| Axial flow cooling fan is widely used in industrial production and people's daily life.It is inevitable that the fan will produce noise in the process of operation,in which aerodynamic noise is the main component.How to reduce the aerodynamic noise of the fan has attracted the attention of many scholars.With the development of bionics,the application of bionics to reduce the aerodynamic noise in the process of fan operation has become a research hotspot in recent years.Based on the bionic flow control theory,the bionic design of NACA0012 airfoil is carried out in this paper.Through numerical simulation and experimental research methods,the noise reduction performance and mechanism of bionic blade are explored.The main research work is as follows:(1)The research status of fan blade noise reduction at home and abroad is described,and the advantages and disadvantages of traditional noise reduction measures and bionic noise reduction methods are compared.Compared with traditional noise reduction methods,bionic noise reduction methods have great advantages in noise reduction ability and future development.The bionic blade with leading edge wavy and trailing edge serrated structure has good noise reduction ability and great development potential.It is confirmed that the bionic blade structure studied in this paper is the leading edge wavy tail edge serrated structure.(2)The numerical simulation method of airfoil blade is described.It is confirmed that the numerical simulation method of airfoil blade aerodynamic noise in this paper is large eddy simulation combined with FW-H acoustic analogy simulation.NACA0012 is selected as the blade of numerical simulation in this paper.The computational domain of blade numerical simulation is determined and four kinds of grids with different grid numbers are divided.Finally,the grid number of 4.64 million is determined as the grid number of numerical simulation in this paper through the grid independence test.The mechanism of NACA0012 airfoil noise generation is studied by numerical simulation.The results show that the boundary layer transition and vortex shedding of the airfoil flow near the trailing edge are caused by the existence of the inverse pressure gradient of the airfoil wall,which is accompanied by the vortex breaking,resulting in the violent pressure fluctuation of the wall.The acoustic feedback loop model is found to produce on the blade wall by comparing the noise theoretical model The effect of the unstable monophonic peak noise on the different position of the blade can be explained better.(3)Through summarizing the research of bionic blade at home and abroad,the bionic design of airfoil blade is carried out,and four bionic blades with different combination structures of leading edge and trailing edge are designed.Through numerical simulation,the noise reduction performance of bionic blade is compared,and the noise reduction mechanism is analyzed.The simulation results show that the bionic blade has obvious noise reduction effect at 0 angle of attack,and the bionic blade can suppress the peak noise of blade,making the blade The chip noise is broadband,and the maximum reduction of total noise is 15.64dB(R4).At 5 angles of attack,the bionic model has a better effect on peak noise suppression,especially the higher frequency peak(957hz),and the noise reduction range for the total sound pressure level is 2.1-3.2dB.The noise reduction mechanism of bionic blade is analyzed as follows: 1)due to the wavy leading edge structure,the bionic blade has the equivalent reverse spanwise flow velocity in the trough area near the front edge of the wall,and the spanwise flow velocity converges at the trough position,so as to accelerate the boundary layer transition and advance the vortex shedding position;2)the leading edge wavy structure and the trailing edge sawtooth structure can make the large-scale and strong spanwise vortex The structure is broken into small-scale and strong vortex structure,so as to reduce the spanwise correlation of the shedding vortex;3)the decrease of the strength and spanwise correlation of the shedding vortex on the wall reduces the amplitude of the wall pressure fluctuation,and the spanwise correlation of the pressure fluctuation near the tail edge;4)the existence of bionic structure can inhibit the transmission of T-S wave and reduce the peak value of T-S wave;all these factors make the Bionic blade produces noise reduction effect.(4)Through the numerical simulation of the rotating interference model,the flow field characteristics of the leading edge wavy structure of the blade cascade to the leading edge interference flow field of the blade were explored.Through the modification of the bionic design of the axial-flow fan blade,the aerodynamic performance and noise performance of the fan are tested by means of test.The test results show that the aerodynamic performance of all bionic models under the lower working conditions has a great loss,among which Bionic-3 has the greatest impact,and the maximum static pressure efficiency will be 19.06% and 18.80% lower respectively.However,in higher operating conditions,the aerodynamic performance of all bionic models has little change compared with the smooth blade fan.Except that the maximum static pressure efficiency of Bionic-3 is reduced by 2.62%,the maximum static pressure efficiency of other models is increased,and the maximum static pressure efficiency of Bionic-5 is increased by 2.00%.(5)Finally,the bionic blade is applied to the small axial-flow fan,and its aerodynamic and noise performance is tested.The results show that: the bionic model 3 has good acoustic performance,the total noise reduction is about 3.02 dB,3.46dB;the noise reduction performance of all bionic models is poor in the high condition(24v3000rpm),and the worst noise reduction performance of Bionic-3 increases about 3.66 dB. |
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