Transfer of Nonvolatile Compounds

Jahne et al. (1984,1987) proposed that hquid film coefficient is better related to mean water surface slope. Frew (1997) has found that the Kl relationship using mean square slope can be used to describe gas transfer with and without surface slicks. The problem with mean surface slope is that it cannot be accurately predicted for water bodies, because most investigators have emphasized the larger and longer waves, and the slope is most significant for the small, short waves. This will likely be the subject of future investigations. 

The evaporation of water is generally used to determine the gas film coefficient. A loss of heat in the water body can also be related to the gas film coefficient because the process of evaporation requires a significant amount of heat, and heat transfer across the water surface is analogous to evaporation if other sources and sinks of heat are taken into account. Although the techniques of Section 8.D can be used to determine the gas film coefficient over water bodies, they are still iterative, location specific, and dependent on fetch or wind duration. For that reason, investigators have developed empirical relationships to characterize gas film coefficient from field measurements of evaporation or temperature. Then, the air-water transfer of a nonvolatile compound is given as 

The relationships developed from field measurements have been made dimensionless with the assumptions that v = 1.33 x 10 m /s and AijO = 2.6 x 10 m /s to facilitate comparisons between relations and avoid dimensional problems. They are given in Table 9.2. The early measurements were to investigate the loss of water from the reservoirs of the Colorado River in the United States, and the later measurements were designed to investigate heat loss from heated water bodies. A revelation occurred in 1969, when Shulyakovskyi brought in buoyancy forces as related to natural convection to explain the heat loss from heated water at low wind velocities. This was picked up by Ryan and Harleman (1973), who realized that natural convection could explain the need for a constant term in front of the relationship for gas film coefficient, as had been found by Brady et al. (1969), Kohler (1954), Rymsha and Dochenko (1958), and Shulyakovskyi (1969). Finally, Adams et al. (1990) rectified 

By using the latent heat of vaporization for water, Lg, from equation (8.86), the previous relations for mass transfer can also be used for heat transfer due to evaporation  

EXAMPLE 9.3 Application of characteristic relations for gas film coefficient 

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