The consideration of aeromechanics plays a vital role in the design of machines that operate under aerodynamic forces, such as turbomachinery, and aircraft. The structure of those machines is subject to aeromechanical dynamics, including forced response and flutter. The strength of aeromechanical interaction depends on the level of coupling between flow and structure. One effect that can lead to strong coupling is the interaction between eigenmodes of the structure and eigenmodes of the flow near coincidence. This paper examines the impact of modal coincidence on the linear dynamic stability of aeromechanical systems for two illustrative canonical examples, one governed by inviscid acoustics, and one by the eigenmode of a wake. Three commonly used analysis techniques are applied and ranked for various levels of coupling: The 1-way coupled work-per-cycle method, a 2-way coupled non-linear modal FSI analysis in time, and an eigenanalysis of the 2-way coupled linear system, based on a state-space representation. It is demonstrated that all three methods agree for low to moderate levels of 2-way coupling, typical in turbomachinery applications. At higher levels, the work-per-cycle assessment is insufficient, whereas the FSI and eigenvalue analysis agree well.

In the subsea oil and gas industry, multiphase pumps and wet gas compressors are engineered choices saving transportation and separation facility costs. In these machines, seals handling multiple phase components must be able to operate without affecting the system efficiency and its dynamic stability. This paper, extending prior work conducted with uniform clearance and wavy surface annular seals, presents measurements of leakage and dynamic force coefficients in a grooved seal whose dimensions are scaled from an impeller wear ring seal in a boiler feed pump. The 14-grooves seal has diameter D = 127 mm, length L = 0.34 D , and clearance c = 0.211 mm; each groove has shallow depth d g ∼2.6 c and length L g ∼ 3.4% L . At a shaft speed of 3.5 krpm (surface speed = 23.3 m/s), a mixture of air in ISO VG 10 oil with inlet gas volume fraction (GVF) ranging from 0 (just oil) to 0.7 (mostly air) lubricates the seal. The pressure ratio (inlet/exit) is 2.9. The flow is laminar since the liquid is viscous and the pressure drop is low. The measured mixture mass flow decreases continuously with an increase in inlet GVF. The seal stiffnesses (direct K and cross coupled k ), added mass ( M ), and direct damping ( C ) coefficients are constant when the supplied mixture is low in gas content, GVF ≤ 0.1. As the gas content increases, 0.2 ≤ GVF ≤ 0.5, the seal direct dynamic stiffness becomes nil with an increase in excitation frequency, whereas k and C reduce steadily with GVF. In general, for GVF ≤ 0.5 the direct damping is invariant with frequency; variations appearing for GVF = 0.7. Compared against a three wave annular seal, the grooved seal offers much lower force coefficients, in particular the viscous damping. Thus, for laminar flow operation (heavy oil) with a low pressure drop as in a wear ring seal, a three wave seal is recommended as it also offers a significant centering stiffness.

A dynamic model is developed for a Micro Gas Turbine (MGT), characterized by an intrinsic free-spool configuration, coupled to large volumes. This is inspired by an experimental facility at the National Energy Technology Laboratory (NETL) called Hyper, which emulates a hybrid MGT and Fuel Cell system. The experiment and model can simulate stable and unstable operating conditions. The model is used to investigate the effects of different volumes on surge events, and to test possible strategies to safely avoid or recover from unstable compressor working conditions. The modelling approach started from the Greitzer lumped parameter approach, and it has been improved with integration of empirical methods and simulated components to better match the real Hyper plant layout and performance. Pressure, flow rate, and frequency plots are shown for the surge behavior comparing two different volume sizes, for cases where gas turbine shaft speed is uncontrolled (open loop) and controlled (closed loop). The ability to recover from a surge event is also demonstrated.

In subsea environments, multiphase pumps and compressors add pressure to the process fluid thus enabling long distance tie back systems that eliminate topside oil and gas separation stations. One challenge to construct a reliable multiphase pump or a reliable wet gas compressor is that the first must handle, without process upset, a mixture whose gas volume fraction (GVF) changes suddenly; while the other must remain stable while working with a liquid volume fraction (LVF) changing over long periods of time. The mixture GVF/LVF affects the static and dynamic forced performance of secondary flow components, namely seals, and which could lead to an increase in both rotor lateral or axial vibrations, thus compromising system reliability and availability. The current research is a planned endeavor towards developing seal configurations amenable to maintain rotor dynamic characteristics during changes in the contents of flow components. This paper extends prior work with uniform clearance annular seals and presents the static and dynamic forced performance of a three-wave surface annular seal designed to deliver a significant centering stiffness. The test element has length L = 43.4 mm, diameter D = 127 mm, and mean radial clearance c m = 0.191 mm. At a shaft speed of 3.5 krpm (23 m/s surface speed), an air in ISO VG 10 oil mixture with an inlet GVF, 0 to 0.9, feeds the seal at 2.5 bara pressure and 37 °C temperature. The mixture mass flow rate decreases continuously with an increase in inlet GVF; shaft speed has little effect on it. Dynamic load tests serve to identify the seal dynamic force coefficients. The liquid seal (GVF = 0) shows frequency independent force coefficients. However, operation with a mixture produces stiffnesses that vary greatly with excitation frequency, in particular the direct one that hardens. The direct damping coefficients are not functions of frequency albeit dropping rapidly in magnitude as the GVF increases. The work also compares the performance of the wavy seal against those of two other seals; one with clearance equal to the mean clearance of the wavy seal, and the other with a large clearance emulating a fully worn wavy seal. The small clearance seal leaks 20% less than the wavy seal, whereas the leakage of the worn seal is twofold that of the wavy seal. For the three seals, the leakage normalized with respect to a pure liquid condition collapses into a single curve. The wavy seal produces the greatest direct stiffness and damping coefficients whereas the worn seal produces the smallest force coefficients. Derived from a homogeneous mixture bulk flow model, predicted force coefficients for the three-wave seal match well with the test data for operation with a pure oil and an inlet GVF 0.2. For operation with GVF > 0.2, the discrepancy between the prediction and experimental data grows rapidly. The extensive test campaign reveals a wavy-surface seal offers a centering stiffness ability, a much desired feature in vertical submersible pumps that suffer from persistent static and dynamic stability issues.

Engine oil lubricated (semi) floating ring bearing (S)FRB systems in passenger vehicle turbochargers (TC) operate at temperatures well above ambient and must withstand large temperature gradients that can lead to severe thermo-mechanical induced stresses. Physical modeling of the thermal energy flow paths and an effective thermal management strategy are paramount to determine safe operating conditions ensuring the TC component mechanical integrity and the robustness of its bearing system. On occasion, the selection of one particular bearing parameter to improve a certain performance characteristic could be detrimental to other performance characteristics of a TC system. The paper details a thermohydrodynamic model to predict the hydrodynamic pressure and temperature fields and the distribution of thermal energy flows in the bearing system. The impact of the lubricant supply conditions (pressure and temperature), bearing film clearances, oil supply grooves on the ring ID surface are quantified. Lubricating a (S)FRB with either a low oil temperature or a high supply pressure increases (shear induced) heat flow. A lube high supply pressure or a large clearance allow for more flow through the inner film working towards drawing more heat flow from the hot journal, yet raises the shear drag power as the oil viscosity remains high. Nonetheless, the peak temperature of the inner film is not influenced much by the changes on the way the oil is supplied into the film as the thermal energy displaced from the hot shaft into the film is overwhelming. Adding axial grooves on the inner side of the (S)FRB improves its dynamic stability, albeit increasing the drawn oil flow as well as the drag power and heat flow from the shaft. The predictive model allows to identify a compromise between different parameters of groove designs thus enabling a bearing system with a low power consumption.

The effect of a pilot flame on the dynamic stability characteristics of a single-nozzle lean-premixed swirl combustor operating on natural gas fuel is investigated. Experiments are performed at a fixed inlet temperature and inlet velocity over a broad range of overall fuel-lean equivalence ratios with varied amounts of pilot fuel. The stability results are presented in the form of a dynamic stability map which is a three-dimensional graph of the normalized rms pressure fluctuation versus the overall equivalence ratio and percent pilot. The dynamic stability map identifies boundaries between stable and unstable combustion and results are presented characterizing the transition across one of these boundaries. Time-averaged two-dimensional chemiluminescence images of the stable flame and phase-averaged two-dimensional chemiluminescence images of the unstable flame are used to gain an understanding of the effect of the pilot on dynamic stability through changes in flame structure.

Oil-free bearings for automotive turbochargers (TCs) offer unique advantages eliminating oil-related catastrophic TC failures (oil coking, severe bearing wear/seizure, and significant oil leakage, for example) while increasing overall system reliability and reducing maintenance costs. The main objective of the current investigation is to advance the technology of the gas foil bearings (GFBs) for automotive TCs by demonstrating their reliability, durability, and static/dynamic force characteristics desirable in extreme speed and temperature conditions. The paper compares drag friction and on-engine performances of an oil-free TC supported on GFBs against an oil-lubricated commercial production TC with identical compressor and turbine wheels. Extensive coast-down and fast acceleration TC rotor speed tests are conducted in a cold-air driven high-speed test cell. Rotor speed coast-down tests demonstrate that the differences in the identified rotational viscous drag coefficients and drag torques between the oil-free and production TCs are quite similar. In addition, rotor acceleration tests show that the acceleration torque of the oil-free TC rotor, when airborne, is larger than the production TC rotor due to the large mass and moment of inertia of the oil-free TC rotor even though air has lower viscosity than the TC lubricant oil. Separate experiments of the oil-free TC installed on a diesel engine demonstrate the reliable dynamic forced performance and superior rotor dynamic stability of the oil-free TC over the oil-lubricated TC. The post on-engine test inspection of the oil-free TC test hardware reveals no evidence of significant surface wear between the rotor and bearings, as well as no dimensional changes in the rotor outer surfaces and bearing top foil inner surfaces. The present experimental characterization and verified robustness of the oil-free TC system continue to extend the GFB knowledge database.

Flame structure is an important aspect of the combustion process which must be considered in the design of gas turbine combustors as it can have a significant effect on the combustor’s static stability (blowoff) and dynamic stability (combustion instability). The relationship between flame structure and flame stability has been studied extensively in single-nozzle combustors. However, relatively few studies have been conducted in multi-nozzle combustor configurations typical of actual gas turbine combustion systems. In this paper, a chemiluminescence-based tomographic reconstruction technique is used to obtain three-dimensional images of the flame structure in a laboratory-scale five-nozzle can combustor. The images reveal the complex three-dimensional structure of this multi-nozzle flame, as well as, the effects of interacting swirling flows, flame-flame interactions and flame-wall interactions on flame structure.

This paper presents an overview of the development of a reliable bearing system for a new line of small turbochargers where the bearing system has to be compatible with a new compressor and turbine design. The first part demonstrates how the increased weight of the turbine, due to a 40 % increase in flow capacity, influences the dynamic stability of the rotor-bearing system. The second part shows how stability can be improved by optimizing important floating ring parameters and by applying different bearing designs, such as profiled bore bearings supported on squeeze film dampers. Test results and stability analyses are included as well as the criteria which led to the decision to choose a squeeze film backed symmetrical 3-lobe bearing for this new turbocharger design.

Fibers such as fabric and bristles can be readily fabricated into a variety of seal configurations that are compliant and responsive to high speed or lightly loaded systems. A linear, circular, or contoured brush seal system is a contact seal consisting of the bristle pattern and hardened interface. When compared to a labyrinth seal the brush seal system is superior and features low leakage, dynamic stability, and permits compliant structures. But in turn the system usually requires a hardened smooth interface, permits only limited pressure drops. Wear life and wear debris for operations with static or dynamic excitation are largely undetermined. A seal system involves control of fluid within specific boundaries. The brush and rub ring (or rub surface) form a seal system. In this paper, design similitudes, a bulk flow model, and rub ring (interface) coatings are discussed. The bulk flow model calculations are based on flows in porous media and filters. The coatings work is based on our experience and expanded to include current practice.

A study of pressure oscillations occurring in small centrifugal compressor systems without a plenum is presented. Active and passive surge control were investigated theoretically and experimentally for systems with various inlet and discharge piping configurations. The determination of static and dynamic stability criteria was based on Greitzer’s (1981) lumped parameter model modified to accommodate capacitance of the piping. Experimentally, passive control using globe valves closely coupled to the compressor prevented the occurrence of surge even with the flow reduced to zero. Active control with a sleeve valve located at the compressor was effective but involved a significant component of passive throttling which reduced the compressor efficiency. With an oscillator connected to a short side-branch at the compressor, effective active control was achieved without throttling. Both methods of active control reduced the flow rate at surge onset by about 30%. In general, the experiments qualitatively confirmed the derived stability criteria.

An aerodynamic investigation of the influence of outlet stator part (vaneless diffuser and return channel) surface roughness on aerodynamic performance of a very low flow coefficient centrifugal stage has been carried out. The stage with design inlet flow coefficient 0.007 was tested within the range of stage Mach number M u2 =0.5–1.1. Then the surface quality of outlet stator part was improved and the tests have been repeated. Aerodynamic performance and losses in both vaneless diffuser and return channel with de-swirl vanes were investigated. The values of isentropic head coefficient increased while those of loss coefficient decreased nearly in the whole range of characteristics in the stage with improved surface quality. The detailed pressure recovery in vaneless diffuser in vicinity of design point measured and calculated by the performance prediction method is compared and discussed. The nonsteady flow phenomena were also investigated. The change of dynamic stability limit by improving of surface quality was observed.

This paper describes the rotor dynamic stability analysis and the PTC-10 Class 1 test of a three body centrifugal compressor train for high pressure natural gas injection service. This train had a full load full pressure string test on hydrocarbon gasses to a final discharge pressure of 500 BAR (7250 PSIA). Each compressor is of the back to back configuration, and is equipped with tilting pad seats, damper bearings, and a honeycomb labyrinth at the division wall with shunt holes. The driver is a gas turbine.

Flexible bearing supports may have a great influence in the calculation of forced response and stability of rotor systems. However, this effect is not always included in rotor analyses since an accurate model of the foundation and pedestals may be difficult and costly to obtain. It is common practice to use either a one degree of freedom model or a full modal analysis to represent the bearing supports. While the one degree of freedom model is easy to set up for computer calculations, it often requires experience to determine values for the stiffness, mass and damping of the model that will accurately represent the support under study. This model, however, fails to capture the dynamics of the system for stability analyses when more than one mode of the support structure is in the range of interest. On the other hand, modal representation provides much more information and can be measured experimentally, but requires measurement of the mode shapes. Even though modal representation can include all the dynamics of the system in the frequency range of interest, it provides much more information than is required for calculation of the rotor response and it is more difficult to use in calculation programs. This paper presents a procedure to include the support characteristics using transfer functions. Transfer functions permit modeling of multi-degree of freedom systems while maintaining the size of a one degree of freedom system (2×2 matrix if rotation at the bearing is not considered). Another advantage of transfer functions is that they can be obtained from existing discrete models, from modal information or can be measured directly. The fixed size of the transfer function matrix permits the method to be easily incorporated into rotor dynamic stability and forced response programs. The method is applied to stability calculations of models of typical industrial machines.

Analysis of the stability in a cracked blade-disk system is proposed. The effect of modal localization on the stability in a rotating blade-disk was studied. A crack near the root of a blade is regarded as a local disorder in this periodically coupled blade system. Hamilton’s principle and Galerkin’s method were used to formulate the equations of motion for the cracked blade-disk. The instability regions of this cracked blade-disk system were specified by employing the multiple scales perturbation method. Numerical results indicate that the rotation speed, shroud stiffness and crack depth in the blades affect the stability regions of this mistuned system significantly.

Combustion oscillations are a serious technical challenge for stationary gas turbine manufacturers. In order to address this issue, the development cycle for a gas turbine combustor typically involves an extensive test program. In many cases, the dynamic stability observed in sub-scale test rigs differs from the commercial engines. If encountered late in the development cycle, these rig-to-engine variations can impact an entire engine program. This paper discusses an experimental approach to measure time-scales (i.e., fuel-transport time and flame response time) that play a critical role in combustion oscillations. If these time-scales can be measured throughout the development cycle, the rig-to-engine variations in combustor performance may be improved. The results described in this paper are collected from a nominal 1MW th single-can combustor rig operating on natural gas. The inlet-air temperature and pressure are 589K (600°F) and 760 kPa (7.5 atm), respectively. The measured time-scales are significantly larger than the bulk fuel-transport time calculated from simple estimates. In order to explain these discrepancies, a simple flame model is presented to show how the flame structure can influence these time-scales. This model is also used to illustrate how these critical time-scales can change as a function of operating condition.

The effect of coriolis force on the stability in a rotational blade-disk with a cracked blade was presented in this paper. A disk comprising of periodically shrouded blades was used to simulate the weakly coupled periodic structure. The mode localization phenomenon introduced by the blade crack on the longitudinal and bending vibrations on the rotating blades are considered. The Galerkin method was used to derive the unperturbation equations for the system. The boundaries of instability zones of the mistuned system were approximated by employing the so called multiple scales method. The effects of coriolis force and the magnitude of crack on the variation of the dynamic stability zones in a cracked blade-disk system are investigated numerically. Numerical results indicate that the coriolis force and the coupling effect between longitudinal and bending vibrations could affect the dynamic stability in a mistuned system significantly.

Squeeze film dampers (SFDs) are used in the high speed turbomachinery industry as a means to reduce vibration amplitude, provide damping, and improve dynamic stability of the rotor bearing system. Past numerical studies analyzing SFDs, have been computationally expensive and time consuming. The present study investigates the feasibility of applying a steady state solver to obtain computational efficiency while ensuring that the parameters of interest are captured. This is done via the application of the Moving Reference Frame (MRF) solver in ANSYS® Fluent. A steady state solver in an absolute frame of reference was used to produce whirling motion of the rotor. The results are validated by comparison to the experimental data of Delgado [1]. The numerical model shows good agreement with these results.

This paper delves upon the Aero-Thermodynamic performance and Mechanical design aspects of microturbines comprising a single shaft radial compressor driven by a single stage radial inflow turbine with a combustor and recuperator sized to directly drive a permanent magnet type high speed generator with an output power in the 5–10KW bracket and commensurate rotational speeds in the 100–200 krpm range. It is initially shown that stipulation of a cycle design point output power, turbine inlet or exit temperatures, and compressor pressure ratio delivering optimum thermal efficiency inherently confines rotational speed selection, and that independent rotational speed choice away from those identified optimum speed regimes may result in cycle thermal efficiency compromises. Confining the cycle analysis within temperature limits of cost competitive superalloys and foil materials reveals that the achievement of optimum thermal efficiency is more dependent on temperature at the turbine exit rather than at inlet. Albeit the choice of rotational speed is of particular importance in the compressor and turbine design it moreover is dominant in the mechanical design of the rotating assembly in terms of high speed bearing life and shaft dynamic stability. As a consequence rotating assembly and bearing design options suitable for direct drive permanent magnet generators are reviewed and recommendations offered as to the prime candidate assemblies for future microturbines in the 5.0 to 10.0 kW power output range.

In this paper, we investigate the impact of heat transfer between the flame and the flame-holder on the dynamic stability characteristics in a 50-kW backward facing step combustor. We conducted tests where we use a backward step block made of two different materials: ceramic and stainless steel whose thermal conductivities are 1.06 and 12 W/m/K, respectively. A set of experiments was conducted using a propane/air mixture at Re = 6500 for the inlet temperature of 300–500 K at atmospheric pressure. We measure the dynamic pressure and flame chemiluminescence to examine distinct stability characteristics using each flame-holder material over a range of operating conditions. We find that for tests with a flame-holder made of ceramic, the onset of instability is significantly delayed in time and, for certain operating conditions, disappears altogether. Stated differently, for certain operating conditions, the combustor can be stabilized by reducing the thermal conductivity of the flame-holder. As the thermal conductivity of the flame-holder increases, the combustor becomes increasingly unstable over a range of operating conditions. These results imply that the dynamic stability characteristics depend strongly on the heat transfer between the flame and the combustor wall near the flame anchoring region.