Bugra Han Ertas

COMPLIANT HYBRID GAS JOURNAL BEARING USING INTEGRAL WIRE MESH DAMPERS

The following work presents a new type of hybrid journal bearing developed for enabling oil-free operation of high performance turbomachinery. The new design integrates compliant hydrostatic-hydrodynamic partitioned bearing pads with two flexibly mounted integral wire mesh dampers (IWMD). The primary aim of the new bearing configuration was to maximize the load carrying capacity and effective damping levels while maintaining adequate compliance to misalignment and variations in rotor geometry. The concept of operation is discussed along with the description of the bearing design. Several experiments using room temperature air as the working fluid were performed that demonstrate proof of concept, which include lift-off tests, bearing load tests, and rotordynamic characterization tests. The experiments demonstrate stable operation to 40,000 rpm (2.8 million DN) of a 2.750 in (70 mm) diameter bearing. In addition to the experimental results, an analytical model is presented for the compliant bearing system. The aeroelastic theory couples the steady state numerical solution of the compressible Reynolds flow equation with a flexible structure possessing translational and rotational compliance. This was achieved by formulating a fluid-structure force balance for each partitioned bearing pad while maintaining a global mass flow balance through the hydrostatic restrictors and bearing lands. Example numerical results for pad pressure profile, film thickness, torque, and leakage are shown.Copyright

Identification and Prediction of Force Coefficients in a Five-Pad and Four-Pad Tilting Pad Bearing for Load-on-Pad and Load-Between-Pad Configurations

This paper presents the identification of the rotordynamic force coefficients for direct lubrication five-pad and four-pad tilting pad bearings. The bearing is 110 mm in diameter with a L/D of 0.4 pad axial length (44 mm). The experiments include load-on-pad and load-between-pad configurations, with 0.5 and 0.6 pivot offsets, for rotor speeds ranging from 7500 rpm to 15,000 rpm. The bearing force coefficients are identified from multiple frequency excitations (20–300 Hz) exerted on the bearing housing by a pair of hydraulic shakers and are presented as a function of the excitation frequency and rotor speed for a 300 kPa unit load. The experimental results also include temperatures at the trailing edge of three pads. The direct force coefficients, identified from curve-fits of the complex dynamic stiffness, are frequency independent if considering an added mass term much smaller than the test device modal mass. The force coefficients from the four-pad bearing load-between-pad configuration show similar coefficients in the loaded and orthogonal directions. On the other hand, as expected, the five-pad bearing load-on-pad shows larger coefficients (∼25%) in the loaded direction. The maximum pad temperature recorded for the 0.5 pivot offset configurations is up to 20°C higher than those associated to the 0.6 offset configuration. Results from a predictive code are within 50% of the experimental results for the direct stiffness coefficients and within 30% for the direct damping coefficients.

Rotordynamic Bearing Dampers for Cryogenic Rocket Engine Turbopumps

It is a common practice for cryogenic turbomachines to utilize stiffly mounted bearings due to the incapability of providing significant damping with conventional methods and designs. With no damping available, the historical motive behind this design practice was to elevate all critical speeds above the maximum running speed. The desire for higher energy density has raised the running speeds of rocket engine turbopumps very near, or even above, the first three critical speeds. A companion paper gives experimental evidence that accurate prediction of these critical speeds with the turbopump rotor on ball bearings with stiff supports is not practically possible. The feasibility of applying a metal mesh damper with soft supports in a cryogenic engine is investigated. To date the focus has been on cryogenic experimental tests, rotordynamic simulations, and possible bearing support design schemes to justify the incorporation of a metal mesh bearing support in a liquid rocket engine fuel turbopump. Success of the design is dependent on the amount of damping that the damper retains at cryogenic temperatures and its ultimate effect on fuel turbopump rotordynamics, especially subsynchronous instability.

Rotordynamic Force Coefficients for Three Types of Annular Gas Seals With Inlet Preswirl and High Differential Pressure Ratio

The following paper presents and compares rotordynamic force coefficients for three types of non-contact annular gas seals, which include a labyrinth (LABY), honeycomb (HC), and a fully partitioned damper seal (FPDS). These three annular seals represent the typical seal types used in process gas centrifugal compressors at the balance piston location or center seal location to limit internal leakage and ensure a robust rotordynamic design. Tests were conducted on 170.6 mm (6.716 in) diameter seals for rotor speeds up to 15 kprm, inlet air pressure of 6.9 bar (100 psi), ambient back pressure, and with inlet gas preswirl. The three seals were designed to have the same nominal clearance and similar axial lengths. Testing was conducted on a controlled motion test rig possessing non-synchronous excitation capability up to 250 Hz. Three different test methods were employed to give confidence in the rotordynamic coefficients, which include static force deflection tests, mechanical impedance tests, and dynamic cavity pressure tests. Results from experiments compare force coefficients for all seal configurations while paying special attention to the crossover frequencies of the effective damping term. All seals possessed negative effective damping at lower excitation frequencies with inlet preswirl, where the straight-through FPDS possessed the lowest cross over frequency of 40 Hz at 15 krpm. The testing also revealed that the preswirl parameter had significantly more influence on effective damping levels and crossover frequencies when compared to rotor speed.

Synchronous Response to Rotor Imbalance Using a Damped Gas Bearing

One type of test performed for evaluating bearings for application into turbomachinery is the synchronous bearing response to rotor imbalance. This paper presents rotordynamic tests on a rotor system using a 70 mm diameter damped gas bearing reaching ultra-high speeds of 50,000 rpm. The main objective of the study was to experimentally evaluate the ability of the damped gas bearing to withstand large rotor excursions and provide adequate damping through critical speed transitions. Two critical speeds were excited through varying amounts and configurations of rotor imbalance while measuring the synchronous rotordynamic response at two different axial locations. The results indicated a well-damped rotor system and demonstrated the ability of the gas bearing to safely withstand rotor vibration levels while subjected to severe imbalance loading. Also, a waterfall plot was used to verify ultra-high-speed stability of the rotor system throughout the speed range of the test vehicle. In addition to the experimental tests, a rotordynamic computer model was developed for the rotor-bearing system. Using the amplitude/ frequency dependent stiffness and damping coefficients for the ball bearing support and the damped gas-bearing support, a pseudononlinear rotordynamic response to imbalance was performed and compared with the experiments.

Rotordynamic Force Coefficients of Pocket Damper Seals

This paper presents measured frequency dependent stiffness and damping coefficients for 12-bladed and 8-bladed pocket damper seals (PDS) subdivided into four different seal configurations. Rotating experimental tests are presented for inlet pressures at 69 bar (1000 psi), a frequency excitation range of 20-300 Hz, and rotor speeds up to 20,200 rpm. The testing method used to determine direct and cross-coupled force coefficients was the mechanical impedance method, which required the measurement of external shaker forces, system accelerations, and motion in two orthogonal directions. In addition to the impedance measurements, dynamic pressure responses were measured for individual seal cavities of the eight-bladed PDS. Results of the frequency dependent force coefficients for the four PDS designs are compared. The conclusions of the tests show that the eight-bladed PDS possessed significantly more positive direct damping and negative direct stiffness than the 12-bladed seal. The results from the dynamic pressure response tests show that the diverging clearance design strongly influences the dynamic pressure phase and force density of the seal cavities. The tests also revealed the measurement of same-sign cross-coupled (cross-axis) stiffness coefficients for all seals, which indicate that the seals do not produce a destabilizing influence on rotor-bearing systems.

Dynamic Characteristics of Shape Memory Alloy Metal Mesh Dampers

*† ‡ The study presented in this paper involved characterizing the amplitude and frequency dependent stiffness and damping coefficients for oil-free metal mesh dampers weaved from several different types of materials. The material test matrix consisted of stainless steel 304, Inconel 600, copper, and nickel-titanium (NiTi) shape memory alloy. The research specifically focuses on NiTi alloy damper and how the stiffness and damping performance compares with the other metal mesh damper materials. Steady state forced vibration and transient vibration tests were used to characterize the stiffness and damping as functions of frequency and vibration amplitude. The results show that vibration amplitude has the opposite influence on the NiTi material damping when compared to the other materials, whereas the influence of vibration amplitude on NiTi stiffness was the same as the other materials. While the conventional metal samples give decreasing damping as vibration amplitude increases, the NiTi samples generated increasing damping as vibration levels increased. In addition to comparisons of stiffness and damping between different materials with the same mesh density, a lower mesh density for NiTi was tested and compared to the higher mesh density damper.

Effect of Static and Dynamic Misalignment on Ball Bearing Radial Stiffness

The primary objective was to determine, through experimental testing and rotordynamic computer simulations, the radial stiffness of an angular contact ball bearing with inner race static misalignment and outer race dynamic misalignment under various axial loads. Two different testing methods were used to determine the radial stiffness of the ball bearing: 1) eigenvalue frequency analysis and 2) critical speed transition tests. Each method used experimental measurements in conjunction with computer simulation to determine the radial ball bearing spring rate. Experimental measurements of the forward eigenvalues, backward eigenvalues, backward critical speeds, and forward critical speeds were matched with computer model simulations to determine the stiffness values for each of the cases. The rotordynamic characteristics of the rotating assembly suggested significant nonlinearity of the test bearings and also revealed signs of bifurcation for higher degrees of static misalignment.

Stabilizing a 46 MW Multistage Utility Steam Turbine Using Integral Squeeze Film Bearing Support Dampers

A 46 MW 5,500 rpm multistage single casing utility steam turbine experienced strong subsynchronous rotordynamic vibration of the first rotor mode; preventing full load operation of the unit. The root cause of the vibration stemmed from steam whirl forces generated at secondary sealing locations in combination with flexible rotor-bearing system. Several attempts were made to eliminate the subsynchronous vibration by modifying bearing geometry and clearances, which came short of enabling full load operation.The following paper presents experimental tests and analytical results focused on stabilizing a 46 MW 6,230kg utility steam turbine experiencing subsynchronous rotordynamic instability. The paper advances an integral squeeze film damper (ISFD) solution, which was implemented to resolve the subsynchronous vibration and allow full load and full speed operation of the machine. The present work addresses the bearing-damper analysis, rotordynamic analysis, and experimental validation through waterfall plots, and synchronous vibration data of the steam turbine rotor. Analytical and experimental results show that using ISFD improved the stability margin by a factor of 12 eliminating the subsynchronous instability and significantly reducing critical speed amplification factors. Additionally, by using ISFD the analysis showed significant reduction in interstage clearance closures during critical speed transitions in comparison to the hard mounted tilting pad bearing configuration.Copyright

The Influence of Same-Sign Cross-Coupled Stiffness on Rotordynamics

The following paper focuses on same-sign stiffness cross coupling from several viewpoints. First, the influence on rotordynamics is analyzed and discussed, followed by a discussion of how past experiments measuring cross-coupled forces on rotating components may have been in error due to an incomplete testing procedure. Finally, experimental tests are presented on a partitioned damper seal and the correct and complete experimental procedure for determining cross-coupled coefficients is described.