| Vibration and noise are important criterions for the contra-rotating auxiliary fan.Being in such harsh environment as humid air and coal dust erosion for a long time,the vibration of the fan becomes more and more severe with unsteady pneumatic force.In addition,vibration is accompanied with noise.Being in the noisy environment not only causes serious harm to human body,but also interferes with the judgement of the operating status.Therefore,it is necessary to research the vibration and noise of the fan combining with internal flow characteristics.The method of theoretical analysis and experimental research was used to study the FBD No8.0 contra-rotating auxiliary fan.The CFD was used to simulate the unsteady flow.Besides,the type B fan performance test system was set up to study the fan performance to verify the validity of the simulation results.The main reasons that caused the unsteady flow were analyzed,the work ability and interference between two stage impellers under different conditions and axial distances were discussed.The increasing of axial distance could reduce the interaction between the two stage impellers,make the air flow smoothly,but excessive axial distance not only extended the axial size of the fan,the loss but also increased,resulting in the decline of efficiency.Therefore,reasonable axial distance should be chosen to guarantee the fan performance at the fan design process.To identify whether the noise was caused by structural resonance,this paper used the Workbench to simulate two stage impellers’ modal under the free state and different conditions of prestress.The LMS Test Lab was utilized to measure the impellers modal at free state to verify the accuracy of simulation.The impellers modal parameters under different conditions were obtained.The aerodynamic and centrifugal force could be ignored for little influence on modal parameters.When the fan ran under variable conditions,the vibration spectrum in various position was analyzed based on modal analysis.The results showed that the vibration signal at inlet was not obvious as working condition changed and the vibration was mainly caused by the fan structure.However,the vibration at the two impellers changed distinctly.Combined the natural frequency with vibration signal,the natural frequency of impellers under different status did not overlap with the fan base frequency and double frequency,which indicated that there was no resonance phenomenon when the fan was on operation.Furthermore,the vibration signal was coincident with pneumatic force which indicated the main factor causing the fan vibration was flow pulsation.The broadband noise at different axial distances and working conditions was simulated by Fluent to analyze distribution of sound power.This paper studied sound power cloud in different cross section when the axial distance was 0.7.Combined with the vorticity distribution,it was obvious that the sound power decreased and then increased with flow decreased.When the axial distance increased,the vorticity and broadband noise were reduced between the two impellers.The fan noise under various axial distances was simulated by FW-H model based on unsteady flow simulation,and then the noise spectrum on design condition was obtained.The noise of the inlet of the fan under different conditions at 0.7 axial distance was tested by LMS acoustic module.Test results showed that the higher part of sound pressure was mainly concentrated in the low frequency range.The frequency of discrete peak was near to that of the impellers fundamental and harmonic.When flow reduced,the noise decreased and then increased.But the quantity of discrete pulse peak reduced gradually.When flow was small,stalling phenomenon would appear associated with the flow simulation results.There was a great quantity of vorticity and turbulence noise increased and gradually occupied the main position.Moreover,the frequency of discrete peak and pulse amplitude of vibration were consistent with the impeller rotating frequency. |
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