Initiation of deflagration

On initiation of deflagrating explosives, local, finite hotspots are developed either through friction between the solid particulates, by the compression of voids or bubbles in the liquid component, or by plastic flow of the material. This in turn produces heat and volatile intermediates which then undergo highly exothermic reactions in the gaseous... 

J.C. Krok, Jet initiation of deflagration and detonation. Ph.D. thesis, Caltech Institnte, Pasadena, 1997, 190 p. 

Liquid Hazards. Pure liquid ethylene oxide will deflagrate given sufficient initiating energy either at or below the surface, and a propagating flame may be produced (266,267). This requites certain minimum temperatures and pressures sensitive to the mode of initiation and system geometry. Under fire exposure conditions, an ethylene oxide pipeline may undergo internal decomposition either by direct initiation of the Hquid, or by formation and subsequent decomposition of a vapor pocket (190). 

The problem of flame arrestment, either of deflagrations or detonations, depends on the properties of the gas mixture involved plus the initial temperature and pressure. Gas mixture combustion properties cannot be quantified for direc t use in flame arrester selection and only general charac teristics can be assigned. For this reason, flame arrester performance must be demonstrated by realistic testing. Such... 

This chapter is organized as follows. First, an overview of experimental research is presented. Experimental research has focused on identifying deflagration-enhancing mechanisms in vapor cloud explosions and on uncovering the conditions for a direct initiation of a vapor cloud detonation. 

A deflagration-detonation transition was first observed in 1985 in a large-scale experiment with an acetylene-air mixture (Moen et al. 1985). More recent investigations (McKay et al. 1988 and Moen et al. 1989) showing that initiation of detonation in a fuel-air mixture by a burning, turbulent, gas jet is possible, provided the jet is large enough. Early indications are that the diameter of the jet must exceed five times the critical tube diameter, that is approximately 65 times the cell size. 

Pressures of deflagration or detonation shock waves build upon the existing system pressure at the time of the initial blast. When a deflagration starts and then builds to a detonation, the resulting peak pressure can be quite high because the final pressure of the detonation builds on the peak pressure of the deflagration. 

Confinement—Deflagration rates of substances such as azo compounds, peroxides, and certain lead oxides may accelerate by pressure increase, especially when the governing decomposition reaction is gas-phase controlled [28]. Initiation of a deflagration at the bottom or at the center of a closed or partially closed vessel may lead to an increase of eh deflagration rate by a factor of more than 100 in comparison with top initiation. Autocatalytic decomposition by a volatile catalyst is enhanced by confinement. 

Deflagration tests run under ambient pressure are relatively rudimentary. They provide information concerning only the propagation rate of deflagration after forced initiation. Examples of these tests are the UN deflagration test [143], dedicated to classification of organic peroxides, and the UN trough test [145], dedicated to classification of fertilizers. 

The UN deflagration test consists of a Dewar vessel with a volume of about 400 cm3. The vessel is filled with preheated material (standard temperature is 50°C if the stability of the substance permits), and the substance is initiated at the top of the vessel with a flame. The propagation of deflagration is recorded by temperature sensors that are located in the substance at preset distances. From the time required for passing two temperature sensors and from the known distance between them, the deflagration velocity can be calculated. 

Control of a deflagration after initiation by a source such as a hot spot, a flame, or a spark, depends on the rate of deflagration, the confinement, and the accumulation of heat from the evolved energy. Very slow deflagrations can sometimes be controlled under nonconfined situations. Under confined conditions, pressure builds up with simultaneous energy accumulation, which increases the deflagration velocity, most likely to an unacceptable level in processing. 

Initiation Bringing an explosive to the state of deflagration or detonation. 

As in consideration of deflagration phenomena, other parameters are of import in detonation research. These parameters—detonation limits, initiation energy, critical tube diameter, quenching diameter, and thickness of the supporting reaction zone—require a knowledge of the wave structure and hence of chemical reaction rates. Lee [6] refers to these parameters as dynamic to distinguish them from the equilibrium static detonation states, which permit the calculation of the detonation velocity by C-J theory. 

Heat of Deflagration. If an explosive, serving as a propellant (such as BkPdr or a smokeless propellant) is initiated by an electric blasting cap, in a calorimetric bomb (similar to that used for determination of heat of combstn) under confinement, but without addition of oxygen, the substance usually behaves as if it were fired in a gun barrel. This is known as "deflagration ... 

Some unconfined high expls can also be ignited to deflagration especially if they are in a molten condition (such as TNT), or spread in a thin layer (such as MF or Diazodinitrophenol). In many cases deflagration develops into detonation [See also Detonation (and Explosion), Initiation of] Refs 1) F.P. Bowden, "The Initiation of Explosion and Its Growth to Detonation , PrRoySoc 204A, 20 ff (1950) la) A.F. Belyaev, ZhPraktKhim 23, 432 ff (1950). 

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