| Annular seals are key components in centrifugal pumps and are mainly used for sealing and thrust-balancing, e.g. impeller wear rings, inter-stage seals, labyrinth seals, balancing drum gaps, etc. The leakage flows in them can generate certain dynamic excitations on rotating shafts and will influence the vibration characteristics and running stability of shaft system.Seal dynamics have got sustainable developments from the last century, and the analysis method has experienced a period of the short seal theory, the finite-length seal theory, and the three-dimensional CFD method. But, current research interests still mainly focus on the linear characteristics of various concentric seals under eccentric perturbations. Seal dynamic characteristics under the conditions of non-uniform intake flows, rotor tilting perturbations, large eccentricities and large disturbances etc, however, are rarely discussed. Besides, for the often-used quasi-steady simulations, the drawbacks of quasi-steady simplification should be studied and confirmed. The studies on these key issues will undoubtedly enrich the theory system of seal dynamics, and thus promote the theory research and engineering analysis of rotor-seal systemTaking pump smooth annular seals as research objects, this dissertation conducted in-depth studies on the dynamic issues mentioned above by means of the transient CFD method and test results. For centrifugal pumps, a nonlinear fluid-structure interaction model was also established and could be used for the rotor-dynamic study of complex rotor/bearing/seal system. The major contents of this dissertation consist of the following five aspects:Firstly, the CFD approach used for dynamic studies of concentric annular seals was introduced. The shortcoming of quasi-steady CFD method was analyzed and then an improved one was put forward. Based on interpolation method, the dynamic mesh model of annular seals was developed, and then the transient CFD method was presented. By the seal test device, fluid forces of a pump wear ring was measured and used to verify the reliability of CFD methods. Based on a liquid seal with large LID ratio, the effect factors of CFD methods in predicting dynamic coefficients were discussed, and the difference between quasi-steady and transient methods was specially investigated.Secondly, with respect to eccentric parturitions, the identification method for tilting dynamic coefficients was presented. Given the time-consuming problem of transient CFD simulations, a new transient method was proposed based on rotor variable-speed whirls. It can determine all the (eccentric and tilting) dynamic coefficients of concentric liquid seals. The reliability of this new method was evaluated by comparing with measured results. In order to highlight fluid excitations due to tilt, seal forces under rotor composite (eccentric and tilting) whirl were investigated, and the effects of LID ratio on the size relationship between eccentric and tilting dynamic coefficients were studied.Thirdly, to study the effects of non-uniform incoming flows on seal dynamic characteristics, the seal model with full upstream zone was established. By transient CFD simulations, stiffness coefficients and fluid-induced forces under the non-uniform flow were investigated. With respect to uniform flows, the linear seal force model, which can consider the effects of non-uniform flows, was proposed.Fourthly, based on the transient CFD method, seal dynamic coefficients and leakage rates under different eccentricities were computed and the experimental validation was performed. The effects of disturbance magnitudes on dynamic coefficients of eccentric seals were discussed to investigate the applicability of linear assumption. To describe nonlinear fluid excitations, a nonlinear seal force model was fitted based on the famous Muszynska model and CFD simulations. By using this nonlinear model and transient simulations, fluid-induced forces under various large disturbances were computed to study the seal dynamic properties under large eccentricities and disturbances.Fifthly, in order to better consider nonlinear seal forces, the fluid-structure coupling model of rotor-seal system was established by means of the control subroutine between two physical fields. And the reliability validation for this coupling model was performed.Research results show that, the developed dynamic mesh model and transient method have good performance and can fit for the dynamic studies of annular seals under various rotor disturbances. The performance of the improved quasi-steady CFD method is better than the traditional one, and the transient CFD method outperforms the other two. The introduction of quasi-steady simplification causes significant under-predictions for direct mass coefficient, but the transient method can predict accurately. The new transient method based on variable-speed whirls can maintain the advantages of the traditional transient method in accuracy and can well improve the time-consuming problem. With the increase of seal L/D ratio, the gap of eccentric and tilting dynamic coefficients is in narrowing trend, especially the direct stiffness coefficient. Namely, for long annular seals, the fluid excitations due to tilting perturbations should not be ignored. Besides the pre-whirl and pressure loss, the flow uniformity at inlet also has great effects on seal stiffness coefficients, and should be especially concerned. Under large eccentricity, seal dynamic characteristics are very sensitive to disturbance magnitudes and are also varying with rotor motion ways. This leads to the low reliability of nonlinear seal force model under large eccentricity. In this case, the fluid-structure interaction method involving both seal flow field and rotor structure field will be a promising solution.The content and conclusions of this dissertation can provide useful references for the theoretical research and engineering analysis on liquid annular seals and rotor/seal system. |
PDF Full Text Request