Numerical Simulation of the Dynamics of Turbulent Swirling Flames

Identification of Flame Transfer Functions using LES/SI
ffect in the same burner, but with a power rating of 70 kW (around 2.33 times
higher than the one used in this study) using URANS. The propagation speed
for the tangential velocity fluctuation for the 70 kW case was 40% higher than
the mean axial velocity on the cross section. From the axial velocity contours
in Fig. 5.27, it is observed that the distribution of axial velocities in the cross
section and along the tube show sectors with maximum velocities around the
double of the mean axial velocity of the cross section and close to the propagation
speed of the tangetial velocity fluctuations [136, 139, 140]. The propagation
velocity of the tangential velocity fluctuation locally is not uniform
and travels convectively with the local convective velocity. This effect can be
observed on Fig. 5.32, where the area averaged tangential velocity on a crosssection
at different positions is shown. For positions further downstream, the
harmonic oscillations become less smooth, indicating some nonuniformity in
the propagation. Furthermore, the mean tangential velocity (taken in this case
by the tangential velocity at time 0 s) is lower at positions more downstream
from the swirler. This is created due to the decay of swirl mentioned before.
In Fig. 5.33, the area averaged axial and tangential velocity fluctuations without
normalization are shown. The axial velocity fluctuation did not present a
decrease in amplitude, which indicates the low level of acoustic dissipation
from the code. The tangential velocity fluctuation presented a decay of 16.4%
in amplitude at the plane z=-10 mm with respect to the one at z=-110 mm,
indicating that the decay in swirl affected also the tangential velocity fluctuations,
which influence the flame response.
In Fig. 5.34, the area averaged swirl number fluctuations are shown. The case
with swirler at position 2 presents higher swirl number fluctuations in spite
of the lower tangential velocity fluctuations. This is created due to the different
phase between axial and tangential velocity fluctuations at the reference
planes as shown in Fig. 5.33.
The results indicate that a flame with similar heat release distribution and flow
field can have a different flame response due to the varied time lag response
to the perturbations. The analysis is extended in section 5.7.2 to identify the
different time lags between perturbations and the discrepancies in the amplitudes
between experimental and identified FTF in Fig. 5.29.
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