Run-to-detonation distance

The experimental stucfy of the shock wave initiation process is primarily directed to the determination of the pressure of the shock wave front, w ich may cause initiation, and to the determination of the relation between the shock wave pressure and the distance (or time) that the shock wave passes through the explosive before being transformed into a full detonation wave. This distance is known as run-to-detonation distance. 

Consequently, the run-to-detonation distance 7 can be calculated according to the equation... 

On the basis of the test data, the relation between the shock wave pressure and run-to-detonation distance is finally obtained. The length of the run distances in difierent types of explosives at the same shock wave pressure is the measure of the explosive sensitivity to the shock wave initiation. 

Figure 4.28 The run to detonation distance as a function of shock pressure.

A model, called Forest Fire after its originator, Charles Forest has been developed for describing the decomposition rates as a function of the experimentally measured distance of run to detonation vs. shock pressure (the Pop plot named after its originator, A. Popolato ) and the reactive and nonreactive Hugoniots. The model can be used to describe the decomposition from shocks formed either by external drivers or by internal pressure gradients formed by the propagation of a burning front. 

The experimental geometries studied using PHERMEX were numerically modeled using the reactive hydrodynamic code 2DL, described in Appendix B. For explosives that had been previously shocked, Craig observed experimentally that the distance of run to detonation for several multiply shocked explosives was determined primarily by the distance after a second shock had overtaken the lower pressure shock wave (the preshock). 

The gap test is strongly two-dimensional as evidenced by the calculated distances of run to detonation in the test samples which were significantly longer than those from the Pop plots at induced pressures near the critical gap thickness. 

Two models were developed to describe the build-up of detonation process. The first model developed is called the impulse model. The build-up of detonation is determined by the distance of run to detonation which determines the steepness of the Taylor wave behind the detonation front and how much carbon can condense and increase the peak detonation pressure and available energy. Since the distance of run is ill-defined in two or three dimensions, Gittings chose to use pressure impulse instead. The pressure impulse is determined as the sum of the pressure of a cell less the lowest effective peak detonation pressure (HEBuildPres) times the time interval. A term PImpulse is saved for each explosive cell and summed over time for each cell. 

An in-line detonation flame arrester must be used whenever there is a possibility of a detonation occurring. This is always a strong possibility in vent manifold (vapor collection) systems, where long pipe runs provide sufficient run-up distances for a deflagration-to-detonation transition to occur. Figure 3-3 shows the installation of in-line arresters of the detonation type in a vent manifold system. 

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

Several studies have shown that the shock sensitivity of granular expls depends on expl particle size. The consensus is that the threshold shock pressure to initiate detonation in a given expl is less for large particles than for small particles. However, the converse is true when one considers run-up distances (or run-up times) to detonation. Thus at some pressure above the threshold for both large and small particles, run-up to detonation is smaller for small particle charges than for large particle charges... 

Algebraic expressions for run-up distances, Xj, and times to detonation, tj, for the shock initiation of high density PETN pressings, taken... 

There are some experimental data available on the effects of tube diameter, initial pressure, and temperature on the run-up distance to detonation for smooth... 

For the initial PA-DBX 1 sample, only 4 of 10 detonators met the specifications of dent depth greater than 0.01 . Other detonators would initiate but would not go high order (e.g. the RDX output charge did not appear to detonate). This could be attributed to a density problem or a run up distance issue. 

C. Run-up to Detonation We have already had occasion to employ time-to-detonation or run-up distance to detonation (also called initiation distance) in some of our discussions. Now we will briefly describe how... 

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