Jets, bouncing

Perhaps the most interesting and surprising discovery about Mercury s surface is the possible presence of ice caps at the planet s north and south poles. The first evidence for this discovery came as the result of studies conducted by researchers at NASA s Jet Propulsion Laboratory in Pasadena, California. The researchers bounced radar signals off the planet s north polar region and analyzed the reflected beams at the Very Large Array radio telescope system in New Mexico. They found that the spectra of the reflected beams were very similar to those for water ice. Since they first reported this result in 1991, further studies have been conducted to determine whether the poles really do contain water ice, although final confirmation has not yet been obtained. 

To minimize the particle bounce off effect, collection surfaces should also be selected carefully. Common types of impaction surfaces include membrane, fiberglass, silver membrane, Teflon and Nuclepore filter, and brass and stainless steel shim stock. Table 2.2 shows an example of the effect of selection of collection surface on the wall losses (Newton et al., 1990). In Table 2.2, the test aerosols are droplets of 1% CsCl plus 1% uranine. Three types of cascade impactors were used, including Mercer, Sierra Radial Slit Jet (SRSJ), and Lovelace Multi-Jet (LMJ). The occurrence of particle bouncing may be indicated by the presence of excess mass on the back-up filter. 

Here we focus oti the impacts onto films with 5 <2 since this condition is often satisfied in most practical spray applications. Similarly to droplet impingement on dry targets, diverse outcomes may occur deposition and coalescence, bounce, formation of a crater, corraia splash or uprising of a central jet. Most studies reported in the Uterature consider corona splash and focus on three issues (1) characterization of crown morphology, (2) establishment of threshold criteria for disintegration and (3) characterization of secondary atomization. 

The use of an impactor with a single circular orifice for each jet provides a convenient method for determining the aerodynamic size distribution of an easily assayed aerosol, such as of a radioactive aerosol. This type of impactor is preferred when small sample rates and short sampling periods are desired since the stage characteristics can be accurately prescribed and particle bounce or re-entrainment can be minimised with the use of an appropriate liquid film on the collectors. 

High-speed movies indicated that the entrained particles tended to bounce back to the jet boundary more readily under high solid loading conditions. This may... 

In the above, the criteria for the jet break-up lengdi and resultant droplet size are discus. The next phenomenon of interest is the vertical impingement of the intact jet or the disintegrated jet in the form of droplet flow. The main question here is whedier die impinging liquid mass spreads out coheiendy over the cavity wall or bounces back into the caviQr space. The phenomena after the drop or jet impingement at the cavity floor can be scaled by the impact Weber number defined by P/V/y 4 c... 

The value of the impingement Weber numb far exceeds the spreading limit of 80. Thus both the coherent jet and droplets spread out upon inpingement to the floor and form a coiium liquid film rather than bouncing back and form a dispersed droplet flow. This indicates that most of the mass that is discharged as a jet and mostly disintegrate into droplets reforms a coherent liquid film upon impingement to the floor. Therefore, for the coiium dispersion in the cavity, the liquid film entrainment becomes the most important mechanism. The duration of tiie entrainment depends on the liquid film residence time in the cavity. Hence flie liquid film motion and transport out of the cavity is also important... 

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