Plate flat, mass transfer from

The mass transfer from a flat plate has been studied by Spalding (S12) for both forced and free convection. 

Another method used is to flow gases over various geometries wet with evaporating liquids. For mass transfer from a flat plate, a porous blotter wet with the liquid serves as the plate. 

Mass transfer in flow parallel to flat plates. The mass transfer and vaporization of liquids from a plate or flat surface to a flowing stream is of interest in the drying of inorganic and biological materials, in evaporation of solvents from paints, for plates in wi nd tunnels, and in flow channels in chemical process equipment. 

EXAMPLE 73-2. Mass Transfer from a Flat Plate... 

Davis, H.R., 1978. Laminar mass transfer from porous tubes and flat plates with wall resistance. Appl. Sci. Res. 34, 127-143. 

TABLE 5-21 Mass Transfer Correlations for a Single Flat Plate or Disk—Transfer to or from Plate to Fluid... 

Figure 9-10 Sketches of film profiles for chemical vapor deposition over a flat plate with reactant gases flowing from left to right. Reaction-limited deposition should produce a uniform film (a), while mass-transfer-limited Roi should produce a thicker film near the leading edge (b).

Zehnder and Trepp [23] studied the evaporation of a-tocopherol and two iso-phythols into a dense gas from a flat liquid plate (Re = 100 to 3000) and observed the surface during evaporation, with an optical fibre device. The regressed mass-transfer equation was as follows ... 

Fractionation, by definition, is simply the mass transfer between a liquid phase and a gas phase in contact with each other. A fractionation column is simply a tall, vertical, cylindrical pressure vessel that contains numerous flat internal metal plates called trays. Each tray allows liquid to flow over it, so the liquid flows from tray to tray by the force of gravity. The liquid thus enters the top tray. The liquid portion not vaporized in the column s trays is taken out in the column s bottom liquid accumulation. Gas enters the column s bottom section and flows through each tray to the top section. Entering vapor pressure is its driving force. Gas not absorbed by the liquid exits the column s top section. 

Wickem. G., Mixed Convection from an Arbitrarily Inclined Semi-Infinite Flat Plate , lnt. J. of Heat and Mass Transfer, Vol. 34, pp. 1935-57, 1991. 

The boundary layer theory is based on the system in Fig. 11, where mass transfer is occurring from a flat plate in the presence of laminar flow. 

In transverse flow along a flat plate the transition from laminar to turbulent flow occurs at Reynolds numbers waL/v between 3 105 and 5 10s wa is the initial flow velocity, L is the length of the plate over which the fluid is flowing. The heat and mass transfer in turbulent flows is more intensive than in laminar. In general, at the same time there is also an increase in the pressure drop. 

Heat and mass transfer apparatus normally consist of channels, frequently tubes, in which a fluid is heated, cooled or changes its composition. While the boundary layers in flow over bodies, for example over a flat plate, can develop freely without influence from neighbouring restrictions, in channels it is completely enclosed and so the boundary layer cannot develop freely. In the following the flow, and then the heat and mass transfer in tubes will be discussed. After this we will study flow through packed and fluidised beds. 

This is the appropriate correlation to use when there is heat or mass (i.e., substitute Nu by Sh) transfer from a sphere immersed in a stagnant film is studied, Nu = 2. The second term in (5.294) accounts for convective mechanisms, and the relation is derived from the solution of the boundary layer equations. For higher Re3molds numbers the Nusselt number is set equal to the relation resulting from the boundary layer analysis of a flat plate ... 

Many numerical and series solutions for the laminar boundary layer model of mass transfer are available for situations such as flow in coeduits under conditions of fully developed or developing concentration or velocity profiles. Skellaed31 provides a particularly good summary of these results. The laminar boundary layer model has been extended to predict tha effects of high mass transfer flux on the mass transfer coefficient from a flat plate. The results of this work ate shown in Fig. 2.4-2 and. in com rest to the other theories, iedicate a Schmith number dependence of Ihe correction factor. 

T. Z. Fahidy, On the Flat Plate Approximation to Laminar Convection From the Outer Surface of a Vertical Cylinder, Int. J. Heat Mass Transfer (17) 159-160,1974. 

Uniform Surface Injection. Although a mass transfer distribution yielding a uniform surface temperature is most efficient, it is much easier to construct a porous surface with a uniform mass transfer distribution. Libby and Chen [34] have considered the effects of uniform foreign gas injection on the temperature distribution of a porous flat plate. For these conditions, however, boundary layer similarity does not hold. Libby and Chen extended the work of Iglisch [35] and Lew and Fanucci [36], where direct numerical solutions of the partial differential equations were employed. An example of the nonuniform surface enthalpy and coolant concentrations resulting from these calculations is shown in Fig. 6.16. 

In typical applications, pure solid naphthalene is melted and poured into a mold so it will have the desired shape such as a flat plate [127], a circular cylinder [128], or a turbine blade [129]. For average mass transfer measurements on a test surface, the section coated with naphthalene can be weighed before and after exposure to air flow to determine the mass transfer rate. Local mass transfer coefficients can be determined from the sublimation depth, which is the difference in surface profiles, measured using a profilometer, before and after each test run. Once the vapor density of naphthalene is known, the local mass transfer coefficient hD can be evaluated from the following expression ... 

In a few limited situations mass-transfer coefficients can be deduced from theoretical principles. One very important case in which an analytical solution of the equations of momentum transfer, heat transfer, and mass transfer has been achieved is that for the laminar boundary layer on a flat plate in steady flow. 

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