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The water surface saturated absolute humidity C sv can also be calculated by using Eq.
(3.41) but using the saturated humidity at the water temperature.
The corresponding thermal energy removed from the water to chamber air by
evaporation is:
Q k A ( C C ) h
(3.42)
ev ev ws sv Caiv fg
Where hfg is the latent heat with unit of (J/kg) which depends on temperature of water
where the vapour comes from [54]:
h 2257000 4182(100 T )
(3.43)
fg w
For the mass convection (evaporation) coefficient k ev , an analogy to heat convection is
applied [55]:
1/ 4
0.54( Gr Sca
)
kevn Dwa
(3.44)
0.73 1/3 0.8 1/3
0.5(0.23Re ws Sca 0.037 Re ws Sca
)
evf wa
k D
(3.45)
Where Sca is Schmidt number for molecular diffusivity of water vapour in air and Dwa is the
mass diffusivity of water molecules from fluid water into air.
D wa , may be expressed as [56]:
D
5
o 1.685
wa 1.97 10 ( )( )
(3.46)
P To
54
P T
Where o P is the standard atmospheric air pressure (101325 Pascals) and To is a specific
standard temperature (256 K) for mass diffusivity empirical equation. However, since
the mass diffusivity does not change very much within the working range, for
simplifying the computation, an average value of 2.71×10 -5 m 2 /s calculated from Eq.
(3.46) is used for chamber evaporation.
Schmidt number for molecular diffusivity of water vapour in air is defined as [56]: