Stagnant film thickness

This approach to the problem is purely theoretical, since this model is based on a characteristic stagnant film thickness which is difficult to estimate or to measure. In addition, this model does not give any information as to the value of A, which must be determined separately by some other method. As a result, it is impossible to estimate the total mass-transfer rate in the disperser with the aid of this model only. 

Fig. 10). Under steady-state conditions and assuming the boundary conditions (note that no assumptions about the stagnant film thickness have been made)... 

The air-sea fluxes are inversely proportional to the thickness of the thin film. From the measurement of gas fluxes and concentration gradients, oceanographers have inferred that the stagnant-film thickness ranges from 10 to 60 gm. As mentioned earlier, direct flux measurements are difficult to make at sea, especially in the presence of... 

The effective segregation coefficient keS is defined in tenns of the stagnant-film thickness as... 

Introducing this value of the stagnant film thickness, they derived a value of the axial dispersion coefficient ... 

In commercial absorption equipment, both the liquid and vapor are usually in turbulent flow and the effective stagnant film thickness AZ is not known. The common practice, therefore, is to rewrite (16-2) in terms of an empirical mass transfer (film) coefficient that replaces both the diffusivity and film thickness. Thus, by definition. 

A overaii increases because, as the LR and BR stagnant film thicknesses decrease, the time to cross each of them decreases due to Atbrf and A lrf being constant. 

Both the gas and the liquid phase can be divided into a stagnant film (thickness 5g and 5i) located near the interface and well-mixed bulk phases without concentration gradients. 

In practice, the analysis of heat transfer by convection is treated anpiricaUy (by direct observation). Convection heat transfer is treated empirically because of the factors that affect the stagnant film thickness such as fluid velocity, fluid viscosity, heat flux, surface roughness, and the type of flow (single phase/double phase). 

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