Explosives blast wave

Explosion blast wave largely on the basis of observed explosions, TNT equivalent model (vid. Sect. 10.6.3.1)... 

The staggered arrangement leads to a favorable height-to-width ratio of the reactor building. This simplifies verification of its stability in the case of loads resulting from external impacts (e.g. earth-quake, explosion blast wave). 

Earthquake Aircraft crash Explosion blast wave... 

In this volume. Dynamics of Detonations and Explosions Explosion Phenomena the papers have been arranged into chapters on vapor cloud explosions, blast wave reflections and interactions, and vapor explosions. Although the brevity of this preface does not permit the editors to do justice to all papers, we offer the following highlights of some of the especially noteworthy contributions. 

The problem of explosion of a vapor cloud is not only that it is potentially very destructive but also that it may occur some distance from the point of vapor release and may thus threaten a considerable area. If the explosion occurs in an unconfined vapor cloud, the energy in the blast wave is generally only a small fraction of the energy theoretically available from the combustion of all the material that constitutes the cloud. The ratio of the actual energy released to that theoretically available from the heat of combustion is referred to as the explosion efficiency. Explosion efficiencies are typically in the range of 1 to 10 percent. A value of 3 percent is often assumed. 

Deflagration A propagating chemical reaction of a substance in which the reaction front advances into the unreacted substance at less than the sonic velocity in the unreacted material. Where a blast wave is produced that has the potential to cause damage, the term explosive deflagration may be used. 

Explosion A rapid or sudden release of energy that causes a pressure discontinuity or blast wave. 

Most people will tolerate greater risk from activities when the threat to life is offset in time from when the risk (and the benefit) is originally accepted. For example, people may feel worse (and usually accept less risk) about a threat of immediate harm (e.g., the blast wave from an explosion) than a threat of latent harm (e.g., an increase in the chance of getting a fatal disease following a 20-year exposure to a hazardous material, like asbestos), even though the risks may be equivalent. 

Deflagration The ehemieal reaetion of a substanee in whieh the reaetion front advanees into the unreaeted substanee at less than sonie veloeity. Where a blast wave is produeed that has the potential to eause damage, the term explosive deflagration is used. 

If the explosion occurs in an unconfined vapor cloud, the energy in the blast wave is only a small fraction of the energy calculated as the product of the cloud mass and the heat of combustion of the cloud material. On this basis, explosion efficiencies are typically in the range of 1-10%. 

In the surrounding atmosphere, a blast wave is experienced as a transient change in gas-dynamic-state parameters pressure, density, and particle velocity. Generally, these parameters increase rapidly, then decrease less rapidly to sub-ambient values (i.e., develop a negative phase). Subsequently, parameters slowly return to atmospheric values (Figure 3.7). The shape of a blast wave is highly dependent on the nature of the explosion process. 

If the combustion process within a gas explosion is relatively slow, then expansion is slow, and the blast consists of a low-amplitude pressure wave that is characterized by a gradual increase in gas-dynamic-state variables (Figure 3.7a). If, on the other hand, combustion is rapid, the blast is characterized by a sudden increase in the gas-dynamic-state variables a shock (Figure 3.7b). The shape of a blast wave changes during propagation because the propagation mechanism is nonlinear. Initial pressure waves tend to steepen to shock waves in the far field, and wave durations tend to increase. 

Making a detailed estimate of the full loading of an object by a blast wave is only possible by use of multidimensional gas-dynamic codes such as BLAST (Van den Berg 1990). However, if the problem is sufficiently simplified, analytic methods may do as well. For such methods, it is sufficient to describe the blast wave somewhere in the field in terms of the side-on peak overpressure and the positive-phase duration. Blast models used for vapor cloud explosion blast modeling (Section 4.3) give the distribution of these blast parameters in the explosion s vicinity. 

The upper half of Figure 3.9 represents how a spherical explosive charge of diameter d produces a blast wave of side-on peak overpressure P and positive-phase duration r" " at a distance R from the charge center. Experimental observations show that an explosive charge of diameter Kd produces a blast wave of identical side-on peak overpressure p and positive-phase duration Kt at a distance KR from the charge center. (This situation is represented in the lower half of Figure 3.9.) Consequently,... 

In the earliest applications of numerical methods for the computation of blast waves, the burst of a pressurized sphere was computed. As the sphere s diameter is reduced and its initial pressure increased, the problem more closely approaches a point-source explosion problem. Brode (1955,1959) used the Lagrangean artificial-viscosity approach, which was the state of the art of that time. He analyzed blasts produced by both aforementioned sources. The decaying blast wave was simulated, and blast wave properties were registered as a function of distance. The code reproduced experimentally observed phenomena, such as overexpansion, subsequent recompression, and the formation of a secondary wave. It was found that the shape of the blast wave at some distance was independent of source properties. 

A similar technique was used by Oppenheim et al. (1977) to analyze the blast waves produced by some gas explosions of a different nature ... 

For these and other purposes, blast-modeling methods are needed in order to quantify the potential explosive power of the fuel present in a particular setting. The potential explosive power of a vapor cloud can be expressed as an equivalent explosive charge whose blast characteristics, that is, the distribution of the blast-wave properties in the environment of the charge, are known. 

The positive-phase duration of the blast wave from a vapor cloud explosion is in the range of 100 and 300 ms. 

The energy term E must be defined to calculate energy-scaled standoff R. The energy term represents the sensible heat that is released by that portion of the cloud contributing to the blast wave. Any of the accepted methods of calculating vapor cloud explosive energy are applicable to the Baker-Strehlow method. These methods include ... 

TNT blast is, however, a poor model for a gas explosion blast. In particular, the shape and positive-phase duration of blast waves induced by gas explosions are poorly represented by TNT blast. Nevertheless, TNT-equivalency methods are satisfactory, so long as far-field damage potential is the major concern. 

If, on the other hand, a vapor cloud s explosive potential is the starting point for, say, advanced design of blast-resistant structures, TNT blast may be a less than satisfactory model. In such cases, the blast wave s shape and positive-phase duration must be considered important parameters, so the use of a more realistic blast model may be required. A fuel-air charge blast model developed through the multienergy concept, as suggested by Van den Berg (1985), results in a more realistic representation of a vapor cloud explosion blast. 

Auton, T. R., and J. H. Pickles. 1978, The calculation of blast waves from the explosion of pancake-shaped vapor clouds. Central Electricity Research Laboratories note No. RD/L/N 210/78. 

Guirao, C. M., G. G. Bach, and J. H. S. Lee. 1979. On the scaling of blast waves from fuel-air explosives. 6th Symp. on Blast Simulation. Cahors, France. 

Munday, G., and L. Cave. 1975. Evaluation of blast wave damage from very large unconfined vapor cloud explosions. International Atomic Energy Agency, Vienna. 

Oppenheim, A. K. 1973. Elementary blast wave theory and computations. Proc. of the Conf. on Mechanisms of Explosions and Blast Waves. Yorktown, Virginia. 

Strehlow, R. A. 1981. Blast wave from deflagrative explosions an acoustic approach. AIChE Loss Prevention. 14 145-152. 

Taylor, P. H. 1985. Vapor cloud explosions—The directional blast wave from an elongated cloud with edge ignition. Comb. Sci. Tech. 44 207-219. 

Van Wingerden, C. J. M. 1988a. Experimental investigation into the strength of blast waves generated by vapor cloud explosions in congested areas. 6th Int. Symp. Loss Prevention and Safety Promotion in the Process Industries. Oslo, Norway, proceedings. 26 1-16. 

Woolfolk, R. W., and C. M. Ablow. 1973. Blast waves for non-ideal explosions. Conference on the Mechanism of Explosions and Blast Waves, Naval Weapons Station. York-town, VA. 

Table 6.10 presents some damage effects. It may give the impression that damage is related only to a blast wave s peak overpressure, but this is not the case. For certain types of structures, impulse and dynamic pressure (wind force), rather than overpressure, determine the extent of damage. Table 6.10 was prepared for blast waves of nuclear explosions, and generally provides conservative predictions for other types of explosions. More information on the damage caused by blast waves can be found in Appendix B. 

Temperature determines whether or not the liquid in a vessel will boil when depressurized. The liquid will not boil if its temperature is below the boiling point at ambient pressure. If the liquid s temperature is above the superheat-limit temperature Tj] (Tsi = 0.897 ), it will boil explosively (BLEVE) when depressurized. Between these temperatures, the liquid will boil violently, but probably not rapidly enough to generate significant blast waves. However, this is not certain, so it is conservative to t sume that explosive boiling will occur (see Section 6.3.2). 

The above procedure produces blast parameters applicable to a completely symmetrical blast wave, such as would result from the explosion of a hemispherical vessel placed directly on the ground. In practice, vessels are either spherical or cylindrical, and placed at some height above the ground. This influences blast parameters. To adjust for these geometry effects, and 7 are multiplied by some adjustment factors derived from experiments with high-explosive charges of various shapes. 

The method presented above is based on the similarity of the blast waves of pressure vessel bursts and high explosives. This similarity holds only at some distance from the explosion. In the near field, the peak overpressure and impulse from a pressure... 

This refinement assumes that an explosion s blast wave will be completely symmetrical. Such a shape would result from the explosion of a hemispherical vessel placed... 

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