Experimental And Numerical Study On Duct Mode Identification Of Fan Noise

With the advent of high bypass ratio turbofan engines,the fan has become a major source of modern commercial aircraft propulsion noise.Therefore,any significant reduction in noise produced by modern aircraft power plants must include provisions for controlling and reducing fan noise.With the continuous development of aeroengine and its fan noise reduction technology,in order to further reduce the engine noise,we need to deepen and refine the understanding of the generation and transmission of the fan noise.In particular,the fine decomposition and identification of the acoustic mode in the fan instake/exhaust duct,has become an importment basis for the design of efficient linear in the engine nacelle and the aerodynamic-acoustic integration design of the fan.The present paper carried out the systematic theoretical and experimental study on the mode identification and prediction,the main research work and main achievements are:(1)The paper established the method for identifying the acoustic mode of fan tonal noise and broadband noise,which can sepatates the modes into incident and reflected parts,and developed the corresponding processing software.The acoustic mode of a high-load single stage axial-flow fan was measured experimentally.The sound field structure of tonal noise and the sound field correction of broadband noise were analyzed,and the correction between different modes of the real fan broadband noise was studied.It indicated that the mode constituents of broadband noise might be partly correlated with each other,and which is fully coherent in the frequency range of 0~50Hz.The related work of the paper deepens the understanding of the sound field structure of the fan noise in the duct,and lays a solid theoretical foundation for the experimental measurement of sound field structure of the fan noise in the duct,and obtains the corresponding technical reserve.(2)The present paper established a hybrid prediction model(flow-field/sound-field)for acoustic mode of fan tonal noise and broadband noise,and the corresponding softwares were compiled.The model can relate detailed design parametres of fan blade to the noise level without any simplification of fan structure in unsteady numerical simulations,and it doesn't require much computer sources because Lighthill's acoustic analogy theory was used.It is found that the hybrid model can accurately capture the effects of the rotor-stator space,fan rotating speed and the tip clearance on the fan noise level by comparing with the results of experiment,and the predictions of empirical model and the analytical model.(3)The paper studied the distribution characteristics of acoustic mode based on the experimental results and numerical results.The results show that the distribution of acoustic energy in the modes is closely related to frequency,propagation direction and the duct geometry.The number of the mode selected will greatly affect the prediciton result of the fan broadband noise.Therefore,the whole propagation modes should be included in the measurement and numerical calculation of the fan broadband noise.In addition,the distribution of the fan noise source was analyzed by numerical calculation,it indicated that the sound source near the leading edge of the stator mainly affects the noise in the frequency range of less than 3 kHz,and the sound source near the trailing edge which caused by the small scale turbulence in the boundary layer mainly affects the noise in the frequency range of higher than 3 kHz.(4)In the pressent paper,the physical mechanism of effects of the design parameters such as the tip clearance and the swept/leaned stator on the aerodynamic performance and noise level of the fan were studied.It was found that the rotor tip clearance increases the fan tonal noise level,beacuse it increases the rotor wake strength and width near the casing region,but has little effect on its phase distribution.The research work showed that the influence of the eigenfunction in the annular duct,the wake characteristics of the real fan and the wake transport process shoud be taken into account in analyzing the noise reduction mechanism of swept/leaned stator.The negative leaned stator will increase the noise level,and the effectiveness of the positive leaned stator is related to the tone harmonic and the direction of noise propagation.Higher the harmonics,better the noise reduction effect.Swept stator reduces the wake strength to the stator leading edge,and also increases indirectly the angle between the wake and the stator.So,it has better noise reduction effect than leaned stator.The swept-leaned stator combined with the advantages of the two,so it has the best noise reductione effect,and also,it has little impact on the the fan aerodynamic performance.