Convection falling films

For example, vaporization may occur as a result of heat absorbed, by radiation and convection, at the surface of a pool of hquid or as a result of heat absorbed by natural convect ion from a hot wall beneath the disengaging surface, in which case the vaporization takes place when the superheated liquid reaches the pool surface. Vaporization also occurs from falling films (the reverse or condensation) or from the flashing of hquids superheated by forced convec tion under pressure. 

A knowledge of the velocity profiles within falling films under various flow conditions would be of very great value, making it possible to calculate the rates of convective heat and mass transfer processes in flowing films without the need for the simplified models which must be used at present. For instance, the analyses of Hatta (H3, H4) and Vyazovov (V8, V9) indicate clearly the differences in the theoretical mass-transfer rates due to the assumption of linear or semiparabolic velocity profiles in smooth... 

Labuntsov (LI), 1956 Heat transfer to falling films (laminar flow) effects of convective heat transfer and inertia forces (neglected in Nusselt theory) considered experimentally and theoretically. 

Forced convection heat transfer is probably the most common mode in the process industries. Forced flows may be internal or external. This subsection briefly introduces correlations for estimating heat-transfer coefficients for flows in tubes and ducts flows across plates, cylinders, and spheres flows through tube banks and packed beds heat transfer to nonevaporating falling films and rotating surfaces. Section 11 introduces several types of heat exchangers, design procedures, overall heat-transfer coefficients, and mean temperature differences. 

R. Numrich, Falling Film Evaporation of Soluble Mixtures, in Convective Flow Boiling, J. C. Chen ed., pp. 335-338, Taylor Francis, Washington, DC, 1996. 

In terms of process technology the most easily realized evaporators are the natural-convection evaporator with heating elements and the thin-film evaporator with rotating wiper blades. When the exhaust vapors of a wiped thin-film evaporator are withdrawn at the bottom, for small heights, a falling-film evaporator can be simulated (Figure 2.3.1-6). Exceptions are cases, especially with evaporators, in which material-, temperature-, and residence-time-dependent changes in the product are ex-... 

The thickness of falling film is small the density difference between interface and the film in the direction perpendicular to the interface is negligible so that the effect of Rayleigh convection can be neglected. Also the process is isothermal, and the Marangoni convection is only due to interfacial concentration difference. 

Under the condition of no Marangoni convection, the mass transfer on falling film is only by diffusion. Zhang et al. [16] and Bird et al. [17] derived the following equation of overall mass transfer coefficient ATuheo based on the penetration theory ... 

This model assumes a stagnant film of thickness 8l at the surface of the liquid next to the gas, while the rest of the liquid below the film boundary is kept uniform in composition by turbulent agitation (W5). Convection is assumed absent in the liquid film, within which the transport of the soluble gas takes place by molecular diffusion alone. The concentration in the film falls from C at the interface toCAo at the inner edge of the film, that is, in the bulk liquid. This simple model leads to... 

When nucleate boiling does occur, correlations similar to those for pool boiling and forced convective boiling may be developed. An example of such a correlation, obtained for fully developed nucleate boiling in falling water films at atmospheric pressure, is that of Fujita and Ueda [362], which is as follows ... 

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