Cloud expansion

For a cloud of unipolar and uniformly distributed charged particles bounded by a surface equidistant from a center of symmetry, the rate of cloud expansion due to the repulsion of the charged particles within the... 

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

The astrochemistty of ions may be divided into topics of interstellar clouds, stellar atmospheres, planetary atmospheres and comets. There are many areas of astrophysics (stars, planetary nebulae, novae, supemovae) where highly ionized species are important, but beyond the scope of ion chemistry . (Still, molecules, including H2O, are observed in solar spectra [155] and a surprise in the study of Supernova 1987A was the identification of molecular species, CO, SiO and possibly ITf[156. 157]. ) In the early universe, after expansion had cooled matter to the point that molecules could fonn, the small fraction of positive and negative ions that remained was crucial to the fomiation of molecules, for example [156]... 

Clouds A mass of droplets of water or other liquids remaining at a more or less constant height. Clouds are usually formed by condensation after warm moist air rises by convection into cooler regions and cools by expansion to below its dew point. 

A BLEVE involving a container of flammable liquid will be accompanied by a fireball if the BLEVE is fire-induced. The rapid vaporization and expansion following loss of containment results in a cloud of almost pure vapor and mist. After ignition, this cloud starts to bum at its surface, where mixing with air is possible. In the buoyancy stage, combustion propagates to the center of the cloud causing a massive fireball. 

Experimental research has shown that a vapor cloud explosion can be described as a process of combustion-driven expansion flow with the turbulent structure of the flow acting as a positive feedback mechanism. Combustion, turbulence, and gas dynamics in this complicated process are closely interrelated. Computational research has explored the theoretical relations among burning speed, flame speed, combustion rates, geometry, and gas dynamics in gas explosions. 

Experiments by Schmidli et al. (1990) were focused on the distribution of mass on rupture of a vessel containing a superheated liquid below its superheat-temperature limit. Flasks (50-ml and 100-mI capacity) were partially filled with butane or propane. Typically, when predetermined conditions were reached, the flask was broken with a hammer. Expansion of the unignited cloud was measured by introduction of a smoke curtain and use of a high speed video camera. Large droplets were visible, but a portion of the fuel formed a liquid pool beneath the flask. Figure 6.5 shows that, as superheat was increased, the portion of fuel that... 

H = visible flame height S = 2.3 X = flame speed = wind speed d = cloud depth g = gravitational acceleration po = fuel-air mixture density pj = density of air r = stoichiometric air-fuel mass ratio a = expansion ratio for stoichiometric combustion under constant pressure (typically 8 for hydrocarbons)... 

This calculation takes into account only the blast from the expansion of vessel contents. In fact, this blast may be followed by one from a vapor cloud explosion. This possibility must be considered separately with the methods presented in earlier chapters. 

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