Showing papers on "Overpressure published in 1984"

Dynamic Parameters of Gaseous Detonations

Abstract: In addition to gases, flammable liquids and solids in the form of fine droplets and dust particles also form explosive mixtures with air. An explosive mixture can, in general, support two modes of combustion. The slow laminar deflagration mode is at one extreme; here the flame propagates at typical velocities of the order 1 m s -1 relative to the unburned gases and there is negligible overpressure development when the explosion is unconfined. At the other extreme is the detonation mode, in which the detonation wave propagates at about 2000 m s -1 accompanied by an overpressure rise of about 20 bars across the wave. The propagation of laminar defiagrations is governed by the molecular diffusion of heat and mass from the reaction zone to the unburned mixture. The propagation of detonations depends on the adiabatic shock compression of the unburned mixtures to elevated temperatures to bring about autoignition. The very strong exponential temperature dependence of chemical reaction rates in general makes possible the rapid combustion in the detonation mode. Two­ phase liquid droplets or dust-air mixtures are similar, but they require more physical processes (e.g. droplet break-up, phase change, mixing, etc.) prior to combustion. Thus, characteristic time or length scales associated with the combustion front are usually much larger than those of homogeneous gaseous fuel-air mixtures. The essential mechanisms of propagation of the combustion waves, however, are similar. In between the two extremes of laminar detlagration and detonation, there is an almost continuous spectrum of burning rates where turbulence plays the dominant role in the combustion process. Due to space limitations, only homogeneous gaseous fuel-air detonations are considered in this article.

Hydrodynamic aspects of caldera‐forming eruptions: Numerical models

Abstract: Comparison of results from a two-dimensional numerical eruption simulation (KACHINA) to calculations based upon a shock tube analog supports the conclusion that the hydrodynamics during the initial minutes of large caldera-forming ash flow eruptions may be dominated by blast wave phenomena. Field evidence for this phenomenology is pyroclastic surge deposits commonly occurring both directly below caldera-related ash flow sheets, on top of a preceding Plinian fall deposit (ground surge), and separating individual ash flow units. We model the eruption of the Tshirege member of the Bandelier Tuff (1.1 Ma B.P.) from the Valles caldera, New Mexico. In the model a magma chamber at 100 MPa (1 kbar) and 800°C is volatile rich, with an average H2O abundance above saturation greater than 8.7 wt % increasing to nearly 100 wt % near the very top of the chamber. Using a shock tube analogy, decompression of the chamber through a wide-open dikelike vent 0.1 km wide and 1 to 5 km long forms a shock wave of 3 MPa (≃3O atm) with a velocity greater than 1.0 km s−1. Steady flow of material erupted from the vent begins after 20 to 100 s based upon a 7-km depth from the ground surface to a reflective (density) boundary in the chamber and a rarefaction wave velocity of 100 to 600 m s−1. The velocity of the ash front behind the shock wave is 300 to 500 m s−1. The shock tube model serves as a basis to evaluate the consistency of the KACHINA code results which are similar to a one-dimensional problem along the symmetry axis. The results of the KACHINA simulation show in some detail the effect of multiple reservoir rarefaction reflections and possibly Prandtl-Meyer expansion in generating compressive wave fronts following the initial shock. The rarefaction resonance not only prolongs unsteady flow in the vent but tends to promote surging flow of ash behind the leading shock. Furthermore, these results are consistent with a blast wave characterized as a shock front followed by one or more pulses of entrained ash. The blast wave shocks ambient air to higher pressures and temperatures, the magnitudes of which depend strongly on the initial chamber overpressure, distance, and direction from the vent. In consideration of volcanic hazards our numerical model shows that a shock wave compressed the atmosphere to pressures of ≃0.2 to 0.7 MPa (2–7 atm) and temperatures of ≃200° to 300°C for distances to 10 km from the Bandelier vent(s).

Dynamic Shear Failure of Shallow-Buried Flat-Roofed Reinforced Concrete Structures Subjected to Blast Loading

Abstract: : Five box structures with span-to-depth (L/d) ratios of 10, 1 percent reinforcement in each face, and concrete strengths of approximately 4000 and 6000 psi, and six box structures with L/d ratios of 7, concrete strength of approximately 7000 psi and steel percentages of 1.2 and 0.75 percent, were tested dynamically at depth of burial equal to L/5. The dynamic overpressure simulated the peak overpressure, rate of pressure decay, and load duration associated with nuclear detonation and was generated using high-explosive primacord in a Foam HEST charge cavity configuration placed over the structure at the ground surface. Results of these tests indicate that current dynamic shear failure criteria significantly underpredict the dynamic shear strength of these structures.

Influence of H 2 Overpressure on the Properties of GaAs Grown by Low-Pressure MOCVD

Abstract: The influence of H2 overpressure on the electrical and optical properties of GaAs epitaxial layer grown by the low-pressure MOCVD of TMG/AsH3/H2 system was studied. It was demonstrated that high H2 overpressure was not necessary in the reaction process to reduce carbon contamination for low-pressure MOCVD. The carbon incorporation process in low-pressure MOCVD was discussed briefly.

Computation of the Space Shuttle solid rocket booster nozzle start-up transient flow

Abstract: The first NASA Space Shuttle flight (STS-1) produced an overpressure wave that exceeded preflight predictions by as much as 5 to 1. This second overpressure wave occurred just after the solid rocket booster (SRB) igniter wave. To understand this overpressure phenomenon, a numerical simulation effort was undertaken. Both the SRB static firing test and STS-1 geometries were studied for two-dimensional, inviscid and viscous flow. The inviscid calculations did not produce significant second overpressure waves. However, the viscous calculations did produce second overpressure waves that qualitatively agree with experiment. These overpressure waves were present in both the static firing test and STS-1 geometries. This second overpressure wave is generated by the motion of the boundary layer separation point and the subsequent radial motion of the exhaust jet during the start-up of the SRB nozzle flow. The presence of the mobile launch platform exhaust hole wale amplifies this wave, but does not appear to be the source of any additional overpressure waves. The lack of good quantitative agreement between theory and experiment indicates that other overpressure sources, not accounted for by this simulation, may be present.

Computational and Experimental Studies of Blockage Effects in a Blast Simulator

Abstract: : The confinement of a target by the walls of a blast simulator produces changes in the loading experienced by the target. The changes increase in magnitude with the blockage ratio, the ratio of target area to the simulator cross-sectional area. Computations were made using the HULL hydrocode in an axisymmetric configuration for non-decaying waves for different blockage ratios for a cylindrical target. It was found that the flow in the constricted region between the target and the shock tube wall was typically accelerated to velocities greater than the steady flow values behind the incident shock in an unobstructed shock tube. The net axial load on the target computed from the hydrocode results could be related directly to the dynamic pressure in the constricted region, which increased as the blockage was increased. This meant that non-decaying shocks which did not produce net loads sufficient to overturn a target in a shock tube at low blockage could be made to do so by increasing blockage while keeping shock overpressure constant. Thus, tests where the blockage is high can be misleading unless corrections are made for the effect of blockage. The step shock computations provided an extreme case for analysis; similar computations were performed for the other extreme case of a rapidly decaying wave. Although critical flow parameters in the constricted region (e.g. , particle velocity and dynamic pressure) were similarly increased for the decaying wave, the effect on the net force was found to be small. This was because of the overpressure gradient across the target and the reduced importance of drag loading compared to diffraction phase loading.

Hydrodynamic ship propulsion

Abstract: For the propulsion of a ship, the sea waves can also be used in addition to wind and power propulsion This force is transmitted to the ship by the hydrodynamic drive in that two surfaces movably arranged in relation to one another alternately generate overpressure and underpressure

Dynamic calibration of shock overpressure transducers

Abstract: Piezoelectric transducers for dynamic overpressure measurements are commonly calibrated with static or quasistatic loads and the calibration is extrapolated to frequencies up to 30% of the resonant frequency of the piezoelectric crystal. Sinusoidal pressure generators are also used for dynamic calibration up to 500 Hz in the range of 3 MPa. This paper describes a method for dynamic calibration using transient overpressures, with rise time of 2 µsec and width 40 µsec, generated by exploding wires in air. The calibration is done in the range of 600 kPa.

Instrument for measuring pressure changes in time of closed-off volumes under overpressure or underpressure

Abstract: The instrument for which a patent is applied for solves the problem which arises in the sectors of submersion and pressure technology, power-station construction, the chemical industry, etc, namely the monitoring of pressurised gases or liquids for very slight variations of this pressure or for the speed at which these variations take place Where gases are concerned, the medium can then be allowed to overflow (Figure 1) into an equalising vessel by way of a suitable measuring instrument, and the flow per unit volume is determined in terms of size and direction at the measuring point The speed of change of pressure is proportional to this measured quantity If the medium to be measured is a liquid, the measuring method must be modified, as shown in Figure 2 The method according to Figure 2 can also be used for measurements of the change of depth of penetration In these cases, however, the speed of change of pressure is not proportional to the measured quantity The measuring method for determining flow per unit volume is chosen in accordance with the set object

Flame Acceleration by a Postflame Local Explosion

Abstract: Delayed local explosion of segregated parts of an explosive gas mixture behind the flame front can lead to flame front acceleration and hence to increased overpressures. The effect of a local explosion on flame speed and overpressure during deflagration of a stoichiometric ethylene-air mixture was investigated in two different experimental configurations. In the first experimental setup, the interaction of a spherical shock front with a planar flame front was studied in a 1-m-diam tube. The effect of flame front acceleration was indicated by an increase of overpressure from 0.1 to 0.5 bar. In a second experimental setup, the interaction of a spherical shock front with a spherical flame front in an unconfined configuration was studied. The flame front, which moved with an initial velocity of about 10 m/s, became strongly folded and torn up during the interaction with the shock front. As a result, a large increase in the overall flame speed was observed. Different parts of the reaction zone attained velocities in the range between 75 and 125 m/s. The folding of the flame front due to the occurrence of the MarksteinTaylor instability is thought to be the main cause for this large increase of combustion rate and overall flame speed.