SANDIA Flame D
Application Challenge AC2-09 © copyright ERCOFTAC 2024
Comparison of Test Data and CFD
In this section comparisons of the CFD results and test data are
organized as follows:
- Comparisons of two different approaches for modeling the turbulence/combustion interaction, namely: steady flamelet model and simplified Conditional Moment Closure (designated as CMC -model in the figures from here on) obtained with the classical Smagorinsky subgrid scale model,
- Comparisons of two subgrid-scale models, namely: classical Smagorinsky subgrid scale model and dynamic Smagorinsky one using the steady flamelet model of turbulence/combustion interaction.
Fig.6. shows mean velocity axial component and mixture fraction along
the centerline for both steady flamelet and CMC approaches. One can see
quite significant discrepancies between both models. Steady flamelet
shows rapid velocity decay in the near field and then the slope is
quite close to the one measured experimentally. On the other hand the
CMC model leads to much smaller velocity decay. The velocity profile
for CMC is closer to experimental data but the slope at the distance
z/D=10 is underpredicted. At the distance z/D=30 both models predict
good velocity decay. As to the mixture fraction both models predict
quite a long distance z/D ≤ 16 for which the mixture fraction is unity
while the experiment showed much more intense mixing in this region. As
a consequence, at the jet centerline for a distance z/D ≤ 16 the numerical
models do not predict reaction and this is reflected in the temperature
profile and combustion products like CO2 shown in Fig.8. However,
further downstream in the fully developed flame the numerical results
are much closer to the experimental data. Especially CMC predicts the
value of temperature maximum and its location with quite high accuracy
(see Fig.7).
|
|
|
Fig. 6. Axial velocity (left) and mixture fraction (right) along the flame axis
|
|
Fig. 7. Temperature along the flame axis
|
The species distributions are shown in Figs 8-10. As the models
predict the flame too far from the nozzle exit, in all species
distributions similar discrepancies are observed at the distance
z/D = 10 – 20. And again, as was observed for the temperature profile, further in
the developed flame the agreement with experimental data is much
better. The distributions for H2O, O2, CO predicted with CMC are nearly
perfect. A bit worse results were obtained for H2.
|
|
|
Fig. 8. Mean mass fraction of CH4 (left) and CO2 (right)
|
|
|
|
Fig. 9. Mean mass fraction of H2O (left) and O2 (right)
|
|
|
|
Fig. 10. Mean mass fraction of CO (left) and O2 (right)
|
Fig.11 shows the fluctuating axial velocity and mixture fraction
components. In the near field the velocity fluctuations predicted by
both models at the distance z/D ≤ 10 are much higher than measured
experimentally which is consistent with much faster velocity decay in
this region shown in Fig.6. Further downstream in the developed flame,
agreement is very good. For the fluctuating component of the mixture
fraction shown in Fig.11 in the near field fluctuations are lower than
observed experimentally and again in the developed flame agreement is
quite good. Maximum value of temperature fluctuations (Fig.12) is
located much closer to the nozzle exit than the maximum value of mean
temperature. CMC leads to higher level of temperature fluctuations.
Fluctuating components of major species are shown in Figs 13-15. Both
models predict reasonable levels of species fluctuation in agreement
with experimental results.
|
|
|
Fig. 11. RMS of axial velocity (left) and mixture fraction (right) along the jet flame axis
|
|
Fig. 12. RMS of temperature along the jet flame axis
|
|
|
|
Fig. 13. RMS of mass fraction of CH4 (left) and CO2 (right)
|
|
|
|
Fig. 14. RMS of mass fraction of H2O (left) and O2 (right)
|
|
|
|
Fig. 15. RMS of mass fraction of CO (left) and H2 (right)
|
Sample radial profiles of mean and fluctuating velocities are shown in
Figs 16 and 17. Both models predict radial velocity profiles in very good
agreement with experiment in all the cross sections. Fluctuating
components are also in good agreement with experimental data. At the last
cross section at the distance z/D = 45 the flamelet model leads to some
overprediction of fluctuating components far from the flame axis. Mean
mixture fraction shown in Fig.18 is close to experimental data. As to the
mixture fraction fluctuating component shown in Fig. 19 agreement with
experimental data is also reasonable. However, a surprising result was
obtained at the distance z/D = 30 where the radial distribution of
fluctuating component of mixture fraction obtained with the CMC is
qualitatively different from experimental profile and from the result
obtained with steady flamelet. In the profile predicted with CMC a local
maximum is observed on the axis that is not present in experimental data.
At the current stage of the research it seems to be difficult to explain
these discrepancies analyzing physics of turbulence/combustion
interaction.
|
|
|
Fig 16. Mean axial velocity radial profile at the cross sections (a) z/D=15, (b) z/D=30, (c) z/D=45
|
|
|
|
Fig 17. Fluctuating axial velocity radial profile at the cross sections (a) z/D=15, (b) z/D=30, (c) z/D=45
|
|
|
|
|
|
|
Fig 16. Mean axial velocity radial profile at the cross sections (a) z/D=15, (b) z/D=30, (c) z/D=45|Fig 17. Fluctuating axial velocity radial profile at the cross sections (a) z/D=15, (b) z/D=30, (c) z/D=45
|
Contributed by: Andrzej Boguslawski — Technical University of Częstochowa
© copyright ERCOFTAC 2024