Best Practice Advice AC2-09: Difference between revisions

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<!--{{Demo_AC_BPA}}-->
<!--{{Demo_AC_BPA}}-->
==Key Fluid Physics==
==Key Fluid Physics==
<!--{{Demo_AC_BPA1}}-->
The non-premixed Sandia D flame is  an  example  of  a  flame  in  the
flamelet regime in which  the Kolmogorov scale is  significantly  larger
than the scales characteristic for the combustion process. In  this  not
demanding test case the models based on flamelet assumption should  lead
to good agreement with experimental data as was shown  in  the
[[Evaluation_AC2-09|Evaluation]] section,
especially in the region  of  developed  flame.  However,  more
discrepancies were observed in the near field were only mixing  of  fuel
and oxidizer is considered.
 
==Application Uncertainties==
==Application Uncertainties==
<!--{{Demo_AC_BPA2}}-->
The flow field in the near field  is  certainly  influenced  by  inlet
conditions. The mean velocity profile  and  fluctuating  component  were
chosen according to the experimental data. However, in the unsteady  LES
calculations the fluctuations were simulated by white noise. This  means
that the fluctuations characteristic for developed turbulent  flow  were
not reproduced at the inlet and this could influence the mixing features
in the near field.  It  is  well  known  that  white  noise  provides  a
fluctuating signal with a very short time scale which is  then  smoothed
at a short distance from the inlet plane. However,  it seems that due to
very low inlet turbulence level the further results in the flame  region
are only weakly influenced by these near field results  as  both  models
analyzed led to reasonable results.
 
==Computational Domain and Boundary Conditions==
==Computational Domain and Boundary Conditions==
<!--{{Demo_AC_BPA3}}-->
The computational domain for Sandia D Flame should extend  far  enough
from the nozzle outlet to  capture  at  least  the  region  of  maximum
temperature, i.e. ''x/D''&nbsp;&ge;&nbsp;50. The numerical tests performed have  shown  that
in order to limit the CPU time the  computational  domain  can  be  made
divergent in the downstream direction, and the lateral  extent  used  in
the current calculations (5.5''D'' and 18.3''D'' at the inlet and  outlet  plane
respectively),  is sufficient not to influence the  flame  structure  by
the  lateral  boundary  conditions.  The  coflow  as  in  experimental
conditions should  be  introduced  at  the  inlet  and  at  the  lateral
boundaries.
 
==Discretisation and Grid Resolution==
==Discretisation and Grid Resolution==
<!--{{Demo_AC_BPA4}}-->
In the BOFFIN code second order discretization was applied in time and
space. The grid refinement studies for the LES calculations showed that
the grid resolution  80×80×160  nodes  in  the  proposed  computational
domain is sufficient and further grid refinement leads to minor changes
of statistically converged parameters. However, no quantitative measure
of the contribution of the subgrid  scale  to  overall  flow  parameter
oscillations was evaluated.
 
==Physical Modelling==
==Physical Modelling==
<!--{{Demo_AC_BPA5}}-->
The results of the  LES  prediction  of  combusting  flows  could  be
affected by many factors. First could  be  the  type  of  subgrid-scale
model. In principle, in non-premixed combustion  the  mixing  intensity
determines the combustion rate so that the quality  of  the  SGS  model
could be of importance. However, comparisons of Smagorinsky model  with
the results obtained with the dynamic model did not show a  significant
influence.
 
On the other hand the LES calculations performed have  shown  that  the
results obtained in the flame region are significantly better than  the
flow behaviour predicted  upstream  of  the  flame.  Another  important
factor is chemical kinetics. The two reduced  mechanisms  used  in  the
current calculations seems to be sufficiently accurate for  prediction
of major species in the Sandia D flame.
 
==Recommendations for Future Work==
==Recommendations for Future Work==
<!--{{Demo_AC_BPA6}}-->
It should be kept in mind that LES of combusting flow is still a  new
and not mature field and thus many different research directions  could
be proposed to understand  the  associated  problems  and  improve  the
modeling quality. On the other hand it is important to stress that  LES
with combustion requires a  sufficiently  fine  mesh  and  consequently
large computer resources  and  CPU  time.  Hence,  the  number  of  LES
predictions leading to new  insight  is  limited  by  current  computer
resources and required CPU time.  A  tentative  list  of  future  works
generally in LES modeling of non-premixed combustion is as follows:
 
*Testing high-order numerical schemes to speed up calculations  allowing coarser meshes like spectral, compact differences, WENO schemes
 
*Systematic studies of reduced kinetics for chosen  flames  allowing  to establish minimum requirements in terms of required number  of  species and reaction steps for typical non-premixed flames
 
*Treating Sandia D Flame as a first step towards extensive LES studies  of  non-premixed flames with more complex structure involving local  extinction phenomena like Sandia E and F flames are needed
 
*Studies of more advanced turbulence/combustion interaction models  like full 1<sup>st</sup> order CMC and/or transported PDF models like  Eulerian  Fields PDF approach.
<br/>
<br/>
----
----
{{ACContribs
{{ACContribs
|authors=Andrzej Boguslawski
|authors=Andrzej Boguslawski, Artur Tyliszczak
|organisation=Technical University of Czestochowa
|organisation=Częstochowa University of Technology
}}
}}
{{ACHeader
{{ACHeader
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© copyright ERCOFTAC {{CURRENTYEAR}}
© copyright ERCOFTAC 2011

Latest revision as of 15:43, 11 February 2017


Front Page

Description

Test Data

CFD Simulations

Evaluation

Best Practice Advice

SANDIA Flame D

Application Challenge AC2-09   © copyright ERCOFTAC 2024

Best Practice Advice

Key Fluid Physics

The non-premixed Sandia D flame is an example of a flame in the flamelet regime in which the Kolmogorov scale is significantly larger than the scales characteristic for the combustion process. In this not demanding test case the models based on flamelet assumption should lead to good agreement with experimental data as was shown in the Evaluation section, especially in the region of developed flame. However, more discrepancies were observed in the near field were only mixing of fuel and oxidizer is considered.

Application Uncertainties

The flow field in the near field is certainly influenced by inlet conditions. The mean velocity profile and fluctuating component were chosen according to the experimental data. However, in the unsteady LES calculations the fluctuations were simulated by white noise. This means that the fluctuations characteristic for developed turbulent flow were not reproduced at the inlet and this could influence the mixing features in the near field. It is well known that white noise provides a fluctuating signal with a very short time scale which is then smoothed at a short distance from the inlet plane. However, it seems that due to very low inlet turbulence level the further results in the flame region are only weakly influenced by these near field results as both models analyzed led to reasonable results.

Computational Domain and Boundary Conditions

The computational domain for Sandia D Flame should extend far enough from the nozzle outlet to capture at least the region of maximum temperature, i.e. x/D ≥ 50. The numerical tests performed have shown that in order to limit the CPU time the computational domain can be made divergent in the downstream direction, and the lateral extent used in the current calculations (5.5D and 18.3D at the inlet and outlet plane respectively), is sufficient not to influence the flame structure by the lateral boundary conditions. The coflow as in experimental conditions should be introduced at the inlet and at the lateral boundaries.

Discretisation and Grid Resolution

In the BOFFIN code second order discretization was applied in time and space. The grid refinement studies for the LES calculations showed that the grid resolution 80×80×160 nodes in the proposed computational domain is sufficient and further grid refinement leads to minor changes of statistically converged parameters. However, no quantitative measure of the contribution of the subgrid scale to overall flow parameter oscillations was evaluated.

Physical Modelling

The results of the LES prediction of combusting flows could be affected by many factors. First could be the type of subgrid-scale model. In principle, in non-premixed combustion the mixing intensity determines the combustion rate so that the quality of the SGS model could be of importance. However, comparisons of Smagorinsky model with the results obtained with the dynamic model did not show a significant influence.

On the other hand the LES calculations performed have shown that the results obtained in the flame region are significantly better than the flow behaviour predicted upstream of the flame. Another important factor is chemical kinetics. The two reduced mechanisms used in the current calculations seems to be sufficiently accurate for prediction of major species in the Sandia D flame.

Recommendations for Future Work

It should be kept in mind that LES of combusting flow is still a new and not mature field and thus many different research directions could be proposed to understand the associated problems and improve the modeling quality. On the other hand it is important to stress that LES with combustion requires a sufficiently fine mesh and consequently large computer resources and CPU time. Hence, the number of LES predictions leading to new insight is limited by current computer resources and required CPU time. A tentative list of future works generally in LES modeling of non-premixed combustion is as follows:

  • Testing high-order numerical schemes to speed up calculations allowing coarser meshes like spectral, compact differences, WENO schemes
  • Systematic studies of reduced kinetics for chosen flames allowing to establish minimum requirements in terms of required number of species and reaction steps for typical non-premixed flames
  • Treating Sandia D Flame as a first step towards extensive LES studies of non-premixed flames with more complex structure involving local extinction phenomena like Sandia E and F flames are needed
  • Studies of more advanced turbulence/combustion interaction models like full 1st order CMC and/or transported PDF models like Eulerian Fields PDF approach.




Contributed by: Andrzej Boguslawski, Artur Tyliszczak — Częstochowa University of Technology

Front Page

Description

Test Data

CFD Simulations

Evaluation

Best Practice Advice


© copyright ERCOFTAC 2011