Environmental Engineering Reference
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of liquid on the ground surface. The vertical axis in the
figure
is the ratio of
predicted to observed droplet diameter, D
pred
/D
obs
.
model as a function of the superheat, ΔT
SH
. Overall, the JIP model exhibits a trend
of over-prediction (up to a factor of 3) at the lower superheats (ΔT
SH
< 30 K) and
under-prediction for the higher superheats, with the RELEASE model showing a
similar trend. The dependence of these trends on the orifice velocity, U
o
, is
investigated in the right side of
Fig. 1.
For both models, a trend of over-prediction
at lower velocities and under-prediction at higher U
o
's is seen, and may be caused
o
by a deficiency in the model physics concerning the treatment of U , a difference
in the measurement methods for the two datasets, and/or a difference between the
methods for determining D.
In most of these and other comparisons of source emission model estimates
with observations there is typically a factor of 2 error in models compared with
research-grade observations. Of course, in a real accident or terrorist action, the
source conditions (e.g., hole size and shape, and location on the vessel; storage
temperature and pressure; wind speed and direction; locations of nearby obstacles)
will be poorly known and thus there will be source emission uncertainties of more
than a factor of 2.
5
5
a)
6.5
b)
6.5
REL JIP Data
CCPS
4
4
Witlox et al. (2007)
Ideal ratio
3
3
2
2
1
1
0
0
0
0
20
40
60
80
10
20
30
40
∆
T
SH
(˚K)
U
°
(m/s)
Fig. 1.
Predicted-to-observed droplet diameter as a function of liquid superheat and orifice
velocity for the JIP and RELEASE (REL) models
3. Hand-Off to the SCIPUFF Transport and Dispersion Model
The dispersion model SCIPUFF currently has some restrictions on what source
emission model input data it can accept during the “hand-off” process. The principal
limitation is that non-vertical jets cannot be accommodated. This restriction is
currently being removed by the SCIPUFF developers. Britter et al. (2009) review
general methods for the hand-off or transition (mostly based on physics-based
