| Axial flow fan with adjustable moving blades is widely used in large power plant for its adaptability and high efficiency,but the noise generated during the operation also has gradually become a non-negligible problem,which worsens working environment and endangers people's health.Therefore,it is necessary to study the noise's mechanism and control technology.In this paper,a three-dimensional computational model of axial flow fan with adjustable moving blades with the rated rational speed of 990 r/min was established.Based on Fluent numerical simulation software,the flow field characteristics of the fan under variable installation angles were studied by standard ?-? turbulence model.On the basis of the steady calculation,LES turbulence model and FW-H acoustic model were applied to find the evolution law of aerodynamic noise under variable installation angles.Firstly,the static pressure characteristics of the impeller internal flow field were discussed along with the installation angle ?? from-12° to 12°.The simulation results indicated that the static pressure of the radial annular flow surface increased with the rise of the radial height at any installation angles.According to the diversity of the static pressure growth rate,the impeller passage was divided into two regions along the radial direction,as Ap increased,the growth rate in H1 region increased from 2.1 Pa/mm to 4.2 Pa/mm,but it tended to be equal in H2 region,with the average growth rate of 6.1 Pa/mm.The static pressure of the axial flow surface showed the same trend with the increase of the axial distance.According to the difference of the static pressure changing law,the impeller passage was divided in to three regions along the axial direction.The static pressure in L1 region dropped gradually,while it increased first and then decreased in L2 region,however it increased in L3 region.Besides,the inflection point of L2 region moved towards the outlet of the impeller with an increase in ??.Secondly,under varying ?? conditions,the changing rule of far-field aerodynamic noise of axial flow fan was researched.The simulation results show that the sound pressure level(SPL)at the inlet,outlet and impeller circumference of the fan increased monotonously,and the variation amplitude of the SPL is the largest when Ap changed from-12° to-8°.The acoustic energy density was introduced to evaluate the noise frequency characteristics,which represents the average sound energy per unit volume.Depending on the frequency difference,the far-field aerodynamic noise was divided into discrete noise and broadband noise.The discrete noise was generated by the pernodic interaction between the blade and the fluid,and the broadband noise was related to turbulent flow.As Al increased,the acoustic energy ratio of the discrete noise to the inlet noise increased initially,follows by a stabilization,and then increased,totally,the ratio value rose from 0.78 to 0.93.Additionally,the ratio of the discrete noise to the outlet noise decreased firstly and then increased,taking ??=0° as turning pointing.Furthermore,the acoustic energy ratio of the discrete noise to both the inlet noise and the outlet noise was always higher than that of the broadband noise at varying?? conditions.As for the circumferential noise,the ratio of the discrete noise was lower than that of the broadband noise only in the case of ??=12°.Lastly,the changing rules of the aerodynamic noise at tip clearance,blade leading-edge and blade trailing-edge were revealed.The simulation results showed that with an increase in ??,the maximum SPL of three regions grew continuously,and the trailing-edge noise was always the lowest.When ??<0°,the tip noise was higher than the leading-edge noise,When ??>0°,the relationship between them became opposite.The frequency of the near-field aerodynamic noise was divided into low frequency(20-500 Hz),intermediate frequency(500-1000 Hz)and high frequency(1000-3000 Hz).With an increase in ??,the acoustic energy ratio of the low-frequency noise to the maximum noise(CP)at the tip region dropped from 0.71 to 0.59,and then rose to 0.77,additionally,that of the intermediate-frequency noise(CP2)and the high-frequency noise(CP3)increased firstly and then decreased.However,CP1was higher than CP2 and CP3,which represented the influence of the discrete noise was larger than that of the broadband noise at tip region.As the Atp rises,Cq1 at the leading-edge region and trailing-edge region was on the decline,and the ratio value at the leading-edge region was lower than that at the trailing-edge region,but CP2 and CP3 were higher,which indicated the influence of the broadband noise at leading-edge region was larger.Sound pressure level and frequency are two characteristic parameters of noise.The research on the changing law of sound pressure level with varying installation angles could determine the working condition and position of the maximum sound pressure level,which could provide guidance for the structural optimization design and active noise reduction technology of axial flow fan.Besides,the research on the spectrum characteristics can obtain the influence of aerodynamic noise under different frequencies,which could lay a foundation for the study of passive aerodynamic noise control technology. |
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