Research On The Impact Of Gas Turbine Tie Rod Rotor Connection Stiffness On Rotordynamics Characteristics

Tie rod rotors are widely used in aero engines and heavy gas turbines due to their low weight,high strength,and easy disassembly.Unlike continuous rotors,tie rod rotors are not a continuous structure.Especially in the case of pretension force relaxation,the results obtained from the analysis of the tie rod rotor by using the continuous rotor modeling are quite different from the experimental results.Therefore,it is necessary to establish a mechanical model of the contact interface of the tie rod rotor,and consider the contact interface connection stiffness in the analysis of its rotor dynamic characteristics,then obtain a modeling method suitable for the tie rod rotor.In order to study the influence of the connection stiffness between the disks on the vibration characteristics of the gas turbine circumferentially distributed rods rotor.This paper first conducts a calculation method of the connection stiffness.After comparing and analyzing three preload loading methods,the preloading force element method is used to accurately apply the preloading force of the tie rod,and then a finite element model is established.Finally,with the help of this method,the relationship between the connecting stiffness and the parameters such as the moment and the structural characteristics of the rod is studied.The finite element calculation results show that the stiffness of the tie rod rotor disk-to-disk connection is at its maximum when the contact surface is not separated.During the continuous increase of the bending moment,the stiffness decreases rapidly.After reaching a critical bending moment,it tends to convergence.In addition to the finite element method,this paper also uses the Hertz contact model and the GW model to jointly derive a theoretical calculation method for bending stiffness based on rough surfaces.This method provides another idea for understanding the properties of contact surfaces.After obtaining bending stiffness,this paper proposes an equivalent method based on six-degree-of-freedom spring element.Firstly,the contact stiffness of the disks and the bending stiffness of the tie rod are equivalently entered into the connection interface at the same time,which is convenient to use a one-dimensional beam element to solve the change of its critical speed and vibration response,and verified that the coincidence between the method and the full three-dimensional finite element method is less than 3%.Combining the finite element method and the equivalent method of connection stiffness,a model that can calculate the effect of axial temperature distribution on the dynamics is studied.The model divides the effect of temperature into two parts.One considers the thermal expansion of the structure caused by temperature.The main cause is the relaxation of the tie rod pre-tensioning force,which results in a reduction in connection stiffness.The second is the change in material properties caused by temperature,which is mainly reflected by the change in elastic modulus,which causes a reduction in the stiffness of the rotor.Combining these two parts,the influence of the axial temperature field on the dynamic characteristics of a rod rotor is solved.Then,the nonlinear dynamic equation of the rotor is rewritten.The harmonic balance method is used to solve the equation and the harmonic response of the rotor is obtained.The accuracy is verified by the classical Newmark method.The influence of the connection stiffness of M701 f rotor on its critical speed,response characteristics and nonlinear characteristics is calculated by using spring element equivalent method.The calculation results show that: When the shear stiffness and bending stiffness of the joint surface decrease by three orders of magnitude,the critical speed decreases and the response increase,the degree of influence can be roughly estimated from the various modes and the response amplitude is greatly affected by the shear stiffness;The Comparison error between the critical speed calculated by this model and the test value is less than 2%,which is significantly higher than that of the continuous model;In the case of anisotropic connection stiffness,two response peaks appear at each order of critical speed,and the axis trajectory becomes elliptical;When considering the change of stiffness with displacement(nonlinear stiffness,soft characteristics of connection stiffness),the response peak of the rotor has a “torsional” characteristic,and the characteristics of the peak show a "wavy" change with the changes of imbalance and dimensionless damping.In order to verify the dynamic characteristics calculation method of tie rod rotor considering connection stiffness proposed in this paper,a series of related tests were designed,including tie rod rotor dynamic test,modal test and bending stiffness test.The rod rotor test specimen is designed,and the test bench and test procedure are introduced in detail.Finally,a state parameter describing the tightness of the contact surface is proposed to quickly and intuitively analyze the changing law of rotor dynamic characteristics during the test.In this paper,a set of research methods for the influence of the disk-disk connection stiffness on the vibration characteristics of the rod rotor are proposed systematically,including calculation of connection stiffness,application of connection stiffness,and calculation of vibration characteristics considering connection stiffness.This set of methods can calculate the effect of the combined structure on the rod rotor,and can calculate the performance change of the discontinuous model under the temperature field.It provides a new idea for the design of China's heavy-duty gas turbine rotors in the future.