Untitled - Aerobib - Universidad Politécnica de Madrid

13.15. DROPLET EVAPORATION 327
4
0.25
P 1
(kg/cm 2 )
3
2
P 1
(kg/cm 2 )
0.20
n−heptane
0.15
0.10
n−exane
0.05
n−octane
0.00
250 275 300 325 350
T (K)
n−heptane
n−octane
n−exane
1
n−decane
n−dodecane
0
250 300 350 400 450 500
T (K)
Figure 13.6: Partial pressure of fuel vapour as a function of temperature.
Therefore a relation exists between p 1s and T s of the form
Figure 13.6 gives Eq. (13.93) for some typical fuels.
f(p 1s , T s ) = 0. (13.93)
Furthermore 14 p 1
p = M a Y
( 1
) , (13.94)
M c Ma
1 + − 1 Y 1
M c
where M c and M a are, respectively, the molar masses of the fuel vapour and of the
gas through which it diffuses.
When (13.94) is particularized on the droplet surface, taking the result into
(13.93), the following relation between Y 1s and T s is obtained
This relation and (13.92) determine Y 1s and T s .
F (Y 1s , T s ) = 0. (13.95)
Once T s is known the evaporation velocity m of the droplet is deduced from
(13.90) by making r = r s and T = T s . Thus obtaining
m = 4πλ (
∞r s
1 − T s
+ q )
l − c p1 T s q l
ln
. (13.96)
c p1 T ∞ c p1 T ∞ c p1 (T ∞ − T s ) + q l
14 See chapter 1.