Condensation vapor drag

For practical values of H and Prf, Eq. 14.33 was found to be near unity, indicating that acceleration and convection effects are negligible. Chen [34] included the effect of vapor drag on the condensate motion by using an approximate expression for the interfacial shear stress. He was able to neglect the vapor boundary layer in the process and obtained the results shown in Fig. 14.8. The influence of interfacial shear stress is negligible at Prandtl numbers of ordinary liquids (nonliquid metals, Pr< > 1). Chen [34] was able to represent his numerical results by the approximate (within 1 percent) expression ... 

FIGURE 14.8 Influence of vapor drag during laminar free convection condensation on a vertical plate [31]. (Reprinted with permission from JSME International Journal, Tokyo, Japan.)... 

Equation 14.112 overpredicts the experimental data of Nandapurkar and Beatty [124] for ethanol, methanol, and R-113 by about 25 percent. Sparrow and Gregg [125] subsequently included the effect of induced vapor drag in their analysis and found that this effect was negligible. Sparrow and Hartnett [126] conducted a similar analysis for condensation on the outside of a rotating cone of half-cone angle p and found that... 

E. M. Sparrow and J. L. Gregg, The Effect of Vapor Drag on Rotating Condensation, J. Heat... 

The term mist generally refers to liquid droplets from submicron size to about 10 /xm. If the diameter exceeds 10 /xm, the aerosol is usually referred to as a spray or simply as droplets. Mists tend to be spherical because of their surface tension and are usually formed by nucleation and the condensation of vapors (6). Larger droplets are formed by bursting of bubbles, by entrainment from surfaces, by spray nozzles, or by splash-type liquid distributors. The large droplets tend to be elongated relative to their direchon of mohon because of the action of drag forces on the drops. 

The velocity of the vapor is low (or zejo) so that it exerts no drag on the condensate (no viscous shear on the liquid-vapor interface). 

A possible explanation for this absorption enhancement may be that under conditions of water condensation, there is a net movement of water vapor toward the water surface, tending to drag extra gas molecules along, whereas in the steady-state condition there exists an equimolar exchange of water molecules both to and from the surface. The gas is absorbed in both cases>but in the non-steady state the amount of gas entering the surface is enhanced somewhat by the condensing H2O molecules. The resistance to absorption is primarily in the liquid phase which is governed by the dissolved gas concentration at the air-water interface. Therefore, if more gas molecules can be packed into the surface, this would tend to enhance the overall absorption process. 

The main processes are electrochemical reactions at electrified metal-electrolyte interfaces reactant diffusion through porous networks proton transport in water and at aggregates of ionomer molecules electron transport in electronic support materials water transport by gasous diffusion, hydraulic permeation, and electro-osmotic drag in partially saturated porous media and vaporization/condensation of water at interfaces between liquid water and gas phase in pores. 

Under steady-state operation, local mechanical equilibrium prevails at all microscopic and macroscopic interfaces in the membrane. It fixes the stationary distribution of absorbed water. The condition of chemical equilibrium is, however, lifted to allow for the flux of water. Continuity of the net water flux in the PEM and across its interfaces with adjacent media adjusts the gradients in water activity or pressure in the system. Water fluxes occur by diffusion, hydraulic permeation, and electro-osmotic drag. At external interfaces, vaporization and condensation proceed at rates that match the net water flux. These mechanisms apply to PEM operation in a working cell, as well as to ex situ water flux measurements that are conducted in order to investigate the transport properties of PEMs. 

A highly water-permeable PEM would facilitate water removal via liquid transport toward the anode, alleviating the problem of cathode flooding and anode dehydration. Erom a system perspective, it is deemed beneficial to make use of internal humidification of CLs and PEM by water that is produced at the cathode. This mode of internal water management obviates the need for external humidifiers. It demands, however, precise control of water permeation rates through the PEM and of vaporization rates in partially saturated porous electrodes. Therefore, it is cmcial to know how relevant parameters of water transport (diffusion, hydraulic permeation, electro-osmotic drag, vaporization, and condensation) depend on PEM morphology and thermodynamic conditions. 

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