Overdriven detonation

Pipeline deflagrations and detonations can be initiated by varions ignition sonrces. The flame proceeds from a slow flame throngh a faster accelerating tnrbnlent flame to a point where a shock wave forms and a detonation transition occnrs, resnlting in an overdriven detonation (see Fignre 4-3). A stable (steady state) detonation follows after the peak overdriven detonation pressnre snbsides. 

Overdriven detonation is the condition that exists during a DDT before a state of stable detonation is reached. Transition occurs over the length of a few pipe diameters and propagation velocities np to 2000 m/s have been measnred for hydrocarbons in air. This is greater than the speed of sonnd as measnred at the flame front. Overdriven detonations are typically accompanied by side-on pressnre ratios (at the pipe wall) in the range of 50-100. A severe test for detonation flame arresters is to adjust the mn-np distance so that DDT occurs at the arrester, subjecting it to the overdriven detonation impulse. 

Overdriven detonations, not long-pipe stable detonations, provide a greater potential for mechanical damage to detonation flame arresters. 

The detonation flame arrester mnst be able to arrest ten deflagrations with and withont a pipe restriction downstream of the flame arrester and five nnrestricted stable and overdriven detonations. The UL standard states, after tests determine the maximnm nnsta-ble (overdriven) detonation, the arrester is to be snbjected to fonr additional nnstable detonations with the length of pipe that resnlted in the maximnm nnstable (overdriven) detonation. The arrester is also to be snbjected to five stable detonations. ... 

The British Standards Institnte standard specification BS 7244 (1990) applies to both deflagradon (end-of-line and in-line) and detonadon flame arresters. For end-of-line deflagradon flame arresters ten tests are reqnired, and for in-line deflagradon flame arresters fifteen tests are reqnired. For detonation flame arresters three tests at increasing lengths of pipe for both deflagration and detonation conditions and ten nnrestricted overdriven detonation tests are reqnired. Endnrance bnrning test procedures are presented, bnt tests are condncted only if specified. 

For describing structural loading functions needed for design analysis, the use of overdriven detonation data representing the net overpressure (run-up side less protected side overpressure) on the arrester element and supporting structure is preferable to data representing only the run-up side, side-on overpressure. However, the run-up side transient history of side-on overpressure for overdriven detonations should provide a conservative estimate for design purposes (see Chapter 6). 

There is a need to develop a standardized test approach to induce an overdriven detonation since the overdriven detonation has the potential of producing the most severe mechanical damage to a flame arrester. 

Overdriven Detonations In Nitromethane , LASL, Univ Calif, LA-5278-MS (1973) 60) L.J. 

Akulintsev et al, On the Possibility of Stimulated Emission of CO Molecula Behind Overdriven Detonation Waves in CS2+02 Mixtures , FizikaGoreniaiVziyva 12, No 5 (1976) 739—44... 

An overdriven detonation (6) after transition and retonation wave (9). 

Steady-state detonation (7) after decay of overdriven detonation wave. 

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. 

Temperature field behind the leading shock at different times obtained by the numerical simulations [21]. 1, transverse detonation 2, strong part of the leading shock (overdriven detonation) 3, weak part of the leading shock (inert) 4, induction zone 5, transverse shock 6, unreacted tail 7, primary unreacted pocket and 8, secondary unreacted pockets. (Courtesy of V. Gamezo.)... 

Thus, it appears that solutions in region I are possible, but only in the transient state, since external effects quickly break down this state. Some investigators have claimed to have measured strong detonations in the transient state. There also exist standing detonations that are strong. Overdriven detonations have been generated by pistons, and some investigators have observed oblique detonations that are overdriven. 

Equation of State of Detonation Products Behind Overdriven Detonation Waves in Composition B , 4thONRSympDeton(1965), pp 47-51... 

In experiments of Skidmore Hart, the overdriven detonation waves were generated by an "explosive driven plate impact technique , which was essentially as follows (Ref 15, p 48) ... 

A metal plate ("driver ) of mild steel or brass was propelled explosively against a similar plate ("target ) on which was resting a sample layer of explosive backed by a further layer of an inert solid. When the driver plated velocity was sufficiently high, this process generated a steady "overdriven detonation wave in the explosive unless (or until) it was overtaken by the rarefaction from the rear of the driver plate. The shock transit times thru each layer of the system were measured to determine tne transmitted shock or detonation velocities. 

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