| The contra-rotating fan machine is widely used in the ventilation system of mines,tunnels,subways and other special Spaces because of its large flow,good performance and high efficiency.In practice,unsteady aerodynamic excitation on the flow field of the contra-rotating fan will be applied to the blade surface.When the frequency of the exciting force is equal to or close to the natural frequency of the blade,it is easy to cause the resonance of the blade and radiate noise outward,and even cause the fracture of the blade.Therefore,it is necessary to deeply analyze the vibration and noise characteristics of blades under unsteady excitation force of contra-rotating fan machine.At the same time,in order to avoid such phenomenon,it is necessary to propose a new blade with good aerodynamic performance and effectively avoid the resonance zone.In this paper,the flow field of a certain type of contra-rotating fan is numerically calculated,the distribution characteristics of blade surface load and the characteristic frequencies of aerodynamic forces of two-stage blades are clarified and the time domain and frequency domain laws of blade tip clearance and pressure pulsation at monitoring points on blade surface are analyzed.The results show that the characteristic frequency of unsteady excitation force is the passing frequency and frequency doubling of two-stage impeller,and the magnitude of excitation force decreases gradually along the flow direction in the first and second channels.The pressure pulsation intensity at the monitoring points on the suction surface of the two-stage blades was higher than that on the pressure surface,and the pressure pulsation intensity at the leading edge of the two-stage blades was higher than that at the trailing edge of the blade.The influences of different speed,flow rate and material factors on blade modal are studied.The results show that the application of centrifugal force and aerodynamic pre-stress increases the stress on the surface of the blade structure,increases the surface stiffness of the blade,and thus increases the natural frequency of the blade mode.The influence of centrifugal force on the structural mode is much greater than that of aerodynamic force,and the material with smaller elastic modulus is greatly affected by the pre-stress.The vibration acceleration of the blade monitoring point was calculated by using the modal-based forced vibration response method with the pressure pulsation of unsteady flow field as the excitation load.The acoustic boundary element method was used to calculate the acoustic radiation with the vibration response results as the boundary condition,and the total sound pressure level of the sound pressure spectrum of the monitoring point was obtained The total acoustic radiation power level values are compared for the flue-induced vibration and radiated noise characteristics of blades under two modes and three different materials.Aiming at the original airfoil of the contra-rotating fan,a new blade with good performance and avoiding the resonance zone was proposed to reduce vibration and noise.The performance,resonance characteristics,fluid-induced vibration and radiated noise characteristics of two blade fans were compared.The results show that the new blade can effectively avoid the resonance range,and the frequency allowance of the new blade at the rated speed increases from 17%to 26%,which is safer than the original blade.The new blade has a good vibration reduction effect,and the vibration reduction effect of the new blade at the secondary blade monitoring point is more obvious.The new blade has good noise reduction effect.Compared with the original blade,the maximum noise reduction of the sound pressure level at the monitoring point of the new blade is 2.67dB,and the radiated sound power level is 1.68dB. |
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