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Correlation between Aerodynamic Noise and Velocity Fluctuation of Tip Leakage Flow of Axial Flow Fan  [PDF]
Hiromitsu Hamakawa, Masatomo Shiotsuki, Takaaki Adachi, Eru Kurihara
Open Journal of Fluid Dynamics (OJFD) , 2012, DOI: 10.4236/ojfd.2012.24A026
Abstract: In the present paper the attention is focused on correlation between fan noise and velocity fluctuations of tip leakage vortex around rotor blade of a low pressure axial flow fan at the maximum pressure operating point. We measured time fluctuating velocity near the rotor tip around the rotor blades by using a hot-wire sensor from a relative flame of refer- ence fixed to the rotor blades. As the results, it is clear that the velocity fluctuation due to tip leakage vortex has weak periodicity and the hump portion appeared in its spectrum. If the flow rate was lower than the design condition, the tip leakage flow became to attach to the following blade and the sound pressure level at frequency of velocity fluctuation of this flow was increased. The correlation measurements between the velocity fluctuation of tip leakage flow and the aerodynamic noise were made using a rotating hot-wire sensor near the rotor tip in the rotating frame. The correlation between the velocity fluctuation due to tip leakage flow and acoustic pressure were increased due to generation of weak acoustic resonance at the maximum pressure operating point.
Numerical Study on Internal Flow of Small Axial Flow Fan with Splitter Blades  [PDF]
Lifu Zhu, Yingzi Jin, Yanping Wang, Li Zhang, Yuzhen Jin
Open Journal of Fluid Dynamics (OJFD) , 2013, DOI: 10.4236/ojfd.2013.32A012
Abstract: The splitter blades are widely used in axial compressors and play an active role in the improvement of the overall performance of compressors. However, little research on the application of splitter blades to small axial flow fans is conducted. This paper designs a splitter blade small axial flow fan (model B) with a small axial flow fan as the prototype fan (model A) by adding short blades at the second half part of the passageway among long blades of model A. The steady simulation for the two models was conducted with the help of RNG k-ε turbulence model provided by software Fluent, and static characteristics and internal flow characteristics of the two models were compared and analyzed. Results show that splitter blades can improve the unsteady flow in the small flow rate region and also have a positive role to increase static pressure rise and efficiency in the higher flow rate region. The variation of static pressure gradient on the meridian plane in model B is well-distributed. The static pressure on the blade surface of model B distributes more uniformly. Splitter blades can suppress the secondary flow from pressure side to suction side in the leading edge because the pressure difference between suction side and pressure side in model B is generally lower than that of model A. And it also can restrain the vortex shedding and flow separation, and further it may be able to get the aerodynamic noise lower because static pressure gradient on the blade surface is well-distributed and the vortex shedding is not developed. Therefore, the performance of the fan with splitter blades is better than that of the prototype fan. The findings of this paper can be a basis for the design of high performance small axial flow fans.
Experimental Characterisation of the Far-Field Noise in Axial Fans Fitted with Shaped Tip End-Plates  [PDF]
S. Bianchi,A. Corsini,A. G. Sheard
ISRN Mechanical Engineering , 2012, DOI: 10.5402/2012/212358
Abstract: 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
Attenuation of Cross-Flow Fan Noise Using Porous Stabilizers  [PDF]
Huanxin Lai,Meng Wang,Chuye Yun,Jin Yao
International Journal of Rotating Machinery , 2011, DOI: 10.1155/2011/528927
Abstract: This paper presents a qualitative analysis of controlling the cross-flow fan noise by using porous stabilizers. The stabilizer was originally a folded plate. It is changed into a porous structure which has a plenum chamber and vent holes on the front wall. In order to investigate the influences of using the porous stabilizers, experiments are carried out to measure the cross-flow fan aerodynamic performances and sound radiation. Meanwhile, the internal flow field of the fan is numerically simulated. The results show that the porous stabilizers have not produced considerable effect on the cross-flow fan's performance curve, but the noise radiated from the fan is strongly affected. This indicates the feasibility of controlling the cross-flow fan noise by using the porous stabilizers with selected porosity. 1. Introduction In order to improve the nowaday indoor environment for daily life and work, air-conditioners are becoming widely used [1]. Because cross-flow fans (CFFs) generally have large capacities of mass flow and are compact in size [2–5], they are often used to move air in the air conditioners. However, a CFF is also well known for the complex flow configuration, such as the irregular geometry of the impeller and the stabilizer. As a result, the internal flows of CFF are very complex, while the eccentric vortex is a typical phenomenon [6]. An illustration of the internal flow-field of a CFF is shown in Figure 1. These complexities have made it difficult to form a universal designing theory for CFF, and therefore, the flow efficiencies of the CFF are relatively poor [7–9], as compared with axial or centrifugal fans. Figure 1: Typical flow pattern in a cross-flow fan. Noise radiation from CFF is a very important issue of concern for indoor environment. In order to attenuate the noise radiation from the CFF, Koo et al. [7] and Tsai et al. [10] used skewed stabilizers and skewed rotors, respectively. Because the spanwise size of a CFF is generally several times larger than the radial size, it is possible to skew the impeller and/or the stabilizer so as to avoid the blades at different axial locations pass the stabilizer simultaneously. On the other hand, Moon et al. [4] tried to modulate the blade passing frequency noise by using random-pitch impellers. All these considerations are aimed at weakening the vortical flow impingements on the stabilizer. Indeed, some effects in cutting down the noise radiation have been achieved by using these methods. Porous media are widely used in sound controlling. Generally, porous materials are applied to wrapping
An Experimental and Numerical Study on Wake Vortex Noise of a Low Speed Propeller Fan  [PDF]
Soichi Sasaki, Ikki Torise, Hidechito Hayashi
Open Journal of Fluid Dynamics (OJFD) , 2012, DOI: 10.4236/ojfd.2012.24A035

In order to clarify the relationship between the aerodynamic noise and the flow regime around the rotating blade of a propeller fan operated at the maximum efficiency point and the off-design point, the characteristics of fans with different solidity impellers were analyzed experimentally. At the off-design point, the broadband noise of the high solidity fan was much larger than that of the low solidity fan because the relative velocity increased according to the solidity and the noise sources increased because of the number of blades. In the case of the low solidity fan, the broadband noise due to wake vortex shedding was generated at the off-design point in the low flow rate domain and the maximum efficiency point because the relative flow around the blade separated easily.

Simulation of Vortex Flows in Axial Flow Fan Using Computational Fluid Dynamics  [PDF]
E.E. Elhadi,Wu Keqi
Information Technology Journal , 2002,
Abstract: This work aims to study and analyze the behavior of flow in axial flow fan. Numerical simulations were carried out based on Navier-stocke equations coupled with k – ε Turbulence model with standard wall function. These simulations were applied to axial flow fan which consists of nine rotor blades and fifteen stator blades. These blades were designed by modified quasi-three dimensional design program code. In the present work, different flow phenomena which occur in axial flow fan were presented and interpreted at different regions with special concern to the rotor exit and stator regions. In order to study the behavior and mechanism of these phenomena, different flow parameters (velocity components, pressure and turbulent intensity) were calculated at different regions and at different flow rate coefficients. It is found that high flow rate causes higher vortex flow than low flow rate, mainly due to existence of the adverse pressure and hence a reverse flow at high flow rate. This is clearly observed in the stator region which negatively affected by increasing the flow rate and the stator became inefficient. It has been also found that, high flow rate causes a vortex flow at stator pressure side while the lower flow rate causes a vortex flow at stator suction side. This study shows that decrease in flow rate causes wider range of wake and vortex at the rotor exit than that caused by the other cases of flow rate. Finally in order to validate the present simulation, the obtained results were compared with the available literature results and it was found that their physical behavior agree quite well.
仿?^翼前缘蜗舌对多翼离心风机 气动性能和噪声的影响
Effects of Bionic Volute Tongue Bioinspired by Leading Edge of Owl Wing on Aerodynamic Performance and Noise of Multi??Blade Centrifugal Fan

- , 2015, DOI: 10.7652/xjtuxb201501003
Abstract: 针对某多翼离心风机,受?^类翅膀前缘结构的启发,设计了一种新型降噪结构――仿?^翼前缘蜗舌。基于逆向工程方法,通过提取长耳?^翅膀气动性能较好的40%翼展方向上的翅膀剖面型线,对风机蜗舌进行了仿生降噪重构设计,并分别对原型风机和仿生蜗舌风机的气动性能和噪声特性进行了数值模拟。研究结果显示,仿?^翼前缘蜗舌风机的风量较原型风机增加了1.9 m3/min,噪声下降了1.6 dB,效率提高了3.8%,表明采用仿生蜗舌有效降低了气流对风机蜗舌的冲击作用,抑制了流动分离的发生,在蜗舌附近区域流场的逆压梯度明显减小,涡的结构、强度和分布都有所改变。
Bionic volute tongue bioinspired by the leading edge of owl wing, a new type of noise??reduction structure, is designed for multi??blade centrifugal fan. Adopting reverse engineering method, the section profile in the position of 40% spanwise of the wing of a long??eared owl, whose aerodynamic performance is relatively excellent, is extracted to restructure the bionic volute tongue. Numerical simulations are respectively conducted to investigate the aerodynamic performance and noise characteristics of the prototypical fan and the developed fan with the bionic volute tongue. Compared with the prototypical fan, the volume flow rate of the bionic fan is increased by 1.9 m3/min, while the noise is decreased by 1??6 dB and the efficiency is increased by 3.8%. The bionic volute tongue bioinspired by the leading edge of owl wing facilitates weakening the impact of the airflow to the volute tongue of multi??blade centrifugal fan, and the flow separation is suppressed effectively. Near the volute tongue, the adverse pressure gradient is reduced significantly, and the corresponding vortex’s structure, distribution and intension are also changed
内凹式蜗舌对多翼离心风机 气动性能和噪声的影响
Study on the Effects of Concave Volute Tongues on Aerodynamic Performance and Noise of a Multi??Blade Centrifugal Fan

- , 2017, DOI: 10.7652/xjtuxb201712019
Abstract: 针对吸油烟机用多翼离心风机设计了内凹弧形、内凹槽形2种内凹式蜗舌,采用实验测量和数值模拟方法研究了内凹式蜗舌对多翼离心风机气动性能和噪声的影响。与传统的原型蜗舌相比,采用内凹式蜗舌在风机出口压力和运行效率基本保持不变的情况下,叶轮出口气流对蜗舌的冲击强度减小,蜗舌处流场的逆压梯度降低,使得风机气动噪声下降。实验结果表明:与原风机相比,带有内凹弧形蜗舌和内凹槽形蜗舌的多翼离心风机的噪声分别下降了1.4 dB和1.7 dB;同时,由于内凹式蜗舌能够抑制流动分离,使风机出口处的有效流通面积增大,蜗舌附近区域的旋涡强度及其影响区域减小,从而增加了多翼离心风机的流量。
Two types of volute tongue are designed for a multi??blade centrifugal fan used in range hoods. One is an inner concave arc volute tongue and the other is an inner concave grooved volute tongue. The effects of the inner concave volute tongues on both the aerodynamic performance and the noise of the multi??blade centrifugal fan are studied by using the measurement and numerical simulation methods. Compared with the traditional tongue, the impact of the air flow discharged by the impeller on the surface of the volute tongue is weakened and the inverse pressure gradient of the flow field near the volute tongue is reduced when the inner concave arc and the inner concave grooved volute tongues are used. Under the condition that the total pressure and operating efficiency of the multi??blade centrifugal fan keep constant, the aerodynamic noise is reduced. Experimental results show that the sound pressure level of the multi??blade centrifugal fans with the inner concave arc volute tongue and the inner concave grooved volute tongue goes down by 1.4 dB and 1.7 dB, respectively. At the same time, the effective flow area at the fan outlet increases because the flow separation occurred near the volute tongue is suppressed. The vortex intensity and the corresponding influence area are reduced. These results show that the flow rate of the multi??blade centrifugal fan with the concave volute tongues effectively increases
Optimization Design and Experimental Study of Low-Pressure Axial Fan with Forward-Skewed Blades  [PDF]
Li Yang,Ouyang Hua,Du Zhao-Hui
International Journal of Rotating Machinery , 2007, DOI: 10.1155/2007/85275
Abstract: This paper presents an experimental study of the optimization of blade skew in low pressure axial fan. Using back propagation (BP) neural network and genetic algorithm (GA), the optimization was performed for a radial blade. An optimized blade is obtained through blade forward skew. Measurement of the two blades was carried out in aerodynamic and aeroacoustic performance. Compared to the radial blade, the optimized blade demonstrated improvements in efficiency, total pressure ratio, stable operating range, and aerodynamic noise. Detailed flow measurement was performed in outlet flow field for investigating the responsible flow mechanisms. The optimized blade can cause a spanwise redistribution of flow toward the blade midspan and reduce tip loading. This results in reduced significantly total pressure loss near hub and shroud endwall region, despite the slight increase of total pressure loss at midspan. In addition, the measured spectrums show that the broadband noise of the impeller is dominant.
Study on the Aerodynamic Performance of Multi??Blade Centrifugal Fan with Bionic Blades Inspired by Carangiform Fish

- , 2018, DOI: 10.7652/xjtuxb201811004
Abstract: 受?科鱼类机动游动姿态和涡流特征启发,设计了一种仿?科鱼体弯曲姿态的多翼离心风机用仿生叶片,采用数值模拟方法,分别研究了常规单圆弧原型叶片和仿鱼形叶片对多翼离心风机气动性能、噪声的影响。通过可视化分析发现,在仿鱼形叶片的叶道内,旋涡强度明显小于原型风机,流场分布更加均匀。仿鱼形叶片的采用有效降低了风机蜗舌处的压力脉动,减弱了叶片与蜗舌之间的非定常相互作用。风机气动噪声计算与分析结果表明,单圆弧原型叶片的风机噪声频率分布于低频声段和中频声段,而仿鱼形叶片的风机噪声频率主要集中在中频声段范围内,这表明两种风机的噪声频率分布规律和噪声传播路径不相同。数值计算结果表明,采用仿鱼形叶片的多翼离心风机气动性能明显得到改善,其中风量增加了12.5%,效率提升了5.65%,同时测点处噪声平均值下降了2.78 dB。
Inspired by the maneuvering swimming posture and vortex characteristics of carangiform fish, a bionic blade of the multi??blade centrifugal fan is designed according to the turning feature of carangiform fish. Numerical method is conducted to investigate the effects of bionic blade and single??arc blade on the aerodynamic performance of multi??blade centrifugal fan. Compared with the original centrifugal fan with the single??arc blades, the flow rate and efficiency of the multi??blade centrifugal fan with the bionic blades are increased by 12??5% and 5??65%, respectively. The vortex intensity in the impeller channel consisting of bionic blades is weaker than that in the original impeller channel with single??arc blades. The pressure fluctuation occurring near the region of volute tongue is suppressed and the periodic unsteady interaction between the impeller and the volute tongue is weakened. When the bionic blade is used, the aerodynamic performance of multi??blade centrifugal fan is improved. Numerical results on the aerodynamic noise show that for the original fan, low??frequency and intermediate??frequency noises have important contributions to the sound pressure level while the aerodynamic noise of the centrifugal fan with the bionic blades mainly concentrates in the intermediate??frequency sound section. The distribution of noise frequency and noise propagation path of the original fan are different from that of the multi??blade centrifugal fan with the bionic blades. Compared with the original fan, the average A??weighted sound pressure level of the centrifugal fan with the bionic blades is reduced by 2.78 dB
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