Reaction Front in Detonation

M,A.Cook et al, JChemPhys 24, 191-201 (1956) (Rate of reaction of TNT in detonation by direct pressure measurements) 22)Dunkle s Syllabus (1957-1958) (See Vol 4 of Encycl, p XLIX) p 126 (Reaction front in detonation) 135-42 (Thermal decomposition of solids) 23)M.A.Cook, "The Science of High Explosives , Reinhold NY(1958), pp 123-42 (Reaction rate in detonation) 174-87 (Thermal decomposition of soli ds) 386-89 (Thermochemistry of detonation and expltr) 24)F.A.Baum, K.P.Sranyukovich B.I.Shekhter "Fizika Vzryva , Moscow (1959), pp 81-108 (Thermochemistry of explosives) 25)K.K.An-dreev A. F. Belyaev, " Teoria Vzryvcha-rykh Veshchestv Moscow(1960), p 49-56 (Thermal expln in gases) p 56—61 (Thermal explosion in solids) 26) Encycl of Expls PATR 2700, Vol 1 (I960), p A501 (Atomic expins, chain reactions in) 27)F.M.Turner,... 

Detonotion, Reaction Front in. It is generally agreed that a detonation is a combination of a shock front and a combustion front (Ref 1, p 126 Ref 2). Where combustion is the detonation reaction, the combustion front can also be called the reaction front. The two fronts do not always have the same velocity. At an interesting stage of the DDT (Deflagration to Detonation Transition), the shock front is still faster than the reaction front behind it (See under Detona-... 

Heat pulse) 91-122 (Deton wave shape and density properties) 123-4 (Reaction rates in deton) 123 (Nozzle theory) 124 (Curved-front theory) 125-28 (Geometrical model) ... 

Two different rates of detonation of nitroglycerine (explosion and deto ation) (Vol. II. pp. 54-55) became the subject of experiments and discussiui for example. Taylor [86], Kauscler [87] and Soviet workers [88,89). It has a been shown that the reaction front in homogeneous liquid explosives becoiv. distorted under conditions close to failure [9,91 ]. 

Deflagration to Detonation Transition A reaction front that starts out with velocities below the speed of sound and subsequently accelerates to velocities higher than the speed of sound in the unreacted material is said to have undergone a Deflagration to Detonation Transition. The possibility of transition is enhanced by confinement/turbulence generators in the path of the reaction front. 

Detonation A propagating chemical reaction of a substance in which the reaction front advances into the unreacted substance at equal to or greater than the sonic velocity in the unreacted material. 

This reaction can oscillate in a well-mixed system. In a quiescent system, diffusion-limited spatial patterns can develop, but these violate the assumption of perfect mixing that is made in this chapter. A well-known chemical oscillator that also develops complex spatial patterns is the Belousov-Zhabotinsky or BZ reaction. Flame fronts and detonations are other batch reactions that violate the assumption of perfect mixing. Their analysis requires treatment of mass or thermal diffusion or the propagation of shock waves. Such reactions are briefly touched upon in Chapter 11 but, by and large, are beyond the scope of this book. 

Detonation An explosion in which the reaction front moves at a speed greater than the speed of sound in the unreacted medium. 

The damage effects from an explosion depend highly on whether the explosion results from a detonation or a deflagration. The difference depends on whether the reaction front propagates above or below the speed of sound in the unreacted gases. For ideal gases the speed of sound or sonic velocity is a function of temperature only and has a value of 344 m/s (1129 ft/s) at 20°C. Fundamentally, the sonic velocity is the speed at which information is transmitted through a gas. 

Figure 6-13 shows the physical differences between a detonation and a deflagration for a combustion reaction that occurs in the gas phase in the open. For a detonation the reaction front moves at a speed greater than the speed of sound. A shock front is found a short distance in front of the reaction front. The reaction front provides the energy for the shock front and continues to drive it at sonic or greater speeds. 

The explosion of a dust or gas (either as a deflagration or a detonation) results in a reaction front moving outward from the ignition source preceded by a shock wave or pressure front. After the combustible material is consumed, the reaction front terminates, but the pressure wave continues its outward movement. A blast wave is composed of the pressure wave and subsequent wind. It is the blast wave that causes most of the damage. 

A physical explosion, for example, a boiler explosion, a pressure vessel failure, or a BLEVE (Boiling Liquid Expanding Vapor Explosion), is not necessarily caused by a chemical reaction. Chemical explosions are characterized as detonations, deflagrations, and thermal explosions. In the case of a detonation or deflagration (e.g., explosive burning), a reaction front is present that proceeds through the material. A detonation proceeds by a shock wave with a velocity exceeding the speed of sound in the unreacted material. A... 

In the steel tube test, the fragmentation pattern is a measure of whether or not a substance detonated, for example, a substance is capable of detonation if the tube is completely fragmented. The velocity of the reaction front is also a measure of detonation, that is, a detonation has occurred if this velocity is higher than the velocity of sound in the substance. When a witness plate is used, a substance is said to be capable of detonation if either a hole is punched through the plate or the tube is fragmented along its entire length. 

For a detonation, the reaction front moves faster than the speed of sound, pushing the pressure wave or shock front immediately ahead of it. For a deflagration, the reaction front moves at a speed less than the speed of sound, resulting in a pressure wave that moves at the speed of sound, moving away from the reaction front. A noticeable difference is found in the resulting pressure-time or pressure-distance plots. 

The reaction zone in a detonation wave is no different from that in other flames, in that it supplies the sustaining energy. A difference does exist in that the detonation front initiates chemical reaction by compression, by diffusion of both heat and species, and thus inherently maintains itself. A further, but not essential, difference worth noting is that the reaction takes place with extreme rapidity in highly compressed and preheated gases. 

DETONATION An event in which the speed of the reaction front exceeds the speed of sound in a material. In this event the velocity of... 

---