Flame propagation, deflagration

According to the evaluation of explosion accidents in the past, only a small portion of the energy of a hydrogen cloud is expected to be liberated in an open air explosion it is estimated to be in the range of 0.1 - 10 %, mostly 1 % [68]. 

The lower flammability limit of a hydrogen-air mixture was observed to be a function of temperature expressed by the empirical Burgess-Wheeler law [131]. The respective value near the boiling point (20 K) is 7.7 voI% and as low as 2.3 vol% at a temperature of 723 K. Because of its strong buoyancy connected with a rapid mixing with air, a hydrogen gas 

Deflagration is the type of combustion with a subsonic burning velocity level. It occurs under non-adiabatic conditions and at densities lower than that of the unbumt mixture. Its propagation mechanism is conduction heating and free radical diffusion. Only fuel-air gas clouds at around stoichiometric mixture imply high flame front velocities and maximum expansion upon ignition, i.e., the ratio of the density of unbumt mixture to that of the reaction products, which is about 7 - 8 for hydrocarbons (in reality smaller by up to 50 % because of non-ideal micro-mixture), but 1 for hydrogen. 

Experience with explosion accidents has shown that whenever hydrogen was involved, a shock wave was created, however, a comparatively weak one because only a portion of the gas cloud actually did explode. One reason might be the pinch effect, a flame deceleration 

The thermal emission of a burning gas cloud can induce further damages in the neighborhood by igniting other objects. The thermal radiation of a hydrogen flame is low (emission coefficient e 0.1) compared with that of conventional gaseous fuels (e ss 1) leading to a much smaller thermal impact. 

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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... 

Provide automatic sprinkler system/inerting gas Provide deflagration vents Provide deflagration suppression system Monitor flammable atmosphere/fire Provide nitrogen blocks (nitrogen injection to stop flame propagation) or other explosion isolation measures... 

This book makes reference to flame arresters, deflagration flame arresters, and detonation flame arresters. Flame arresters is the generic term for both deflagration and detonation flame arresters. Deflagration flame arresters are nsed when a flame only propagates at snbsonic velocity, whereas detonation arresters are nsed when a flame can propagate at all velocities inclnding snpersonic velocities. 

Chatrathi et al. (2001) recently reported some experiments on flame propagation in indnstrial scale piping. They presented data on deflagration propagation in three sizes of pipes (6-inches, 10-inches, and 16-inches) and three fnels (propane, ethylene, and hydrogen). The effects of bends were evalnated, bnt other piping system components were not evaln-ated. The conclnsions from this work are as follows ... 

The flame propagation direction affects the type of flame arrester selected. An end-of-line or in-line deflagration flame arrester used for the protection of an individual tank may be of a unidirectional design because the flame will only propagate from the atmosphere towards the tank interior. A bidirectional flame arrester design, however, is needed for an in-line application in a vapor recovery (vent manifold) system because the vapors must be able to flow from the tank interior into the manifold, or from the manifold into the tank interior. Consequently, flame may propagate in either direction. 

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. 

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... 

Deflagration initiation. A relatively weak energy source, such as an electric spark, ignites the mixture and a laminar flame is first formed. The mechanism of laminar flame propagation is via molecular transport of energy and free radicals from the reaction zone to the unburnt mixture ahead of it. 

Choked flame (CJ deflagration)—high-speed flame propagating with the velocity close to sound speed in the combustion products (600-1200 m/s)... 

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. 

Under very light confinement in hemispherical geometry only deflagrations were observed when the fuels (see Table 3) were mixed with air and initiated by high-energy sparks. Flame propagation velocity was increased by turbulence... 

The so-called diffusion theories of flame propagation, as exemplified by the work of Tanford and Pease 38), emphasize the transport of mass, in that concentration of an active radical is assumed to be the rate-controlling property. Its use seems to be fairly limited in that only a few specific reactions have been successfully studied with this theory. What is more interesting, however, is that this theory forms the counterpart to the thermal theory. These two extreme views bracket the actual case, and their study allows a consideration of each of two of the basic flame mechanisms, unencumbered by the other. Actual deflagration depends on both the transport of heat and the transport of mass, and a successful theory should contain both phenomena. 

Nevertheless, the most typical general feature of a reaction is the existence of fronts of chemical transformation which are able to propagate, without being extinguished, in a hot mixture with a constant velocity at subsonic speed for a laminar flame (or deflagration front), at supersonic speed for a detonation wave (see below for a more detailed discussion of this paper). 

For decelerating flames, flames propagating downward, or burner-stabilized flames with the flow upward, the body-force effects are stabilizing. Because of other mechanisms of instability, to be discussed later, the ease with which stable laminar deflagrations are observed in the laboratory may be attributable largely to the stabilizing influence of buoyancy. Normal... 

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