Investigation On Aerodynamic Noise Property And Performance Optimization Of Small Axial Flow Fan

Small axial flow fans are commonly used as preferred heat dissipating components in cooling system of electronic communication products,such as servers and projectors,whose performance and noise conditions will directly influence its working life and the comfort of its operation environment.With the upgrading of electronic communication product,the higher requirements to fan performance and noise level are needed.So an investigation on the aerodynamic performance and noise characteristic has the very strong practical significance to solve the problem of heat dissipating and noise of electronic communication products.In this thesis,the small axial flow fans are selected as the research object,and the aerodynamic performance and noise characteristics of numerical calculation and experimental study are carried out.Firstly,the static characteristics and internal flow pattern of the fans are discussed,and the influences of blade number on static characteristics and internal flow field of the fans are analyzed,and then the characteristics of temperature field under the rated flow conditions and the internal flow characteristics under different flow conditions are revealed.Secondly,the aerodynamic noise characteristics under the rated conditions based on the internal flow characteristics are deduced and the far field noise prediction method suitable for small axial flow fan is developed according to the above noise characteristics,and then the influence of blade number on fan aerodynamic noise is analyzed.Thirdly,a multi-objective optimization strategy to fan performance is put forward and the optimization function is established that fan efficiency and its noise are the optimization targets and structural parameters such as blade numbers and hub ratio etc.are the optimization variables.Fourthly,a series of structural noise reduction measures are proposed and then noise reduction coefficient is raised to quantify fan noise reduction effect.Finally,the experimental devices for aerodynamic-acoustic performance of small axial flow fans and the testing apparatus for the internal flow field of the fans are applied to measure the static characteristics,noise and the velocities of internal flow field of different structural fans,confirming the feasibility of the calculation methods and the effectiveness of this paper’s conclusion.The main contents and conclusions of this thesis are shown as follows:1.The internal flow characteristics and the distribution patterns of the internal temperature field of small axial fans are determined.The fan with the optimum efficiency is selected based on the comparison of several types of fan static characteristics,and its distribution rule of the internal flow field under the different flow conditions and the internal temperature field under the rated flow conditions is analyzed.The internal flow characteristics of the fan under the different flow conditions are determined by the combined methods of pressure distribution,velocity streamline,vorticity distribution and vortex motion.The mutual influences between the internal flow field and the temperature field of the fan are discussed.The variation trends of fan temperature field at different moments are qualitatively analyzed and heat convection capacity of the internal flow field is quantitatively discussed by introducing heat convection coefficient.2.The numerical calculations of noise characteristics and aerodynamic noise prediction of the fan are developed.The theory of vortex sound and broadband noise model are used to confirm aerodynamic noise source of the small axial flow fan mainly distributing near the blade,and vortex shedding noise near the trailing edge and tip leakage noise are identified as the main aerodynamic noise source of the small axial flow fan.Through analyzing pressure fluctuation power spectrum of the internal flow field of the fan,the main frequency range of fan noise source is confirmed between 30Hz-200Hz.blade passing frequency and its harmonics,and the main band of fan noise close to far field is between 30Hz-200Hz.The major source of the aerodynamic noise whose band between 30Hz-200Hz is mainly due to the vortex shedding caused by the boundary layer separation near the blade through the Strouhal method.The feature width of blade wake flow is numerically calculated by introducing the helicity,and its sample value is selected from the exit section of 7mm away from fan center point.Fukano vortex shedding noise prediction model based on the above sample values is applied to carry out the aerodynamic noise prediction close to far field of the small axial flow fan.The difference of the noise between the predictive value and the experimental value is 0.39dB,which proved the feasibility of the above predictive method.3.The multi-objective optimization strategy to fan performance of small axial flow fan is proposed.Some structural parameters are selected to constitute a orthogonal test scheme in which fan efficiency is the optimization objective only,and several structural parameters that mainly affected the above optimization objective will compose the multi-objective optimization variables.The target response function between the multi-objective optimization variables and optimization objective is established by combining response surface method and the entropy weight method,and structural parameters corresponding to the maximum response function construct the optimal fan.The experimental results show that the optimal fan not only enhances the static characteristic of the fan but also obviously reduces the noise.4.Noise reduction coefficient is proposed to qualify the noise reduction effect of fan noise reduction measure.A series of noise reduction measures of the structure such as splitter blade,blade root hollow,blade trailing edge serrated and blade pressure surface flange are applied to the prototype so that not only less loss of fan static characteristic is ensured but also the local noise reduction effect of the fan is achieved.It is found that the best noise reduction measure of the above structures is the splitter blade,followed by the blade pressure surface flange,then by the blade trailing edge serrated,the last by the blade root hollow.The physical quantity of noise reduction coefficient is proposed and its correctness to qualify the noise reduction effect of the above noise reduction measure is verified by designing the noise reduction measure and the contrast test.5.The static characteristic experiment,the noise experiment and the internal flow experiment of small axial flow fans are launched.The validities of the numerical calculations of steady state and noise of the fans are affirmed by the experiments.The aerodynamic noise characteristics of the fan close to far field is verified by the experiments.The influences of blade number on fan static characteristics are obtained.The internal flow status of the optimal fan is better than that of fan prototype.It is ascertained that the aerodynamic noise of the small axial flow fan is the typical dipole source through the speed-sound pressure curves and directional distribution experiment.The noise test problems of small axial flow fan under different flow conditions are solved by combining the method of three spline interpolation and the increment function of the flow rate,and sound pressure level of fan noise increases at first and then decreased with increasing the flow rate.