Modern gas turbine rotors are usually designed as combined rotors in which disks are tied together by one central tie rod or several tie rods distributed along the circumference. This structure has the advantages in cooling design, light weight and processing assembly, however it brings some problems and challenges in predicting the dynamic characteristics of rotor. No matter how many tie rods are used to fasten the disks, the rotor is not an integral or continuous structure any more. The contact effects between contact faces and the pre-tightening forces of tie rods have great influence on the rotor’s dynamic behaviors. Traditional methods to calculate the critical speed in rotor dynamics such as Transfer Matrix Method and 2-D Finite Element Method (FEM), based on the integral and continuous rotor, fail to consider factors of the contact effects and the pre-tightening forces of tie rods. Although the 3-D FEM can exactly calculate the critical speed, it is still time and resource consuming to establish and calculate such complex three-dimensional structures, even on the most advanced computers at present. In this paper, the traditional 2-D FEM is improved by considering the contact effects and pre-tightening forces of tie rods. Contact faces in the rotor are dealt as elements with equivalent stiffness but without mass, thus the rotor-bearing system of gas turbine are composed of contact elements, elastic elements, rigid disk elements and bearing elements. According to the improved 2-D FEM, a program is developed to calculate the critical speed and unbalance response of gas turbine rotors. The equivalent stiffness, serving as an important input parameter in the program and elements in the stiffness matrix of the system, is mainly determined by the contact stiffness between contact faces and the pre-tightening force. To find out relationships between them, GW (Greenwood and Williamson) statistical model is used and the equivalent stiffness of complex contact faces is obtained. According to the results, certain curves showing the relationship between equivalent stiffness of contact surface and pre-tightening force are obtained. By these curves and the program, we can easily calculate the dynamic characteristics of gas turbine rotors with satisfying accuracy and less time. To validate this method, the critical speed of a real rotor of a certain gas turbine was calculated with the program and curves, and the results agree well with the measured data.
Surgical pulmonary valve replacement (PVR) is one of the most common approaches in the management of patients with congenital heart disease. The discouraging long term outcomes of standard prostheses for PVR, including homografts and bioprosthetic, constrained their indications. Recent developments in the expanded-polytetrafluoroethylene (ePTFE) pulmonary prosthetic valves provide promising alternatives. In this study, the hemodynamic characteristics of bileaflet and trileaflet ePFET valve designs were experimentally evaluated. The in-vitro tests were performed under the right ventricle (RV) flow conditions by using an in-vitro RV circulatory system and particle image velocimetry (PIV). The leaflet kinetics, transvalvular pressure gradients, effective orifice areas, regurgitant fractions, energy losses, velocity fields and Reynolds shear stress (RSS) in both prostheses were evaluated. The opening of the bileaflet and trileaflet valve cost 0.060 s and 0.088 s, respectively. The closing of the former takes 0.140 s, in contrast to 0.176 s of the latter. The trans-valvular pressure is 6.8 mmHg in the bileaflet valve vs. 7.9 mmHg in the trileaflet valve. The effective orifice area is 1.83 cm2 in bileaflet valve and 1.72 cm2 in bileaflet valve. The regurgitant fraction and energy loss of bileaflet are 7.13% an 82 mJ, which are 7.84% and 101.64 mJ in its bileaflet counterpart. The maximum RSS of 48.0 Pa and 49.2 Pa occur at the systole peak in bileaflet and trileaflet valve, respectively. A higher average RSS level is found in the bileaflet valve. The results form this preliminary study indicates that the current bileaflet prosthetic valve design is capable of providing a better overall hemodynamic performance than the trileaflet design.
Tie‐bolt rotors are composed of several disks fastened by tie bolts where contact properties have a great influence on the modal behavior. In this work, a linear spring‐damper element is used to consider the contact stiffness and damping in a tie‐bolt rotor. A tie‐bolt rotor model is developed using the beam element and the zero‐length contact element. Experimental modal testing is performed under different preloads of tie bolts. Model updating is carried out to tune the contact parameters using the Particle Swarm Optimization algorithm. Furthermore, a global eigenvalue evaluation is carried out to demonstrate the impact of the lumped spring‐damper element on the modal results. Results show that a larger pretension reduces the influence of contact damping on modal parameters. Compared to antisymmetric modes, symmetric modes are more sensitive to the change of contact damping.
The shafting of large steam turbine generator set is composed of several rotors which are connected by couplings. The computing method of shafting with different structure under specific installation requirement is studied in this paper. Based on three-moment equation, shafting alignment mathematical model is established. The computing method of bearing elevations and loads under corresponding installation requirements, where bending moment of each coupling is zero and there exist preset sag and gap in some couplings, is proposed, respectively. Bearing elevations and loads of shafting with different structure under specific installation requirement are calculated; calculation results are compared with installation data measured on site which verifies the validity and accuracy of the proposed shafting alignment computing method. The above work provides a reliable approach to analyze shafting alignment and could guide installation on site.
Tie-bolt rotors are commonly applied in aeroengines and gas turbines, and interference fits ensure accurate positioning for disk assembling. This paper presents experimental and numerical studies on the stiffness and damping properties of a tie-bolt rotor with interference fits under different preloads. An FE model incorporating thin-layer elements is used to carry out model updating to predict the contact stiffness and damping characteristics. A two-step model updating approach of the tie-bolt rotor using the response surface optimization is implemented. Finally, a quasistatic FE simulation concerning the interference fits and sensitivity analysis is performed to validate the experimental results. Press fit induces a bilinear contact stiffness. Also, modal damping demonstrates a more sensitive and nonmonotonic behavior versus preload.
Tie-bolt rotors are commonly applied in heavy-duty gas turbines due to a better stiffness-mass ratio behavior compared to traditional integral rotors. In order to achieve a steady transformation from mechanical energy to electricity, the clamping force of the bolt is critical to keep multiple disks as a whole unit. However, the tensile stress of the bolt is changing during rotor assembly as well as operational conditions that should be considered carefully for the rotor safety. Moreover, subjected to mechanical as well as thermal loads, the stress of the rotor disks is a complicated mixture by different applied forces. In this manuscript, the preload variation during the process of rotor assembling is studied analytically with the help of the stiffness analysis. Then, a steady thermal analysis is performed with appropriate boundary conditions. Making use of the thermal-mechanical approach, a steady-stress distribution under different load combinations is achieved. Furthermore, a thorough insight into the stress level as well as the contact pressure of Hirth serrations is presented.
In this paper, an ultra-wideband dipole antenna for borehole radar system is designed. By loading resistor and ferrite on the two arms of the dipole antenna to attenuate the current and expand the operation bandwidth, the bandwidth of 221-700MHz is achieved, and the relative bandwidth of the antenna is 104%. The time domain radiation waveform shows that the antenna has high peak-to-peak value of radiation signal and short tailing duration. Therefore, the antenna is suitable for borehole radar applications.
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