Investigation of flow dynamics and decrease of vortex noise inside volute of centrifugal fans

The characteristics of three-dimension flow dynamics inside the volute of a G4-73No.8D centrifugal fan was simulated numerically using a computational fluid dynamics(CFD) technique. The generation, evolution, and noise of the vortex were investigated when cylinder-shaped and cone-cylinder-shaped anti-vortex rings were added separately. Numerical results showed that large-scale vortices were broken effectively and the flow fields inside the fan were more uniform with the two anti-vortex rings installed. Experimental results indicated A-sound level and spectrum noise of the refitted fan decreased and the two anti-vortex rings were effective in decreasing vortex noise. The cone-cylinder-shaped anti-vortex ring was more effective than the cylinder-shaped one in breaking large-scale vortexes and decreasing vortex noise.     

Numerical analysis of aerodynamic noise radiated from cross flow fan

The flow field in a cross flow fan was simulated by solving the 2-D unsteady Reynolds-averaged Navier-Stokes equations. The calculated pressure fluctuations of the blades, the vortex wall, and the rear wall were then used as noise sources to calculate the sound field. The Ffowcs Williams-Hawkings (FW-H) equation was employed to predict the noise field caused by these sources. The predictions show that the rear wall and the vortex wall sources contribute significantly to the total noise and that both the predicted aerodynamic performance and noise agree well with the experimental results. __________ Translated from J Tsinghua Univ (Sci & Tech), 2007, 47(2): 236–239 [译自: 清华大学学报(自然科学版)]     

Flow fields with tip leakage vortex in a small axial cooling fan

In order to improve the fan characteristics, especially efficiency and noise level of a small axial cooling fan with a large tip clearance, the internal flow measurements with tip leakage vortex were carried out at fan rotor outlet us- ing an I-type hot-wire probe. The probe was set toward two directions, parallel and normal to the meridional plane of test fan, and the two directional velocity components were measured. From the result of fan test it was found that the test fan didn＇t have the unstable characteristic with a positive gradient on its pressure - flow-rate curve. From the results of velocity measurement it was observed that the tip leakage vortex exited at maximum efficiency flow-rate and large flow-rate conditions. However, at small flow-rate conditions the tip leakage vortex was not observed and it was found that the flow field were enlarged toward radial outwards     

离心风机内泄漏数值优化研究

利用流体分析软件ANSYS CFX对某一高效离心风机进行了内部三维数值模拟,得到其流体动力学特征和内泄漏损失特征.根据理论公式对风机泄漏量进行了估算,比较理论计算泄漏量与数值模拟计算泄漏量.首先,在设计工况点对未加装防涡圈的风机进行数值计算,发现在蜗壳内部有一些大的旋涡,并观察到一些重要的流动现象.然后,对加装防涡圏后的离心风机进行整机数值模拟,结果表明,加装防涡圏后蜗壳内部的漩涡明显减小,漩涡强度减弱,流场得到改善,泄漏损失明显减小.最后,对不同间隙的加装防涡圈后的风机进行数值模拟,结果显示,随着间隙的减小,泄漏量明显减小,所以在保证安全运行的条件下,间隙应尽可能小.     

Noise reduction for centrifugal fan with non-isometric forward-swept blade impeller

To reduce the noise of the T9-19No.4A centrifugal fan, whose impeller has equidistant forward-swept blades, two new impellers with different blade spacing were designed and an experimental study was conducted. Both the fan’s aerodynamic performance and noise were measured when the two redesigned impellers were compared with the original ones. The test results are discussed in detail and the effect of the noise reduction method for a centrifugal fan using impellers with non-isometric forward-swept blades was analyzed, which can serve as a reference for researches on reduction of fan noise. __________ Translated from Fluid Machinery, 2007, 35(9): 1–4, 37 [译自: 流体机械]     

Numerical simulation of air flow field in high-pressure fan with splitter blades

For a deeper understanding of the flow characteristics in the high-pressure centrifugal blower of a fan of Model 9–26 with splitter blades, a three dimensional (3-D) numerical simulation of air flows in the fan was conducted with FLUENT software. The standard k-ε turbulent model and unstructured grids were used. The computational fluid dynamics (CFD) results showed that the performance of a fan could be improved by adding the splitter blades in the channel among the leaf blades. Under operational conditions, with the presence of splitter blades, the air flow rate of the fan increased about 5% and the total pressure at the outlet of the fan increased about 10% on average. It was also found that the length of the splitter blades affected the air flow and pressure drop. There is an optimal value for the length. The simulation results provide helpful information for improving the fan performance. __________ Translated from Fluid Machinery, 2007, 35(10): 29–32 [译自:流体机械]     

Prediction of aerodynamic noise in a ring fan based on wake characteristics

A ring fan is a propeller fan that applies an axial-flow impeller with a ring-shaped shroud on the blade tip side. In this study, the entire flow field of the ring fan is simulated using computational fluid dynamics (CFD); the accuracy of the CFD is verified through a comparison with the aerodynamic characteristics of a propeller fan of current model. Moreover, the aerodynamic noise generated by the fan is predicted on the basis of the wake characteristics. The aerodynamic characteristic of the ring fan based on CFD can represent qualitatively the variation in the measured value. The main flow domain of the ring fan is formed at the tip side of the blade because blade tip vortex is not formed at that location. Therefore, the relative velocity of the ring fan is increased by the circumferential velocity. The sound pressure levels of the ring fan within the frequency band of less than 200 Hz are larger than that of the propeller fan. In the analysis of the wake characteristics, it revealed that Karman vortex shedding occurred in the main flow domain in the frequency domain lower than 200 Hz; the aerodynamic noise of the ring fan in the vortex shedding frequency enlarges due to increase in the relative velocity and the velocity fluctuation.     

Air-borne noise of thermal module and system for notebook personal computers:experimental study

Thermal performance is the most important issue to be considered when a thermal module is designed for a notebook personal computer (PC). Because the fan causes air-borne noise and affects the user’s comfort, the acoustic characteristics of the module attract more attention. Experiments were conducted to study the noise sources, the noise characteristic and the main factors influencing the noise level. The difference between the air-borne noise of the thermal module and the whole computer system was analyzed and its propagating characteristics were derived. The influence of I/O ports on the air-borne noise was also studied experimentally. __________ Translated from Journal of Shanghai Jiaotong University, 2007, 41(3): 465–468 [译自: 上海交通大学学报]     

Diagnosis method based on wavelet coefficient scale relativity correlation dimension for fault

Correlation dimension as a tool to describe machinery condition is introduced. Vibration signals of the fan under different working conditions are analyzed using a threshold filtering algorithm based on the region relativity of the wavelet coefficients for reducing noise. The result shows that the characteristics of the signal could be preserved completely. The correlation dimension is able to identify conditions of the fan with faults compared with the normal condition, thereby providing an effective technology for condition monitoring and fault diagnosis of mechanical equipment. __________ Translated from Journal of North China Electric Power University, 2007, 34(1): 59–62 [译自: 华北电力大学学报]     

Numerical investigation and analysis of heat transfer enhancement in channel by longitudinal vortex based on field synergy principle

3-D numerical simulations were presented for laminar flow and heat transfer characteristics in a rectangular channel with vortex generators. The effects of Reynolds number (from 800 to 3 000), the attack angle of vortex generator (from 15° to 90°) and the shape of vortex generator were examined. The numerical results were analyzed based on the field synergy principle. It is found that the inherent mechanism of the heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, that is, the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. The longitudinal vortex improves the field synergy of the large downstream region of longitudinal vortex generator (LVG) and the region near (LVG); however, transverse vortex only improves the synergy of the region near vortex generator. Thus, longitudinal vortex can enhance the integral heat transfer of the flow field, while transverse vortex can only enhance the local heat transfer. The synergy angle decreases with the increase of Reynolds number for the channel with LVG to differ from the result obtained from the plain channel, and the triangle winglet performs better than the rectanglar one under the same surface area condition. __________ Translated from Journal of Xi’an Jiaotong University, 2006, 40(9): 996–1000 [译自: 西安交通大学学报]     

Effect of inlet box on performance of axial flow fans

Numerical investigations on 3D flow fields in an axial flow fan with and without an inlet box have been extensively conducted, focusing on the variation of fan performance caused by the internal flow fields and the velocity evenness at the exit of the inlet box. It is interesting to find that although the inlet box is well designed in accordance with basic design principles, there is a flow separation region in it. Furthermore, this flow separation and the resulting uneven velocity distribution at the exit lead to some decrease in the efficiency and an increase in the total pressure rise of the fan. This research shows that the inlet box needs further improvement and such a check on the flow fields is of value for the design of inlet boxes. __________ Translated from Journal of Engineering Thermophysics, 2007, 28 (Suppl.1): 161–164 [译自: 工程热物理学报]     

Numerical investigation of aerodynamic performance influenced by circumferentially pre-swirling coming flow on the crossover and de-swirling cascade of a multistage centrifugal compressor

Numerical investigation is implemented on aerodynamic performance inside the crossover and de-swirling cascade of a multistage centrifugal compressor. The emphasis is put on the aerodynamic performance influenced by the circumferentially pre-swirling coming flow. The results indicate that flow separation occurs inside the crossover, and the separation area may be changed with different circumferentially pre-swirling coming flow. Decreased pre-swirling intensity of the coming flow may effectively restrain the flow separation and make the outflow from the crossover more uniform, which helps to improve the aerodynamic performance of the successive de-swirling cascade. The flow inside the de-swirling cascade is a non-uniform swirling flow with large separation. The complex secondary flow occurs along the main flow and experiences a process of generating, developing, dissipating and collapsing. __________ Translated from Power Engineering, 2007, 27(1): 24–28, 49 [译自: 动力工程]     

Influence of the karman vortex street on the broadband noise generated from a multiblade fan

Introduction Multiblade fans are widelyused in domestic airconditioners, commercial sanitary apparatuses, etc. In the case of these fans, decreasing the fan noise is one of the important technical issues for its comfortable operation. The spectra of the fan noise showed that the noise is composed of a few types of aerodynamic noise. This noise comprises not only a discrete frequency noise due to the interference with the wake and the tongue of the scroll casing but also a broadband noise dist…     

Influence of diffuser on aerodynamic noise of a forward curved fan

In order to clarify the influence of a diffuser on the characteristics of a forward curved fan,the influence of the bare ratio and the outlet angle on the characteristics of the fan were measured through an experiment performed with an actual fan as well as through a numerical simulation,respectively.The mechanism of the discrete frequency noise generated by the separated flow of the diffuser was analyzed.The optimized bare ratio was approximately 17%.The flow separated inside of the diffuser generated discrete frequency noise owing to the interaction between the reversed flow from the diffuser and the impeller rotating at the blade passing frequency.The diffuser outlet angle influenced the pressure ratio more than that by the bare ratio.Furthermore,it was confirmed that restraining the separation in the diffuser effectively decreases the fan noise.     

Numerical analysis of aerodynamic noise radiated from cross flow fan

The flow field in a cross flow fan was simulated by solving the 2-D unsteady Reynolds-averaged Navier-Stokes equations. The calculated pressure fluctuations of the blades, the vortex wall, and the rear wall were then used as noise sources to calculate the sound field. The Ffowcs Williams-Hawkings (FW-H) equation was employed to predict the noise field caused by these sources. The predictions show that the rear wall and the vortex wall sources contribute significantly to the total noise and that both the predicted aerodynamic performance and noise agree well with the experimental results.     

An improved prediction model of vortex shedding noise from blades of fans

The main source of the noise of an axial flow fan is the fluctuating pressure field on blade surfaces caused by the shedding of vortices at the trailing edge of blades. An analytical model to predict the vortex shedding noise generated at the trailing edge of blades of axial flow fans was proposed by Lee in 1993. In this model, for mathematical convenience, an idealized vortex street is considered. However, the agreement between the analytical results and the experimental data needs to be improved because of the simplification about the Karman vortex street in the wake of blade. In the present study, a modified model is proposed based on the prediction model by Lee. The boundary layer theory is used to analyze and calculate the boundary layer development on both the pressure and the suction sides of blades. Considering the effect of boundary layer separation on the location of noise source, the predicted overall sound pressure level compares favorably with the experimental data of an axial fan. In the calculation of A-weighted sound pressure level (L_A), considering the effect of static pressure on radiate energy, the predicted broadband noise with the modified model compares favorably with the experimental data of a multiblade centrifugal fan.     

Application of high-turning bowed compressor stator to redesign of highly loaded fan stage

A redesign of a highly loaded fan stage by using high-turning bowed compressor stator was conducted. The original tandem stator was replaced by the highly loaded bowed stator which was applicable to highly subsonic flow conditions. 3D contouring technique and local modification of blade were applied to the design of the bowed blade in order to improve the aerodynamic performance and the matching of the rotor and stator blade rows. Performance curves at different rotating speeds and performances at different operating points for both the original fan stage and redesigned fan stage were obtained by numerical simulations. The results show that the highly loaded bowed stator can be used not only to improve the structure and the aerodynamic performances at various operating points of the compressor stage but also to provide high performances at off-design conditions. It is believed that the highly loaded bowed stator can advance the design of high-performance compressor. __________ Translated from Journal of Propulsion Technology, 2007, 28(1): 26–31 [译自: 推进技术]     

冷却塔风机支承结构的流体力学优化

对一冷却塔风机支承结构附近的流场进行CFD分析。支承结构造成的流体绕流现象在风机进口处形成涡流，造成风机风量减少，噪声增加，浪费能源。期望通过数值模拟计算结果为解决以上问题提出理论上的可行性指导，达到节约能源的最终目的。主要措施：在宽梁上增加额外送风；在平台上下方各加一导流罩装置。计算结果显示，这两项措施对风机进口处流场的改善起到了一定的作用。     

Application of digital holography to circle flow bed boiler measurement

The spatial distribution of particles in the boiler is very important in the study on the circle flow bed boiler (CFB). Digital in-line holography technique was applied to obtain the spatial and diameter distribution of the particles inside the boiler. A HE-NE laser was used to illuminate the particles inside the CFB through two glass windows and the in-line diffraction pattern was recorded by a CCD camera. The diffraction can be interpreted as a convolution between a family of wavelet functions and the object function. So the three-dimensional (3D) images of the particles in the two-phase flow were reconstructed by the convolution between diffraction pattern and wavelet functions. The particle diameters and 3D coordinates were calculated from the reconstructed 3D images by a series of image-processing methods, followed by a discussion of the experimental results. Translated from Proceedings of the CSEE, 2005, 25(15): 111–115 [译自: 中国电机工程学报]     

仿生轴流风机气动噪声特性的实验研究

针对轴流风机工作时产生的气动噪声问题,运用仿生学原理对某轴流风机叶轮进行了仿生改型设计,分别得到了尾缘锯齿式单结构仿生叶轮的轴流风机和前缘波齿、尾缘锯齿及表面脊状三结构耦合仿生叶轮的轴流风机。对两类风机以及原型风机进行了气动与噪声实验,获得了风机的气动性能与辐射噪声的频谱特性。测试结果表明：两类仿生风机的全压在全流量范围内均有不同程度的下降,最高下降达27%,但尾缘锯齿风机可以提高中小流量工况下的效率,而三结构耦合仿生风机效率低于原型风机;两类仿生风机产生的辐射噪声A声级均低于原型光滑叶轮风机,且尾缘锯齿风机降噪效果优于耦合仿生风机,并且比A声级最大降噪值为1.58 dB;尾缘锯齿沿展向的分布长度越长,效率越高,降噪效果也越佳。