Conductances for Heat and Mass Transfer
Air
Leaf
Temperature
20" C
30" C
wind speed
3 rnls
vapor concentration
15 mmoVmol
carbon dioxide
350 pmollmol
70 pmollmol
concentration
oxygen concentration 2 10.0 mmoVmol 2 10.2 mmollmol
Assume the leaf has a characteristic dimension in the direction of wind
flow of d = 0.042 m and that surface conductances (stomatal) to water
vapor, C02, and 0 2 transfer are 0.2,0.13, and 0.18 rnol m
2 s-'.
Solution. The water vapor concentration at the leaf surface is the saturation concentration at leaf temperature. From Table A.3 the vapor pressure
is 4.2 kPa, so the mole fraction (at 10 1 H a ) is 43 mmol/mol. The Reynolds
number is Re = udlv = 3 d s x 0.042 d1.51 x
= 8.3 x lo3.
The flow apparently is laminar. All of the conductances except heat involve two conductors in series, the stomatal and the boundary layer. The
conductances all involve:
The conductances are:
1
gv =
1
= 0.172 mol m-2s-1
0.147x8.47 + a
1
gc =
1
1
= 0.114 mol m-2s-1
0.11x8.47 + 0.13
1
go =
I
=0.155m0lrn-~s-'.
0.13x8.47 +
The fluxes are:
rnol
H = c p g ~ ( T s
- Ta) = 1.14 - x 29;3 - (30°C - 20°C)
m 2 s
mol C
rnol
E = gu(Cus - Cua) = 0.172- x (0.042 - 0.015)
m
2 s
mmol
= 4.6 -
m 2 s
rnol
Fc = gc(Ccs - Cca) = 0.114-(70 x
- 350 x
m 2 s