Detonation confined explosion models

As described in Chapter 5, section 5.4, the cylinder test consists of detonating a cylinder of explosive confined by copper and measuring the velocity of the expanding copper wall until it fractures. The cylinder test is commonly used to evaluate explosive performance using the JWL fitting form. The numerical model required to interpret cylinder wall expansion experiments must include a realistic description of build-up of detonation, Forest Fire burn and resulting wave curvature. That first became possible with the development of the NOBEL code. All previous calibrations of the JWL equation of state from cylinder test expansion data used explosive models without the essential detonation build-up to and of detonation. 

The consequence of the second approach is that, if detonation of unconfined parts of a vapor cloud can be ruled out, the cloud s explosive potential is not primarily determined by the fuel-air mixture in itself, but instead by the nature of the fuel-release environment. The multienergy model is based on the concept that explosive combustion can develop only in an intensely turbulent mixture or in obstructed and/or partially confined areas of the cloud. Hence, a vapor cloud explosion is modeled as a number of subexplosions corresponding to the number of areas within the cloud which bum under intensely turbulent conditions. 

Cook (1958), 91 3 (Steady-state detonation head for solid unconfined and confined charges) 93-7 (Experimental detonation head in gases) 97-9 (Experimental detonation head in condensed explosives) 120-22 (Detonation head model proposed in 1943) and 128 (Detonation head in ideal detonation with maximum velocity transient)... 

The TNT equivalent model is often used as a simple method of estimating the mass of TNT per mass unit fuel gas whose detonation results in the same blast wave at the same distance. One kg of TNT translates into an energy of 4520 kJ. The equivalent for hydrogen is 2.22 kg TNT per Nm gas. The weakness of this model is to ignore the pressure-time characteristic differences between a gas cloud and a detonative TNT explosion. In the short range, the model overestimates the pressure. Furthermore the model does not take into account the influence of turbulence and confinement [113]. 

McGuire, R.R. and Tarver, C.M. (1981) Chemical Decomposition Models for the Thermal Explosion of Confined HMX, TATB, RDX and TNT Explosives Proceedings of the 7th Symposium (International) on Detonation, US Naval Academy, Annapolis, MD, 56-64. 

All of the methods (except the TNT equivalency) require an estimate of the vapor concentration— this can be difficult to determine in a congested process area. The TNT equivalency model is easy to use. In the TNT approach a mass of fuel and a corresponding explosion efficiency must be selected. A weakness is the substantial physical difference between TNT detonations and VCE deflagrations. The TNO and Baker-Strehlow methods arc based on interpretations of actual VCE incidents—these models require additional data on the plant geometry to determine the confinement volume. The TNO method requires an estimate of the blast strength while the Baker-Strehlow method requires an estimate of the flame speed. 

The relative shock sensitivities of explosive compositions are commonly assessed by means of gap tests. In these tests, the shock from a standard donor explosive is transmitted to the test explosive through an inert barrier (the so called gap ). The shock sensitivity of the test explosive is characterized by the gap thickness for which the probability of detonation is 50%. In reference 26, the Los Alamos standard gap test and the Naval Ordnance Laboratory (NOL) large scale gap test were modeled using the 2DE code with Forest Fire burn rates. The Los Alamos gap test uses Dural for the inert barrier while the NOL gap test uses Plexiglas. The test explosive is unconflned in the Los Alamos gap test. In the NOL gap test the test explosive is confined with steel. The model showed good agreement between the calculated and experimental gap test values for PBX-9404, PBX-9502, Pentolite, Composition B, and an HMX based propellant, VTQ-2. 

As described in Chapter 4, explosive failure or propagation as a function of diameter, pulse width of initiating shock, wave curvature, confinement and propagation of detonation along surfaces are all dominated by the heterogeneous shock initiation mechanism. The Pop Plot and the resulting Forest Fire decomposition of heterogeneous explosive rate have been used to model all these explosive phenomena. 

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