Analysis of the mist lift process for mist flow open-cycle OTEC Page: 9 of 13
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injected into this almost stagnant vapor and are
slowed by drag forces and gravity until they stop, so
that rainout occurs after only a few metres. The
steep jump at the low-flow-rate side of each of the
curves corresponds to the flow rate at which just
enough vapor is generated by flashing of the warm
water to accelerate the drops up the tube just before
they stop due to the force of gravity. This condition
leads to a maximum lift height. As the flow rate is
increased further, the predicted pressure at the
bottom of the lift tube decreases, which leads to
increased flashdown of the warm water and higher vapor
velocities at the inlet. Since the temperature at the
entrance decreases as the flow rate increases, the
condenser temperature is reached at a lower ..height and
the model predicts a decreased lift height. However,
the velocities of the vapor and droplets are not zero
when the temperature of the condenser is reached, and
kinetic energy is available for recovery by
coasting. The high-flow-rate cutoff of each of the
curves corresponds to the flow rate at which the
flashdown temperature of the water is calculated to be
the temperature of the condenser immediately upon its
entrance; the calculation is terminated at that point.
The results of the multigroup model are shown in
Fig. 3 as dashed curves. In the multigroup model, the
growth of the drops in the spectrum causes the drops
to rain out in a short distance unless there is enough
flashing of the warm water to generate a substantial
amount of vapor to sustain the drops as they grow.
The predicted lift for the low-flow-rate portion of
each curve, where the flashdown is small, is therefore
much less for the multigroup model than for the sin-
gle-group model. As the flow rate is increased, the
amount of flashdown and, hence, the amount of vapor
generated increase. Thus, the vapor velocity in-
creases, and the drops are lifted more by the vapor
S.8'{
J -a.
P0 - 0.1 bar
11
11
11
gS
1SLngle-group Model
----- MuLt group ModeLInLet Temperature
Lift Tube Dometer
Area of Injectors25 C
10 m 2
0.3 mP0 - 0.2 bor
I,
1a
11
11
11
1 Ibefore raining out. This process yields an increasing
lift height before rainout with increasing flow rate,
as shown in Fig. 3. This increase in. predicted lift
height with flow rate continues to a point at which
the flashdown temperature becomes nearly equal to the
condenser temperature. At this point, the multigroup
model predicts the greatest lift height. Beyond the
point of maximum predicted lift height, the multigroup
model results are similar to the single-group model
results;. the predicted lift decreases because the tem-
perature of the condenser is soon reached. However,
the drops still have kinetic energy at that point.
In Fig. 3, the reduction in the maximum lift
height predicted by the single-group model for the
higher inlet pressure is the result of a combination
of increased inlet losses and exit kinetic energy
losses at the higher inlet pressure. The multigroup
model predicts an increase in maximum lift height at
the higher inlet pressure because the injection
velocities increase with the inlet pressure, and thus
the ballistic height is increased.
To assess the effect of the coalescence efficiency
model on the predictions of the multigroup model, the
original results were compared with the results
obtained with the coalescence efficiency set at unity
and at one-half of the value obtained from Abbott.
Figure 4 summarizes the results of this parametric
study.
With a coalescence efficiency of one (i.e., all
collisions resulting in coalescence), the results do
not greatly differ from the original results. This is
expected for the drops while they are small because
the coalescence efficiency is very near unity for
drops less than 1 mm in diameter. The slight increase
in lift height achieved with a coalescence efficiency
of one is due to the formation of larger drops that
are not slowed as much by the low-velocity vapor at
the entrance.J
4.-
-J1100 1200 1300 1700 1800 100
Moss Flow Rote (kg/) (note ecole change)FLg. 3. ComparLson of Single-group and MuLtL-
group Rseult.8
------ Normal CooLesoens EffLietncy Model
--- - - Coo escence Eff i~cenj - 1.
.----------- CoaLescence EfficLency - 1/2 Normol
8 Inlet Temperature 25 C
Inlet Preseure 0.1 bar
Lift Tube Dameter 10 m 2
Area of Injectors 0.3 m
4
S1
1100 11SO 1200 ta0 100
Mass Flow Rate (kg/e)
FLg. 4. LLift Height Predicted by MuLtigroup Model
for Varioue Values of the Coolescence EfficLency.4
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Davenport, R. L. Analysis of the mist lift process for mist flow open-cycle OTEC, article, June 1, 1980; Golden, Colorado. (https://digital.library.unt.edu/ark:/67531/metadc1059384/m1/9/: accessed April 26, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.