Environmental Engineering Reference
In-Depth Information
Fig. 4 Heat release rate
contour in the mid-span plane
for a Re = 18,000 and
b Re = 33,000
Fig. 5 Contours of ARS in
mid-span plane for
a Re = 18,000 and
b Re = 33,000
(Fig. 5 a), whereas for Re = 33,000, the combustion occurs predominantly in the
large-scale vortical structures that are rolled up relatively near to the step (Fig. 5 b).
Close examination of the ARS
field directly reveals that the regions of active ARS
are adjoined by regions of active combustion. This is because the ARS promotes
mixing, which aids in the combustion of the reactants.
Another interesting feature of a combustor in instability mode is that the
ame
length reduces dramatically when compared to its stable mode operation. In order to
quantify this change from the computations, the
fl
fl
flame length in the combusting
95 % of the total heat release
rate occurs. Figure 6 shows the variation of the average
field is computed as the axial distance over which 5
-
fl
flame length as a function
of Re. Dramatic decrease in the
flame length is observed as the Re is increased,
moving the combustor from no lock-on conditions to lock-on mode. The shortening
of the combustion zone is predominantly attributed to the large-scale mixing and
subsequently ef
fl
cient combustion due to the action ARS. In fact, the lock-on is
made possible by the shortening of the
fl
flame, such that the characteristic
fl
flow time
scale evolves to approach the acoustic time scale to enable resonance.
In order to further substantiate the role of the ARS in churning large-scale
vortical structures, the
flow components of velocity are monitored at x/h = 4 in the
mid-span plane at the centerline of the duct for Re = 18,000 and 33,000. Figure 7
fl
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