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Simulation of Swirling Flow in a Burner

CFD simulation of a swirl burner and Linear-Eddy-modeling of pollutant formation in lean premixed combustion

Modern gas turbines often operate under lean premixed conditions to obtain combustion at low temperatures being able to produce low pollutant emissions such as NOx and CO at the same time. Strong swirling flows with high levels of turbulence intensity are used to stabilize the flame downstream of the burner in the combustion chamber. The disadvantage of lean combustion combined with high swirling flows is the high sensitivity of the flame to external disturbances which may induce thermoacoustic instabilities which in turn may lead to undesirable effects like high emissions, noise and mechanical destruction.

Effekt eines Wirbels auf ein skalares Feld (triplet map, eindimensional)
 
Simulierte Flamme des untersuchten Drallbrenners
 

Passive and active control mechanisms are capable of preventing thermoacoustic instabilities. One possibility of active control is fuel-staging which adapts the fuel injection configuration to the combustor operating parameters to adjust convective time delays and the mixing quality of the burner. This project, which is financially supported by the German Science Foundation (DFG), is working together with the SFB 557, project B9, which investigates fuel staging control experimentally.

Within the project the mixing behavior of fuel and air in the combustor with fuel-staging and the pollutant formation for varying parameters are investigated numerically. Thereby, the cold and reacting strong swirling flow fields are simulated with the commercial CFD-programs Fluent & CFX. Turbulence-chemistry-interaction and the influence of mixing fluctuations on emissions are calculated with the one-dimensional Linear- Eddy-Model (LEM) as 'stand-alone model'.

The Linear-Eddy-Model (LEM) developed by A. Kerstein (1989/1990) describes molecular mixing in turbulent flows on a one-dimensional domain. Molecular diffusion is implemented deterministically, punctuated by a sequence of instantaneous, statistically independent 'rearrangement events' ('triplet maps') representing turbulent stirring. The rearrangement events make it possible to transfer three dimensional turbulent structures (eddies) to one dimension.

The advantage of using the Linear-Eddy-Model in comparison to DNS ('Direct Numerical Simulation'), LES ('Large Eddy Simulation') or RANS ('Reynolds Averaged Navier-Stokes-Equations') is the efficiency in time and concurrently high accuracy. In contrast to LES or RANS, LEM resolves all turbulent scales. Due to its one dimensional nature its computing cost is at the same time much lower than that of DNS.

The Linear Eddy-Model can also be used as a subgrid model for LES or RANS allowing for simulations of three-dimensional domains including all scales.

The numerical results are validated with the experimental findings of the SFB 557 B9 project.

Contact:

  • Christina Schrödinger
  • Michael Oevermann
  • Oliver Paschereit

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