Theoretical And Experimental Study On Vibration Reduction Of Flow Control Seal In Turbomachinery

Annular seal fluid-induced vibration is the important problem restricting the development of modern turbomachinery towards high parameters and large capacity.Studying the origin of fluid-induced vibration,mastering the main influencing factors of fluid-induced vibration,and carrying out the research of vibration reduction technology are of great theoretical significance and engineering value.At first,the calculation formula of the tangential force generated in the labyrinth seal(LS)is established based on the bulk flow theory.The origin and influencing factors of the tangential force are analyzed.It is found that the circumferential flow of fluid is the origin of the tangential force.The inlet preswirl is the main factor affecting the tangential force.The high inlet preswirl generates the destabilizing tangential force,and the low inlet preswirl is beneficial for the stable operation of rotor.The novel vibration reduction seal should effectively control the inlet preswirl and circumferential flow inside the seal cavities.Based on the concept of flow control,the helix-comb seal(HCS)with inducement effect is proposed.A series of helical deflectors are inserted equally spaced around the circumferential in the annular cavities.The helix angle direction is set to oppose the rotational direction of rotor.The computational fluid dynamics method is used to calculate the flow field inside the HCS.The flow field results show that the negative circumferential flow is induced inside the seal cavities under the guiding role of helical deflectors.The circumferential flow of the fluid in the seal clearance is also suppressed,and its flow direction is almost parallel to the axial direction.To facilitate the evaluation of flow field control effect of the HCS,its flow field results are compared with those of the LS with negative-swirl brakes.Results show that the negative-swirl brakes can effectively reduce the inlet preswirl.But the effect of rotor rotation on the circumferential flow of fluid cannot be suppressed by the negative-swirl brakes.The control effect of the negative-swirl brakes becomes weaker along the axial direction.Compared with the negative-swirl brakes,the HCS can conduct the global control of the fluid inside the seal.The effects of inlet preswirl and rotor rotation can be effectively suppressed by the HCS.The HCS has the better flow field control effect.On the basis of demonstrating the HCS can effectively control the circumferential flow,its vibration reduction performance is investigated by theoretical calculation and experimental test.At first,the 3D eccentric model is established to investigate the effects of structure parameters on the tangential force of the HCS.Results show that the tangential force decreases with the increasing helical deflector number and helical deflector height.The minimum tangential force can be obtained with the helix angle of 15 deg.Then,the HCS with the minimum tangential force is used for transient calculation.Leakage results show that the HCS and LS have the similar sealing performance.The rotordynamic coefficients results show that the cross-coupled stiffness of the HCS is largely reduced(significantly negative)by the negative circumferential flow.Take the case of f=20 Hz as an example,the cross-coupled stiffness of the HCS decreases by 745% relative to that of the LS.At a high preswirl ratio,the cross-coupled stiffness of the LS is positive,whereas the cross-coupled stiffness of the HCS remains negative.The isolated helical groove structure enhances the gas compression effect,and the damping characteristics of the HCS is increased.The direct damping of the HCS is4.5～5.4 times as strong as that of the LS.Compared with the LS with negative-swirl brakes,the HCS can more effectively reduce the cross-coupled stiffness and largely increase the direct damping.The HCS has the better vibration reduction performance.On the basis of theoretical research,the rotordynamic characteristics of the HCS are investigated by experimental test.Based on the frequency response function method,the rotordynamic characteristics of the HCS are measured and compared with those of the LS in cases of different frequencies,pressure differences,and rotational speeds.The experimental test and comparisons further confirm that the HCS has the outstanding vibration reduction performance.In turbomachinery,the tangential force generated in the shroud seal has the important influence on the rotor stability.To reduce the shroud seal force,the HCS is used as the shroud seal.The 3D eccentric calculation model including the shroud seal and rotor blades is established.The calculation results show that compared with the LS,the HCS can reduce the shroud seal force,blade force,and total tangential force by 64.2%,27.6%,and 61.9%,respectively.The stability of turbomachinery is improved.In addition,the HCS reduces the influence of increasing pressure difference on the tangential force.This leads to that the turbomachinery can keep stable under large pressure difference.It is very time-consuming to calculate the frequency-dependent rotordynamic coefficients using the transient method.To reduce the computational time,the orbit decomposition method based on small perturbation and linear superposition theory is proposed.Compared with the transient method,the orbit decomposition method has the similar calculation accuracy with significantly reduced computational time.Elliptical orbit parameter analysis reveals that the effect of whirl amplitude on the calculated rotordynamic coefficients of the orbit decomposition method is very small when the whirl amplitude is less than 20% of the seal clearance.The accuracy of the orbit decomposition method decreases when the whirl amplitude is larger than 20% of the seal clearance.The effect of ellipticity on the calculated rotordynamic coefficients of the orbit decomposition method is small.Finally,the accuracy of the orbit decomposition method is further demonstrated with the experimental data.