Fan Noise Reduction by Cascade Clocking for Next Generation Supersonic Transport

叶栅时钟是一种通过适当调整下游定子叶片相对于上游定子叶片的周向位置来减少多级轴流压气机总压损失的概念。首席研究员发现，同样的概念也适用于下一代超音速运输机推进系统中使用的多级风扇。在这种情况下，风扇的涵道比相对较低，风扇的压力比较高，需要二级或三级风扇。为了将级联时钟概念应用于音调降噪，多级的动叶数量应该相同，静叶数量也应该相同。那么，在多个阶段，音调频率是相同的，并且音调的周向模态是相同的。根据叠加原理，有可能将第一级产生的音调压力波与第二级产生的音调压力波相抵消。本课题的目的是在实验上证实用级联时钟的概念来降低噪声的可能性，并在数值上找出优化级联时钟的必要条件。实验采用两级轴流压气机，第一级和第二级各有32片动叶，两级各有38片静叶。它具有定子叶片连续计时和动叶每1/8节距计时的能力。结果表明，定子叶片时钟可使噪声降低18 dB。降低噪音的水平取决于压缩机的转速为一个固定的转子叶片设置为两个阶段。然而，转子叶片时钟和定子叶片时钟的组合可以给出上述最大的减少。数值计算很好地解释了实验得到的串级时钟时的噪声级变化。少

Cascade clocking is a concept to reduce the total pressure loss incurred in a multi-stage axial compressor by adjusting properly the circumferential position of the downstream stator vane relative to the upstream stator vane. The head investigator discovered the fact that the same concept can be applied to reduce fan tone noise in the case of a multi-stage fan such as used for the propulsion system of the next generation supersonic transport where the fan bypass ratio is low accordingly, the fan pressure ratio is high and a two or three stage fan is required.In order to apply the cascade clocking concept to tone noise reduction, the number of rotor blades should be the same for multiple stages and also the number of stator vanes should be the same. Then, the tone frequency is the same for multiple stages and the circumferential mode of the tone becomes identical. There is a possibility to cancel the pressure wave of the tone generated by the first stage by the pressure wave of the tone … More generated by the second stage based on the principle of superposition.The purpose of this project is to confirm experimentally the possibility to reduce tone noise by the concept of cascade clocking and also to find out numerically the condition necessary for the cascade clocking to be optimized.A two-stage axial compressor having 32 rotor blades for the first and the second stages and 38 stator vanes for both stages were used for the experiment. It has the ability of stator vane clocking in continuous fashion and that of rotor blade clocking at every 1/8 pitch.Results show that stator vane clocking can reduce the tone noise by as much as 18 dB. The level of noise reduction depends on rotational speed of the compressor for a fixed rotor blade setting for both stages. However, the combination of rotor blade clocking and stator vane clocking can give the maximum reduction mentioned above. Numerical calculation well explains the tone noise level variation against cascade clocking as obtained by experiments. Less