Investigation On The Instability And The Mechanism Of Its Induced Noise In An Axial-flow Fan

Noise performance is one of key parameters of axial-flow fans overall performances,and in some fields,for instance chemical engineering,ships and military equipment,the most important evaluation criteria for axial-flow fans is noise performance.The internal three-dimensional flow of axial-flow fan are complex,especially at the off-design flow rates,it involving flow instabilities in the axial-flow fan.This leads to the decrease of axial-flow fan aerodynamic performance and other unstable factors such as vibration and noise.In this paper,the theoretical analysis,experimental investigation and numerical simulation were used to study the characteristics of instabilities and mechanism of flow-induced noise of axial-flow fans under partial flow rates.The purpose is to establish a good relationship between the instability and the aerodynamic performance or noise performance for axial-flow fans.The main research contents of this dissertation are below:(1)The isolating airfoil design method was used to design the impeller blades of the axial-flow fan.Then the impeller was optimized by orthogonal design method based on CFD technology.The optimal scheme of impeller with good aerodynamic performance is obtained.The results show that the total pressure of the axial-flow fan increases gradually with the increase of hub ratio,while the efficiency decreases.The aerodynamic performance of the axial-flow fan is deteriorating as the number of the blades increases or decreases.Further,the flow in the blade passage is improved after optimization.The flow in the blade inlet is symmetrical,and there are no turbulent vortices in the passage and no wake vortex in blade outlet region.The pressure gradient of blade suction surface is small.There is no flow separation on the blade surface.The results of optimized design conforms to design assumptions.(2)The phenomena of instabilities flow in the axial-flow fan and their characteristics are summarized in detail.In addition,the air dynamic noise source type of the axial-flow fan is analyzed.Furthermore,the relationship between different source types noise and internal flow is discussed.And the mechanism and propagation characteristics of several unstable flows in the axial-flow fan are also discussed.Moreover,a numerical prediction method for aerodynamic noise of the axial-flow fan is established.(3)The test rig of aerodynamic performance and noise performance of the axial-flow fan is set up.The experimental scheme and data acquisition methods are proposed base on LMS system to collect the data for the unsteady instability experiment and flow-induced noise test.The aerodynamic performance and aerodynamic noise performance of the axial-flow fan have been measured.At the same time,the unsteady dynamic signals induced by the instabilities are obtained,which include the fan outlet pressure fluctuation and noise signals.(4)The aerodynamic performance,pressure fluctuation,sound pressure level and spectra of noise by experiment are obtained.The good relationships between the internal instabilities and the pressure fluctuation and noise characteristics are established.The total pressure curve shows obvious saddle-shaped curve.When the total pressure decreases to the lowest in the unstable conditions region,the sound pressure level of noise reaches the maximum.Further,the spectra of pressure fluctuation and noise show the second peak at the frequency is 0.74 times of blade passing frequency when the flow rate is 0.7Q_d,meaning that flow instabilities occur in the model fan and induce noise.When the flow rate decreases to 0.6Q_d,the pressure fluctuation and noise spectra show the highest peak at 0.74 times of the blade passing frequency.The intense of flow instabilities become stong or reak,and the noise induced by them is higher than noise induced by blade and duct interaction.The noise of model fan has mainly affected by instabilities at partial flow rates.(5)A unsteady calculation method based on SST SAS turbulence model was applied to obtain instabilities in the model fan,and an acoustic finite element method combinating computational fluid dynamics with computational aeroacoustics was applied to calculate the aerodynamic noise sound field of the model fan.Then,the pressure fluctuation and noise spectra on monitoring points are deeply discussed.The results show that the instabilities and sound field are accurate predicted by the unsteady calculation method and acoustic finite element method respectively,in comparison with the experimental results.At the flow rate of 0.7Q_d,the tip clearance leakage vortices appear in the blade tip regions,and are rapidly extended towards the tangential direction,then break.The shape of them represents fragmentary.The revolution of leakage vorties induces noise.At the flow rate of 0.6Q_d,the size of the tip clearance leakage vorties becomes larger and intense.The shape of them shows flake and distribution of them in each passage is almost symmetrical.As the location of leakage vorties is quite closed to the duct,the leakage vorties suffer intense extrusion from the duct and produce a strong aerodynamic noise.This is the reason why the sound pressure level of model fan aerodynamic noise is highest at that flow rate.With the flow rate decreasing,the leakage vorties expand along in both the tangental and radial direction,the distribution of them is asymmetry,and the leakage vorties are mainly concentrated in several passages.The shape of them along the axial direction shows ribbon.The revolution of the leakage vorties leads to the occurance of rotating stall in some passages.Due to the distance between the leakage vorties core and the duct surface is increased,the leakage vorties suffered extrusion from the duct become weakened,so the aerodynamic noise is decreasing.(6)In order to investigate the characteristics of rotating stall,the flow in a vaneless diffuser are studied by both experiment based on Laboratoire de Mécanique de Lille of France and 2D unsteady numerical simulation used software STAR CCM.The results show that the pressure fluctuation spectra shows obvious peaks at the low-frequency(the frequency is equal to 0.5 to 2 times of the rotating frequency of centrifugal impeller)under the occurrence of rotating stalls conditions.The amplitude of the low-frequency pressure fluctuation increases with flow angle at the diffuser inlet decreasing.The high amplitude of the low-frequency pressure fluctuation is one of characteristics of rotating stall.The number of rotating stalls is different as the inlet flow angle of the vaneless diffuser changes.Four rotating stalls are obtained in the vaneless diffuser when the inlet flow angle change from 6.1odecreases to 4.3o.Moreover,when the inlet flow angle decreases to the lowest value of 2.2o,there is three rotating stalls be found in the vaneless diffuser.The propagation speed of rotating stall is lower than the rotation speed of the centrifugal impeller.