大功率汽轮机低压部分末级动叶的振动状态由蒸汽的流量来决定

汽轮机在运行时会遇到低压段末级动叶在非设计条件下的工况。在这些工况中,有些是不可避免的(如空载运行、低负荷),然而这些正是机组(在凝汽器中扩压)损坏的原因。在小容量的蒸汽流量时,由于末级动叶片受到强烈的、不稳定的空气动力学的力的影响,因此就必然会增大在这些叶片上的动应力。当小容量的蒸汽流量时,汽轮机低压段末级中就产生空气动力学不稳定的问题。

Research on Automatic Test System of Engine Blade Natural Frequency

Blades are one of the important components on aircraft engines.If they break due to vibration failure,the normal operation of the entire engine will be offected.Therefore,it is necessary to measure their natural frequency before installing them on the engine to avoid resonance.At present,most blade vibration testing systems require manual operation by operators,which has high requirements for operators and the testing process is also very cumbersome.Therefore,the testing efficiency is low and cannot meet the needs of efficient testing.To solve the current problems of low testing efficiency and high operational requirements,a high-precision and high-efficiency automatic test system is designed.The testing accuracy of this system can reach ±1%,and the testing efficiency is improved by 37% compared to manual testing.Firstly,the influence of compression force and vibration exciter position on natural frequency test is analyzed by amplitude-frequency curve,so as to calibrate servo cylinder and fourdimensional motion platform.Secondly,the sine wave signal is used as the excitation to sweep the blade linearly,and the natural frequency is determined by the amplitude peak in the frequency domain.Finally,the accuracy experiment and efficiency experiment are carried out on the developed test system,whose results verify its high efficiency and high precision.

Flow-induced Noise and Vibration Analysis of a Piping Elbow with/without a Guide Vane

The effect of a guide vane installed at the elbow on flow-induced noise and vibration is investigated in the range of Reynolds numbers from 1.70×10^5 to 6.81×10^5, and the position of guide vane is determined by publications. The turbulent flow in the piping elbow is simulated with large eddy simulation （LES）. Following this, a hybrid method of combining LES and Lighthill＇s acoustic analogy theory is used to simulate the hydrodynamic noise and sound sources are solved as volume sources in code Actran. In addition, the flow-induced vibration of the piping elbow is investigated based on a fluid-structure interaction （FSI） code. The LES results indicate that the range of vortex zone in the elbow without the guide vane is larger than the case with the guide vane, and the guide vane is effective in reducing flow-induced noise and vibration in the 90° piping elbow at different Reynolds numbers.

Numerical Investigation of Flutter Stability in Subsonic Space Turbine Blisk with Emphasis on Cut-on/Cut-off Modes and Interblade Phase Angles

The so-called blisks,i.e.integrally bladed disks,are characterized by very low viscous material damping and make the flutter prediction much more critical.In that framework,a two-dimensional numerical study of a space turbine blisk featuring complex deformation of blades and high eigenfrequency(>40kHz)is performed.The simulations are based on unsteady Reynolds Averaged Navier Stokes computations linearized in the frequency domain and consist in the superposition of an unsteady linear(in time)pressure field,generated by a harmonic perturbation,upon a steady nonlinear(in space)flow.The aerodynamic damping coefficient is calculated over a range of nodal diameters,and the blades are predicted aeroelastically stable.However,violent changes occur and are rather critical since sudden and large deviations in stability appear.In that context,the nature of the waves propagating from the cascade are evaluated.Such an approach provides fundamental knowledge about the perturbations which can either propagate to the far-field(cut-on mode)or decay(cut-off mode).It is expected that the ability of the flow to damp or to amplify the blade motion is strongly affected by the way unsteady perturbations are transferred from the cascade to the far-field.The nature of the waves are first assessed from the aforementioned linearized results,then they are evaluated analytically and finally compared.A good agreement is found despite the strong assumptions of the analytical model.The results show a clear correlation between the cut-on/cut-off conditions and stability.The least stable configuration corresponds to cut-off mode at the inlet and no wave at the outlet.Without outgoing waves from the cascade,the blade is prone to be less stable:the energy from the blades vibration is necessarily dissipated or sent out by the cascade.

L_p-L_q DECAY ESTIMATES FOR HYPERBOLIC EQUATIONS WITH OSCILLATIONS IN COEFFICIENTS

This work is concerned with the proof of Lp-Lq decay estimates for solutions of the Cauchy problem for utt-λ2（t）b2（t） △ u =0. The coefficient consists of an increasing smooth function λ and an oscillating smooth and bounded function b which are uniformly separated from zero. The authors’ main interest is devoted to the critical case where one has an interesting interplay between the growing and the oscillating part.