Conductance Ratios
pmol
= -32m2s
mol
Fo = go(Cos - CO,) = 0.155- x (0.2102 - 0.2100)
m
2 s
pmol
= 3 1 - .
m
2 s
The negative sign on the carbon dioxide flux means that C02 is being
taken up by the leaf. The fact that the oxygen and C02 fluxes are nearly
equal in size and opposite in sign is the result of the stoichiometry of
the photosynthesis reaction. One oxygen molecule is produced per C02
molecule.
7.10 Combined Forced and Free Convection
Almost all convective heat transfer processes in nature involve both forced
and free convection. Usually one or the other process dominates and the
conductance used is that calculated for the dominant process. The criterion normally used to determine which process is dominant is to determine
the ratio ~ r / R e ~ .
If this ratio is small, forced convection dominates. When
the ratio is large, the opposite is true. When the ratio is near one, both
forced and free convection must be considered, and the value of the resistance depends on whether the direction of the forced convection flow
is such that it enhances or diminishes free convection. Additional detail
on the mixed regime can be found in Kreith (1965).
Example 7.5. Find Gr for the previous example, and compute the ratio
cr/Re2.
Solution. From Table 7.3:
Forced convection is obviously dominant.
7.1 1 Conductance Ratios
Situations often arise when the conductance for one species is known and
we need to know the conductance for another. For example, we can easily
determine the stomatal conductance for water vapor, but cannot determine
it for COz. Is it possible to calculate one conductance if another is known?
Considering any of the molecular diffusion equations (Eqs (7.4) through
(7.8)), the ratio of the conductances is equal to the ratio of the diffusivities
pmol
= -32m2s
mol
Fo = go(Cos - CO,) = 0.155- x (0.2102 - 0.2100)
m
2 s
pmol
= 3 1 - .
m
2 s
The negative sign on the carbon dioxide flux means that C02 is being
taken up by the leaf. The fact that the oxygen and C02 fluxes are nearly
equal in size and opposite in sign is the result of the stoichiometry of
the photosynthesis reaction. One oxygen molecule is produced per C02
molecule.
7.10 Combined Forced and Free Convection
Almost all convective heat transfer processes in nature involve both forced
and free convection. Usually one or the other process dominates and the
conductance used is that calculated for the dominant process. The criterion normally used to determine which process is dominant is to determine
the ratio ~ r / R e ~ .
If this ratio is small, forced convection dominates. When
the ratio is large, the opposite is true. When the ratio is near one, both
forced and free convection must be considered, and the value of the resistance depends on whether the direction of the forced convection flow
is such that it enhances or diminishes free convection. Additional detail
on the mixed regime can be found in Kreith (1965).
Example 7.5. Find Gr for the previous example, and compute the ratio
cr/Re2.
Solution. From Table 7.3:
Forced convection is obviously dominant.
7.1 1 Conductance Ratios
Situations often arise when the conductance for one species is known and
we need to know the conductance for another. For example, we can easily
determine the stomatal conductance for water vapor, but cannot determine
it for COz. Is it possible to calculate one conductance if another is known?
Considering any of the molecular diffusion equations (Eqs (7.4) through
(7.8)), the ratio of the conductances is equal to the ratio of the diffusivities