THESE de DOCTORAT - cerfacs

76 Chapter 5: Assessment of combustion noise in a premixed swirled combustor
Numerical scheme Lax-Wendroff
2 nd artificial viscosity 0.05
4 th artificial viscosity 0.01
LES model
Combustion model
Smagorinsky
Dynamic Thickened Flame
Table 5.2: Main parameters of LES computation
and oulets. Inlets and outlets are characterized by their acoustic impedance Ẑ = ˆp/( ¯ρ ¯cû · n).
Following [91] it is possible to relate this acoustic quantity to the NSCBC parameters (see section
4.2.1) by the Eq. (4.11) and (4.13)
ˆR = Ẑ + 1
Ẑ − 1 = 1
1 − i 2ω K
(5.2)
in which K is the relaxation coefficient. Classically, outlets to the atmosphere are modeled by
zeroing pressure fluctuations → p ′ = 0 (| ˆR| = 1 and θ = π). Another path was followed in
the present study, extending the computational domain to some region of the atmosphere after
the burner outlet. By doing so, the true impedance of the outlet is implicitely accounted for.
On the other hand, the acoustic impedance at the inlets of the EC2 combustor was measured
experimentally by Lamraoui [43, 44]. These values were used to characterize the reflection
coefficient ˆR in the LES performed through the relation (5.2). Similar results were obtained
when a simplified acoustic condition (u ′ = 0) at the inlets was applied, proving that acoustic
conditions at the inlets are not critical in the present LES study. The LES computational grid is
shown in Fig. 5.3
Inlet
Inlet
Outlet
Inlet
Inlet
Figure 5.3: Computational grid of EC2 combustor
Two different meshes were used to compute the flow and flame dynamics of the EC2 combustor;
their main characteristics are given in table 5.3. The ‘coarse’ mesh has 3 millions of cells
whereas the ‘refined’ mesh is made of 10 million cells. The quality of the two LES performed