| ABSTRACTAnnular seals are the key component to restrain the leakage of steam turbine,aeroengine,gas turbine,compressor,and improve the operation efficiency of the unit.The labyrinth seal is widely used in turbine machinery due to its high structural reliability and excellent sealing performance.With the continuous increase of the operation parameters of the unit,the problem of fluid-induced vibration caused by the seal becomes prominent.Therefore,it is of great theoretical significance and engineering value to carry out the research on the mechanism of fluid-induced vibration caused by the labyrinth seal and its suppression method.Firstly,the mechanism of fluid-induced vibration caused by the labyrinth seal is studied by using CFD method.The influence of key parameters such as inlet pressure,rotor speed,number of seal teeth,whirling frequency and inlet preswirl on the characteristics of fluid force are analyzed.The fluid force acting on the rotor of the seal section is decomposed by using the seal cavity as a unit,and the contribution of each cavity to the overall fluid force is obtained,the reason of fluid-induced vibration is analyzed.Based on the above research,the concepts of “crossover teeth number” and “critical stability” are put forward in this paper,which can directly evaluate the stability of labyrinth seal system.The mechanism of inlet preswirl is explained,and the criterion for the selection of the preswirl direction is also determined.Secondly,in order to improve the accuracy and applicability of the identification for the dynamic characteristic coefficients of annular gas seals,an identification method using infinitesimal theory is proposed.The transient solution combined with moving grid method was unitized to obtain the fluid force at a specific position under different whirling frequencies.Then,the infinitesimal method is applied to obtain the rotordynamic coefficients.This method can be used to identify the seal-rotor system with any elliptical trajectory,whirling frequencies and eccentricities.In order to verify the accuracy of the identification method,the dynamic characteristic coefficients of labyrinth seal and smooth annular seal under different eccentricities and whirling frequencies are identified,and the identification results are compared with the experimental results.The predicted direct stiffness coefficient,direct damping coefficient,cross stiffness coefficient and effective damping coefficient are consistent with the experimental results.The research on the dynamic characteristics of labyrinth seal under the condition of eccentricities shows that the high eccentricity trends to destroy the stability of the seal system,especially at the low whirling frequency.Finally,in order to restrain fluid-induced vibration caused by the labyrinth seal,a new type of anti-stagnant labyrinth seal is proposed.Through the design of a micro anti-stagnant nozzles on the labyrinth seal teeth,the adjacent seal cavity is connected,and the reverse jet is generated by the pressure difference between the adjacent cavities,and the circumferential flow inside the seal cavities are adjusted to improve the stability of the system.The dynamic characteristic coefficients of the new structure are identified and the results are compared with the traditional labyrinth seal.The results show that the counter swirl produced by the antistagnant labyrinth seal has a significant effect on restraining the circumferential flow inside the seal cavities.Under high whirling frequency,the effective damping coefficient of the new structure can reach 200% of the original structure,which greatly improves the stability of the seal system. |
