Gravitational force, condensation

Another type of condensation, called dropwise, occurs when the wall is not uniformly wetted by the condensate, with the result that the condensate appears in many small droplets at various points on the surface. There is a growth or individual droplets, a coalescence of adjacent droplets, and finally a formation of a liviilet. Adhesional force is overcome by gravitational force, and the rivulet flows quickly to the bottom of the surface, capturing and absorbing all droplets in its path and leaving dry surface in its wake. 

The rate of condensation on a vertical surface is controlled by the force of gravity acting on the condensed liquid film. A consideration of Eq. (11.20) shows for example that for a vertical plate the mean heat transfer rate from the plate with laminar flow in the film is proportional to gw. Attempts have therefore been made to increase condensation rates by using centrifugal forces instead of the gravitational force to drain the condensed liquid film from the cold surface [55], The simplest example of this would be condensation on the upper surface of a cooled circular plate rotating in a horizontal plane. This situation is shown in Fig. 11.23. A Nusselt-type analysis of this situation will be considered in the present section. 

For a vertical condensing liquid film, scale analysis of the 2-dimensional constant-property conservation equations of mass, momentum and energy, with appropriate boundary conditions, shows that Rafii is a measure of the slenderness ratio L/d of the film (18). Here L is taken as the streamwise length of the condensation film and d an average film thickness. This interpretation is valid in the limit where the momentum equation is a balance of viscous and gravitational forces. Under such conditions, a velocity scale is... 

In order to improve the heat transfer in a reactor, use can be made of gravitational forces. This concept is used in the spinning disc reactor (SDR) as developed at Newcastle University. The reaction mixture flows in a thin layer in axial direction over a rotating disc. A typical heat transfer coefficient is 10 kW/m2K. This reactor however is dedicated for liquid-liquid reactions. Especially condensation reactions can be enhanced by removing the gaseous by-products thus shifting the chemical equilibrium to the right. 

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. 

At film condensation, the heat has to be transported through the film. If the film is laminar and the film surface is at thermodynamic equilibrium, then the heat is transferred by conduction. The transfer coefficient can be calculated, if the film thickness is known depending on the film length. In case of a laminar film the velocity profile is defined by an equihbrium between viscous and gravitational forces, see Chap. 3. Considering the conservation laws for mass and energy allows to derive the heat transfer coefficient on a theoretical basis. 

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... 

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. 

Gen4 or HYPERION s safety system can remove the decay heat in two ways (1) dumping the steam to the condenser and (2) if first decay heat removal way is not sufficient, back up decay heat removal system is used. This system utilizes natural circulation of primary coolant through bypass path in the core. The surface of Gen-IV module is cooled with latent of heat via water sprays provided by emergency cooling tank. The water inventory in this tank can be injected due to gravitational force to remove the decay heat for 2 weeks. The second system works as a passive safety system. 

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