Research On The Dynamic Characteristics Of The Spiral-grooved Dynamic Pressure Annular Seal And The Stability Of The Pump Rotor

The annular seal is a local structure widely used in fluid machinery such as pumps and hydraulic turbines,which is also a key part related to the efficiency and performance of the fluid machinery.As a typical structure of the ring seal,pump annular seal is mainly used to meet the tolerance requirements of the static and dynamic components and to reduce the medium leakage from the high pressure side to the low pressure side by reducing the annular gap distance.With continuous improvement of the processing technology,the annular gap can be made smaller and smaller,which can effectively improve the volume efficiency of the pump.However,the state of the annular gap liquid film is directly related to the stability of the pump rotor.How to keep the two cylindrical surfaces of the annular seal rotating in a non-contact manner to avoid collision and wear has become an outstanding problem that needs urgent research.A dynamic pressure annular seal structure with ‘seal dam’ and ‘short spiral shallow groove’ is proposed under the micro-scale condition,which is referred to as the spiral-grooved dynamic pressure annular seal.The numerical simulation method was used to study the sealing performance and dynamic characteristics of the spiral-grooved dynamic pressure annular seal,whose influence on the critical speed and stability of the rotor was further explored.The main research contents and conclusions of the paper are as follows:1.The establishment process of the dynamic model of the spiral-grooved dynamic pressure annular seal was introduced.The state of the fluid flow in annular gap was discussed.The grid independence test and the reliability verification of the calculation model were performed.The laminar flow and turbulence model were respectively used to analyze the influence of the different rotational speeds and eccentricity on the dynamic characteristics and the sealing performance of the spiral-grooved dynamic pressure annular seal,including the pressure distribution,main stiffness characteristics,reaction force,offset angle,leakage and the friction torque.2.An eccentric whirl dynamic model of the spiral-grooved dynamic pressure annular seal was established.A multi-rotation coordinate system method was used to steadily solve the dynamic characteristic coefficients of the smooth and spiral-grooved dynamic pressure annular seal.The law and the internal mechanism of the stiffness and damping coefficientswere studied under different conditions like gap size,inlet and outlet pressure and rotational speed.3.The rotor system dynamic model was established.Taking the shaft of a condensate pump as an example,the rotor dynamic characteristics were analyzed using the finite element software SAMCEF ROTOR.The effect of the annular seal on the dynamic characteristics of the rotor system was analyzed using the linear coupling method of dynamic characteristic coefficients.The ‘dry’ and ‘wet’ state of the rotor system were calculated respectively to study the influence of the spiral-grooved dynamic pressure annular seal on the critical speed and stability of the shaft system,which were specifically analyzed about the effect of the different positions of the spiral-grooved dynamic pressure annular seal.The results of the study show that whether laminar or turbulent model are used,similar dynamic characteristics and sealing performance of the spiral-grooved dynamic pressure annular seal can be obtained.The spiral groove generates dynamic pressure effect and pumping effect.The low pressure area and the high pressure area are respectively generated at the sharp corners of the groove stair and the groove dam side.When the rotor is eccentric,the spiral groove can increase the main stiffness,reset force and reduce the offset angle,which can improve the rotor centering performance.The leakage can be greatly reduced and the influence on friction torque can be ignored.For the dynamic characteristics coefficients of the annular seal,which is larger when the annular gap is smaller.The spiral groove has a greater influence with smaller gap and increases the main stiffness coefficients and the cross damping coefficients.The absolute value of the spiral-grooved annular seal stiffness coefficients decrease with the increasing pressure difference of the annular seal,but the damping coefficients are less affected by the pressure difference.With larger pressure difference,the stiffness coefficients of the spiral-grooved annular seal tend to that of the smooth annular seal.The rotational speed has a great influence on the dynamic characteristic coefficients of the spiral-grooved annular seal.The main stiffness coefficient and the main damping coefficient increase with the increase of the rotational speed.The cross-damping coefficient increases first and then decreases slightly with the increasing rotational speed.For a condensate pump shaft system,the spiral-grooved dynamic pressure annular seal will not change the original vibration modes of the pump rotor,but compared with the smooth annular seal,the first fourorders critical speeds can be increased,whose forward whirl second and fourth orders increase more.While the maximum amplitude of the fifth and sixth orders will decrease with the increase of the main stiffness of the spiral-grooved dynamic pressure annular seal.The maximum amplitude of the second-order forward whirl decreases with the increase of the rotating speed,and the decreasing amplitude increases with the increase of the stiffness of the spiral-grooved annular seal.When the spiral-grooved dynamic pressure annular seal used at the first stage impeller,which has the greatest influence on the first and second order critical speeds of the rotor.However,the use of spiral-grooved dynamic pressure annular seal at different stage impeller has little effect on the maximum amplitude of the entire shaft.The research content and conclusions of this paper will provide the basis and support for the theoretical research and engineering analysis of the dynamic characteristics and sealing performance of the liquid annular seal and the rotor system dynamics.