The authors investigate the far-field noise emissions of a datum fan blade fitted with tip end-plate geometries, originally designed to control the leakage vortex swirl level. The end-plate geometries influence the tip-leakage flow, vortex formation, and swirl level. In doing so, the end-plate geometries influence the sound-power levels. After an evaluation of fan rotors' aerodynamic performance, the study compares the rotors' far-field noise signature characterised in terms of sound-power and pressure-level spectra to enable and assess the end-plate acoustic pay-off. The investigation attempts to establish a cause-and-effect relationship between the tip-flow dynamics and the radiated sound fields, exploring the diverse directivity patterns. The authors found a tonal reduction, due to the enhanced blade-tip end-plates and clarified the relevance of the tip features influencing the radial distribution of the noise sources using coherence analysis. The modified multiple-vortex breakdown end-plate design was effective in reducing the broadband noise, giving an improvement in the frequency range of the turbulent noise. 1. Introduction Researchers have studied the link between the aerodynamic features of the fan rotor and its acoustic emissions extensively. In particular, Wright [1] and Cumpsty’s [2] works have enhanced the understanding of axial-turbomachinery aeroacoustics. Cumpsty [2] concluded that, with the exception of the low-frequency range of high-speed machines, the mechanism that determines broadband noise in subsonic fans is the same as that in supersonic tip-speed fans and compressors. According to Wright [1], this is due to the prominence of rotor noise originating from turbulent boundary layers. Researchers have identified a variety of mechanisms as causing noise signatures. The dominant sources, they believe, are the rotor blades, which generate noise as a result of turbulent wake shedding from the interaction between the end-wall boundary layer and the rotor tip. In view of the aerodynamic effect that tip-leakage flow exerts on wake and secondary flows, the industry widely recognises this mechanism as one of the most significant sources of noise [3, 4]. The advent of stringent environmental regulations with respect to noise production has stimulated academics and practitioners alike to pursue the development of concepts and technologies that are likely to reduce fan noise either by attenuating noise propagation or by controlling the noise at source. In this regard, researchers have not given the deserved attention to the control and
References
[1]
S. E. Wright, “The acoustic spectrum of axial flow machines,” Journal of Sound and Vibration, vol. 45, no. 2, pp. 165–223, 1976.
[2]
N. A. Cumpsty, “Review: a critical review of turbomachinery noise,” Journal of Fluids Engineering, vol. 99, no. 2, pp. 278–293, 1977.
[3]
T. Fukano, Y. Takamatsu, and Y. Kodama, “The effects of tip clearance on the noise of low pressure axial and mixed flow fans,” Journal of Sound and Vibration, vol. 105, no. 2, pp. 291–308, 1986.
[4]
F. Holste and W. Neise, “Noise source identification in a propfan model by means of acoustical near field measurements,” Journal of Sound and Vibration, vol. 203, no. 4, pp. 641–665, 1997.
[5]
H. Marcinowski, “Einfluss des Laufradspalts und der Luftfuehrung bei einem Kuehlgeblaese axialer Bauart,” Motortechnische Zeitschrift, vol. 14, pp. 259–262, 1953.
[6]
B. D. Mugridge and C. L. Morfey, “Sources of noise in axial flow fan,” Journal of the Acoustical Society of America, vol. 51, no. 5, pp. 1411–1426, 1972.
[7]
R. E. Longhouse, “Control of tip-vortex noise of axial flow fans by rotating shrouds,” Journal of Sound and Vibration, vol. 58, no. 2, pp. 201–214, 1978.
[8]
T. Fukano and C. M. Jang, “Tip clearance noise of axial flow fans operating at design and off-design condition,” Journal of Sound and Vibration, vol. 275, no. 3–5, pp. 1027–1050, 2004.
[9]
F. Kameier and W. Neise, “Rotating blade flow instability as a source of noise in axial turbomachines,” Journal of Sound and Vibration, vol. 203, no. 5, pp. 833–853, 1997.
[10]
D. A. Quinlan and P. H. Bent, “High frequency noise generation in small axial flow fans,” Journal of Sound and Vibration, vol. 218, no. 2, pp. 177–204, 1998.
[11]
C. E. Jensen, “Axial-flow Fan,” 1986, Patent No. US 4,630,993.
[12]
C. M. L. Longet, “Axial Flow Fan with Noise Reducing Means,” 2003, US Patent 2003/0123987 A1.
[13]
M. Mimura, “Axial Flow Fan,” 2003, US Patent 6,648,598 B2.
[14]
R. B. Uselton, L. J. Cook, and T. Wright, “Fan with Reduced Noise Generation,” 2005, US Patent 2005/0147496 A1.
[15]
S. Bianchi, A. Corsini, F. Rispoli, and A. G. Sheard, “Detection of aerodynamic noise sources in low-speed axial fans with tip end-plates,” Proceedings of the Institution of Mechanical Engineers C, vol. 223, no. 6, pp. 1379–1392, 2009.
[16]
A. Corsini, F. Rispoli, and A. G. Sheard, “Development of improved blade tip end-plate concepts for low-noise operation in industrial fans,” in Proceedings of the 13th International Conference on Modelling Fluid Flow Technologies, pp. 50–61, Budapest, Hungary, September 2006.
[17]
A. Corsini, F. Rispoli, and A.G. Sheard, “A Meridional Fan,” 2008, Patent No. GB 0800582.9.
[18]
A. Corsini, F. Rispoli, and A. G. Sheard, “Development of improved blade tip endplate concepts for low-noise operation in industrial fans,” Proceedings of the Institution of Mechanical Engineers A, vol. 221, no. 5, pp. 669–681, 2007.
[19]
A. Corsini and A. G. Sheard, “Tip end-plate concept based on leakage vortex rotation number control,” Journal of Computational and Applied Mechanics, vol. 8, no. 1, pp. 21–37, 2007.
[20]
M. Inoue, M. Kuroumaru, and M. Fukuhara, “Behavior of tip leakage flow behind an axial compressor rotor,” Journal of Engineering for Gas Turbines and Power, vol. 108, no. 1, pp. 7–14, 1986.
[21]
A. Corsini and F. Rispoli, “Using sweep to extend the stall-free operational range in axial fan rotors,” Proceedings of the Institution of Mechanical Engineers A, vol. 218, no. 3, pp. 129–139, 2004.
[22]
S. Bianchi, A. Corsini, F. Rispoli, and A. G. Sheard, “Experimental aeroacoustic studies on improved tip geometries for passive noise signature control in low-speed axial fan,” Transactions of the ASME, Journal of Vibration & Acoustics, vol. 131, Article ID 061007, 2009.
[23]
A. Corsini, F. Rispoli, and A. G. Sheard, “Shaping of tip end-plate to control leakage vortex swirl in axial flow fans,” Transactions of ASME, Journal of Turbomachinery, vol. 132, Article ID 031005, pp. 1–9, 2010.
[24]
L. J. Leggat and T. E. Siddon, “Experimental study of the aeroacoustic mechanism of rotor-vortex interactions,” Journal of the Acoustical Society of America, vol. 64, no. 4, pp. 1070–1077, 1978.
[25]
B. Magliozzi, D. B. Hanson, B. V. Johnson, and F. B. Metzger, “Noise and wake structure measurements in a subsonic tip speed fan,” Tech. Rep. CR-2323, NASA, 1973.
[26]
M. R. Khourrami and M. Choudari, “A novel approach for reducing rotor tip-clearance induced noise in turbofan engines,” in Proceedings of the 7th AIAA/CEAS AeroacousticsConference, Maastricht The Netherlands, May 2001.
[27]
S. Bianchi, A. G. Sheard, A. Corsini, and F. Rispoli, “Far-field radiation of aerodynamic sound sources in axial fans fitted with passive noise control features,” Transactions of the ASME, Journal of Vibration & Acoustics, vol. 133, Article ID 051001, 2011.
[28]
P. D. Shah, D. Mobed, Z. Spakovszky, and T. F. Brooks, “Aero-acoustics of drag generating swirling exhaust flows,” in Proceedings of the 13th AIAA/CEAS Aeroacoustics Conference (28th AIAA Aeroacoustics Conference), Rome, Italy, 2007, Paper No. AIAA-2007-3714.
