Gravitational force vapor

At the top distributed solutions flow down the inside of evaporator tubes as a falling film by gravitational force, vapor is evaporated from the liquid film, Z = 5-7 m, d approximately 50 mm, liquid load ca. 0.7-3 m /(h m), mean residence time 1-3 min, to calculate the heat transfer see [7.21]. 

In settling processes, particles are separated from a fluid by gravitational forces acting on the particles. The particles can be liquid drops or solid particles. The fluid can be a gas, vapor or liquid. 

The properties of liquid metals can cause flow instability (oscillation) because of vapor pressure—temperature relationship. Most liquid metals, especially alkali metals, show a greater change in saturation temperature, corresponding to a given change of pressure, than does water. In a vertical system under gravitational force, the change of static pressure could appreciably alter the saturation temperature such that explosion -type flow oscillation would occur that would result in liquid... 

This first term in this equation is the result of the gravitational force on the liquid in the control volume, the second term is due to the viscous shear force, while the third represents the pressure exerted on the control volume due to the gravitational force on vapor. 

This equation indicates that the shear force is equal to the net gravitational force, i.e., the gravitational force on the liquid less the gravitational force that would have acted on the control volume had it contained vapor and not liquid. 

Allowable flows of vapor through the froth are correlated on the basis of liquid entrainment. Balancing of the drag force of the vapor on a representative drop of liquid against the gravitation force on the drop has led to the relationship. 

In order to stimulate condensate motion under zero-G conditions, other forces must replace the gravitational force. This may be done by centrifugal forces, vapor shear forces, surface tension forces, suction forces, and forces created by an electric field. McEver and Hwangbo [133] and Valenzuela et al. [134] describe how surface tension forces may be used to drain a condenser surface in space. Tanasawa [1] reviews electrohydrodynamics (EHD) enhancement of condensation. Bologa et al. [135] showed experimentally that an electric field deforms the liquid-vapor interface, creating local capillary forces that enhance the heat transfer. 

Stratified. During condensation within horizontal tubes, when the vapor velocity is very low (i.e., jf is less than 0.5), the flow will be dominated by gravitational forces, and stratifica-... 

An interfacial shear may be very important in so-called shear-controlled condensation because downward interfacial shear reduces the critical Re number for onset of turbulence. In such situations, the correlations must include interfacial shear stress, and the determination of the heat transfer coefficient follows the Nusselt-type analysis for zero interfacial shear [76], According to Butterworth [81], data and analyses involving interfacial shear stress are scarce and not comprehensive enough to cover all important circumstances. The calculations should be performed for the local heat transfer coefficient, thus involving step-by-step procedures in any condenser design. The correlations for local heat transfer coefficients are presented in [81] for cases where interfacial shear swamps any gravitational forces in the film or where both vapor shear and gravity are important. 

In the last two sections we considered hydrometeors, drops and ice particles in clouds, fog, mist and precipitation. This section deals with the formation of interfacial water, either from water vapor (dew and frost) or from hydrometeors (rime and interception). These forms of atmospheric water need the contact with a surface soils and vegetation but also artificial surfaces. Some meteorologists classify these phenomena as belonging to precipitation - we will not (see Chapter 4.4 for more details). Precipitation is physically a sedimentation process due to gravitational force see Fig. 2.40 for different deposition processes. 

The heat pipe is a device that utilizes evaporation heat transfer in the evaporator and condensation heat transfer in the condenser in which the vapor flow from the evaporator to the condenser is caused by the vapor pressure difference, and the liquid flow from the cmidenser to the evaporator is produced by capillary force, gravitational force, electrostatic force, or other forces directly acting on it. A micro heat pipe is so small that the mean curvature of the liquid-vapor interface is comparable in magnitude to the reciprocal of the hydraulic radius of the total flow channel. Mathematically, the definition of micro heat pipe can be expressed as... 

Thermally gentle treatment of a solution is enabled by continuous (one pass) evaporators (Fig. 7-21). The solution flows as a film through the evaporation zone, driven by gravitational force, and may be additionally whipped, or dragged up by the vapor. A small pressure drop, low liquid holdup and hence a small mean solution residence... 

As mentioned above, spray occurs under large vapor momentum (Mh<4) but small liquid gravitational force h. By considering density difference in vapor and liquid, the equation for spray factor describing the relative momentum was developed by Lockett (1986) as... 

However, how can we know the limit for vapor capacity at which the column starts to flood This question can be answered by making a force balance between vapor and liquid on the tray. Assume the bulk of liquid with a volume v and the liquid gravitational force is v — p )g. At the same time, vapor flows upward with velocity of u and thus vapor momentum is f palp ll) where/is the friction coefficient between vapor and liquid. 

In Fig. 4.8-3a vapor at is condensing on a wall whose temperature isT , K. The condensate is flowing downward in laminar flow. Assuming unit thickness, the mass of the element with liquid density p, in Fig. 4.8-3b is (S — y)(dx l)p,. The downward force on this element is the gravitational force minus the buoyancy force or (S — yXdx) x (Pi — p )g where p is the density of the saturated vapor. This force is balanced by the viscous-shear force at the plane y of p, dv/dy) (dx- 1). Equating these forces. 

A concern some users have expressed over the years is whether gases can unmix or stratify due to gravitational forces. It has been demonstrated conclusively that once a gas mixture is homogeneous, it remains so and does not separate due to gravity. However, mixtures containing vapors of condensable components, if subjected to a temperature below their dew point, will experience condensation of the condensable component. 

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