Experimental investigation of the off-design sound emission of low-pressure axial fans with different fan blade skew

Transcription

1 Experimental investigation of the off-design sound emission of low-pressure axial fans with different fan blade skew Florian Krömer, Stefan Becker Institute of Process Machinery and Systems Engineering, Friedrich-Alexander University Erlangen- Nürnberg, Germany. Summary In this study, we experimentally investigated the sound emission of a backward- and a forward-skewed low-pressure axial fan at design and off-design operating conditions. We observed that the forwardskewed fans had a better aerodynamic performance as well as a lower sound emission at operating points close to the design point. Below and beyond this range, the aerodynamic properties of the fans were similar; however the backward-skewed fan had a lower sound emission than the forward-skewed fan. Hence, the installation systems and the induced pressure losses need always to be considered for choosing a low-noise fan design. PACS no h, Ra 1. Introduction The design of low-pressure axial fans usually relies on design parameters for a specific operating point, such as the volume flow rate and the pressure rise. However, in complex ventilation systems, these design parameters are often hard to anticipate at the time the fan design needs to be selected. This often leads to an off-design operating point of the axial fan, which in turn affects the aerodynamic performance and particularly the sound emission. The radiated sound of axial fans incorporates tonal, narrowband and broadband components [1 3]. Tonal sound is mainly generated by unsteady loading noise, induced by unsteady, i.e. fluctuating, blade forces [1, 3]. Fluctuating blade forces arise under an unsteady and/or inhomogeneous flow field upstream of the fan. This applies to nearly all installation systems of axial fans, even under ideal testing conditions. Characteristic tonal component are observed in the acoustic spectrum at the blade passing frequency (BPF) f BPF = n z b, with the rotational speed n and the number of fan blades z b, and harmonics. An additional tonal sound source is trailing edge noise, which arises from vortex shedding on the fan blade trailing edges [4 6]. The vortex shedding frequency is determined by the trailing edge thickness. Other tonal sound sources in axial fans are steady loading noise (Gutin noise) and thickness noise. However, particularly (c) European Acoustics Association for low-pressure axial fans, the contribution of these sound sources to the overall emitted sound is of minor importance [1, 6, 7]. If peaks or humps arise in the acoustic spectrum that do not correspond to the BPF or harmonics, they are called subharmonic components. Two different sound generation mechanisms lead to subharmonic component in low-pressure axial fans: tip noise and rotating stall [1, 3, 8]. Subharmonic tip noise has a narrowband characteristic, i.e. humps instead of sharp tonal peaks are observed in the acoustic spectrum. This sound is generated by coherent flow structures in the tip region and their interaction with the fan blades [8 11]. Rotating stall is induced by flow separations on the fan blade [3, 12]. If separation occurs on one fan blade, the blade channel is blocked and the volume flow is directed towards the neighboring blade channels. Owing to the rotating movement of these disturbances, subharmonic narrowband sound is emitted at f 0.7f BPF and harmonics. A very dominant broadband sound source is induced by turbulence ingestion noise. Thereby, sound sources are generated during the interaction of the inflow with the fan blades. A varying total blade loading arises if the integral length scale of the inflow Λ is larger than the chord length l c [4, 6, 12]. In this case, the acoustic wavelength of the emitted sound λ is usually larger than the chord length l c, i.e. the fan blade is acoustically compact and no specific sound source region can be identified on the fan blades. If the integral length scale is small than the chord length, pressure fluctuations are generated on the fan blade leading Copyright 2018 EAA HELINA ISSN: All rights reserved

2 edges [4, 6, 12]. Both mechanisms are a very effective broadband sound source. Besides having a tonal characteristic, trailing edge noise also leads to a broadband sound emission - induced by convected eddies past the fan blade trailing edges [4 6]. A governing parameter for the magnitude of broadband trailing edge noise is the velocity perpendicular to the trailing edge [12 15]. The tip vortex, initiated by the backflow from the pressure side to the suction side, also creates broadband sound similarly to an airfoil that is fixed only on one side along the blade span [4, 5, 16]. Broadband noise is further generated by pressure fluctuations beneath the turbulent boundary layer this is called boundary layer noise. However, similarly to Gutin noise and thickness noise, this sound source is only of minor importance [13, 17]. An effective measure to reduce axial fan noise is the application of fan blade skew, i.e. a combination of fan blade sweep and dihedral [15, 18, 19]. This has an impact on several sound generation mechanisms [15, 20 26]. In general, dominant sound sources are anticipated in the outer part of the fan blade as the circumferential velocity increases from the hub to the tip [27, 28]. Consequently, it has been shown that the greatest impact on the sound emission is assigned to the fan blade skew applied to the outer part of the fan blade [29]. There have been several studies on the sound emission of axial fans with skewed and unskewed fan blades, however most of those studies were focused on the sound emission at or close to the fan design point. Cai et al. [30] investigated the aerodynamic performance and the sound emission of a forward- and unskewed axial fan at design and off-design operating conditions. They found that the forward-skewed fan had a better aerodynamic performance and a lower sound emission at all considered operating conditions. Fukano and Jang [28] observed a high impact of the operating point on the tip leakage flow behavior with an intensified tip gap flow for volume flow rates below the design point, accompanied with an increase in the sound emission. With this study, we aimed at experimentally comparing the sound emission of a backward- and a forward-skewed low-pressure axial fan at design and off-design operating conditions. To achieve comparable results, all evaluations were made at identical operating conditions, i.e. matching volume flow rate and pressure rise. This will extend the current knowledge of axial fan sound generation mechanisms, in particular with different types of fan blade skew, at several operating regimes. In this study, a forward-skewed fan (F) (Figure 1) and a backward-skewed fan (B) (Figure 2) were investigated. They were designed with the blade element theory for low-solidity fans [31, 32]. A detailed description of the design process can be found in [15]. Each fan had nine fan blades, an outer diameter d fan = 495 mm and a hub diameter d hub = mm. A NACA 4510 airfoil [33] was used for the fan blades. Fan B had a linearly decreasing sweep angle from λ hub = 0 to λ tip = 55 and fan F a linearly increasing sweep angle from λ hub = 0 to λ tip = 55. They were installed in a duct with a diameter d duct = 500 mm, i.e. the tip gap size was s tip = 2.5 mm. Both fan were designed with a controlled vortex distribution [12, 15]. The design rotational speed was n = 1486 rpm, the design volume flow rate V = 1.4 m 3 /s and the design total-to-static pressure rise p ts = 150 Pa (fan B) and p ts = 140 Pa (fan F). The rotational speed was adapted during testing to ensure identical volume flow rate and total-to-static pressure difference values at each operating point. Figure 1: Forward-skewed fan F. 2. Fan design Figure 2: Backward-skewed fan B

3 butterfly damper standardised bellmouth inlet absorber 2445 (depth: 2445) flow straightener pressure tap test fan x 2 x 3 bellmouth inlet x 1 auxiliary fan diffusor torque meter motor 3635 Figure 3: Standardized inlet test chamber. 3. Experimental setup All investigations were carried out in a standardized inlet test chamber, based on ISO 5801 [34], with absorbing walls, ceiling and floor (Figure 3). The fans were installed in a short duct with an inlet bellmouth on the suction side and a diffusor on the pressure side. The fan mount (at the diffusor exit) was supported by four non-centric struts, see [15] for more details. The sound field was captured with seven 1/2 inch microphones that were arranged in a semicircle around the inlet bellmouth at the same height as the fan s rotational axis (Figure 4). microphone 3: +30 microphone 4: 0 microphone 1: +90 microphone 2: +60 microphone 5: microphone 6: microphone 7: 90 Figure 4: Microphone positions. 4. Characteristic curves Aerodynamic and acoustic characteristic curves of fans B and F are shown in Figure 5. As the fan rotational speed was adapted to achieve identical operating points, the pressure rise in Figure 5a of both fans is identical. The efficiency curves of both fans are similar for low volume flow rates V < 0.8 m 3 /s and high volume flow rates V > 1.6 m 3 /s. For V (0.8 m 3 /s, 1.6 m 3 /s), the efficiency values of fan F are substantially higher than that of fan B. The maximum efficiency is achieved at the design point and at slightly lower volume flow rates (fan F) and at the design point and slightly higher volume flow rates (fan B). Higher efficiency values and overall better efficiency margin of fan F can be explained with the forward-skewed fan blades and their beneficial impact on the fan aerodynamics [1, 20, 35, 36]. Similarly to the efficiency benefits, fan F has a considerably lower sound emission than fan B in the range V (1 m 3 /s, 1.5 m 3 /s). In contrast, lower averaged sound pressure levels are observed for fan B at low volume flow rates V < 0.9 m 3 /s and high volume flow rates V > 1.5 m 3 /s. The impact of the operating point on the acoustic spectra will be discussed in the next section. For this, three characteristic operating points are selected: Operating point 1 at V 1 = 0.7 m 3 /s: This corresponds to an operating point at which stall is anticipated. Operating point 2 at V 2 = 1.4 m 3 /s: This corresponds to the design volume flow rate; here, the fan blades are expected to be highly loaded but without the occurrence of stall

4 pressure diff. pts in Pa fan F fan B volume flow rate V in m 3 /s efficiency ηts in % fan F fan B volume flow rate V in m 3 /s av. sound pr. lev. Lp in db fan F fan B volume flow rate V in m 3 /s (a) (b) (c) Figure 5: Aerodynamic and acoustic characteristic curves: total-to-static pressure difference (a), total-to-static efficiency (b) and averaged sound pressure level (c). Operating point 3m at V 3 = 1.8 m 3 /s: This corresponds to an operating point at which the fan blades are only moderately loaded; stall does not occur here. 5. Sound field Averaged sound pressure for fans B and F are shown in Figure 6 for three operating points at V 1 = 0.7 m 3 /s, V 2 = 1.4 m 3 /s and V 3 = 1.8 m 3 /s. At V 1 = 0.7 m 3 /s, broadband levels for fan F are more pronounced than for fan B. Furthermore, hardly any tonal components can be seen for fan F, whereas tonal components at f BPF occur for fan B. It is expected that tonal components are masked by the broadband sound in case of fan F. For both fans, dominant subharmonic humps can be seen below f BPF at f = 0.69f BPF (fan F)/f = 0.71f BPF (fan B) and also at f = 1.38f BPF (fan F)/f = 1.42f BPF (fan B). The hump at f 0.7f BPF is substantially higher for fan F than for fan B. As discussed in Section 1, these humps can be related to rotating stall or to a combination of rotating stall and subharmonic tip noise. Obviously, these sound sources are more pronounced for fan F than for fan B under deep stall operating conditions. At the design volume flow rate V = 1.4 m 3 /s, there is a substantial difference between the spectra for fans B and F. For fan B, very dominant subharmonic components at f = 0.69f BPF and f = 1.38f BPF arise. They can be related to narrowband tip noise. This noise mechanism is usually more pronounced for backward-skewed than for forward-skewed fans [20, 35, 37, 38]. As a highly unsteady flow field is anticipated owing to the tip gap flow for fan B, unsteady blade loading is increased. As a consequence, tonal components at f BPF and harmonics are increased for fan B compared with fan F. Furthermore, low-frequency broadband sound is increased for fan B, induced by turbulence ingestion noise, due to the unsteady flow field in the tip region. In the range f > 2 khz, broadband noise of fan F becomes more dominant. This occurs owing differences in the fan blade curvature at the trailing edge. Owing to the geometry, the velocity perpendicular to the trailing edge is increased for fan F compared with fan B. This leads to a more pronounced high-frequency broadband sound radiation. At 1.8 m 3 /s, no narrowband subharmonic sound components can be seen. Wit decreasing pressure rise, the tip gap flow, and in turn subharmonic tip noise, is reduced. The low-frequency broadband sound is nearly identical for both fans. The levels of the tonal peaks at f BPF are also very similar. Substantial differences can be seen only for the high-frequency broadband noise from f = 1.8 khz. Similarly to the operating point at V = 1.4 m 3 /s, the broadband sound for fan F is more dominant than for fan B. This can again be related to the occurrence of higher velocities perpendicular to the trailing edge in case of fan F, due to the trailing edge curvature. 6. Conclusions The sound field of fans with backward- and forwardskewed fan blades was investigated experimentally at design and off-design operating conditions. Thereby, the rotational speed of the fans was adapted individually so that each fan had an identical pressure rise at all points along the characteristic curves. The fan characteristic curves revealed that the forward-skewed fan F had a higher efficiency in the usual operating range, i.e. close to the fan design point. In the same range, the forward-skewed fan F had also a substantially lower sound emission than the backward-skewed fan B. Below and beyond this range,

5 av. sound pressure level Lp in db av. sound pressure level Lp in db fan B fan F frequency f in Hz (a) V = 0.7 m 3 /s fan B fan F frequency f in Hz (b) V = 1.4 m 3 /s The acoustic spectra showed that for very low volume flow rates, broadband sound components are more dominant for the forward-skewed fan F than for the backward-skewed fan B. Additionally, narrowband humps had higher levels for fan F than for fan B. This lead to a lower overall sound emission of fan B compared with fan F. At the design point, narrowband subharmonic components could be seen only for the backward-skewed fan B. Additionally, tonal levels at f BPF and low-frequency broadband components were elevated for fan B, which was linked to a highly unsteady flow field in the tip region. Consequently, the overall sound emission of the backwardskewed fan was substantially higher than that of the forward-skewed fan F. A high volume flow rates, the low-frequency part of the spectra (f > 1.8 khz) was nearly identical. Major differences were seen only for the high-frequency part. Here, broadband sound levels of the forward-skewed fan F were higher than for the backward-skewed fan B, which is the reason for the higher overall sound emission for fan F compared with fan B. This study showed that the forward-skewed fan had a better performance, both in terms of aerodynamics and aeroacoustics, under operating conditions close to the fan design point. However, if the fan is operated under extreme off-design conditions, the backwardskewed fan had a lower sound emission. This shows the significance of the consideration of all design boundary conditions for the selection of a suitable fan design for technical systems. References av. sound pressure level Lp in db fan B fan F frequency f in Hz (c) V = 1.8 m 3 /s Figure 6: Averaged sound pressure spectra. i.e. a low and high volume flow rates, the backwardskewed fan B had a lower sound emission than the forward-skewed fan F. [1] Wright, T. and Simmons, W.: Blade sweep for low speed axial fans. Journal of Turbomachinery, 112: , doi: / [2] Sharland, I. J.: Sources of noise in axial flow fans. Journal of Sound and Vibration, 1(3): , doi: / X(64) [3] Bommes, L.; Fricke, J. and Grundmann, R. (editors): Ventilatoren. Vulkan, Essen, 2nd edition, [4] Wagner, S.; Bareiß, R. and Guidati, G.: Wind turbine noise. Springer, Berlin, doi: / [5] Brooks, T. F.; Pope, S. D. and Marcolini, M. A.: Airfoil self-noise and prediction. NASA Reference Publication 1218, [6] Blake, W. K.: Mechanics of flow-induced sound and vibration: Complex flow-structure interactions, volume 2. Academic Press, New York, [7] Wright, T. and Gerhart, P. M.: Fluid machinery: Application, selection and design. CRC Press, Boca Raton, 2nd edition, doi: /b [8] Moreau, S. and Marlène, S.: Sub-harmonic broadband humps and tip noise in low-speed ring fans

6 Journal of the Acoustical Society of America, 139 (1): , doi: / [9] Kameier, F. and Neise, W.: Experimental study of tip clearance losses and noise in axial turbomachines and their reduction. Journal of Turbomachinery, 119(3): , doi: / [10] Mailach, R.; Lehmann, I. and Vogeler, K.: Rotating instabilities in an axial compressor originating from the fluctuating blade tip vortex. Journal of Turbomachinery, 123(3), doi: / [11] Zhu, T.; Lallier-Daniels, D.; Sajosé, M.; Moreau, S. and Carolus, T.: Rotating coherent flow structures as a source for narrowband tip clearance noise from axial fans. Journal of Sound and Vibration, 417: , doi: /j.jsv [12] Carolus, T.: Ventilatoren - Aerodynamischer Entwurf, Schallvorhersage, Konstruktion. Springer Vieweg, Wiesbaden, 3rd edition, doi: / [13] Hayden, R. E.: Some advances in design techniques for low noise operation of propellers and fans. In Proceedings of the National Conference on Noise Control Engineering, Hampton, USA, [14] Brown, N. A.: The use of skewed blades for ship propellers and truck fans. In Proceedings of Noise and Fluids Engineering, Winter annual meeting of ASME, pages , Atlanta, USA, [15] Krömer, F. J.: Sound emission of lowpressure axial fans under distorted inflow conditions. Dissertation., Friedrich-Alexander University Erlangen-Nürnberg, FAU University Press, March [16] Giez, J.; Vion, L.; Roger, M. and Moreau, S.: Effect of the edge-and-tip vortex on airfoil selfnoise and turbulence impingement noise. In Proceedings of 22nd AIAA/CEAS Aeroacoustics Conference, AIAA Paper , Lyon, France, May doi: / [17] Meecham, W. C.: On noise produced by boundary-layer turbulence. Journal of the Acoustical Society of America, 35: , doi: / [18] Smith, L. H. and Yeh, H.: Sweep and dihedral effects in axial-flow turbomachinery. Journal of Basic Engineering, 85(3): , doi: / [19] Beiler, M.: Untersuchung der dreidimensionalen Strömung durch Axialventilatoren mit gekrümmten Schaufeln. PhD thesis, University Siegen, Germany, [20] Vad, J.: Aerodynamic effects of blade sweep and skew in low-speed axial flow rotors at the design flow rate: an overview. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, 222(1):69 85, doi: / JPE471. [21] Carolus, T. and Beiler, M.: Skewed blades in low pressure fans: a survey of noise reduction mechanisms. In Proceedings of 3rd AIAA/CEAS Aeroacoustics Conference, AIAA Paper , pages 47 53, Atlanta, USA, May doi: / [22] Yang, L.; Hua, O. and Zhao-Hui, D.: Optimization design and experimental study of lowpressure axial fan with forward-skewed blades. International Journal of Rotating Machinery, pages 1 10, doi: /2007/ [23] Kerschen, E. and Envia, A.: Noise generation by a finite swept airfoil. AIAA Paper , [24] Hanson, D. B.: Influence of propeller design parameters on far-field harmonic noise in forward flight. AIAA Journal, 18(11): , doi: / [25] Zenger, F. J.; Renz, A.; Becher, M. and Becker, S.: Experimental investigation of the noise emission of axial fans under distorted inflow conditions. Journal of Sound and Vibration, 383: , doi: /j.jsv [26] Cumming, R. A.; Morgan, W. B. and Boswell, R. J.: Highly skewed propellers. In Proceedings of the Annual Meeting of the Society of Naval Architects and Marine Engineers (SNAME), pages , New York, USA, November [27] Agboola, F. A. and Wright, T.: The effects of axial fan noise control by blade sweep on the radial component of velocity. In Proceedings of 5th AIAA/CEAS Aeroacoustics Conference, AIAA Paper , Bellevue, USA, May doi: / [28] Fukano, T. and Jang, C.-M.: Tip clearance noise of axial flow fans operating at design and offdesign condition. Journal of Sound and Vibration, 275: , doi: /S X(03) [29] Krömer, F.; Müller, J. and Becker, S.: Investigation of aeroacoustic properties of lowpressure axial fans with different blade stacking. AIAA Journal, 56(4): , doi: /1.J [30] Cai, N.; Xu, J. and Benaissa, A.: Aerodynamic and aeroacoustic performance of a skewed rotor. In Proceedings of ASME Turbo Expo 2003: Turbomachinery Technical Conference and Exposition, GT , Atlanta, USA, June doi: /GT [31] Dixon, S. L. and Hall, C. A.: Fluid mechanics and thermodynamics of turbomachinery. Elsevier, Amsterdam, 7th edition, [32] Pfleiderer, C. and Petermann, H.: Strömungsmaschinen. Springer, 7th edition, doi: /b

7 [33] Abbott, I. H. and Von Doenhoff, A. E.: Theory of wing sections: Including a summary of airfoil data. Dove Publications, New York, [34] International Organization for Standardization. ISO 5801:2007 Industrial fans - Performance testing using standardized airways, [35] Corsini, A. and Rispoli, F.: Using sweep to extend the stall-free operational range in axial fan rotors. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, 218: , doi: / [36] Gallimore, S. J.; Bolger, J. J.; Cumpsty, N. A.; Taylor, M. J.; Wright, P. I. and Place, J. M. M.: The use of sweep and dihedral in multistage axial flow compressor blading - part 1: University research and methods development. Journal of Turbomachinery, 124(4): , doi: / [37] Paxton, C.; Gryn, P.; Hines, E.; Perez, U. and Zha, G.-C.: High efficiency forward swept propellers at low speed. In Proceedings of 41st Aerospace Sciences Meeting and Exhibit, AIAA Paper , Reno, USA, January doi: / [38] Li, Y.; Ouyang, H. and Du, Z.-h.: Experimental research on aerodynamic performance and exit flow field of low pressure axial flow fan with circumferential skewed blades. Journal of Hydrodynamics, 19(5): , doi: /S (07)

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