Deflagration and Detonation

With proper initiation, chemical explosives (as opposed to mechanical or atomic explosives) undergo violent decomposition to produce heat, gas, and rapid expansion of matter. 

The practical effect wiU depend on the speed at which the decomposition takes place as well as on the amount of gas and heat released. We can distinguish two important cases, as follows  

Chemical reaction proceeds through the material at a rate less than or equal to the speed of sound in the unreacted material. This is known as a deflagration . 

Both deflagrations and detonations can produce what a lay observer might describe as an explosion . Nonetheless, a detonation is a special type of explosion with specific physical characteristics. It is initiated by the heat accompanying shock compression it liberates sufficient energy, before any expansion occurs, to sustain the shock wave. The shock wave propagates into the unreacted material at supersonic speed, typically 1500—9000 m/s. We discuss the practical differences between the effects of detonation and deflagration in Chapter 11 on post-blast issues. 

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In-line flame arresters for both detonations and deflagrations and end-of-line flame arresters can be tested and approved for atmo-... 

On DSC examination it exhibits a large decomposition exotherm at 200-250°C, and is a sensitive detonating and deflagrating explosive. 

Figure 6-13 Comparison of detonation and deflagration gas dynamics. The explosion is initiated to the far left.

The pressure fronts produced by detonations and deflagrations are markedly different. A detonation produces a shock front, with an abrupt pressure rise, a maximum pressure of greater than 10 atm, and total duration of typically less than 1 ms. The pressure front resulting from a deflagration is characteristically wide (many milliseconds in duration), flat (without an abrupt... 

The first aim of a thermal stability screening test (e.g., DSC/DTA) is to obtain data on the potential for exothermic decomposition and on the enthalpy of decomposition (AHd). These data, together with the initial theoretical hazard evaluation, are used in reviewing the energetic properties of the substance (Box 4) and the detonation and deflagration hazards of the substance (Boxes 7 and 8). The screening tests also provide data on the thermal stability of the substance or mixture, on the runaway potential, on the oxidation properties, and to a lesser extent, on the kinetics of the reaction (Box 10). 

Detonation and Deflagration The difference between a detonation and deflagration depends on how fast the pressure wave moves out from the blast origin. If the pressure wave moves at a speed less than the speed of sound in the ambient gas, then a deflagration results. If the pressure wave moves at a speed greater than the speed of sound in the ambient gas, then a detonation results. 

The difference in behavior between a detonation and deflagration results in a significant difference in the damage. For a detonation, the damage is usually localized. However, for a deflagration, the damage is more widespread. 

Computations of the thermochemical values of various combinations of oxidizers and fuels can be found in the JANAF tables.FI Practical computations are carried out by the use of computer programs such as the cited NASA program.PI Table 3.2 shows an example of a computation comparing the detonation and deflagration characteristics of the gaseous mixture 2H2 -1- O2. 

L.D. Pitts, "Electrical Probe Technique for Measurement of Detonation and Deflagration Velocities , 4thONRSympDeton(1965), 616-26. In these experiments the probe consisted of a length of resistance wire sandwiched between two strips of insulating material. After placing the probe adjacent to the wall of the metallic test cylinder, a constant current was forced thru the probe. Detonation, or deflagration front pressure... 

Detonation (and Deflagration) in Gases Determination of Ignition Points. H.F. Coward describes in Kirk Othmer 5(1950), 966-83 the following experimental procedures ... 

These tangents, known in the US Western Europe as Rayleigh Lines, are called in this work Rayleigh-Mikhel son Lines. The waves corresponding to these slopes are called Chap man-Jouguet detonations and deflagrations, respectively... 

Fig Hugoniot curve iff111 = of reaction products, with sections corresponding to strong, Chapman—Jouguet, and weak detonations and deflagrations... 

Detonation and deflagration) 110) J. Herschkowitz, "The Chapman-Jouguet Plane for a Granular Explosive , PATM 1474(1964) (Based on the deton vel of a granular mixt of K perchlorate and powdered A1 confined in a Lucite tube and an ideal deton velocity calcd by the Ruby computer, H. found that the C-J plane is ca 0.9cm behind the plane at which the expln reaction begins) 111) W.H. Rinken-bach, formerly of PicArsn, Private communication, Oct 1964) 112) F.J, Cheselske, "In-... 

Results are shown in Fig 5. The exptl deflagration data are for H2/02 deflagrations (Ref 3a). Fishburn suggests that the exptl large-scale deflagration may in fact be a partial detonation For the above conditions, Fishburn s computed static impulses for detonation and deflagration are shown in Fig 6. Note that impulses are essentially identical for R/Rc> 2... 

Electrical probe technique for measurement of detonation and deflagration 4 D341... 

MORRISON, ADAMSON, AND WEIR—DETONATIVE AND DEFLAGRATIVE COMBUSTION... 

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