| Non-contact seal is a common type of seal in rotary machinery for conveying fluid medium.In the centrifugal pump,the annular seal is usually used as a part to reduce the internal leakage and balance the axial force of the impeller.Under the condition of high rotational speed and high pressure in high speed centrifugal pump,the rotor and the seal clearance fluid interact strongly due to its smaller seal clearance.This kind of interactive effect can be described as stiffness and damping effect for the rotor.In a certain conditions,the coupling effect can make the rotor instability,leading to abnormal vibration of structural failure.At present,most studies focused on the influence of seal on the performance of pump.Some researchers studied the seal dynamic characteristics of inter-stage seal in multistage pump.Few studies have been made on the dynamic characteristics of centrifugal pump seals with high speed and high pressure difference.In this paper,the three-dimensional transient CFD method is used to calculate the seal's dynamic characteristics and the fluid exciting force characteristics.The main research contents and conclusions are as follows:1.Through the research on the dynamic characteristics and sealing performance of short liquid seal(L/D<1)which is commonly used in the pump,it is found that reducing the seal clearance or increasing the seal length will enhance the coupling effect between the seal and the rotor.The direct stiffness of the seal is more sensitive to the size of the seal clearance,while the direct damping,cross coupling stiffness and cross coupling damping are more sensitive to the axial length of the seal.At the same time,the sensitivity of seal leakage to seal clearance is higher than seal length.2.Because of the unsymmetrical pressure chamber structure of single volute high speed centrifugal pump,the inlet flow of the annular seal is unsymmetrical,then,the additional radial forces are imposed on the rotor.Under the circular whirling track,the whirl radius and whirl ratio of the rotor have great influence on the fluctuation amplitude of the self-excited force of the fluid in the annular seal.The seal size has a significant effect on the fluid exciting force.Reducing the axial length of the annular seal is beneficial to reducing the exciting force of the ring seal without severely weakening the sealing performance.In addition,the dynamic coefficients of the seal are obviously different when the rotor is eccentric in different directions.3.The fluid in the seal inlet not only has the axial velocity,but also the circumferential pre-swirl velocity caused by the rotation of the impeller.By studying the influence of the circumferential pre-swirl velocity of the seal inlet on the dynamic characteristics of the seal,it is found that the pre-swirl velocity can enhance the circulation inside the seal,increase the cross coupling stiffness of the seal,and then reduce the stability of the rotor,that is to say,the smaller pre-swirl velocity can enhance the stability of the rotor.4.After comparing three different types of seals,it is found that labyrinth seal has a better throttling effect in terms of sealing performance,while axial groove seal is slightly poor because of the shorter minimum gap length.For the dynamic characteristics of the seal,the direct stiffness and damping coefficients of the axial groove seal are almost twice higher than that of the labyrinth seal,and the axial groove seal has a lower cross coupling stiffness under the high pre-swirl speed.Hence,the axial groove seal has a good dynamic characteristics of the seal compared with the labyrinth seal,and the annular seal is between the two.The structural characteristics of axial groove seal can effectively reduce the internal circulation of the seal,and can reduce the influence of the inlet pre-swirl speed on the cross coupling stiffness.Its high direct stiffness and damping coefficients can improve the critical speed of the rotor in high-speed centrifugal pump,and maintain good rotor stability. |
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