| As a kind of energy saving technology by recovery of residual pressure, centrifugal pump as turbine (PAT) has been widely used in many fields, such as petro, chemistry and so on. To improve the efficiency of energy recovery, PAT is gradually developing for high-power. And, the flow-induced noise becomes one of the most important issues that cause negative effect on reliability. The flow-induced noise consists of flow-borne noise and flow-induced structure noise from dipole source. It is a complicated problem involving hydrodynamic fluid load, structure response and vibroacoustic coupling. Therefore, developing researches on (i.e., prediction of flow-induced noise characteristics, principles and propagation laws of flow-borne noise and flow-induced structure noise, as well as distinguishment of main source in interior and exterior noise) will provide theoretical basis and technical support for noise reduction of PAT, as well as valuable reference for related research of similar turbomachinery.This dissertation deals with flow-induced noise problems in PAT. Specifically, vibration responses of shell structure and vibroacoustic characteristics are systematically investigated by incorporating theoretical analysis, numerical simulation and experimental measurement. An integrated optimal design toward high hydraulic performance and low acoustic level is subsequently proposed, based on which optimal and modified design of prototype PAT is performed. The main work and creative achievements are summarized below.1. Advances in prediction methods for flow-borne noise and flow-induced structure noise were systematically summarized. The advantages, disadvantages and applicability of the existing Boundary Element Method (BEM) and Finite Element Method (FEM) based on acoustic analogy theory were deeply discussed. Through comparison of their respective solutions to vibroacoustic coupling problems, the final suitable solving approaches for interior and exterior acoustic of PAT were determined. In addition, multi-objective optimization methods, considering both hydraulic and noise performance, were generalized.2. An experimental investigation on flow-induced noise of PAT was carried out, with focus placed upon the sound transmission characteristic from inlet to outlet. The results show that the spectrum of flow noise is mainly concentrated on low and medium frequency bands, with enhanced low-frequency broadband spectrum behavior under small flow rates. Flow noise is easier to spread toward outlet in water. The free modal parameters for rotor and casing were firstly recognized by introducing experimental modal analysis. Using Modal Assurance Criterion (MAC), the precision of experimental modes was checked to validate the accuracy of finite element method, with good agreement achieved. It is demonstrated that the average relative error of natural frequency between calculation and experiment is less than5.12%under approximate mode of vibration, providing basis for further researches on vibroacoustic characteristics built upon structure mode.3. On the basis of Richardson expansion, the errors caused by grids and turbulence models were quantitatively estimated to account for their impacts on numerical simulation. The pressure fluctuation and exciting force characteristics as well as the relations between uneven circumferential distribution and hydraulic exciting force were emphatically studied. It is found that the unsteady flow in each blade is characterized by periodic fluctuation whose frequency is the same as the rotation frequency of impeller, and the periodic fluctuation in the volute is related to blade number. Fluid viscosity reduces radial force acting on the impeller and casing, while the flow in the sidewall of impeller passage as well as the clearance of impeller and casing enhances them. The uneven circumferential distribution of velocity and static pressure is the main reason for unbalanced unsteady hydraulic exciting force.4. Analytical models were firstly developed for flow-borne noise and flow-induced structure noise in the interior/exterior acoustic fields. On the basis of this, the spectrum characteristics of each noise source and their contribution to the total noise were distinguished. Unlike previous simultaneous solution of interior/exterior acoustics with IBEM, the proposed models ensure exact handling of exterior acoustic problem with unbounded physical region by considering various media’s characteristics between inner and outer surfaces. It is found that within interior acoustic, the radiated energy of flow-induced structure noise is less than that of flow-borne noise generated by dipole source, whereas at exterior acoustic, flow-induced structure noise provides an upper value. Experimental measurements were subsequently performed to validate the reliability of flow-borne noise considering structure due to casing source. It is demonstrated that it can reflect joint action of multiple sources. The energy focused regions of spectrum versus frequency curves agree well with each other, especially in larger flow conditions. The maximum error is only6.9%at Blade-Passing Frequency (BPF) and its harmonics. 5. An optimization method combining design of experiment (DOE) and response surface was established to jointly improve hydraulic and acoustic performance of PAT. Through comparison of impeller parameters’impact on energy and noise performance, the geometric parameters with great influence on acoustic were filtered based on sensitivity analysis. Furthermore, with the efficiency and A-weighted sound pressure level (OASPL) as target, the multiple regression models connecting variables and multi-objective functions were constructed. Optimization for high efficiency and low OASPL of PAT was carried out with focused research on the interaction among significant parameters. The results show that the factors including impeller inlet diameter, blade outlet width, blade number as well as blade wrap angle have great influence on noise. According to the experiments, after optimization using this proposed method, one third octave OASPL of the PAT is reduced by4.25dB(A) with efficiency increase of1.98percentage points.6. From the relation between in-phase hydrodynamic action and radiated noise, based on the blade and tongue’s paying their leaning share, the angle formulae associated with counter-leaning blade and tongue in ideal condition were derived, and noise-reduction active-control approaches were put forward by leaning blade, leaning tongue and adding splitter blades, without losing hydraulic performance. Researches show that the leaned blade can keep PAT’s original performance, the leaned tongue can significantly improve PAT’s efficiency over an overall flow range, and adding splitter blades increase the efficiency in larger flow rates. They can all reduce the peak value at fundamental frequency and total sound energy. Experimental results suggest that the proposed noise-reduction methods can improve PAT’s hydraulic and noise performance together. It shows best effect under three combined actions. |
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