The Characteristic Analysis Of Pulsation Dampers In Hydraulic Systems

Noise and pressure pulsation are becoming more and more severe with the increasing de-mand of high pressure and large flow rate in hydraulic systems. Attenuating pressure ripple is one of the effective methods to decrease vibration and noise. Scientists and engineers working on fluid power systems have been studying on the pressure pulsation since the 19th century, and have invented lots of pulsation dampers. However, due to the development of hydraulic systems, dampers become smaller, wider band-width, active and integrated, and it brings new challenges. Smaller volume and wider band-width are expected so strongly that dampers'shape needs change, therefore, the classical analytic methods may become incompetent.With the development of computer technology, numerical analysis has been becoming one of the state-of-art analytic methods in scientific researches and engineering design. CFD (Com-putational fluid dynamics) and visual post-processing are helpful to analysis fluid flow in hy-draulic elements, which give researchers visual information that cannot be seen in the real opera-tion. CFD technology has been playing an important role in performance optimization of pumps and valves. These technologies would make a great progress in developing pulsation dampers if they can be applied in analysis of dampers. Therefore, this thesis deals with the numerical sim-ulation method for frequency characteristic analysis of dampers, and the characteristic analysis of typical single-cavity, multi-cavity dampers and atypical dampers.First, the thesis details the method for frequency characteristic analysis of dampers. The basic model in fluid dynamics is presented, which is used for transient simulation with a open source and object-oriented CFD code, OpenFOAM, and m sequence acts as a transient signal on the inlet. The stability criterion is build to determine whether the computation is convergent and steady, and computational procedure is presented. Parameters in time domain and frequency domain are coordinated through Fourier transform and Shannon sampling theorem. The known water hammer experiment made by Holmboe and Rouleau is used to validate the basic model, and the classical distributed parameter model is used to verify the numerical model of frequency characteristic of pipes. The results show that the numerical analysis is effective and accurate.Second, factors affecting frequency characteristic of pipes were discussed. Viscosity makes less change on resonance frequency but amplitude near the frequency:with fluid viscosity de-creasing, the amplitude of frequency characteristic increases in the vicinity of the resonance frequency. Gas-mixing in liquid fluid decreases the wave speed of pressure, so it reduces the resonant frequency of pipes. Average operating pressure makes little change on pipe frequency characteristic, and increasing the peak-to-peak value of m sequence pressure on the inlet makes little change on pipe frequency characteristic but raises wave peak of pressure and speed in pipes. Average flow rate has the same feature to average working pressure, and peak-to-peak value of such flow rate makes the same trait to peak-to-peak m sequence pressure. Decreasing diameter of pipe lowers its resonance frequency slightly.Thirdly, single-cavity Helmholtz dampers were analyzed. Structure parameters of dampers influencing frequency characteristic were discussed with mathematical model based on lumped parameter method. Dampers with different shapes of cross-section of cavity, square, rectangle and ellipse, were computed with OpenFOAM, and the result shows that it changes frequency characteristic of dampers little. Dampers with different diameters of the cavity were simulated, stimulated by step pressure signal, and the result shows:if too small is the diameter of cavity, the pressure in main pipe will be extremely high or extremely low, which would aggravate vibration; if too large is it, the lower pressure in the pipe will be extremely low. So, the step pressure impact should be avoided. Frequency characteristic of dampers with different diameter-to-length ratio of cavity with same volume was simulated numerically, and the result shows: when diameter-to-length ratio of cavity becomes smaller, the resonance frequency of the damper decreases if the diameter-to-length is small enough. So if we use the small diameter-to-length ratio with small volume, the resonance frequency could be the same with the dampers which have large diameter-to-length ratio and large volume of cavity. The newly designed dampers with small diameter-to-length ratio of cavity were simulated, which volumes are smaller, and the result shows that smaller dampers can get the same resonance frequency, but the attenuation amplitude is reduced. Tapered neck increases the resonance frequency, according to the result from numerical analysis of them. Asymmetric configuration of cavity to neck decreases the resonance frequency a little. The experiment validates the damping effect of pulsation dampers and shows that dampers decrease the noise too.Fourthly, multi-cavity dampers were analyzed theoretically and numerically. Calculation of resonance frequencies of the series dual-cavity damper has been derived with the lumped parameter method. The example shows it has small errors compared to the distributed parame-ter method. Asymmetric configuration of cavities of dual-cavity dampers decreases resonance frequencies a little. A triple-cavity damper has three resonance frequencies. But the amplitude on the third resonance frequency of the triple-cavity damper is below 0 dB, in other words it has no effect on attenuating pulsation at the frequency. However, the parallel damper has good performance on different frequencies and its performance is very extensible, since the whole damping characteristic is linear summary of those single-cavity dampers which acts on the sys- tem independently.Finally, atypical dampers were discussed. The analyses of atypical dampers show CFD method has good adaptability for complex dampers. The analysis of helix and zigzag dampers shows, atypical dampers benefit damper minimization and integration. The simulation results of a zigzag damper with inter-connection pipes and multi-pipe dampers show, the inter-connection pipes can change or expand the damper's resonance frequencies, so adding inter-connection pipes can make pulsation dampers become wider band-width. Therefore, atypical dampers will be dominant in the future.