation. 

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 ... 

The chronology of the most remarkable contributions to combustion in the early stages of its development is as follows. In 1815, Sir Humphry Davy developed the miner s safety lamp. In 1826, Michael Faraday gave a series of lectures and wrote The Chemical History of Candle. In 1855, Robert Bunsen developed his premixed gas burner and measured flame temperatures and flame speed. Francois-Ernest Mallard and Emile Le Chatelier studied flame propagation and proposed the first flame structure theory in 1883. At the same time, the first evidence of detonation was discovered in 1879-1881 by Marcellin Berthelot and Paul Vieille this was immediately confirmed in 1881 by Mallard and Le Chatelier. In 1899-1905, David Chapman and Emile Jouguet developed the theory of deflagration and detonation and calculated the speed of detonation. In 1900, Paul Vieille provided the physical explanation of detonation... 

In 1957, a flame propagating in a long tube under conditions resulting in a deflagration to detonation transition (DDT) was given the name "tulip" by Salamandra et al. [7]. This term was subsequently commonly applied in detonation studies to describe this typical shape [8,9]. Figure 5.3.2 shows a few... 

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 ... 

Quasi-detonation—flame propagates with the velocity between the sound speed in the combustion products and CJ value... 

F. Pintgen, C.A. Eckett, J.M. Austin, and J.E. Shepherd, Direct observations of reaction zone structure in propagating detonations. Combust. Flame, 133, 211-229, 2003. 

Turbulence is required for the flame front to accelerate to the speeds required for a VCE otherwise, a flash fire will result. This turbulence is typically formed by the interaction between the flame front and obstacles such as process structures or equipment. Turbulence also results from material released explosively or via pressure jets. The blast effects produced by VCEs can vary greatly and are strongly dependent on flame speed. In most cases, the mode of flame propagation is deflagration. Under extraordinary conditions, a detonation with more severe blast effects might occur. In the absence of turbulence, under laminar or near-laminar conditions, flame speeds are too low to produce significant blast overpressure. In such a case, the cloud will merely bum as a flash fire. 

In mixtures near the limit, the shock wave and the flame separate momentarily, and the gases behind the shock are then the seat of vibratory phenomena, not only transverse but also longitudinal (of the same frequency as in the burnt gases). It would appear from this that such phenomena, but at even higher frequency, exist in the gas layer separating the shock wave and the flame of detonations propagating under conditions far removed from the limits, and that they play an important role in the coupling of the shock and the flame... 

The ensuing flame propagation, being unstable, continually accelerates along the tube, and with sufficient tube length, produces a detonation. Such an exptl procedure affords few controls, and the flexibility of such a system is quite limited. However the so-called "shock tube possesses all the desirable qualities that are needed for a detailed study of detonative processes (Ref 8, pp 1 2)... 

There exists a common belief that the high linear velocity of detonation propagation, which is thousands of times higher than the normal velocity of flame propagation, indicates a rapid chemical reaction rate. 

If we compare detonation with other combustion phenomena we find more similarity to self-ignition than to flame propagation in a detonation wave the original mixture enters the reaction undiluted by products. 

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