The transition from deflagration to detonation

Indirect initiation involves a transition from deflagration to detonation. This process has been studied extensively for waves propagating in tubes, and many reviews are available [14]-[28]. The detonation develops 

The importance of the buildup of pressure waves in promoting transition to detonation indicates that confining a combustible mixture by walls aids in the development of a detonation. Detonations are more difficult to initiate in unconfined combustibles [169]. If a transition from deflagration to detonation is to occur in the open, then unusually large flame accelerations are needed, and these are more difficult to achieve without confinement. Numerical integrations of the conservation equations demonstrate that 

In solid or liquid explosives, reactive molecules are continually interacting, and limitations on detonation structures associated with molecular mean free paths no longer apply. It becomes entirely possible for significant release of chemical energy to occur within the structure of the leading shock. This fact motivates new approaches to studies of detonation structure on the basis of molecular dynamics [189], [190]. Although the fundamental complexities that are encountered make the problem difficult, further pursuit of these lines of investigation seems desirable. 

Background information concerning detonations in solid and liquid explosives is available in a number of books [11], [209]-[215]. Additional information may be found not only in the proceedings of the combustion symposia and of the international colloquia on gasdynamics of explosions and reactive systems but also in the proceedings of the international symposia on detonation, published by the U.S. Office of Naval Research. These last symposia have been held periodically, approximately once every 5 years. 

Sprays of liquid fuels in gaseous oxidizers can also support detonations. Reasonably extensive theoretical [216]-[223] and experimental [219], [224]-[230] studies of detonations in sprays have been reported in the literature, and reviews are available [231]-[234]. Since the liquid fuels must vaporize and mix with the oxidizer before combustion can occur, it is 

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Macek (Ref 5) used Pentolite (and DINA) to study the transition from deflagration to detonation (DDT). He found that in Pentolite,... 

As broad as the coverage of this symposium appears, there is much propellant chemistry which has not been included. The experimental determination of thermodynamic properties such as heats of formation and equilibrium constants as well as the calculations of theoretical performance have been presented at other symposia. The applied chemistry related to modifying polymers, and hence mechanical and burning properties of solids, have other forums. The actual firing of solid motors and determination of thrust and efficiency have been omitted while the research into combustion instability and the transition from deflagration to detonation are only alluded to. 

Macek (Ref 5) used Pentolite (and DINA) to study the transition from deflagration to detonation (DDT). He found that in Pentolite, heavily confined and ignited by a hot wire, a low-velocity regime (1—2mm/microsec) precedes steady detonation for 30—SOmicrosec. Compression waves precede the burning front in this pre-detonation region and appear to coalesce into a shock wave... 

Brehm, N., Mayinger, F. (1989). A contribution to the phenomenon of the transition from deflagration to detonation. VDI-Forschungsheft No. 653/1989, pp. 1-36. (website http //www.thermo-a.mw.tu-muenchen.de/lehrstuhl/foschung / eder gerlach.html). 

Current state-of-the-art in the understanding of these phenomena, as well as progress made in achieving empirical and quantitative descriptions of these combustion processes, are reviewed. The specific topics discussed are i) the maximum attainable turbulent flame speed in an obstacle array, ii) computer simulation of turbulent flame accelerations, iii) correlation between the detonation cell size and the dynamic parameters of fuel-air detonations, and iv) the transition from deflagration to detonation. Future directions in the investigation of these problems are also discussed. 

Basically it may be stated that turbulence enhancing circumstances such as obstacles, building stmctures, and confinements as well as high ignition energies favour the transition from deflagration to detonation. The same is true for high initial pressures and temperatures. 

For heterogeneous, commercial-type explosives the velocity of detonation increases and then decreases as the compaction density of the explosive composition increases. The compaction of heterogeneous explosives makes the transition from deflagration to detonation very difficult. 

We can answer the first question fairly well (at least with regard to hydrogeniair mixtures) and also the third question. The second question concerns the transition from deflagration to detonation and is still not completely understood after more than 50 years of investigation. We can say that, in most postulated reactor accident scenarios, deflagrations are much more likely than detonations. 

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