Formation of Gas Cavities

Formation of Gas Cavities in Liquid-saturated Porous Medium... 

Also, the preparation of honeycomb plastics and gas-filled fibrous materials does not require foaming. Formation of gas cavities in the former is carried out by glueing of previoudy prepared sheet materials or by extmsion of thermoplastics through special nozzles in the latter case by glueing or filling with polymeric binders of fibrous substances containing capillaries and pores. 

When gas is sparged into most agitators at a given speed, N, the power decreases (see Figure 15.5) because of the formation of gas cavities behind the... 

A common observation from in vivo experiments in biodegradable Mg research is the local formation of gas cavities which accompanies the implant corrosion. However, there are contradictory reports on the occurrence of gas cavities subcntaneously while intravasal application showed no local gas accumulation. An explanation for this observation might be based on the diffusion and solnbility coefficient of hydrogen in biological tissues which has been widely reviewed (Lango et al, 1996). The solubility of hydrogen... 

In order to compute the dynamics of the liquid-gas phase boundary in spatially complex porous structures, other numerical techniques such as the volume-of-fluid (VOF) method should be used instead [7]. The VOF method relies on a description of the spatial distribution of the phases by time-dependent volume fractions defined in each cell of the computational grid. This method has already been utilized to simulate problems involving highly complicated free surface flow such as, capillary condensation [27], formation of gas cavities [28], and layering growth in wet granulation [29]. 

From the condition 21a it immediately follows that if the clathrate is formed in the presence of a number of compounds which are potential solutes, i.e., sufficiently small to have 0 for some i, all these compounds contribute to its stability. As has already been pointed out by Barrer and Stuart4 this at once explains the stabilizing influence of "Hilfsgase" such as air, C02, or H2S on the formation of gas hydrates discussed by Villard49 and von Stackelberg and Meinhold.47 If there is only one solute, Eq. 21a with the = sign determines the minimum vapor pressure fiA necessary to make the clathrate stable relative to Qa. Since all cavities contribute to the stabilization, one cannot say that this minimum pressure is controlled by a specific type of cavity. 

It is well known that the critical impeller speed for solid suspensions is higher in the presence of a gas, depending mainly on the superficial gas velocity (Rewatkar et al., 1991). This is because of a decrease in the impeller power draw due to the formation of ventilated cavities behind the impeller blades on gassing. For example, for Rushton turbines, /)T//)a - 2-3.3 ... 

Cavitation is the formation of gaseous cavities in a medium upon ultrasound exposure. The primary cause of cavitation is ultrasound-induced pressure variation in the medium. Cavitation involves either the rapid growth and collapse of a bubble (inertial cavitation) or the slow oscillatory motion of a bubble in an ultrasound field (stable cavitation). Collapse of cavitation bubbles releases a shock wave that can cause structural alteration in the surrounding tissue [13]. Tissues contain air pockets trapped in the fibrous structures that act as nuclei for cavitation upon ultrasound exposure. The cavitational effects vary inversely with ultrasound frequency and directly with ultrasound intensity. Cavitation might be important when low-frequency ultrasound is used, when gassy fluids are exposed, or when small gas-filled spaces are exposed. 

For the upflow impeller, the Pq/Po vs. impeller speed curve is shown in Figure 11.44b. The flow generated by this impeller and the gas flow are complementary to each other, and thus formation of large cavities is prevented (Bujalski et al., 1988). Mhetras et al. (1994) have correlated the critical impeller speed for complete dispersion with the gas flow number as... 

The decrease in power with gassing is not just an effect of the lower average density of the gas-liquid dispersion, since the gas holdup is generally 10 percent or less when PgjPo is reduced to 0.5. The decrease in power is associated with the formation of gas pockets behind the turbine blades." Bubbles are captured in the centrifugal field of vortices that form behind the horizontal edges of the blades, and coalescence leads to large cavities that interfere with normal liquid flow. 

Cavitation is the process of nucleation in a liquid and subsequent formation of gas/vapor phase when the pressure falls below a critical value. Researchers studying cavitation address topics ranging from the initial appearance of cavities in liquids (cavitation inception) to the development of large-scale cavities (supercavitation). 

When applied to rubbers, the cavitation usually corresponds to the effect of formation and unrestricted growth of voids in gas-saturated rubber samples after a sudden depressurization [126,127]. In general, this has a broader sense and may be understood as the phenomena related to the formation and dynamics of cavities in continuous media [88,102]. In materials science, for example, it means a fracture mode characterized by formation of internal cavities [128]. In acoustics, the cavitation denotes the phenomena related to the dynamics of bubbles in sonically irradiated hquids [129]. 

Heavens Field suggest that both gas cavities, sealed off during fusion of the sample, and high flow rate shearing near the impacting striker edge contribute to hot spot formation during PETN impact initiation... 

Theory Collapse of gas/vapour cavities in an acoustic field produces extremely high pressures and temperatures capable of causing the emission of light from the core of the collapsing cavity (sonoluminescence) and also the formation of oxidising radical species that can react in the solution with molecules, such as luminol, to produce a secondary, chemical luminescence. 

Although microwave-heated organic reactions can be smoothly conducted in open vessels, it is often of interest to work with closed systems, especially if superheating and high-pressure conditions are desired. When working under pressure it is strongly recommended to use reactors equipped with efficient temperature feedback coupled to the power control and/or to use pressure-relief devices in the reaction vessels to avoid vessel rupture. Another potential hazard is the formation of electric arcs in the cavity [2], Closed vessels can be sealed under an inert gas atmosphere to reduce the risk of explosions. 

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