THESE de DOCTORAT - cerfacs
THESE de DOCTORAT - cerfacs THESE de DOCTORAT - cerfacs
74 Chapter 5: Assessment of combustion noise in a premixed swirled combustor Air + C 3 H 8 Manifolds D = 30 mm Air + C 3 H 8 (a) Hollow cylinder - Actual test rig (b) Hollow cylinder simplified for LES Figure 5.1: Schematic view of a tranversal section through the premixer and circular manifolds. α, defined as the ratio of the fuel massflow in the furthest stage from the chamber (stage 1) to the total fuel injected massflow. α = ṁ f ,1 ṁ f ,1 + ṁ f ,2 (5.1) These instabilities are characterized by an important noise radiation due to the intense acoustic levels reached within the chamber and the premixer. The regime in the present study is given in Table 5.1. The gas mixture in the plenum is considered as perfectly premixed with a global equivalence ratio of φ g = 0.832. α ṁ air,1 ṁ f ,1 ṁ air,2 ṁ f ,2 φ 1 φ 2 φ g 14.5 % 20 0.20 20 1.20 0.238 1.428 0.832 Table 5.1: Present operating regime (Mass flow ṁ in m 3 /h). The combustion chamber is made of two quartz windows for flame visualizations, and two refractory concrete plates (top and bottom), which can be equipped either with small quartz windows for Particle Image Velocimetry (PIV) laser measurements or with transducer ports for acoustic measurements. The three PIV planes we are comparing with are shown in Fig. 5.2. The combustion chamber and the premixer are also equipped with seven microphones (denoted M1 to M7 in Fig. 5.2) placed at equal distances along the combustor.
5.3 Combustion noise Analysis 75 Microphones 7mm 27mm 17mm PIV Planes Figure 5.2: Two staged swirled premixed combustor. (Courtesy of École Centrale Paris) 5.3 Combustion noise Analysis 5.3.1 Direct Approach The AVBP solver, developed at CERFACS, is used for the LES computations [3]. In this tool, the full compressible Navier Stokes equations are solved on hybrid (structured and unstructured) grids with second order spatial and temporal accuracy. Subgrid scale stresses are described by the Smagorinksy model. The flame/turbulence interactions are modeled by the Dynamic Thickened Flame (DTF) model [17]. This combustion model has been used in numerous studies of turbulent combustors [90, 56, 96, 92] in which it has been shown to well predict ignition, blow-off and flash-back of flames as well as acoustic-flame interactions in specific configurations. The spatial discretization in AVBP is based on the finite volume method with a cell-vertex approach, combined to a numerical scheme derived from the Lax-Wendroff scheme. AVBP has been validated/used for a considerable number of configurations[90, 82, 57]. The main parameters of the LES computation are summarized in table 5.2 Boundary conditions in AVBP are treated by the Navier Stokes Characteristic Boundary Conditions (NSCBC) method [72]. This method is already a standard technique to control waves crossing the boundaries [74, 42, 91]. It consists in decomposing the variation of flow variables on boundaries into terms due to ingoing and outgoing waves. While walls are always taken as totally reflecting surfaces (no velocity fluctuations normal to the surface are allowed → u ′ = 0) with a reflection coefficient ˆR = | ˆR|e jθ (| ˆR| = 1 and θ = 0), the same cannot be stated for inlet
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5.3 Combustion noise Analysis 75<br />
Microphones<br />
7mm 27mm<br />
17mm<br />
PIV Planes<br />
Figure 5.2: Two staged swirled premixed combustor. (Courtesy of École Centrale Paris)<br />
5.3 Combustion noise Analysis<br />
5.3.1 Direct Approach<br />
The AVBP solver, <strong>de</strong>veloped at CERFACS, is used for the LES computations [3]. In this tool, the<br />
full compressible Navier Stokes equations are solved on hybrid (structured and unstructured)<br />
grids with second or<strong>de</strong>r spatial and temporal accuracy. Subgrid scale stresses are <strong>de</strong>scribed<br />
by the Smagorinksy mo<strong>de</strong>l. The flame/turbulence interactions are mo<strong>de</strong>led by the Dynamic<br />
Thickened Flame (DTF) mo<strong>de</strong>l [17]. This combustion mo<strong>de</strong>l has been used in numerous studies<br />
of turbulent combustors [90, 56, 96, 92] in which it has been shown to well predict ignition,<br />
blow-off and flash-back of flames as well as acoustic-flame interactions in specific configurations.<br />
The spatial discretization in AVBP is based on the finite volume method with a cell-vertex<br />
approach, combined to a numerical scheme <strong>de</strong>rived from the Lax-Wendroff scheme. AVBP has<br />
been validated/used for a consi<strong>de</strong>rable number of configurations[90, 82, 57]. The main parameters<br />
of the LES computation are summarized in table 5.2<br />
Boundary conditions in AVBP are treated by the Navier Stokes Characteristic Boundary Conditions<br />
(NSCBC) method [72]. This method is already a standard technique to control waves<br />
crossing the boundaries [74, 42, 91]. It consists in <strong>de</strong>composing the variation of flow variables<br />
on boundaries into terms due to ingoing and outgoing waves. While walls are always taken as<br />
totally reflecting surfaces (no velocity fluctuations normal to the surface are allowed → u ′ = 0)<br />
with a reflection coefficient ˆR = | ˆR|e jθ (| ˆR| = 1 and θ = 0), the same cannot be stated for inlet
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