Particles impact velocity

Average impact velocity, particle stream shape (by nozzle design), impingement angle of the stream to the surface, duration of exposure, temperature of the specimen and/or jet... 

Stressing by Impact. Size reduction is achieved by the impact of a particle against a soHd surface (Fig. 3c) or another particle (Fig. 3d). The particle can be accelerated to impact against the surface, or the surface can be accelerated to impact the particle, as in an impact mill. The momentum transferred is limited by the mass of the particle and the achievable impact velocity. 

Although it is entirely possible for erosion-corrosion to occur in the absence of entrained particulate, it is common to find erosion-corrosion accelerated by a dilute dispersion of fine particulate matter (sand, silt, gas bubbles) entrained in the fluid. The character of the particulate, and even the fluid itself, substantially influences the effect. Eight major characteristics are influential particle shape, particle size, particle density, particle hardness, particle size distribution, angle of impact, impact velocity, and fluid viscosity. 

Test data are available for two experiments at different impact velocities in this configuration. In one of the tests the projectile impact velocity was 1.54 km/s, while in the second the impact velocity was 2.10 km/s. This test was simulated with the WONDY [60] one-dimensional Lagrangian wave code, and Fig. 9.21 compares calculated and measured particle velocity histories at the sample/window interface for the two tests [61]. Other test parameters are listed at the top of each plot in the figure. 

We now describe the current techniques of deposition. A coating process involves several parameters. There is the nature of the substrate a crystal or an amorphous material, the quality of its polishing and its temperature. There are also the characteristics of the source, as temperature and emission law, and those of the medium in between, as its pressure and composition. In evaporation process the energy of particles is 0.1 eV, or 1100 K their impact velocity is in the range of m.s . With sputtering techniques, the energy lies in between 10-50 eV and the impact velocity is in the range of km.s . ... 

The particles must hit the accumulating bed with a high impact velocity. 

Fig. 23. Experimental photos (left) and simulated images (right) of the 2.1mm acetone droplet impact on 5.5-mm particle at 250 °C. Impact velocity V = 45cm/s.

The measured driver plate impact velocity or the shock velocity in the target plate, whose shock properties are known, defined the incident shock strength. An impedance match at the target plate-explosive interface using the measured overdriven deton velocity then defined the corresponding detonation pressure and particle velocity... 

Hypervelocity Gun. A patent by Clark Boltz (1959) claims a device consisting of a compression chamber piston energy absorber that produces shear projectiles at velocities of 10,000 to 15,000 ft/sec which is claimed to be 5 times greater than velocities that are produced by. impact. Applications for this device include simulation of possible effects of meteor particles on missiles in outer space and general studies of high-velocity particles colliding with various materials... 

From Eq. (8.8) it is evident that particle momentum depending on mass and velocity of the particles is important to control the delivery. The particle acceleration and impact velocity are defined by particle properties such as size, density, and morphology and device properties such as pressure of the compressed gas source, nozzle geometry, and others. 

To give the particles the required momentum, they should be densely packed and rigid and have a well-defined narrow particle size distribution. Friable and oblique particles are not desirable because the penetration depth will increase if the particle characteristic is more variable (Hickey 2001). Studies have been performed with particles ranging from 20 to 40 J,m in size and 1.1 to 7.9 g/cm3 in density impacting human cadaver skin (Kendall et al. 2000). Velocities of up to 260 m/s were applied to particles of this size range. For many applications, smaller particles of about 1 to 4 j,m diameter may be required for an optimized delivery. To deliver particles of this size into the skin, higher densities and impact velocities are required. For this reason, gold particles are used as a carrier material for the delivery of plasmid DNA vaccines (Kendall et al. 2001). 

The experimental setups of two out of the three installations used to date are shown in Fig. 4. The installation on the left-hand side is used to stress single particles under normal impact loads. It was originally developed by Schonert and is described in Marktscheffel and Schonert [15], The particles are fed one by one into the center of a rotor (2) by means of vibration (1). In the rotor, the particles are accelerated in radial channels and finally hit the impact ring (3) under an impact angle of 90°. The impact velocity is determined by the number of revolutions of the rotor. Particles and attrition debris are collected in the impact chamber (4) and can be discharged through a tube at the bottom of this chamber. The experiments were carried out under vacuum conditions to eliminate any effects due to viscous drag. 

For dilute phase conveying numerical simulations with a commercial computational fluid dynamics code were carried out. The analysis of particle wall impact conditions in a pipe bend showed that they take place under low wall impact angles of 5-35° which results in low normal (5-25 m/s) and high tangential (33-44 m/s) impact velocity components. These findings lead to the conclusion that not only normal stresses caused by the impacts are important in dilute phase conveying but that sliding friction stresses play an important role as well. 

Collisions between particles with smooth surfaces may be reasonably approximated as elastic impact of frictionless spheres. Assume that the deformation process during a collision is quasi-static so that the Hertzian contact theory can be applied to establish the relations among impact velocities, material properties, impact duration, elastic deformation, and impact force. 

Example 4.1 Determine the collisional heat transfer coefficients under each of the following conditions (1) collisions of a cloud of hot particles with a cold particle (2) collisions of a cloud of cold particles with a hot wall. Assume the particles are in random motion with the average impact velocity of 0.1 m/s. All the particles are spherical and of the same diameter of 100 fim. The particles and wall are made of steel with v = 0.3, E = 2 x 105 MPa, Pp = 7,000 kg/m3, Kp = 30 W/m K, and c = 500 J/kg K. The particle volume fraction is 0.4. 

The fractures on a plane surface, created by the collisions of hard spherical particles at low-impact velocities, may form a conical crack according to the Hertzian quasi-static stress theory. In a multiple-impact situation, the conical cracks meet those extending from neighboring impact sites, and then the brittle material becomes detached. Once appreciable damage is done, the cracking mechanism may be altered because the particles no longer strike on a plane surface nevertheless the brittle removal continues by the successive formation and intersection of cracks. 

Filtration is a physical separation whereby particles are removed from the fluid and retained by the filters. Three basic collection mechanisms involving fibers are inertial impaction, interception, and diffusion. In collection by inertial impaction, the particles with large inertia deviate from the gas streamlines around the fiber collector and collide with the fiber collector. In collection by interception, the particles with small inertia nearly follow the streamline around the fiber collector and are partially or completely immersed in the boundary layer region. Subsequently, the particle velocity decreases and the particles graze the barrier and stop on the surface of the collector. Collection by diffusion is very important for fine particles. In this collection mechanism, particles with a zig-zag Brownian motion in the immediate vicinity of the collector are collected on the surface of the collector. The efficiency of collection by diffusion increases with decreasing size of particles and suspension flow rate. There are also several other collection mechanisms such as gravitational sedimentation, induced electrostatic precipitation, and van der Waals deposition their contributions in filtration may also be important in some processes. 

Particle bounce. When particles bounce off the collection surface, they may be carried to subsequent stages, where they may stick or again bounce off. The result is that subsequent stages collect more mass than is appropriate, and the inferred particle-size distribution is biased towards the smaller particles. Apparently, because of increasing velocity, particles that bounce off one stage continue to bounce off the subsequent stages and are finally collected on the afterfilter. As discussed below, such collection can severely limit the utility of afterfilter data. Typically, sticky substances are applied to impaction surfaces to reduce particle bounce. Compounds that can be "wicked" by the collected particles tend to be the most effective. 

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