Combustion to-detonation transition

The first investigations of the combustion phenomenon in the middle of the last century were accomplished upon gaseous mixtures. Significant advances were achieved in the field of combustion of gaseous and solid explosives in the middle of the 20th century, vs iile only recently the investigation of the combustion and combustion-to-detonation transition process of solid explosives has been developed to a greater extent. 

The combustion-to-detonation transition process is of special importance. Namely, it has been found that the combustion of primary explosives is unstable and easily transformed into the detonation. In the case of propellants, combustion is very stable and the transition to detonation may be reached only under very extreme conditions. In practice these conditions are rarely realised. 

The possibility of the combustion-to-detonation transition in the case of high explosives is somewhere between the former two cases. 

Some parameters, such as combustion temperature and the combustion-to-detonation transition process, may be determined using the same principles that are described for methods applied for the determination of the detonation parameters (see Chapter 4). 

For viewing the initiation path of high explosive, combustion-to-detonation transition, determination of the failure diameter of the explosive charge, transmission of the detonation process, and for studying the establishment of stable detonation, a special types of probes combined with a fast oscilloscope are used. 

The experiments from [32] and also from [33] reveal that practical recommendations depend on the geometric dimensions of the facility. In [33] the duct had a 0.195 x 0.144 m cross-section and a 2.44 m length was used instead of 2.44 x 1.8 m cross section duct. The experiments in the small-scale facility showed that mixtures were shown to be safe from the combustion to detonation transition with up to 20% H2 content in a smooth wall duct and up to 17% H2 in a duct with obstacles. It is easy to see the difference in data when compared with the large-scale experiments. Quasi-detonation combustion regimes depend on facility design features and the initial pressure it distinguishes them from normal flame propagation and detonation. 

Although the status of many 3D codes makes it possible to carry out detailed scenario calculations, further work is needed. This is particularly so for 1) development and verification of the porosity/distributed resistance model for explosion propagation in high density obstacle fields 2) improvement of the turbulent combustion model, and 3) development of a model for deflagration to detonation transition. More data are needed to enable verification of the model in high density geometries. This is particularly needed for onshore process plant geometries. 

Model of Transition from Combustion to Detonation in Porous Explosives... 

VI. Preparation VII. Detonation Characteristics VIII. Thermal Decomposition IX. Combustion DDT (deflagration-to-detonation transition) X. References. The major emphasis will be placed on Sections VII, VIII and IX... 

Fig 15 Model of transition from combustion to detonation in porous explosives (Ref 68)... 

Soloukhin, R.I., Perekhod goreniya v detonatsiru v gazakh (Transition from combustion to detonation in gases), P.M.T.F., 4,128,1961. 

Frank-Kamenetskii) 417-28 (Transition of combustion to detonation in gases)... 

Combustion to Explosion, Transition. See under Detonation (or Explosion), Development (or Transition) from Combustion (or Burning) or from Deflagration... 

In conclusion of this rept, Gibson, et al state that deflagration to detonation transition seems to be related to the physical characteristics of an explosive or propellant bed Gibson et al (Refs 12a 12b) also conducted at the BurMines a research which was broader than transition from combustion and deflagration to detonation. The prime objective of the reseat ch was to evaluate mechanisms involved in the initiation and growth of detonation in systems that ate capable of rapid exothermal decomposition. 

Refs 1) A.F. Belyaev, ZhPriklKhim 23, 432-39(1950) Engl transin entitled The Ignition of Explosives and the Transition from Combustion to Detonation , Consultants... 

Continuous oscillographic method for measuring the velocity and conductivity of stable and transient shocks in solid cast explosives) 17) Zel dovich Kompaneets(I960), 191 205 (Combustion and deflagration to detonation transition)... 

Andreev Belyaev (I960), 14l-44(Tran-sition of combustion to detonation in gases) 193 (Transition of combustion to detonation in condensed explosives) 19) N. Griffiths G.M. Groocock, JCS I960, 4154-62 CA 55, 6865(1961) (Transition from combustion to detonation of solid HE s) 20) R.W. 

CA 58, 3263(1963) (Thermal decomposition and combustion processes of explosives) 26a) Vlad Sima, TechChem(Prague) 12(2), 66-9(1962) CA 61, 526(1964) (Determination of the degree of danger in explosives plants) 26b) A. Macek, ChemRevs 62, 50-2(1962) (Deflagration to detonation transition)... 

Shekhter (1959), 417-28 (Transition of combustion to detonation in gases) 5) G.D. Salamandra et al, Formation of Detonation Wave During Combustion of Gas in Combustion Tube , 7thSympCombstn(1959), pp 851-55 6) Andreev Belyaev (i960),... 

Cook attributes the "flashacross during the DDT (Deflagration to Detonation Transition), in which the shock front in an explosive is overtaken by the combustion... 

Detonation, Predetonation Phase. This is an intermediate phase in the DDT (Deflagration to Detonation Transition) between deflagration (or combustion) and detonation Oppenheim (Ref 3, p 475) describes how during this phase a combustion front is accelerated by a shock process until the shock front is overtaken and a (CJ) Chapman-Jouguet detonation sets in. 

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-... 

Overcompressed detonation waves were also observed by Troshin (Ref 55, p 789). He observed it not only in transfer of deton from the broad to narrow tube, but also immediately after transition of combustion to detonation. 

Explosion of combustible substances and transition from combustion to detonation 4 D245—D252 6 E369... 

Until very recently the origination of detonation in the thermal ignition of a mixture, i.e., the transition from slow combustion to detonation, remained completely unexplained. 

The distance at which the transition from combustion to detonation takes place reaches several dozen tube diameters. This distance coincides in order of magnitude with the length over which, according to hydrodynamic data, the velocity profile is established.16... 

The extraordinarily striking fact that transition from combustion to detonation is eased in rough tubes was discovered by Shchelkin, who was led in this by certain ideas about the role of gas turbulization. 

The pressure and temperature change in a compression wave corresponding to a change in the combustion rate occurs extraordinarily rapidly. If this change were the cause of the flame acceleration, the transition from combustion to detonation would occur at a distance which exceeds the width of the reaction zone in the flame by only a few times, i.e., at a distance of not more than a few millimeters. 

The reader interested in new physical concepts related to the combustion of powders, the theory of ignition and the limit of combustion, may omit without any loss 3, which is devoted to more formal problems, and 4, in which the problem of transition of combustion to detonation could not be developed to the point of comparison with experiment because of the absence of much necessary data and information on the role of other factors. 

Thus we may foresee that when the heating wave velocity exceeds the combustion velocity, this leads to transition from combustion to detonation. 

The condition of transition from combustion to detonation may be calculated in advance on the basis of experimental data with minimal knowledge of the reaction mechanism. In doing so we use (1) the dependence of the combustion velocity on the pressure, measured in experiments as u = u1pm ... 

Belyaev considered this question in unpublished calculations and showed that at atmospheric pressure and for low-boiling EM the number of molecules which evaporate in the surface layer is larger than the number of molecules which react in a layer 1 cm thick, i.e., the ratio of the evaporation probability to the probability of reaction is larger than the ratio of the number of molecules in a 1 cm thick layer to the number of molecules in the surface layer. However, the situation changes as the point of transition from combustion to detonation at high pressure is approached. 

Taking account of an exothermic reaction in the liquid phase leads to the impossibility of steady combustion at a high boiling temperature. When the limit is attained, transition from combustion to detonation occurs. Formulas are given for calculating the conditions of transition. 

IV. Characteristics of Steady Detonation V, Initiation Behavior VI. Deflagration-to-Detonation Transition (DDT) and Combustion VII. Decomposition and Kinetics VIII. Analytical Methods IX. Waste Disposal X. Toxicity XI. RDX Detonators XII. Military Specifications and XIII. References. Major emphasis will be placed on Sections IV, V, and VI... 

A commonly accepted hypothesis utilized in explaining this tendency toward detonation in solid proplnts is the grain deflagration or surface combustion mechanism. Processes which enhance internal surface formation such as internal gas evolution and creep induced strains can be expected also to increase the tendency toward deflagration to detonation transition (DDT). The DDT phenomenon in solid proplnts was examined in some detail in Encycl Vol 4. It is believed that the build-up of internal pressures... 

---