THESE de DOCTORAT - cerfacs
THESE de DOCTORAT - cerfacs THESE de DOCTORAT - cerfacs
92 Chapter 5: Assessment of combustion noise in a premixed swirled combustor after extracting hydrodynamics is placed relatively far from that one predicted by the hybrid approach, as can be observed from Fig. 5.25. Pressure Fluctuation (Pa) M5 M6 M7 40 Premixer Combustion chamber 20 0 −20 −40 −0.2 −0.1 0 0.1 0.2 x (m) 0.3 0.4 0.5 (a) Pressure Wave from Eq. (2.63) Hybrid Computation Pressure Fluctuation (Pa) M5 M6 M7 40 Premixer Combustion chamber 20 0 −20 −40 −0.2 −0.1 0 0.1 0.2 x (m) 0.3 0.4 0.5 (b) Pressure fluctuation from LES after filtering Direct Computation Figure 5.25: Longitudinal pressure Waves oscillating at 954 Hz Pressure Fluctuation (Pa) M5 M6 M7 40 Premixer Combustion chamber 20 0 −20 −40 −0.2 −0.1 0 0.1 0.2 x (m) 0.3 0.4 0.5 (a) Pressure Wave from Eq. (2.63) Hybrid Computation Pressure Fluctuation (Pa) M5 M6 M7 40 Premixer Combustion chamber 20 0 −20 −40 −0.2 −0.1 0 0.1 0.2 x (m) 0.3 0.4 0.5 (b) Pressure fluctuation from LES after filtering Direct Computation Figure 5.26: Longitudinal pressure Waves oscillating at 1658 Hz Considering now the acoustic energy, it is observed from Figs. 5.27 and 5.28 that the values predicted by the hybrid approach are in a good agreement to those computed by LES after extracting the acoustic content from the complete pressure fluctuations. It is noticeable then that the energy coming from hydrodynamic fluctuations has been removed. Note that some caution must be taken when computing the Fourier transform of the divergence of the velocity (See T1 Eq. 5.20). If a rectangular window is applied to the temporal signal, a really good correspondance is seen only for frequencies lower than 400 Hz (not shown). On the other hand, if a Gaussian window is applied, a good agreement is seen over the entire frequency band, except at a very low frequencies. This is the case for the spectra shown (Figs. 5.21, 5.22, 5.27 and 5.28) where the signal does not capture the good acoustic level before 200 Hz.
5.6 Conclusions 93 Ac. Energy (J) − micro 5 Hybrid Computation Direct Computation: LES Direct Computation; LES (Filtered) 10 −5 0 500 1000 1500 2000 2500 10 0 Frequency (Hz) Ac. Energy (J) − micro 6 Hybrid Computation Direct Computation: LES Direct Computation; LES (Filtered) 10 −5 0 500 1000 1500 2000 2500 10 0 Frequency (Hz) Figure 5.27: Acoustic energy. Direct and hybrid approaches Ac. Energy (J)− micro 7 Hybrid Computation Direct Computation: LES Direct Computation; LES (Filtered) 10 −5 0 500 1000 1500 2000 2500 10 0 Frequency (Hz) Figure 5.28: Acoustic energy. Direct and hybrid approaches 5.6 Conclusions Combustion noise of a premixed swirled combustor has been assessed by two different numerical approaches: a direct computation, in which the noise produced by the flame is calculated together with the flow and flame dynamics, and a hybrid computation, in which the acoustic field is evaluated from the sources of noise in a separate step. Classical comparisons between mean and fluctuating (rms) velocity fields were performed between two LES on a coarse mesh (3 millions cells) and a refined mesh (10 millions cells), and PIV measurements. Mean velocity fields (axial and radial) were well predicted by both LES cases, whereas only the refined mesh succeed in recovering the proper rms velocity fields. It was then observed that satisfactorily predicting the velocity fluctuating field does not mean to reproduce correct flame dynamics and heat release. On the one hand, the mean heat release corresponding to the experimental thermal power is well captured by both LES. On the other hand, significative differences are found between the two simulations when looking at the shape of the instantaneous heat release and its rate of change. As a consequence, a correct estimate of the combustion noise radiation is not reached either. Several phenomena might be the cause of such a misprediction. A lack in numerical resolution can be one possible explanation: computing the small turbulent length scales in the shear flow region might be significant since these eddies might have a non-negligible influence on the flame dynamics and, as a con-
- Page 52 and 53: 42 Chapter 3: Development of a nume
- Page 54 and 55: 44 Chapter 3: Development of a nume
- Page 56 and 57: 46 Chapter 3: Development of a nume
- Page 58 and 59: 48 Chapter 3: Development of a nume
- Page 60 and 61: 50 Chapter 3: Development of a nume
- Page 62 and 63: 52 Chapter 3: Development of a nume
- Page 64 and 65: 54 Chapter 3: Development of a nume
- Page 66 and 67: 56 Chapter 3: Development of a nume
- Page 68 and 69: 4 Validation of the acoustic code A
- Page 70 and 71: 60 Chapter 4: Validation of the aco
- Page 72 and 73: 62 Chapter 4: Validation of the aco
- Page 74 and 75: 64 Chapter 4: Validation of the aco
- Page 76 and 77: 66 Chapter 4: Validation of the aco
- Page 78 and 79: 68 Chapter 4: Validation of the aco
- Page 80 and 81: 70 Chapter 4: Validation of the aco
- Page 82 and 83: 5 Assessment of combustion noise in
- Page 84 and 85: 74 Chapter 5: Assessment of combust
- Page 86 and 87: 76 Chapter 5: Assessment of combust
- Page 88 and 89: 78 Chapter 5: Assessment of combust
- Page 90 and 91: 80 Chapter 5: Assessment of combust
- Page 92 and 93: 82 Chapter 5: Assessment of combust
- Page 94 and 95: 84 Chapter 5: Assessment of combust
- Page 96 and 97: 86 Chapter 5: Assessment of combust
- Page 98 and 99: 88 Chapter 5: Assessment of combust
- Page 100 and 101: 90 Chapter 5: Assessment of combust
- Page 104 and 105: 94 Chapter 5: Assessment of combust
- Page 106 and 107: 96 Chapter 6: Boundary conditions f
- Page 108 and 109: 98 Chapter 6: Boundary conditions f
- Page 110 and 111: 100 Chapter 6: Boundary conditions
- Page 112 and 113: 102 Chapter 6: Boundary conditions
- Page 114 and 115: 104 Chapter 6: Boundary conditions
- Page 116 and 117: 106 Chapter 6: Boundary conditions
- Page 118 and 119: 108 Chapter 6: Boundary conditions
- Page 120 and 121: 110 Chapter 6: Boundary conditions
- Page 122 and 123: 112 Chapter 6: Boundary conditions
- Page 124 and 125: 114 Chapter 6: Boundary conditions
- Page 126 and 127: 116 Chapter 6: Boundary conditions
- Page 128 and 129: 118 Chapter 6: Boundary conditions
- Page 130 and 131: 7 Computation of the Reflection Coe
- Page 132 and 133: Chapter 7: Computation of the Refle
- Page 134 and 135: Chapter 7: Computation of the Refle
- Page 136 and 137: Chapter 7: Computation of the Refle
- Page 138 and 139: Bibliography [1] AGARWAL, A., MORRI
- Page 140 and 141: 130 BIBLIOGRAPHY [30] FRAYSSÉ, V.,
- Page 142 and 143: 132 BIBLIOGRAPHY [62] NICOUD, F., B
- Page 144 and 145: 134 BIBLIOGRAPHY [93] SENSIAU, C. S
- Page 146 and 147: 136 Chapter A: About the π ′ c =
- Page 148 and 149: 138 Chapter A: About the π ′ c =
- Page 150 and 151: 140 Chapter B: Publications
5.6 Conclusions 93<br />
Ac. Energy (J) − micro 5<br />
Hybrid Computation<br />
Direct Computation: LES<br />
Direct Computation; LES (Filtered)<br />
10 −5<br />
0 500 1000 1500 2000 2500<br />
10 0 Frequency (Hz)<br />
Ac. Energy (J) − micro 6<br />
Hybrid Computation<br />
Direct Computation: LES<br />
Direct Computation; LES (Filtered)<br />
10 −5<br />
0 500 1000 1500 2000 2500<br />
10 0 Frequency (Hz)<br />
Figure 5.27: Acoustic energy. Direct and hybrid approaches<br />
Ac. Energy (J)− micro 7<br />
Hybrid Computation<br />
Direct Computation: LES<br />
Direct Computation; LES (Filtered)<br />
10 −5<br />
0 500 1000 1500 2000 2500<br />
10 0 Frequency (Hz)<br />
Figure 5.28: Acoustic energy. Direct and hybrid approaches<br />
5.6 Conclusions<br />
Combustion noise of a premixed swirled combustor has been assessed by two different numerical<br />
approaches: a direct computation, in which the noise produced by the flame is calculated<br />
together with the flow and flame dynamics, and a hybrid computation, in which the acoustic<br />
field is evaluated from the sources of noise in a separate step.<br />
Classical comparisons between mean and fluctuating (rms) velocity fields were performed between<br />
two LES on a coarse mesh (3 millions cells) and a refined mesh (10 millions cells), and<br />
PIV measurements. Mean velocity fields (axial and radial) were well predicted by both LES<br />
cases, whereas only the refined mesh succeed in recovering the proper rms velocity fields. It<br />
was then observed that satisfactorily predicting the velocity fluctuating field does not mean<br />
to reproduce correct flame dynamics and heat release. On the one hand, the mean heat release<br />
corresponding to the experimental thermal power is well captured by both LES. On the<br />
other hand, significative differences are found between the two simulations when looking at<br />
the shape of the instantaneous heat release and its rate of change. As a consequence, a correct<br />
estimate of the combustion noise radiation is not reached either. Several phenomena might be<br />
the cause of such a misprediction. A lack in numerical resolution can be one possible explanation:<br />
computing the small turbulent length scales in the shear flow region might be significant<br />
since these eddies might have a non-negligible influence on the flame dynamics and, as a con-
Change language