Finite Element Method - Application using Fastflo
Swirling turbulent flame inside a burner
An axisymmetric burner is illustrated schematically in the figure
below. At the bottom section, fuel is pumped into the burner through a jet
located along the axis, whilst air comes into the combustion chamber
through an annulus around the fuel jet. At the annulus inlet, the air has
a tangential velocity of average value 11.7 m/s, and the average value of
the axial velocity is 13.7 m/s. The Reynolds number in this case is Re =
139,081, based on the radius of the burner and the air's average axial
velocity. The fuel is natural gas consisting of 91% methane by mass. The
walls of the burner are all cooled to 400oC.
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Schematic diagram of the axisymmetric burner |
Computations
The combusting turbulent flow inside the burner is
regarded as axi-symmetric and steady. A hybrid method with SIMPLE-type
algorithm and artificial compressibility is used to solve for the velocity
and pressure. For turbulence and combustion effects, the following models
are available in Fastflo to compute the turbulent combusting flow:
- RNG-based k-e turbulence model
- conventional k-e turbulence model
- mixed-is-burnt combustion model
- eddy dissipation combustion model
- P1 spherical harmonics radiation model
As the combusting flow has a low Mach number, there are
two approaches for eliminating the acoustic wave effect. One approach is
to rescale the Navier-Stokes momentum equation, the other approach is to
apply a perturbation expansion to the momentum equation. These two
approaches have both been tried for the test case.
Results presented here were obtained from the
conventional k-e turbulence model and
the eddy dissipation combustion model. As the flow is axi-symmetric, only
half of the cross-section needs to be resolved. This cross-section was
discretised with 4,028 bi-linear quadrilateral elements. A converged
solution was reached within 500 iterations which took about 5 hours CPU
time on a DEC 3000 Alpha workstation.
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| Temperature |
Turbulence |
swirl |
O2 |
CO2 |
Results
The computed velocity vectors illustrate the complex nature of the flow
inside the furnace. A large flow reversal forms on top of the fuel jet.
Due to the swirling velocity, the air rapidly moves from the annulus inlet
toward the side wall. The isothermal contour plot shown above illustrates
the large temperature variation within the furnace. This figure also shows
the existence of a combustion front where temperature reaches 1800K. The
turbulent kinetic energy shows two high turbulence regions - one close to
the inlet, and another at the burner outlet. The mass fraction of O2
and CO2 formed during combustion are also presented above.
Given the complex nature of this swirling turbulent combusting flow,
comparisons have demonstrated that the temperature computed from Fastflo
along the burner axis agrees reasonably well with experiment.
Reference
Z. Zhu and A.N. Stokes, Computation of swirling turbulent diffusion
flames with a finite element method, Proc 12th Australasian Fluid
Mechanics Conference (University of Sydney, 1995), 525-528.
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