Evaluation AC2-09
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).
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| Fig. 6. Axial velocity (left) and mixture fraction (right) along the flame axis | ||
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| 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.
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| Fig. 8. Mean mass fraction of CH4 (left) and CO2 (right) | ||
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| Fig. 9. Mean mass fraction of H2O (left) and O2 (right) | ||
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| 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.
Contributed by: Andrzej Boguslawski — Technical University of Częstochowa
© copyright ERCOFTAC 2024