Confined explosion models

The logic of confined explosion modeling showing the stepwise procedure is provided in Figure 3.30. 

The physical models described in Chapter 2 generate a variety of incident outcomes that arc caused by release of hazardous material or energy. Dispersion models (Section 2.3) estimate concentrations and/or doses of dispersed vapor vapor cloud explosions (VCE) (Section 3.1), physical c q)losion models (Section 3.3), fireball models (Section 3.4), and confined explosion models (Section 3.5) estimate shock wave overpressures and fragment velocities. Pool fire models (Section 3.6), jet fire models (Section 3.7), BLEVE models (Section 3.4) and flash fire models (Section 3.2) predict radiant flux. These models rely on the general principle that severity of outcome is a function of distance from the source of release. 

In what follows mainly unconfined explosions are discussed (for confined explosions see Sect. 2.1.1.9). A number of models exist (vid. [48, 49]) ... 

Confined Explosions. Confined explosions occur when a flammable mixfure in a confined space is ignited. A typical example is the case of a flammable liquids tank. When these tanks are emptied, residu liquid may evaporate and form a flammable mixture in the tank. If ignited, a confined explosion would be produced. The modeling of confined explosion effects is analogous to the modeUng of BLEVEs. Here the explosion energy released is obtained from the enthalpy of combustion. 

TABLE 3.5. Confinement Considerations for the Baker-Strehlow Vapor Cloud Explosion Model (Baker, 1996)... 

Baker-StrehUnv Model An explosion model based on the effect of confinement on the flame speed. 

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. 

A logic model that graphically portrays the range of outcomes from the combinations of events and circumstances in an accident sequence. For example, a flammable vapor release may result in a fire, an explosion, or in no consequence depending on meteorological conditions, the degree of confinement, the presence of ignition sources, etc. These trees are often shown with the probability of each outcome at each branch of the pathway... 

A more deterministic estimate of a vapor cloud s blast-damage potential is possible only if the actual conditions within the cloud are considered. This is the starting point in the multienergy concept for vapor cloud explosion blast modeling (Van den Berg 1985). Harris and Wickens (1989) make use of this concept by suggesting that blast effects be modeled by applying a 20% TNT equivalency only to that portion of the vapor cloud which is partially confined and/or obstructed. 

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. 

If, on the other hand, the multienergy model is employed, the total quantity of fuel present in a cloud is of minor importance. Instead, the environment is investigated with respect to potential blast-generative capabilities. Fuel-air jets and partially confined and/or obstructed areas are identified as sources of strong blast. The explosive power of a vapor cloud is determined primarily by the energy of fuel present in these blast-generating areas. 

By modeling site-specific conditions Inventories of material having explosion or fire potential that can be released Plant volumes and degree of confinement or obstruction (if Multienergy or similar method is used) if vapor cloud explosion is a concern... 

Preliminaries. The combustion of suspended dusts and powders is quite complex and only imperfectly understood. The complexity stems from both fundamental and practical considerations. On the fundamental side, the ignition of suspensions of finely divided solids is influenced by hard-to-quantify factors such as the time-varying concentration of solids, the chemical activity and morphology of the particulate, and the degree of confinement provided by the vessel. On the practical side, industrial conditions are seldom sufficiently well-controlled or characterized to justify application of existing theoretical models. For all the above reasons, this chapter can provide only a very abbreviated coverage of ignition basics. The reader is referred to other sources for in-depth treatment of dust and powder explosions (Bodurtha, 1980 Bartknecht, 1981 Bartknecht, 1987). 

REDIFEM—This fire model has applications including steady state releases of compressible gas/vapor, incompressible liquid and transient release from a gas vessel, Gaussian Plume models, continuous free momentum, BLEVE, and confined and unconfined vapor cloud explosions. REDIEEM is reported to have internal validation with ISO 9001 and checked against PHAST and ERED. 

A) Thermal Induction Time Test consists of measuring the induction time of a highly confined sample (ca 40mg) of explosive, or explosive-inert mixture, as a function of temperature. The LASL version of the test is described by J. Zinn R.N. Rogers in JPhysChem 66, 2646(1962) it is modeled after the test developed by H. Henkin R. McGill in IEC 44, 1391(1952)... 

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)... 

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