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An Investigation on the Unsteady Flow in a Centrifugal Blower
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List of Tables Table 2.1 Specifications of the test blower Table 2.2 Cases for grid test Table 2.3 CFD conditions - vi -
List of Figures Fig. 2.1 Fig. 2.2 Fig. 2.3 Fig. 2.4 Fig. 2.5 Fig. 2.6 Cross-section of the test blower Total pressure and static pressure in volute along azimuth angle (grid test) Computational grid for numerical simulations Spectral analysis of wall pressure fluctuation at the impeller outlet (time step test) Schematic illustration of compression system Schematic diagram of applying 1D system model to 3D-CFD Fig. 3.1 Fig. 3.2 Performance of the test blower at the volute outlet and the impeller outlet A-weighted sound pressure level of wall pressure fluctuation at the impeller outlet and MIC signal Fig. 4.1 Fig. 4.2 Fig. 4.3 Fig. 4.4 Fig. 4.5 Fig. 4.6 Fig. 4.7 A-wighted noise spectra at the impeller outlet @1.0Q d Pressure fluctuation at the impeller outlet @1.0Q d (from CFD and EXP) Velocity distribution at the impeller mid span @1.0Q d Radial velocity at the impeller outlet @1.0Q d A-wighted noise spectra at the impeller outlet @1.0Q d (with system model) Radial velocity at the impeller outlet @1.0Q d (with system model) Pressure distribution at the diffuser mid span @1.0Q d - vii -
Fig. 4.8 Fig. 4.9 Radial velocity at the diffuser mid span @1.0Q d A-wighted noise spectra at the impeller outlet @0.85Q d Fig. 4.10 Velocity distribution at the impeller mid span @0.85Q d Fig. 4.11 Radial velocity at the impeller outlet @0.85Q d Fig. 4.12 A-wighted noise spectra at the impeller outlet @0.65Q d Fig. 4.13 Velocity distribution at the impeller mid span @0.65Q d Fig. 4.14 Radial velocity at the impeller outlet @0.85Q d Fig. 4.15 A-wighted noise spectra at the impeller outlet @0.45Q d Fig. 4.16 Velocity distribution at the impeller mid span @0.45Q d Fig. 4.17 Radial velocity at the impeller outlet @0.45Q d Fig. 4.18 Back flow regions at the diffuser hub @0.45Q d Fig. 4.13 Back flow regions at the diffuser hub @0.45Q d (with system model) Fig. 4.13 A-wighted noise spectra at the impeller outlet @0.45Q d (with system model) Fig. 5.1 Fig. 5.2 Fig. 5.3 Aerodynamic response of the blower to change of throttle line (1D simulation) Aerodynamic response of the blower to change of throttle line (3D simulation with 1D system model) Fluctuation of pressure and mass flow rate at the volute outlet responding to change of throttle line from 0.55Q d to 0.5Q d Fig. 5.4 Fig. 5.5 Fig. 5.6 Predicted surge point by 1D and 3D Relation between surge point and B Relation between surge frequency and ω H Fig. A.1 A-weighted noise spectra at the impeller outlet @1.0Qd - viii -
with the variation of the number of data Fig. A.2 A-weighted noise spectra at the impeller outlet @1.0Qd with the variation of unsteady time steps Fig. A.3 Pressure fluctuation at the impeller outlet @1.0Qd with Rectangular window and Gaussian window Fig. A.4 A-weighted noise spectra at the impeller outlet @1.0Qd with the variation of windows Fig. A.5 Raw and modified data of pressure fluctuation at the impeller outlet @1.0Qd Fig. A.6 A-weighted noise spectra at the impeller outlet @1.0Qd with the raw data and the modified data - ix -
Roman Symbols Speed of sound (m/s) Cross sectional area of duct (m 2 ) Breadth (m) Pressure rise through blower (Pa) Pressure drop through throttle (Pa) Frequency (Hz) Shaft rotating Frequency (Hz) Length of duct (m) Sound pressure level (db) Static pressure (Pa) Mass flow rate (kg/s) Radius (m) Impeller tip speed (m/s) Volume of Plenum (m 3 ) Greek Symbols Circumferential location ( ) Density (kg/m 3 ) Time for one pitch rotation (s) L/A (m -1 ) Flow coefficient - x -
Pressure coefficient Subscripts Blower Throttle Atmosphere Results at design point Plenum Impeller outlet Diffuser outlet Reference Results with system model Surge condition Superscripts Results of n th time step Results of (n+1) th time step Abbreviations EXP Blade passing frequency Computational fluid dynamics Experiment Fast fourier transform Sound pressure level - xi -
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ε ε ε - 4 -
χ χ - 5 -
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Table 2.1 Specifications of the test blower. Design RPM 40,000 Design mass flow rate [kg/s] 0.1756 Number of Impeller blades 18 (9 main, 9 splitter) Impeller diameter [mm] 140 b imp. / r imp. 0.0643 Diffuser type parallel wall vaneless r diff. / r imp. 1.286 Inlet blade angle @ shroud 60 Exit blade angle 55 Volute type overhung Volute exit diameter [mm] 72-7 -
Fig. 2.1 Cross-section of the test blower. Table 2.2 Cases for grid test. Case No. Number of nodes Impeller Diffuser and Volute Total 1 2,700k 300k 3,000k 2 2,700k 700k 3,400k 3 1,400k 700k 2,100k 4 1,400k 1400k 2,800k - 8 -
Fig. 2.2 Total pressure and static pressure in volute along azimuth angle. (Grid test) - 9 -
Fig. 2.3 Computational grid for numerical simulations. Fig. 2.4 Spectral analysis of wall pressure fluctuation @ the impeller outlet (time step test) - 10 -
Table 2.3 CFD conditions. Grid / Solver CFX Turbogrid 11, Star-CD / CFX11 y+ 20-70 Grid size (No. of Nodes) 2,100k Turbulence model Simulation type Unsteady time step Inlet B.C. Outlet B.C. Interface k-ε model Unsteady 8 steps / a pitch Total pressure Total temperature Flow vector Mass flow rate Frozen rotor - 11 -
Q 2 Throttle Plenum V P p P a Q 1 Blower Turbine Fig. 2.5 Schematic illustration of compression system Fig. 2.6 Schematic diagram of applying 1D system model to 3D-CFD - 12 -
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Volute Out Impeller Out Fig. 3.1 Performance curve of the test blower at the volute outlet and the impeller outlet. - 15 -
(d) (b) (c) (a) Fig. 3.2 A-weighted sound pressure level of wall pressure fluctuation at the impeller outlet and MIC signal. - 16 -
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BPF Fig. 4.1 A-weighted noise spectra at the impeller outlet @1.0Q d - 21 -
Fig. 4.2 Pressure fluctuation at the impeller outlet @1.0Q d (from CFD and EXP) - 22 -
Fig. 4.3 Velocity distribution at the impeller mid span @1.0Q d Fig. 4.4 Radial velocity at the impeller outlet @1.0Q d - 23 -
Fig. 4.5 A-weighted noise spectra at the impeller outlet @1.0Q d (with system model) Fig. 4.6 Radial velocity at the impeller outlet @1.0Q d (with system model) - 24 -
Fig. 4.7 Pressure distribution at the diffuser mid span @1.0Q d - 25 -
Fig. 4.8 Radial velocity at the diffuser mid span @1.0Q d Fig. 4.9 A-weighted noise spectra at the impeller outlet @0.85Q d - 26 -
Fig. 4.10 Velocity distribution at the impeller mid span @0.85Q d - 27 -
Fig. 4.11 Radial velocity at the impeller outlet @0.85Q d Fig. 4.12 A-weighted noise spectra at the impeller outlet @ 0.65Q d - 28 -
Fig. 4.13 Velocity distribution at the impeller mid span @0.65Q d Fig. 4.14 Radial velocity at the impeller outlet @0.85Q d - 29 -
612Hz 3000Hz Fig. 4.15 A-weighted noise spectra at the impeller outlet @ 0.45Q d Fig. 4.16 Velocity distribution at the impeller mid span @0.45Q d - 30 -
Fig. 4.17 Radial velocity at the impeller outlet @0.45Q d - 31 -
Fig. 4.18 Back flow regions at the diffuser hub @0.45Q d - 32 -
Fig. 4.19 Back flow regions at the diffuser hub @0.45Q d (with system model) 3333Hz Fig. 4.20 A-weighted noise spectra at the impeller outlet @ 0.45Q d (with system model) - 33 -
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ω ω - 36 -
ω ω - 37 -
(a) 1.0Q d 0.85Q d (b) 0.42Q d 0.34Q d (b) 0.42Q d 0.33Q d Fig. 5.1 Aero dynamic response of blower to change of throttle line (1D simulation) - 38 -
Fig. 5.2 Aero dynamic response of blower to change of throttle line (3D simulation with 1D system model) - 39 -
Fig. 5.3 Fluctuation of pressure and mass flow rate at the volute outlet responding to change of throttle line from 0.55Q d to 0.5Q d - 40 -
0.326Q d 0.33Q d 0.55Q d Fig. 5.4 Predicted surge point by 1D and 3D Fig. 5.5 Relation between surge point and B - 41 -
Fig. 5.6 Relation between surge frequency and ω H - 42 -
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H. J. Feld, S. Aschenbrenner, R. Girsberger, Investigation of Acoustic Phenomena at the Inlet and the Outlet of a Centrifugal Compressor for Pressure Ratio 4.5, Proc. of ASME Turbo Expo, GT-0314, 2001 T. Raitor, W. Neise, Sound generation in centrifugal compressors, Journal of Sound and Vibration, 314, pp.738-756, 2008 W. Kim, N. Hur, W.-H. Jeon, Numerical Analysis of Unsteady Flow Field and Aeroacoustic Noise of an Axial Flow Fan, KSCFE, Vol.15, No.4, pp.60-66, 2010 T. Hase, N. Yamasaki, T. Ooishi, Numerical Simulation for Fan Broadband Noise Prediction, Journal of Thermal Science, Vol.20, No.1, pp.58-63, 2011 I.-S. Park, C.-H.Sohn, J.-K. Oh, Characteristics of Flow-Induced Noise in the Suction Nozzle of a Vacuum Cleaner with a Double-Blade Fan, KSME-B, Vol.35, No.2, pp.205-213, 2011 M. Lee, S. Kang, N. Hur, J. Park, A Detached Eddy Simulation for the Flow Noise of a Cross-Flow Fan, KFMA2011, Kyounju Korea, pp.216-217, 2011 D. Sakaguchi, M. Ishida, H. Ueki, H. Hayami, Analysis of noise generated by low solidity cascade diffuser in a centrifugal blower, GT2008-50750, 2008 Y. Liu, B. Liu, L. Lu, Investigation of Unsteady Impeller-Diffuser Interaction in a Transonic Centrifugal Compressor, GT2010-22737, 2010 A. Abdelwahab, Numerical Investigation of the Unsteady Flow Fields in Centrifugal Compressor Diffusers, GT2010-22489, 2010-45 -
R. Hunziker, H. P. Dickmann, R. Emmrich, Numerical and Experimental Investigation of a Centrifugal Compressor with an Inducer Casing Bleed System, ATI-CST-0225/01, 2001 H. P. Dickmann, T. S. Wimmel, J. Szwedowicz, D. Filsinger, C. H. Roduner, Unsteady Flow in a Turbocharger Centrifugal Compressor: 3D-CFD-Simulation and Numerical and Experimental Analysis of Impeller Blade Vibration, GT2005-68235, 2005 J. N. Everitt, Z. S. Spakovszky, An Investigation of Stall Inception in Centrifugal Compressor Vaned Diffusers, GT2011-46332, 2011 E. M. Greitzer, Surge and Rotating Stall in Axial Flow Compressors, Journal of Engineeing for Power, 1976 J. E. Ffowcs Williams, X. Y. Huang, Active stabilization of compressor surge, J. Fluid Mech.vol204, pp.245-262, 1989 Taebin Jeong, Chanyoung Lee, Kyung-Ku Ha, Shin-Hyoung Kang, An investigation on the Flow-induced Noise Characteristics of Centrifugal compressor, ACGT2012-1117, 2012 H. Sun, S. Lee, Numerical Prediction of Centrifugal Compressor Noise, Journal of Sound and Vibration 269:421-430, 2004-46 -
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Fig. A.1 A-weighted noise spectra at the impeller outlet @1.0Q d with the variation of the number of data Fig. A.2 A-weighted noise spectra at the impeller outlet @1.0Q d with the variation of unsteady time steps - 49 -
Fig. A.3 Pressure fluctuation at the impeller outlet @1.0Qd with Rectangular window and Gaussian window Fig. A.4 A-weighted noise spectra at the impeller outlet @1.0Q d with the variation of windows - 50 -
Fig. A.5 Raw and modified data of pressure fluctuation at the impeller outlet @1.0Q d Fig. A.6 A-weighted noise spectra at the impeller outlet @1.0Q d with the raw data and the modified data - 51 -
An Investigation on the Unsteady Flow in a Centrifugal Blower Seoul National University School of Mechanical and Aerospace Engineering Chanyoung Lee Abstract In this study, 3-D unsteady numerical calculation of a centrifugal blower has been conducted to investigate on the unsteady flow in a centrifugal blower. A 1-D model of compression system is adopted to the 3-D calculation to consider effects of all components of the system. Noise level was predicted by measuring wall pressure fluctuation at the impeller outlet and the results were compared with the experimental data. The wall pressure fluctuations at the impeller outlet were transformed into the frequency domain by Fourier decomposition to find the relationship between flow behaviors and noise characteristics. The sound pressure level (SPL) which is obtained from wall pressure fluctuation at the impeller outlet shows similar trend with the overall sound level of the blower. The sound spectra show that there are some specific peak frequencies at each mass flow rate and they are related with the flow behaviors. At the design point, BPF and its harmonic components are the main sources of noise. As the flow rate - 52 -
decreases periodic separations begin to occur at certain locations of the impeller blade and nonuniformity increases in the circumferential direction. As a result, noise components of 2~4kHz increase and they lead to rise in overall noise. At the operating point of less mass flow rate, the separations in the impeller grow to overall blades and more uniform flow along circumference is established at the impeller outlet. So the tonal noise and 2~4kHz components decrease. At low flow rate near stall, the low frequency broad band noise that has no specific frequency covers other tonal noise. The rotating stall at the impeller outlet and its related noise frequency are found. Although there are some differences in the frequency between the CFD and the experimental results, the phenomenon itself can be found in both results. In the results of adopting 1-D system model the stall characteristics are differently predicted. So it can be expected that more reasonable results can be got using real spec of the compression system. By adopting system model, surge point where the fluctuations of the pressure and the mass flow rate diverge is predicted. The system variables of system such as the length and the sectional area of the duct or the volume of the plenum affect the surge point and the surge frequency. The Surge point is related to the dimensionless number B and the surge frequency has a linear relationship with the frequency of Helmholtz resonance of the system. Keywords : Centrifugal blower, Compression system, Flow-induced noise, Stall, Surge Student Number : 2011-20743 - 53 -