Void vapor pressure

In the more general and more difficult case, either the punc ture is initially in the liquid space, or in a line attached to it, or the liquid swells to reach the punc ture or punc tured line, giving two-phase or all-liquid discharge. For these cases the discharge model solutions must treat four regimes, which are defined by the initial void (vapor) frac tion Ot and by the pressure ratios ... 

Formation of vapor bubbles in rapidly flowing or turbulent water causing risk of pumping failure and erosion and/or corrosion. Due to an increase in velocity at the pump head resulting in a localized pressure reduction and the subsequent collapse of the vapor into voids or cavities. Where FW temperatures are high (over perhaps 195-205 °F) the pump velocity can reduce FW vapor pressure below that corresponding to the temperature of the liquid and cavitation can occur accompanied by some noise. Warning of severe pump cavitation is often indicated by a heavy noise. 

Curing of Polyimlde Resin. Thermoset processing involves a large number of simultaneous and interacting phenomena, notably transient and coupled heat and mass transfer. This makes an empirical approach to process optimization difficult. For instance, it is often difficult to ascertain the time at which pressure should be applied to consolidate the laminate. If the pressure is applied too early, the low resin viscosity will lead to excessive bleed and flash. But if the pressure is applied too late, the diluent vapor pressure will be too high or the resin molecular mobility too low to prevent void formation. This example will outline the utility of our finite element code in providing an analytical model for these cure processes. 

Argon at liquid-nitrogen temperature exhibits an equilibrium pressure of 187 torrs. It offers the advantage of a lower vapor pressure than nitrogen, which will reduce the void volume error while retaining ease of pressure measurements. However, the cross-sectional area of argon is not well established and appears to vary according to the surface on which it is adsorbed. 

Let us first consider the synergistic elfect that water has on void stabilization. It is likely that a distribution of air voids occurs at ply interfaces because of pockets, wrinkles, ply ends, and particulate bridging. The pressure inside these voids is not sufficient to prevent their collapse upon subsequent pressurization and compaction. As water vapor diffuses into the voids or when water vapor voids are nucleated, however, there will be an equilibrium water vapor pressure (and therefore partial pressure in the air-water void) at any one temperature that, under constant total volume conditions, will cause the total pressure in the void to rise above that of a pure air void. When the void pressure equals or exceeds the surrounding resin hydrostatic pressure plus the surface tension forces, the void becomes stable and can even grow. Equation 6.5 expresses this relationship... 

Vapor Pressure. The vapor pressure of water within the void is dependent on temperature according to the Clausius-Clapeyron Equation. 

Equation 6.32 was derived from the requirement that void growth by diffusion at any temperature cannot occur if the pressure within the void is greater than the saturated vapor pressure at that temperature (i.e., if Csat > C ). 

The second ramp portion of this cure cycle is critical from a void nucleation and growth standpoint. During this ramp, the temperature is high, the resin pressure can be near its minimum, and the volatile vapor pressure is high and rising with temperature. These are the ideal conditions for void formation and growth. 

An appreciation of the importance of hydrostatic resin pressure must be developed to understand void growth fully. Because of the load-carrying capability of the fiber bed in a composite layup, the hydrostatic resin pressure needed to suppress void formation and growth is typically only a fraction of the applied autoclave pressure. The hydrostatic resin pressure is critical because it is the pressure that helps to keep volatiles dissolved in solution. If the resin pressure drops below the volatile vapor pressure, then the volatiles will come out of solution and form voids. 

Void formation and growth in addition curing composite laminates is primarily due to entrapped volatiles. Higher temperatures result in higher volatile pressures. Void growth will potentially occur if the void pressure (i.e., the volatile vapor pressure) exceeds the actual pressure on the resin (i.e., the hydrostatic resin pressure) while the resin is a liquid (Fig. 10.9). The prevailing relationship, therefore, is ... 

If a volatile component is present in the cured propellant, another effect must be considered. In polysulfide formulations a molecule of water is generated each time a polysulfide bond is formed. The vapor pressure of the ammonium perchlorate propellant formulation becomes that of an ammonia-ammonium perchlorate saturated solution. Ammonia and water can be driven from the formulation, and the water condenses on cold surfaces. If the condensate returns to the propellant surface, perchlorate is leached from the surface of the propellant. This perchlorate may later recrystallize on the surface. A surface void of perchlorate is very difficult to ignite, while a perchlorate-rich surface produces the... 

When the molecules of water vapor come in contact with the room-temperature water in the saucepan, they condense, leaving a very low pressure inside the can. The much greater surrounding atmospheric pressure crushes the can. Here you see dramatically how pressure is reduced by condensation. This occurs because liquid water occupies much less volume than does the same mass of water vapor. As the vapor molecules come together to form the liquid, they leave a void (low pressure). This activity also shows how the atmospheric pressure surrounding us is very real and significant. 

The fluid phase that fills the voids between particles can be multiphase, such as oil-and-water or water-and-air. Molecules at the interface between the two fluids experience asymmetric time-average van der Waals forces. This results in a curved interface that tends to decrease in surface area of the interface. The pressure difference between the two fluids A/j = v, — 11,2 depends on the curvature of the interface characterized by radii r and r-2, and the surface tension, If (Table 2). In fluid-air interfaces, the vapor pressure is affected by the curvature of the air-water interface as expressed in Kelvin s equation. Curvature affects solubility in liquid-liquid interfaces. Unique force equilibrium conditions also develop near the tripartite point where the interface between the two fluids approaches the solid surface of a particle. The resulting contact angle 0 captures this interaction. 

Wet/Wet Pressing, Representative press cycles, conceptual schematics of mass transfer in the sheet, and density profiles through the thickness of the sheet are portrayed in Figure 7, Pressing of wet mats starts with a steady pressure rise to 400 psi platen pressure so as to compress the mat to minimum void volume and express water retained from cold pressing. Platen steam pressures up to 400 psig heat the mat and reduce water viscosity and raise its vapor pressure. This high-presstire inversion cycle is followed by a period of low platen pressure intended to dry the sheet to anhydrous condition. 

Type H3 hysteresis loop, which does not level off near the saturation vapor pressure, is characteristic of the mesoporous materials being comprised of agglomerates of plate-like particles with slit-shaped pores.79,86 Type H4 loop, which features parallel and almost horizontal branches, is attributable to the adsorption/ desorption in narrow slit-like pores. However, Type H4 loop was recently reported for MCM-41 being comprised of particles with internal voids of irregular shape and broad PSD,90 and also... 

In hot mixing or elevated-temperature curing of an epoxy system, vapor pressure could also be of concern relative to the quality of the adhesive bond. If the components in an epoxy system become too hot, boiling can occur, resulting in gas bubbles. If gas bubbles become trapped in the cured adhesive film, they can lead to reduction of cohesive strength and stress risers. For many adhesive applications, particularly those in the electrical and electronic industries (due to possible ionization of air voids), complete removal of any gas bubbles from the epoxy is essential. 

P0 = pure component vapor pressure (in this case, water), and Pj = partial pressure of water in the gas mixture in the void. 

Refer to the portion of Figure 4.6 where the source tank fluid level is below the center line of the impeller in either the system connected in parallel or in the system connected in series. At the surface of the wet well (point 1), the pressure acting on the liquid is equal to the atmospheric pressure Patm minus the vapor pressure of the liquid Py. This pressure is, thus, the atmospheric pressure corrected for the vapor pressure and is the pressure pushing on the liquid surface. Imagine the suction pipe devoid of liquid if this is the case, then this pressure will push the fluid up the suction pipe. This is actually what happens as soon as the impeller starts moving and pulling the liquid up. As soon as a space is evacuated by the impeller in the suction pipe, liquid rushes up to flU the void this is not possible, however, without a positive NPSH to push the liquid. Note that before the impeller can do its job, the fluid must, first, reach it. Thus, the need for a driving force at the inlet side. 

During desorption, as the relative vapor pressure is reduced, pore solids in which capillary condensation occurs often show a hysteresis loop. The simplest interpretation of this phenomenon is given by the ink-bottle model (6). In the framework of this model (Fig. 12) the adsorption and desorption processes are controlled, respectively, by the void and neck sizes. Thus, desorption from a given pore occurs at a lower pressure than adsorption. 

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