Vortex shedding noise of low tip speed,axial flow fans

Noise and performance tests were conducted on three low tip speed, half-stage, axial flow fans to determine the nature of the vortex shedding noise mechanism. Each fan was 356 mm in diameter and had eight equally spaced, variable pitch blades. The noise measurements were made in a free field environment and the fan back pressure and speed were varied during the tests. An acenaphthene coating on the blades was used to determine the regions of laminar and turbulent flow.Vortex shedding can be a significant source of noise when the fan is operated in a lightly loaded condition. Essentially it is due to instabilities in the laminar boundary layer on the suction side of the blade where these instabilities are in the form of Tollmien-Schlichting (T-S) waves. These instabilities interact with the trailing edge of the blade and generate acoustic waves which radiate from the trailing edge and form a feedback loop with the source of the instabilities. Vortex shedding noise can contribute as much as 5 dB in overall noise level and up to 22 dB at higher frequencies (8–14 kHz).Serrations located at the leading edge, at the mid-chord, or near the trailing edge on the suction side were found to reduce the vortex shedding noise significantly. The mid-chord location was found to be the most satisfactory because, as well as eliminating the noise, the serrations provided a 3% improvement in peak efficiency. This improvement occurred because separation of the laminar boundary layer was prevented on the suction side. On the other hand, serrations placed at the other two locations tended to degrade fan performance.     

不同叶轮形式下离心泵噪声特性对比研究

针对具有无短叶片和有短叶片两种叶轮形式的离心泵,对设计状态下离心泵内部流场进行了全三维、非定常数值模拟,对比分析了其非定常流场特性和噪声辐射特性。流场分析表明:叶轮叶片和蜗舌的相互作用造成了叶片表面强烈的压力脉动,对长短叶片的叶轮形式,在局部增加长叶片表面压力脉动的同时,短叶片表面的压力脉动保持较低水平;同时能够有效降低泵体进口压力脉动,但出口压力脉动有所增强。以叶轮叶片表面作为声源辐射面,对比分析了两种叶轮的偶极子噪声辐射特性,结果表明:长短叶片结构通过改变声能在频域上的分布,从而能有效降低总声压级。      

空调风机叶道内旋涡流动分析及进气口偏心的影响

本文采用CFD方法,详细分析了空调用多翼离心风机叶道中的气流分布以及进气口偏心的影响。研究表明,风机叶道内存在大范围的气流分离现象。在后盘附近,存在分离现象的叶道约占2/3,主要分布在蜗壳内侧;而在“前盘”附近,几乎所有叶道都存在分离现象。在“前盘”附近,蜗舌下方的叶道中气流几乎停滞,蜗舌下游叶道为回流和尾缘旋涡所充满,至临近蜗壳出口侧,前缘旋涡逐步形成、发展并融合尾缘旋涡,最后衰减、消失。风机进气口向蜗壳内侧偏置适当距离, “前盘”附近叶道旋涡分布范围明显减小。     

叶顶端翼提升主动襟翼叶型气动性能的数值研究

本文在振荡来流条件下,数值模拟叶顶端翼对加装主动Gurney襟翼的垂直轴风力机叶片非定常气动特性的影响。采用NACA0015翼型的直叶片,并在尾缘前6%弦长位置安装主动襟翼。在最大出力工况(折合频率为0.1)下,对比原型叶片,加装主动襟翼叶片的切向力系数提高了4.47%,安装有叶顶端翼的主动襟翼叶片的切向力系数提高21.18%。通过比较叶片端部涡结构分布,发现叶顶端翼不仅阻止了叶片压力面及吸力面的叶梢涡分支在尾缘处汇合,同时也隔断了主动襟翼产生的角涡与叶梢涡的融合,有效的降低了叶片端部损失,提升了风力机的整体性能。     

Numerical and experimental investigations of the unsteady aerodynamics and aero-acoustics characteristics of a backward curved blade centrifugal fan

A numerical study of the aerodynamic and aeroacoustic behaviors of a backward curved blade centrifugal fan was conducted under two important flow conditions: BEP and 1.3 × BEP. Three-dimensional numerical simulations of the complete unsteady flow field for the whole impeller-volute configuration were used to determine the aeroacoustic sources. To locate the unsteady flow and perturbations, the near field wall pressure fluctuations at different strategic points on the volute were computed using the URANS approach. Thus the intensities and positions of the aeroacoustic sources were identified by analyzing frequency spectra. The aeroacoustic sources caused by fluctuations in the interactions of the flows leaving the impeller and volute were close to the volute tongue, and the most effective noise sources related to the flow rate were near the impeller shrouds. In addition, the unsteady flow variables provided by CFD calculations were used as inputs in the Ffowcs Williams-Hawkings equation to estimate the noise tones of the fan. The aeroacoustic calculation results showed that the volute noise was much larger than the blade noise, and the noise mainly propagated from the outlet duct of the fan. Moreover, to account for the noise propagation, three calculation methods were used by applying different solid boundaries. Compared with the other methods, the FEM method, which accounted for the complex solid boundaries, produced good agreement and showed that the complex solid boundaries cannot be neglected in aeroacoustic predictions. The calculation results showed good agreement with the experimental results.     

A computational study of the interaction noise from a small axial-flow fan

Small axial-flow fans used for computer cooling and many other appliances feature a rotor driven by a downstream motor held by several cylindrical struts. This study focuses on the aerodynamic mechanism of rotor-strut interaction for an isolated fan. The three-dimensional, unsteady flow field is calculated using FLUENT, and the sound radiation predicted by acoustic analogy is compared with measurement data. Striking differences are found between the pressure oscillations in various parts of the structural surfaces during an interaction event. The suction surface of the blade experiences a sudden increase in pressure when the blade trailing edge sweeps past a strut, while the process of pressure decrease on the pressure side of the blade is rather gradual during the interaction. The contribution of the latter towards the total thrust force on the structure is cancelled out significantly by that on the strut. In terms of the acoustic contributions from the rotor and strut, the upstream rotor dominates and this feature differs from the usual rotor-stator interaction acoustics in which the downstream part is responsible for most of the noise. It is therefore argued that the dominant interaction mechanism is potential flow in nature.     

计及几何参数变化的离心压气机特性分析

本文采用CFD软件对微型燃机的离心叶轮进行数值模拟,讨论了叶片数及分流叶片位置对叶轮性能的影响,并进行了流场分析。在本文研究的情况中,叶片数增加使得性能曲线左移,单个叶片载荷减小,损失增加,叶轮效率下降, 但是增压效果得到改善。分流叶片位置靠近主叶片压力面时,性能曲线右移,流通能力提高,同时会使分流叶片的载荷增大,当分流叶片靠近主叶片压力面时,则情况相反。     

Measurement of aerodynamic noise and unsteady flow field around a symmetrical airfoil

The purpose of this paper is to study the physics of aerodynamic noise generation from the symmetrical airfoil of NACA 0018 in a uniform flow. The relationship between the noise spectrum and the unsteady flow field around the airfoil is studied in an acoustic wind tunnel using flow visualization and PIV analysis. The discrete frequency noise was generated from the airfoil inclined at small angle of attack to the free stream. The flow visualization result indicates the presence of attached boundary layer over the suction side and the separated shear layer over the rear pressure side of the airfoil, when the discrete frequency noise is observed. It is found from the PIV analysis that a large magnitude of vorticity is generated periodically from the pressure side of the trailing edge and it develops into an asymmetrical vortex street in the wake of the airfoil. The periodicity of the shedding vortices was found to agree with that of the frequency of the generated noise.     

离心压气机叶排间涡脱落及气流脉动行为研究

对某一带叶片扩压器跨声速离心压气机内部三维非定常流场进行了数值模拟,依据计算结果描述了叶轮叶片尾缘尾迹涡脱落行为,给出了叶轮尾缘脉动压力频谱特性图和叶轮出口位置脉动速度动能分布图。研究结果表明,主叶片与分流叶片尾缘尾迹涡脱落过程交替进行;转静干涉和转子叶片尾缘的涡脱落行为是造成叶轮尾缘气流脉动行为的主要原因,随着时间的...     

前缘弯掠斜流转子叶顶间隙流动特性数值分析

本文对前缘弯掠斜流转子叶顶间隙内的流动特性进行了数值分析。结果表明:叶顶间隙气流与主流发生卷吸而生成泄漏涡。泄漏涡作用的区域具有较低的压力分布。在叶片通道内,泄漏涡沿着与转子旋向相反的方向朝相邻叶片的压力面移动。大间隙时的泄漏涡比小间隙时强烈。低流量时泄漏涡的作用区域比高流量时大。在各种流量特性下,叶顶尾缘近吸力面区域都存在着二次间隙流。     

叶片尾缘掠型对离心压气机特性影响研究

针对叶片尾缘掠型对离心压气机特性影响开展了数值仿真与试验验证的研究工作,详细分析了尾缘掠型对压气机特性的影响规律和压气机叶轮内部流动特征,研究结果表明:尾缘掠型能够提升压气机出口的总压,使离心压气机压比变高,自由掠叶型相比于斜掠叶型,压比会进一步提升.叶片尾缘掠型能够有效抑制尾缘流动分离与叶间泄漏效应的结合效果,降低流...     

A numerical study on the flow and sound fields of centrifugal impeller located near a wedge

Centrifugal fans are widely used and the noise generated by these machines causes one of the serious problems. In general, the centrifugal fan noise is often dominated by tones at blade passage frequency and its higher harmonics. This is a consequence of the strong interaction between the flow discharged from the impeller and the cut-off in the casing. However, only a few researches have been carried out on predicting the noise because of the difficulty in obtaining detailed information about the flow field and considering the scattering effect of the casing. The objective of this study is to understand the generation mechanism of sound and to develop a prediction method for the unsteady flow field and the acoustic pressure field of the centrifugal impeller. A discrete vortex method is used to model the centrifugal impeller and a wedge and to calculate the flow field. The force of each element on the blade is calculated by the unsteady Bernoulli equation. Lowson's method is used to predict the acoustic source. In order to consider the scattering and diffraction effects of the casing, Kirchhoff-Helmholtz boundary element method (BEM) is developed. The source of Kirchhoff-Helmholtz BEM is newly developed, so the sound field of the centrifugal fan can be obtained. A centrifugal impeller and wedge are used in the numerical calculation and the results are compared with the experimental data. Reasonable results are obtained not only for the peak frequencies but also for the amplitudes of the tonal sound. The radiated acoustic field shows the diffraction and scattering effect of the wedge.     

在大转角透平叶栅中叶片反弯曲对通道涡及静压场的影响

本文采用微型球头五孔探针对低展弦比大转角透平直叶片栅和反弯曲叶片栅栅内外流场进行了详尽测量,并开设静压孔测取了叶栅端壁和叶片表面的静压分布。根据实验结果,作者全面分析了叶片反弯曲对通道涡及静压场的影响,并明确提出：在大转角透平叶栅中,叶片反弯曲后,由叶片力在吸力面前部产生的沿叶高方向的反“Ｃ”型静压分布是控制通道涡减弱的关键,这也是叶片反弯曲作用的本质所在。     

叶片尾缘内冷通道中最佳强化传热的针肋排列结构研究

本文应用湍流模型对涡轮叶片尾缘针肋通道的换热与流动进行了二维数值模拟研究。为了研究通道内针肋排列 方式对换热与流动的影响,对三种不同的针肋排列方式的通道进行了数值模拟计算。比较了顺排和叉排的区别,并提出了 一种沿流向叉排的针肋排列方式,且对各种排列的传热和阻力特性进行了综合分析和比较。     

叶片开缝的离心风机流场研究

本文提出在离心叶轮轮盖与叶片之间开缝,使得叶片压力面侧的高压气体射流到吸力面侧的低速尾迹区,从而直接给叶轮内的低速流体提供能量,减弱了叶轮内由于二次流所导致的射流-尾迹结构,并可用于消除或解决部分负荷时常发生的离心叶轮积灰问题.对某离心风机整机的数值模拟表明,在设计流量和小流量情况下,叶轮开缝后使叶片表面分离区域减小,整个流道速度分布更为均匀,改善了叶轮内部流场的流动状况,提高了离心叶轮性能和整机性能.研究表明,本方法对于提高离心式流体机械性能是有效的.     

大小叶片贯流风机内流特性分析与实验研究

为降低空调用贯流风机的噪声,改善音质,通过采用直叶片贯流风轮达到斜扭叶片贯流风轮的音质和低噪声特性,从而降低贯流风轮的制造成本,本文设计了大小叶片交错组合的新型非等距贯流风机,并采用滑动网格对其内流特性进行了非定常数值模拟,同时对其气动噪声特性进行了实验研究.大小叶片贯流风机的偏心涡基本稳定在叶轮中心与蜗舌相连的切线上,位于叶轮内圆周附近.风轮非定常运转时,偏心涡的涡核位置在直径为2mm的圆所围成的区域内变化.大小叶片交错组合的贯流风轮改变了叶轮与蜗舌的间距,有效地降低叶片通过频率噪声并改善了音质.采用大小叶片交错组合的贯流风轮能够达到斜扭叶片贯流风机的降噪效果.     

离心压气机叶轮内部流动的数值研究:分流叶片的作用

本文对一台单级压比为4/1的离心压气机叶轮内部流动,进行了数值模拟的研究。计算方法基于Jameson格式,湍流模型选择Baldwin-Lomax模型。计算结果为分析叶轮流道内二次流的形成与发展,提供了详细的流动结构。通过对不同通流截面速度分布的比较,发现分流叶片可以延缓横向二次流的发展,降低叶片吸力面扩压程度,减小叶轮出口尾迹的强度与范围,对提高叶轮效率起到决定性的作用。     

Experimental and numerical investigation of the unsteady flow field and tone generation in an isolated centrifugal fan impeller

In spite of a low circumferential Mach number the sound of isolated centrifugal fan impellers is sometimes dominated by distinctive tones at blade passing frequency (BPF) and integer multiples. This paper reports on an experimental and numerical investigation intended to unveil the tone generating mechanism. The sound spectra from three impellers operating at a large range of speed were measured and decomposed into Strouhal and Helmholtz number dependent functions. This led to the preliminary conclusion that the BPF related tones are exclusively flow-induced. Based on hot-wire and blade pressure fluctuation measurements and a subsequent correlation analysis, coherent flow structures different from those associated with the principal azimuthal flow pattern due to the blades were detected. Eventually a numerical three-dimensional unsteady flow simulation revealed an inlet vortex. It takes on a helical form, with the vortex core slowly varying its position with respect to the impeller center. As the blades cut through that quasi-stationary helical vortex they encounter blade force fluctuations, producing the BPF tones. Slow spin of the vortex core and slow variation of vortex strength were identified as the reasons for amplitude modulation of the BPF tone.     

诱导轮泵的汽蚀特性和内部流场的研究

对二个叶片的平板螺旋形诱导轮泵的汽蚀性能，叶轮内的空泡发展和内部流动进行了测试。发现随着吸入压力的降低，二个叶片上原来同样发展的空泡之间有时会失去平衡，出现一个叶片的空泡激剧缩小，而另一个叶片的空泡随之增大这种交错叶片空泡的现象。这种现象有时会使诱导轮的理论扬程和实际扬程急速下降。利用上述测试结果对该现象引起杨程下降的机理作了探讨。     

缝隙抽吸对液膜撕裂特性影响的试验研究

本文在空气-水膜两相流动试验装置上,利用高速摄影机对空心静叶缝隙抽吸下的静叶出口边液膜撕裂特性进行了试验研究,结果表明:缝隙抽吸可以将静叶表面上的大部分流动液膜去除掉,水份在叶片出口边之前形不成完整的流动液膜,仅表现为溪状流动,静叶出口边的撕裂形式主要为丝线状撕裂,撕裂形成的水滴数目和直径将明显地减小。