Numerical And Experimental Study On Coupling Analysis Of Multi-stage Rotors And Annular Seals

Multi-stage centrifugal pumps employ several annular clearance channels, such as seal rings and balance pistons, which not only cause leakage and decreases in efficiency in the pumps but also obviously influence the vibration characteristics of their rotor systems. In the1970s, the rotating speed of the rotor of a high-pressure oil pump in an aerospace plane was unable to achieve the desired value due to the large vibration of the rotor. This problem was solved by replacing the circumferentially grooved seals by smooth seals. Thus, detailed studies of influence of annular seals on rotor behaviors in multi-stage centrifugal pumps are necessary.Taking the multi-stage rotors which considering annular seals as the research object of this dissertation, the influence of annular seals on the multi-stage rotors was studied during the steady and transient process. An experiment facility was established to test the fluid induced forces of annular seals. The major contents of the current dissertation consist of the following five aspects.First, the annular seal model and a few key parameters including the wall friction factor, the pre-swirl coefficient and inlet pressure-loss coefficient were introduced. Based on CFD methods annular clearance channel was modeled and grid independence was verified on this model. Numerical results of annular seal flow were obtained due to different inner wall velocity, inlet axial velocity and clearance sizes. An annular seal was modeled based on the seal parameters of the test facility, and dynamic coefficients of various seal parameters were calculated to see the influence of these parameters on dynamic Coefficients.Second, there is still no fluid forces test facility of pump seals in the domestic. The author designed a test facility to test the radial fluid induced forces when the shaft under different eccentricity and whirling motion. The design concept and operation mechanism of the test facility were described and the adjusting mechanism of the whirling motion was emphasized. Based on the perturbation method, the fluid induced force was calculated using seal parameters of the test facility. Numerical and experiment results were compared to verify the reliability of theoretical methods, which established the foundation of the fluid-structure interaction method developed in this dissertation.Third, to predict the vibration performance of multi-stage rotors directly, a fluid-structure interaction method was developed in the current paper, and the numerical method was verified by experiments conducted on a model rotor. In a typical FSI process, rotor systems were modeled based on the node-element method, and the motion equations were expressed in a type of matrix. To consider the influence of annular seals, dynamic coefficients of annular seals were introduced into the motion equations through matrix transformation. The test results of the model rotor showed good agreement with the calculated results.Fourth, this dissertation studied the influence of annular seals on the rotors transient response during the start-up period of centrifugal pump. A single rotor system and three states of annular seal flow were modeled. These models were solved using numerical integration and finite difference methods. Numerical results were in good agreement with the experimental results. A fluid structure interaction method was developed. This method overcomes some shortcomings of the traditional FSI method by improving the data transfer process between two domains.Fifth, in traditional rotor dynamics analysis it’s generally believed that the stiffness coefficient plays a key role in determining the characteristics of the rotor system, and the effect of damping coefficient is not obvious. Due to the large number and large LID ratio of seal rings in the multi-stage pumps, it’s found that damping coefficients have an important influence on the critical speed of the multi-stage rotors. Taking single rotors and multi-stage rotors as examples, the influence of damping coefficients on rotor performance was analyzed. The influence of LID ratio on the stability of rotor systems was studied by calculating the unstable rotating speed of rotors.Research results indicate that, all dynamic coefficients increased with the increase of seal channel’s LID ratio, while the increase of cross stiffness and damping coefficients were most obvious. The increase of seal clearance significantly reduced the stiffness and damping coefficients. The inlet pre-swirl caused the increase of cross stiffness and cross damping coefficients, and the decrease of principal stiffness coefficients. Test results of fluid induced forces were larger than the theoretical results, which indicated that the theoretical results underestimate the supporting role of seals on the rotor system. Based on the fluid structure interaction method developed in this paper, the numerical results of "wet" critical speed showed a good agreement with experimental results. The difference between calculated and experimental results was less than5%when the seal clearance was in the range of0.3-0.5mm. When considering the supporting role of seals in the rotor system, the change of critical speed was small but its corresponding response amplitude significantly reduced. In the transient process, vibration amplitude and critical speed largely changed when the acceleration of the rotor system increased. The increase of damping coefficients caused the decease of critical speed and vibration amplitude. When LID ratio of seal channels increased to a certain range, the rotor may become unstable under a certain rotating speed. The content and conclusions of the current dissertation can provide references for the performance prediction, design optimization of the rotor systems in a multistage pump.