Study Of Fluid Dynamic Characteristics Of Journal Bearings And Seals In Rotating Machinery

Journal bearings and seals are key components in rotating machinery. With the development of high parameters in recent years, oil film oscillation and air-excited vibration have a strong effect on the rotor stability. So carrying out the study of the journal bearings and seals dynamic characteristics is of significantly theoretical and engineering values.A study on the theoretical and experimental identification methods of journal bearing and seal dynamic characteristics was carried out. The paper analyzed the working principle of the journal bearings, deduced the Reynolds equation, analyzed the boundary condition for the Reynolds equation, and studied the theoretical model of journal bearing dynamic characteristics. The mechanism of the seal air-induced vibration and the seal dynamic characteristic theoretical model were analyzed. The seal test rig was designed and built to identify the dynamic characteristics using the unbalanced synchronous excitation method. The computer fluid dynamics(CFD) moving mesh method was put forward to solve the journal bearing and seal dynamic characteristics.The journal bearing dynamic characteristic coefficients are important parameters of the rotor stability. The dynamic characteristic CFD models of the cylinder bearings, elliptical bearings, and dislocated bearings were set up. On the basis of verifying the accuracy of the CFD model, the effects of ellipticity, dislocation rate, eccentricity and rotational speed on the dynamic characteristics were carried out. The results show that the elliptic and dislocated bearings can form two hydrodynamic lubrication zones, respectively, which increase the damping coefficients. The journal bearings can improve the rotor stability by increasing the ellipticity or dislocation rate.The cause of the seal instability includes hydrodynamic effects, spiral flow effect, Alford effect, three dimensional flow effect and so on. Since the seal hydrodynamic effect and the seal air-induced force increases under the condition of high parameters, the new negative dislocated seal was put forward based on the dislocated journal bearing theory. The negative dislocated seal was designed with dislocated form on the circumferential horizontal plane. The fluid in it flows from the small clearance to big one. The negative dislocated seal forms two divergent air wedge, eliminates the seal hydrodynamic effects, and decreases the air-induced force. The paper set up the negative dislocated seal dynamic characteristic model using CFD moving mesh method, and analyzed the impact of eccentricity and dislocate ratio on the seal dynamic characteristics. The results show that the negative dislocated seal can suppress the seal hydrodynamic effects, decrease the cross-coupled stiffness coefficients, increase the direct damping coefficients, the effective stiffness and effective damping. The negative dislocated seals can increase the seal stability.The cross-coupled stiffness generated by spiral flow effect of the labyrinth seal is the major reason for inducing a load-dependent instability vibration. The studies show that the anti-swirl structure can effectively decrease the seal spiral flow effect, but there are few experimental analyses about it. The paper designed five seals with and without swirl vanes and four seals with and without shunt injection. Numerical and experimental studies on the performances of anti-swirl seal static and dynamic characteristics were investigated. The anti-swirl seal static characteristic CFD numerical model was set up to analyze the leakage, tangential velocity and pressure distribution. The test rig was designed and constructed to test the leakage and the dynamic characteristics with and without anti-swirl structure using unbalanced synchronous excitation method. The results show that the seals with anti-swirl structure have lower fluid tangential velocity, smaller circumferential pressure difference, and less fluid circumferential pressure difference. The swirl brakes decrease the seal leakage, while the shunt injection increases the seal leakage. The anti-swirl structure can effectively decrease the seal cross-coupled stiffness coefficients, increase the seal direct damping coefficients, the seal effective damping, and improve the seal stability.