Velocity-Impact Processes

The simulation shown in Fig. 10 is an impact of a saturated water droplet of 2.3 mm in diameter onto a surface of 400°C with an impact velocity of 65 cm/s, corresponding to a Weber number of 15. This simulation and all others presented in this study are conducted on uniform meshes (Ax — Ay — Az = A). The mesh resolution of the simulation shown in Fig. 10 was 0.08 mm in grid size, although different resolutions are also tested and the results are compared in Figs. 11 and 12. The average time-step in this case is around 5 ps. It takes 4000 iterations to simulate a real time of 20 ms of the impact process. The simulation... 

The impact process of a 3.8 mm water droplet under the conditions experimentally studied by Chen and Hsu (1995) is simulated and the simulation results are shown in Figs. 16 and 17. Their experiments involve water-droplet impact on a heated Inconel plate with Ni coating. The surface temperature in this simulation is set as 400 °C with the initial temperature of the droplet given as 20 °C. The impact velocity is lOOcm/s, which gives a Weber number of 54. Fig. 16 shows the calculated temperature distributions within the droplet and within the solid surface. The isotherm corresponding to 21 °C is plotted inside the droplet to represent the extent of the thermal boundary layer of the droplet that is affected by the heating of the solid surface. It can be seen that, in the droplet spreading process (0-7.0 ms), the bulk of the liquid droplet remains at its initial temperature and the thermal boundary layer is very thin. As the liquid film spreads on the solid surface, the heat-transfer rate on the liquid side of the droplet-vapor interface can be evaluated by... 

We have seen in the previous sections how burning or deflagration can be initiated in an explosive. If the decomposition reaction is completed at shock velocities in the explosive, that is called a detonation. The initiation of chemical reaction in a detonation is similar to what we saw with low-velocity impact. The shock front compresses the unreacted explosive material, causing local shear failure and inelastic flow (Ref. 7). These processes create hot spots that grow into complete reaction. The difference in the case of detonation is that the ensuing reaction is completed at a much higher rate. 

Samples may be desorbed and ionized by an impact process that involves bombardment of the sample with high-velocity atoms, ions, fission fragments, or photons of relatively high energy. The impact deposits energy into the sample, either directly or via a matrix, and leads to both sample molecule transfer into the gas phase and ionization. 

All the information of interest is contained in the acoustic source function. The theory works well for low-velocity impacts where the collisions are elastic but may be extended with some modifications to include plastic deformation. The method is nonempiri-cal (requiring no independent calibration). However, the measurement requires a calibrated broadband sensor, the particles strike a well-characterized metal plate, at a known velocity, and the burst acoustic emission signals must be separated in time by 1 ms. In process analysis, it is unlikely that all (or any) of these requirements can be met. 

Particles of >10 nm diameter are subject to significant sedimentation rates as a result of gravitational forces (the velocity may be estimated crudely by use of Stoke s Law). Consequently their atmospheric lifetime is severely limited by the gravitational settling process, and by impaction upon surfaces. Particles of <10 jum diameter are removed only relatively slowly from the atmosphere, those greater than ca. 0.3 /im by impaction processes and the smaller particles by diffusive deposition. The deposition flux may be estimated from the deposition velocity and some data are presented in Section 4.2.2. 

HIVF. The Hypersonic Velocity Impact Fusion thermal spray coating process. Hob. The so-called ceramic hobs (flat surfaces for supporting cooking ware on cookers) are usually made from glass ceramic sheets. 

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