Investigations On The Leakage Flow And Rotordynamic Characteristics Of The Pocket Damper Seal

Rotating seals are critical components in turbomachinery serving a significant purpose for the control of the leakage flow through rotor-stator clearances from high pressure regions to low pressure regions.Although the primary function of rotating seals is to prevent fluid leakage,rotating seals can also produce stabilizing or destabilizing fluid induced forces that can have a strong influence on the rotor system stability of high-performance tubomachinery.Due to the comparable leakage reduction capacity and significantly improved vibration suppression performance compared to conventional labyrinth seals,pocket damper seals(PDS)have been used in place of labyrinth seals to attenuate rotor vibration response and increase rotordynamic stability in several multistage,high-pressure centrifugal compressors.Therefore,the main objective of this thesis is to investigate the leakage and rotordynamic characteristics of the PDS,and to obtained a more detailed insight into the sealing and damping mechanisms.Firstly,with regard to the problem that sealing clearances of rotating seals in high-parameter turbomachinery will deviate from the design value at high rotational speed,a numerical method was firstly proposed for investigations on the leakage flow characteristics of pocket damper seals(PDS)at high rotational speed.The method is based on a coupling between finite element analysis(FEA)and computational fluid dynamics(CFD)and takes into consideration rotor growth with high rotational speed.Numerical investigations were carried out for the effects of operational conditions and geometric parameters on the leakage characteristics of the PDS,and the corresponding leakage characteristic curves were obtained.The leakage flow rates of a published experimental PDS were conducted at three different rotational speeds and three different pressure drops.The predicted leakage flow rates agree well with the experimental data.After the accuracy and reliability of the utilized numerical methods have been demonstrated,the influences of pressure ratios,sealing clearances and rotational speeds on the leakage characteristics of the PDS were investigated in detail.For comparison,numerical investigations of the leakage characteristics were also carried out on the conventional labyrinth seal.Numerical results show that taking rotor radius growth into account using the FEA method will significantly improve the prediction accuracy of leakage flow rates at high rotational speeds.Compared to pressure ratios and rotational speeds,the sealing clearance is a more important factor which significantly affect the leakage flow rate.And the influence of the partition wall on the leakage flow rate is very little.The leakage flow rate increases with the decreasing pressure ratio and asymptotically approaches a maximum value corresponding to the choked flow condition.The decreased leakage flow rate with increasing rotational speed is partly due to the viscous dissipation effects,but is also influenced by the geometric effects.The partition walls greatly reduce the circumferential flow in the seal cavity of the PDS.Secondly,an ideal gas one-control-volume isothermal Bulk Flow model and the corresponding numerical algorithms were developed to determine the rotordynamic coefficients of the PDS.Based on the developed numerical codes with independence intelligent property right,investigations on the rotordynamic characteristics of the PDS were carried out.The frequency-dependent rotordynamic coefficients of four types of experimental pocket damper seals were calculated using the present codes.The prediction results were in well agreement with the experimental data,which confirms that the present numerical method can reliably predict the frequency-dependent rotordynamic coefficients of the PDS.Eight types of leakage prediction models which are widely used for labyrinth seals were selected to investigate the effects of leakage models on the prediction accuracy of the present Bulk Flow codes,and an optimal leakage model was obtained.Then,based on the modified in-housed Bulk Flow codes,the effects of three important geometrical design parameters,namely tooth number,sealing clearance and cavity depth,on the rotordynamic characteristics of the PDS were studied.The prediction results show that there is a optimal value for tooth number and cavity depth where the PDS possess the maximum damping.Smaller sealing clearance is more beneficial for the overall performance of the PDS.Thirdly,a multi-frequency whirling model and the corresponding transient CFD numerical methods were firstly proposed for computations of frequency-dependent rotordynamic coefficients of rotating gas seals.This numerical approach include three mathematical models of rotor whirling orbits(one-dimensional whirling orbit model,circular whirling orbit model and elliptical whirling orbit model),the CFD method for solving the transient fluid response forces,and the frequency-domain identification method of rotordynamic coefficients.Based on the present multi-frequency elliptical whirling orbit model,the frequency-dependent rotordynamic coefficients of three types of rotating gas seals,including a labyrinth seal,a fully-partitioned pocket damper seal and a hole-pattern seal,were computed at two rotational speeds,two inlet preswirl velocities and thirteen or fourteen frequencies of 20-300 Hz.The predicted rotordynamic coefficients of the three rotating gas seals are all well agreement with the experimental data.The accuracy and availability of the multi-frequency whirling model and the transient CFD numerical methods were demonstrated.In addition,investigations on the effects of three mathematical models of rotor whirling orbit and four turbulence models were also carried out.The numerical results show that unlike the past single frequency whirling models,which require a separate transient solution for each frequency,performing the transient solution using the present multi-frequency whirling model yields results for multiple frequencies,therefore requiring only one transient solution.The present multi-frequency whirling model has several advantages such as high prediction accuracy,fast computational speed and general suitability to different types of rotating gas seals.The prediction results of the present multi-frequency whirling model are not affected by rotor whirling orbits.The standard k-e turbulence model with scalable wall functions is more suitable for the prediction of rotordynamic coefficients of rotating gas seals.Finally,based on the developed multi-frequency elliptical whirling model and the transient CFD numerical methods,investigations on the effects of operational conditions(pressure ratio,rotational speed and inlet preswirl velocity)and geometric parameters(partition wall number and cavity depth)on the rotordynamic characteristics of the PDS were carried out.The numerical results show that the inlet pressure has more significant influence on rotordynamic characteristics of the PDS when comparing with the sealing outlet pressure.The increased inlet pressure for pocket damper seals results in a significant increase in the effective damping.If the system has a very high inlet pressure or large pressure drop,installing a PDS can have more desirable effects.Increasing the rotational speed and inlet preswirl velocity creates appreciable increase in the absolute value of the cross-coupling stiffness and obvious decrease in the effective damping.The inlet preswirl velocity has a much stronger destabilizing effect when comparing with the rotational speed.Unlike the direct coefficients behavior of the labyrinth seal,the PDS yields significantly higher direct stiffness and direct damping,and shows strong frequency-dependent force coefficients.Increasing the partition wall number results in a significant increase in direct stiffness,but has limited desirable effect on the effective damping.The PDS possess larger direct stiffness and smaller effective damping when comparing with the straight smooth annular seal.There is a optimal value for the cavity depth,where the PDS has both of larger direct stiffness and higher effective damping.In this dissertation,the transient leakage flow and rotordynamic characteristics of the PDS at high rotational speed were studied systemically and deeply.The developed computational models and numerical methods,such as FEA/CFD numerical method for investigations of leakage characteristics,Bukl Flow analysis method and transient CFD numerical methods based on the multi-frequency whirling model for predictions of rotordynamic characteristics,can provide theoretical basis and technical support for future improving the designing ability of the PDS to enhance the operational efficiency and stability of the turbomachinery.