| The adjustable-blade axial-flow fan has the advantages of small radial size,large flow,small starting torque and strong adaptability.It is the main choice for large-scale boiler blowers,primary fans,heat dissipation equipment and ventilation systems.However,the noise pollution of axial-flow fans is generated under operating condition which is closely related to fluid-soild coupling.In this way,it is necessary to investigate the noise characteristics and control technology based on fluid-soild coupling.This paper reveals the characteristics of fluid-soild coupling and structure-borne noise under variable installation angle of moving blades conditions of Δβ=-8°,-4°,0°,+4°and+8°.In addition,the research on noise control technology is conducted which has great theoretical significance and engineering application value.First,based on the simulation of steady flow and modal analysis,the characteristics of fluid-soild coupling are explored under variable installation angle of moving blades conditions.The results show that,the deformation distribution is similar with the excitation of fluid under variable installation angle of moving blades conditions.The deformation positions of the first-order,second-order,and fifth-order are mainly concentrated at the variable section between the static impeller and the diffuser,while the deformation positions of the third-order and the fourth-order are located at the diffuser and the outlet pipe.The first twenty natural frequencies increase gradually with the number of orders under variable installation angle of moving blades conditions.The natural frequencies of each order are the smallest under the rated conditions which the first five natural frequency is 28.975Hz,28.975Hz,34.616Hz,34.616Hz and 34.379Hz,respectively.As the installation angle of moving blades is deflected in the positive or negative direction,the natural frequencies of each oder increase by about 1 Hz.Secondly,the fluid pressure of the transient large-eddy simulation is extracted as the acoustic excitation source.Then,the simulation of harmonic response is conducted to analysis the structure-borne noise considerding fluid-soild coupling under variable installation angle of moving blades conditions,which reveals its spatial distribution,the spectral characteristics of sound pressure level and sound power level,and the proportion of sound energy.The results show that the values of vorticity and total pressure at monitoring surfaces decrease with the blade height droping under the same installation angle of moving blades conditions.From 90%to 50%of the blade height,the maximum increment of maximum vorticity is 9653s-1,and the maximum increment of maximum total pressure is 4811Pa.When the installation angle of moving blades is deviated from-8° to+8°,the total sound pressure level of aerodynamic noise and structure-borne noise at each monitoring point gradually increases.Between adjacent installation angle of moving blades conditions,the increment of total sound pressure level of structure-borne noise is maximum at current collector,which is 2.4dB with Δβdeflecting from+4° to+8°.In the range of 0～3000Hz,different peaks of the fluid pressure,the sound pressure level and sound power level of structure-borne noise appear at the fundamental frequency 363Hz and its octaves.In addition,the proportion of sound energy of structure-borne noise exceeds 0.85 in the range of 0～1000Hz.Finally,based on the equivalent porous material model,the noise reduction method of three porous sound-absorbing materials is proposed for the structure-borne noise of the axial-flow fan.The results show that in the range of 0～350mm,the total noise reduction rises with the increase of thickness.The maximum total noise reduction of the polyurethane foam,glass fiber and cotton felt at the monitoring point of inlet is 41dB,40dB and 30dB,which the sound pressure level is 85dB,86dB and 96dB,respectively.Correspondingly,the maximum total noise reduction at the monitoring point of outlet is 51dB,51dB and 38dB,which the sound pressure level is 75dB,75dB and 88dB,respectively.In the range of 400～500mm,the total noise reduction of the three materials no longer rises with the increase of material thickness at monitoring points of inlet and outlet.In addition,in the range of 0～400mm,the circumferential average sound pressure level at each position of the fan gradually reduces under the effect of polyurethane foam and glass fiber,which the total noise reduction is considerable.However,the noise reduction effect is weakened after the thickness reaches 400mm.On the one hand,the characteristics of fluid-soild coupling and the structure-borne noise considering fluid-soild coupling are revealed of the adjustable-blade axial-flow fan under variable installation angle of moving blades conditions.On the other hand,the noise reduction method of different sound-absorbing materials is disscussed to obtain the law between noise reduction and thickness of the material.This paper lays the foundation for the application and research about noise control technology of axial-flow fans,and has guiding significance for the optimization and development of low-noise and environmentally-friendly axial-flow fans. |
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