In CFD of reacting flows, the low-Mach number formulation is often used in LES or DNS as the time-step requirement is much more relaxed compared to the full compressible formulation. In turn, this allows for the consideration of more complex kinetics, which can lead to better prediction of the flame itself and turbulence-flame interactions. However in return, the acoustics are neglected.
Are the benefits of such simulations purely fundamental (i.e. better description of physical phenomena for various reasons?) For most of applications I can think off, either the incompressible assumption breaks down (i.e. piston engines, high speed flows) or the acoustic term is essential to ensure flame stability (jet combustion engine).
Please see the link
http://epubs.siam.org/doi/abs/10.1137/050644100
Please see the link
http://epubs.siam.org/doi/abs/10.1137/050644100
simulation of practical-scale combustion devices is an immense undertaking. The problem is inherently multi-scale both in time and space, the fuel is often turbulent, and the combustion process may involve hundreds of species and thousands of chemical reactions. Traditional direct numerical simulation (DNS) approaches based on explicit numerical methods for the compressible flow equations on uniform grids require very fine spatial grids to resolve the local flame structure
In addition, they require small time steps to resolve the acoustic and chemical time scales inherent in the model. DNS is normally reserved in combustion applications for small idealized problems geared at the fundamental nature of turbulence/chemistry interactions
For engineering design applications on the other hand computational models have been developed to approximate underresolved physics. But these models are incomplete, do not have general applicability, and certainly provide no means of exploring fundamental fluid/chemistry interactions.
Thank you for the answers.
My question was more on engineering/industrial applications to low-mach number combustion. What are industrial combustion systems that may benefit from low-Mach number simulations (LMS)?
For example, for plane combustor, the acoustic-flame interaction are essential to capture in order to ensure stability of a given geometry. For this case, LMS may be use with a skeletal chemical mechanism at a reasonable cost to obtain a preliminary understanding of the flame. But in the end, compressible simulations are required.
Domestic burners display nasty flame instabilities at low stochiometric ratios. Of course the BBQ is another very important application.
Large industrial burners using biomass to generate power can also display dangerous acoustically driven instabilities (low Mach numbers).
This is an interesting topic and, in my opinion still an open field of research... Acoustic wave propagation can play role in some instability so that you have to be aware when you plan to use a low-Mach model instead of a fully compressible one. Often I read of practical combustion problems where such instabilities have to be studied.
The full compressible model can be used for low-Mach flow, provided a suitable preconditioning is used.
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