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78 Chapter 5: Assessment of combustion noise in a premixed swirled combustor a) LES 3 million Cells b) LES 10 million Cells Figure 5.4: Instantaneous Field of Pope’s Criterion. The black line stands for the isocontour line Q LES = 0.8
5.3 Combustion noise Analysis 79 50 x = 7 mm x = 17 mm 50 x = 27 mm 50 x = 7 mm 50 x = 17 mm 50 x = 27 mm 50 40 40 40 40 40 40 30 30 30 30 30 30 20 20 20 20 20 20 z (mm) 10 0 −10 10 0 −10 10 0 −10 10 0 −10 10 0 −10 10 0 −10 −20 −20 −20 −20 −20 −20 −30 −30 −30 −30 −30 −30 −40 −40 −40 −40 −40 −40 −50 −20 0 20 40 −50 −50 −20 0 20 40 −20 0 20 40 −50 −50 −50 −20 0 20 −20 0 20 −20 0 20 (a) Mean Axial Velocity (m/s) (b) Mean Radial Velocity (m/s) Figure 5.5: Velocity Profiles: ◦ Experimental PIV measurements – – – LES 3 million cells, —— LES 10 million cells can be observed in Fig. 5.7(a). Different values in the variations of heat release are however obtained for each LES. Strong and more regular fluctuations of heat release are obtained with the ‘coarse’ mesh while smaller and less periodic fluctuations are given by the ‘refined’ mesh computation. It is likely that the coarser mesh does not capture enough small turbulent scales and trigger too large turbulent eddies. These large coherent structures might clearly have an influence on the flame dynamics and thus in the large fluctuations of heat release. The value of the rate of change of heat release integrated over the whole volume of the combustor has also been computed for the two different meshes and is shown in Fig. 5.7(b). On the finer mesh a quieter flame is obtained, considering the smaller values of rate of change of heat release compared to those obtained from the coarse mesh. As a consequence, smaller rms pressure values should be expected on the finer mesh. Acoustics in the chamber is characterized by the Sound Pressure Level (SPL) at a given point rather than rms values of the pressure. Figure 5.8 compares the SPL values at microphone 7 (see the location of M7 in Fig. 5.2) from the refined and coarse meshes to the experimental measurements. Both LES clearly overestimate the sound levels with a significant improvement with the finer resolution. It is then found that in order to correctly evaluate the dynamics of a flame and the acoustics generated by this one it is not enough to satisfactorily model the fluctuating velocity field as shown in Figs. 5.5 and 5.6. As stated before, computing acoustic pressure fluctuations is very challenging, since these values are very small compared to the aerodynamic fields. Several additional phenomena can play an important role and be the (partial) cause of the observed differences with the experimental data. First, the performed LES assumes a perfect premixed mixture of air and fuel in the reactive
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UNIVERSITE MONTPELLIER II SCIENCES
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An expert is a man who has made all
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Y gracias familia mía!!, que así
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Abstract Today, much of the current
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4 CONTENTS 3.4.1 Preconditioning .
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List of Figures 1.1 Main sources of
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8 LIST OF FIGURES 5.19 Acoustic ene
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List of Tables 1.1 EU recommended r
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14 Chapter 1: General Introduction
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2 Computation of noise generated by
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22 Chapter 2: Computation of noise
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Bibliography [1] AGARWAL, A., MORRI
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130 BIBLIOGRAPHY [30] FRAYSSÉ, V.,
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132 BIBLIOGRAPHY [62] NICOUD, F., B
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134 BIBLIOGRAPHY [93] SENSIAU, C. S
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136 Chapter A: About the π ′ c =
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140 Chapter B: Publications
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142 Chapter B: Publications Computa
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146 Chapter B: Publications Microph
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150 Chapter B: Publications SPL (dB
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154 Chapter B: Publications [7] A.
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