| Turbomachinery plays a very important role in national economy and national defense construction.Development of turbomachinery is taken as a long-term strategic task of China.In view of the country's urgent demands for contra-rotating fans with high efficiency,high safety,low noise and environmental-friendly effects,the limitation of aerodynamic design of contra-rotating fans in steady theoretical methods stands out gradually.Therefore,it is significant to study the complicated unsteady flow,blade vibration and noise characteristics of the contra-rotating fan.Aiming at the complicated unsteady flow problem of contra-rotating fan,the paper studied the unsteady flow field under the effect of dual-stage rotor blade and rotor-rotor interference,and the blade vibration and aerodynamic characteristics of the contra-rotating fan are analyzed.Aiming at the problem of complicated unsteady flow,the numerical simulation is carried out in this paper by adopting the SST k-? turbulence model to solve the constant Navier-Stokes equations with constant and unsteady Renault mean Navier-Stokes equations.The standard computational example NASA37 rotor was taken as the example to verify the numerical simulation method.Firstly,the flow mechanism of the tip clearance in the contra-rotating fans is studied.The research of the unsteady characteristics of tip leakage flow of dual-stage rotor found that:the blade tip leakage vortex of the front-stage rotor presented the obvious cyclic motion,and the loads of the front-stage rotor suffered from strength alternate changes on the front-stage blade edge in the blade tip area;the blade tip leakage vortex of the front-stage edge of rear-stage did not display obvious motion cyclicity,and the loads blade were always large.Under the influence of the plugging action,there is a shift in the relationship of the secondary vortices between the front-stage and rear-stage rotor.It is found in the research of spatial three-dimensional structure of the blade tip leakage flow that,under the design conditions,the blade tip leakage vortexes of the two stages of rotors displayed a concentrated vortex structure.In the near-stall condition,the secondary leakage of the dual-stage rotor is serious,and the vortex structure of the leakage flow of the front-stage leaf leakage disappears;the vortex structure at the rear-stage is relatively obvious,and there is a leading edge overflow phenomenon at the rear-stage.On this basis,the paper researches the effects of rotation speed matching on the fan stall margin and the blade tip leakage flow characteristic.Research results show that the changing the front-stage speed has a greater effect on the stall boundary and the spacing flow field of the fan.The flow rate corresponding to the stall boundary decreases as the dual-stage speed ratio(the ratio of the rear-stage speed to the front-stage speed)increases or decreases.In the near-stall condition,the change of the rear-stage speed alone has little effect on the flow field characteristics of the dual-stage rotor blade leakage flow.When the front-stage speed is changed separately,the flow field of the leakage flow of the front-stage blade is improved,but the flow field of the leakage flow at the rear-stage leaves is deteriorated.Based on strong rotor-rotor interference effects inside a counter-rotating fan,the paper researches unsteady effects inside the fan,and also elaborately studies the effects of rotor-rotor interference on unsteady flow field characteristic.Results show that:due to the unsteady effect,the flow loss of front-stage rotor increased,and the flow loss of rear-stage rotor was restrained.The upstream wake enhanced the control of rear-stage flow loss because of the wake recovery effect and negative jet flow effect.With increase of axial spacing,the unsteady effect was weakened,and the flow loss was basically not affected by the unsteady effect.It is found in unsteady aerodynamic force that the effect of potential flow interference on front-stage aerodynamic force exceeded the effect of upstream wake and potential flow interference on rear-stage aerodynamic force.When the flow rate decreased,both the mean value and fluctuation amplitude of two stages of unsteady aerodynamic force increased,wherein the effect of flow rate on rear-stage aerodynamic force was more obvious.When the axial spacing increased,the mean values of two stages of unsteady aerodynamic force varied very slightly,but the fluctuation amplitudes varied obviously.It is found in research on unsteadiness of two stages of flow fields that:with regard to the front-stage rotor,areas with high static-pressure unsteadiness were located on the side of a blade pressure face close to the tail edge due to the potential flow interference;with regard to the rear-stage rotor,the static-pressure unsteadiness of the blade front edge was very high under effects of wake and potential flow.Under different axial spacing,dominant frequencies of static-pressure fluctuations at two stages of monitoring points were corresponding to blade passing frequencies.and the frequencies were doubled due to the relative rotation of two stages of rotors.Under the axial spacing of 0.3 chord(0.3 multiple of chord length at the front-stage blade root),the two stages of static-pressure fluctuations had the largest amplitude and a large frequency division amplitude.Based on a finite element method and a boundary element method,the paper conducts numerical computation of vibration characteristics and aerodynamic noise characteristics of the contra-rotating fan respectively.The results show that the inherent characteristics of the blade are mainly affected by the centrifugal inertial load.Under the design conditions,the natural frequency of first 6 orders of front-stage rotor is greater than that of rear-stage rotor.Compared with the rear-stage rotor,the inherent characteristics of the front-stage rotor blades are more affected by the applied load.When the flow rate increases,the total deformation and the maximum equivalent stress of the two-stage blades are gradually reduced,and the flow rate has a greater effect on the deformation and equivalent stress characteristics of the rear-stage blades.The axial spacing has little effect on the natural frequency,equivalent stress and deformation distribution of dual-stage blades,and has some influence on the maximum total deformation of dual-stage blades.The noise sound pressure level of the contra-rotating fans decreases as the axial spacing increases,and the sound pressure level have a large extent decreases when the axial spacing are 0.5 chord and 1.1 chord.In general,the greater the axial spacing is,the lower the level of aerodynamic noise.Considering the aerodynamic performance and aerodynamic characteristics of the wind turbine,the most suitable axial clearance for the cyclone is 0.5 chord.Based on comprehensive consideration of aerodynamic performance and aerodynamic noise characteristics,the most suitable axial spacing for the contra-rotating fan in the present paper is 0.5 chord. |
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