Transition of Detonation

Detonation, Transition of. The term transition of detonation has been used by Evans to describe the transfer of detonation from one expl to another, in particular when the expls differed widely in deton velocities. In experiments described,two cylindrical sticks were placed end to end in good contact. These experiments differ from sympathetic detonation where two expls are placed at a distance (gap) from each. other and not in contact as in transition 

When an expl of deton velocity ca 8000m/se c was detonated in contact with an expl of 5000 m/sec, the velocity of the first expl was carried over into the second expl for a distance of less than a few mm from the boundary. The same effect was shown when the transition was from the low component to the high. This result was, however, not obtd if, for example, the second component was loosely packed and therefore of low density. In this case the transition in velocity was gradual and took several centimeters to reach a characteristic steady velocity 

The distance at which transition takes place is called carry-over distance . Its method of determination by means of the rapid photographic camera developed in 1944 by RRL (Road Research.Lab) is described in Ref and the results of tests are shown in Figs 7 to 13 of paper. See previous item Ref W.M Evans, Some Characteristics of Detonation , PrRoySoc 204A, pp 15-17(1950) (Transition of detonation) 

Detonation (and Explosion), Transmission of. Same as Detonation (and Explosion), Propagation of 

Detonation and Two-Phase Flow is discussed at ARS Propellants, Combustion and Liquid Rocket Conference in 1961. Published by Academic Press, NY (1962) 

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In transition of detonation from a rigid-wall tube into a larger volume, the detonation will cease if the tube is of smaller diameter than the critical one for a weakly confined or unconfined expl. Whereas, when the diam is greater than the critical one, an outward-gping deton wave will arise in a certain time period after transition, and the explosive will detonate in the larger volume... 

Transition of Detonation From a Narrow Tube to a Wider Tube... 

The other detonability length scale is the detonation cell width, X (also called cell size) which is the transverse dimension of diamond shaped cells generated by the transverse wave stmctnre at a detonation front. It has a fish scale pattern (see Figure 4-4). Detonation cell widths are nsnally measured by the traces (soot) deposited on smoke foils inserted in test vessels or piping surfaces. The more reactive the gas-air mixture, the smaller is the cell size. The same is tme for chemical indnction length as a qualitative measure of detonability. The cell width, X, is a parameter that is of practical importance. The transition from dehagration to detonation, propagation, and transmission of a detonation, can to some extent be eval-... 

Sichel, M. 1992. Transition to Detonation-Role of Explosion within an Explosion. In Major Research Topics in Combustion (Flussaini, Y. A., Kumat, A., and Voight, R. G., eds.). Springer-Verlag, Berlin. 

Most investigators used tubes open only at the end opposite the point of ignition. For tubes with very large aspect ratios (length/diameter), the positive feedback mechanism resulted in a transition to detonation for many fuels, even when the tubes were unobstructed. Introduction of obstacles into tubes reduced considerably the distance required for transition to detonation. 

One experiment (Moen et al. 1985) revealed that jet ignition of a lean acetylene-air mixture (5.2% v/v) in a 4-m-long, 2-m-diameter bag can produce the transition to 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. 

The preceding section described the state of transition expected in a deflagration process when the mixture in front of the flame is sufficiently preconditioned by a combination of compression effects and local quenching by turbulent mixing. However, additional factors determine whether the onset of detonation can actually occur and whether the onset of detonation will be followed by a self-sustaining detonation wave. 

Sherman, M. P., S. R. Tiezsen, W. B. Bendick, W. Fisk, and M. Carcassi. 1985. The effect of transverse venting on flame acceleration and transition to detonation in a large channel. Paper presented at the 10th Int. Coll, on Dynamics of Explosions and Reactive Systems. Berkeley, California. 

Exploding bridgewire initiation of PETN was studied by Blackburn Reithel (Ref 9). They observed that transition to detonation is aided by fineness (actually increased specific surface) of the PETN particles... 

Comparison between tulips, (a) Image of an actual tulip flower that has been rotated and sized for comparison (b) the tulip shape noted by Salamandra et al. [7] in flames on their transition to detonation and (c) the inverted flame shape identified by Ellis and Wheeler [5] in closed tubes that is now being called a tulip flame. The image to the right is simply a negative of that to its left. 

Transition to detonation caused by instabilities near the flame front, the flame interactions with a shock wave, another flame or a wall, or the explosion of a previously quenched pocket of combustible gas... 

Figure 8.4.5 presents the streak, direct photograph illustrating the stages of transition to detonation after a weak ignition and flame acceleration phase. Four main regions may be identified ... 

The bottom part of Figure 8.4.5 illustrates the pressure histories associated with the transition to detonation events in an unobstructed channel ... 

Fast deflagration—the flame position is much closer to the precursor shock wave. Overdriven detonation—a transition to detonation that has just occurred and the detonation is significantly overdriven with the peak pressure, well in excess (2-3 times) of the value usually associated with a steady Chapman-Jouget (CJ) detonation. This peak pressure generated during the transition process is a particular point of concern in the industry. 

Transition to detonation in charmels without obstacles was recently successfully simulated numerically [11,12]. In these simulations, it was shown that shock compression of the unreacted mixture forms the hot spots resulting from shock-shock, shock-wall, and shock-vortex interactions. The hot spots contain temperature gradients that produce spontaneous reaction waves and detonations. 

From the practical point of view, the most important aspects of the accelerated flame phenomenon are with respect to the steady-state propagation of very highspeed flames, transition to detonation, and propagation of sub-CJ detonations (quasi-detonations). 

For very rough tubes, the flame acceleration is much more rapid as shown in the previous section. Transition to detonation is also clearly marked by a local explosion and abrupt change in the wave speed. The wall roughness controls the propagation of the wave by providing [5] ... 

Gelatine explosives, initiated by commercial detonators, will normally fire at the low velocity of detonation initially, although this may well build up quite quickly into the high velocity. For some applications a high velocity of detonation is essential. This can be ensured by the addition of barium sulphate, or other material with density exceeding 2-8, in a fine form. Such additives have the property of ensuring rapid transition to the high velocity of detonation. This is, for example, of particular importance when the explosive is to be fired under a hydrostatic head, as in submarine work. 

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