Blast wave pressure

The safety distance between LH2 and LOX tanks at the Tanegashima Space Center was set to be 167 m according to the US Air Force Standard for blast wave pressure [83]. 

In addition to [12, 19-21], data on blast propagation in a tube or a space stuffed with rock debris and filled with hydrogen -i- air mixture in spaces between the debris and their pores can be found in [23]. Table 5.1 presents blast wave pressure P values for various detonation regimes for different sizes of rock debris. The table also gives blast wave velocity D and P2 after the blast. 

Fig. 9.3 Relative blast wave pressure (a) and scaled positive impulse (b) caused by the detonation of various hemi-spherical volumes filled with 30% H2 HAM mixture. Records of overpressure 1 and impulse 2 for a 300-m hemi-sphere at 15.61-m distance (c) [7]...

Pressure and impulse in the pressure wave determine the danger level resulting from an explosion of hydrogenous combustible mixtures. The main measurable blast wave parameters are presented in the diagram (Fig. 10.1). The blast wave pressure in a gas explosion is a function of the energy release rate and it reaches a maximum at the detonation mode of combustion [1 ]. 

Fuel-air gas explosive effects (blast wave pressure and impulse) depend on the resultant energy release [7-28]. The blast wave pressure amplitude mainly relates to the energy release rate [10, 13, 24, 25]. The pressure impulse weakly depends on the... 

Mechanical effects of a blast wave interaction with a target depend on the blast wave pressure amplitude and the blast wave impulse. A damage effect boundary in blast wave pressure-blast wave impulse coordinates can be found for any target... 

Using measured incident blast wave parameter data, the basic dependence of the TNT pressure equivalent Kp of a gas explosion on the relative blast wave pressure... 

Fig. 10.8 Diagram of blast wave pressure/impulse resulting from H2 + air cloud explosion ( H2 - 100 kg) for various combustion modes line 1 - detonation line 2 - constant volume explosion line 3 — deflagration with 200 m/s velocity line 4 - deflagration with 100 m/s velocity 5 - boundary of deflagration regimes...

The described cases are characteristic for a central explosion initiation in the cloud. For non-central, asymmetrical explosion initiation, an additional pressure release is anticipated on the descending branch of demolition load [11, 34, 40]. The known dependence of the gas blast wave pressure on the distance and combustion velocity [10] can be transformed into a chart of approximate potential pressure loads (Fig. 10.10). The differently shaded regions illustrate the areas of blast wave overpressure (AP) - relative distance (RRq ) where a compression waves (A area), compression wave with a shock-wave precursor (B area) and blast waves (C area) can be generated. 

In the Sachs variables, Fig. 10.11 presents the relative blast wave pressure amplitude (at AP+) and the rarefaction wave (at AP ) versus distance R The pressure difference ratio at the blast wave front/rarefaction phase is... 

Relevant relations are presented in Fig. 10.16a, b. As a rule, a motor fuel explosion with high steam pressure is characterized by a higher blast wave pressure impulse, which is explained by the better completion of the blast transition process. 

The incident blast wave pressure profile with the description of rarefaction and compression phases [8-11] can have the form ... 

A notable characteristic of the plane explosion in a duct is the lack of a negative pressure phase, if there is no water steam condensation, the same as it is for a H2 + O2 mixture explosion. The blast wave pressure impulse in a duct is practically constant throughout the length R/Rq 100 provided that there is no loss due to friction. In combination with moderate attenuation of the front there is evidence of compression phase expansion as the wave is moves though the duct. 

The calculation data [41] for detonation of gas layers (curve 1), columns (curve 2) and spheres (curve 3) can be presented by the damage diagram in pressure-impulse coordinates (Fig. 10.24). It is easy to compare the dangerous effect resulting from gas clouds of various shapes if one of the targets has a boundary of equi-probable destruction corresponding to curve 4. It is seen that the plane explosion causes a destructive blast wave pressure of the lowest level (position data of intersection points of curves 1, 2, 3 with curve 4). 

Fig. 10.28 Blast wave pressure duration versus scaled distance for a TNT underwater explosion (1) and a CaHg + 5O2 mixture detonation in a gas cavity line 2 - at q = 12 MJ/m ) [45]...

Blast Characteristics Accurate calculation of the magnitude of the blast wave from an exploding pressure vessel is not possible, but it may be estimated from several approximate methods that are available. 

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

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. 

An object struck by a blast wave experiences a loading. This loading has two aspects. First, the incident wave induces a transient pressure distribution over the... 

In summary, an object s blast loading has two components. The first is a transient pressure distribution induced by the overpressure of the blast wave. This component of blast loading is determined primarily by reflection and lateral rarefaction of the reflected overpressure. The height and duration of reflected overpressure are determined by the peak side-on overpressure of the blast wave and the lateral dimensions of the object, respectively. The Blast loading of objects with substantial... 

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. 

The pressure vessel under consideration in this subsection is spherical and is located far from surfaces that might reflect the shock wave. Furthermore, it is assumed that the vessel will fracture into many massless fragments, that the energy required to mpture the vessel is negligible, and that the gas inside the vessel behaves as an ideal gas. The first consequence of these assumptions is that the blast wave is perfectly spherical, thus permitting the use of one-dimensional calculations. Second, all energy stored in the compressed gas is available to drive the blast wave. Certain equations can then be derived in combination with the assumption of ideal gas behavior. 

When a pressure vessel is not a sphere, or if the vessel does not fracture evenly, the resulting blast wave will be nonspherical. This, of course, is the case in almost every actual pressure vessel burst. Loss of symmetry means that detailed calculations... 

In previous sections, it has been assumed that all energy within a pressure vessel is available to drive the blast wave. In fact, energy must be spent to rupture the vessel and propel its fragments. In some cases, the vessel expands before bursting, thus absorbing additional energy. Should a vessel also contain liquid or solids, a fraction of the available energy may be spent in its propulsion. 

Blast waves close to the source of pressure vessel bursts differ greatly from those from TNT blasts. 

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. 

In this section, three methods for calculating the blast parameters of pressure vessel bursts and BLEVEs will be presented. All methods are related that is, one basic method and two variations are presented. The choice of method depends upon phase of the vessel s contents and distance to the blast wave s target, as illustrated in Figure 6.19. 

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). 

Baker et al. (1975) developed a method, presented below, for predicting blast effects fiom the rupture of gas-filled pressure vessels. They include a method for calculating the overpressure and impulse of blast waves from the rupture of spherical or cylindri-... 

The general procedure of the basic method is shown in Figure 6.20. This method is suitable for calculations of bursts of spherical and cylindrical pressure vessels which are filled with an ideal gas, placed on a flat surface, and distant from other obstacles which might interfere with the blast wave. 

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... 

In Steps 2 and 3, the vessel s nondimensional radius and the blast wave s nondimen-sional peak pressure at that radius were calculated. As a blast wave travels outward, its pressure decreases rapidly. The relationship between the peak pressure and the distance R depends upon initial conditions. Accordingly, Figure 6.21 contains several curves. Locate the correct curve by plotting (R, P ) in the figure, as illustrated in Figure 6.28. 

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