Showing papers in "Journal of Engineering for Gas Turbines and Power-transactions of The Asme in 2018"

Thermoacoustics of Can-Annular Combustors

Abstract: Can-annular combustors consist of a set of independent cans, connected on the upstream side to the combustor plenum, and on the downstream side to the turbine inlet, where a transition duct links the round geometry of each can with the annular segment of the turbine inlet. Each transition duct is open on the sides towards the adjacent transition ducts, so that neighbouring cans are acoustically connected through a so called cross-talk open area. This theoretical, numerical and experimental work discusses the effect that this communication has on the thermoacoustic frequencies of the combustor. We show how this communication gives rise to axial and azimuthal modes, and that these correspond to particularly synchronised states of axial thermoacoustic oscillations in each individual can. We show that these combustors typically show clusters of thermoacoustic modes with very close frequencies and that a slight loss of rotational symmetry, e.g. a different acoustic response of certain cans, can lead to mode localization. We corroborate the predictions of azimuthal modes, clusters of eigenmodes and mode localization with experimental evidence.

Characterization of a Supersonic Turbine Downstream of a Rotating Detonation Combustor

Abstract: Rotating detonation combustors (RDCs) offer theoretically a significant total pressure increase, which may result in enhanced cycle efficiency. The fluctuating exhaust of RDC, however, induces low supersonic flow and large flow angle fluctuations at several kHz, which affects the performance of the downstream turbine. In this paper, a numerical methodology is proposed to characterize a supersonic turbine exposed to fluctuations from RDC without any dilution. The inlet conditions of the turbine were extracted from a three-dimensional (3D) unsteady Reynolds-averaged Navier–Stokes simulation of a nozzle attached to a rotating detonation combustor, optimized for minimum flow fluctuations and a mass-flow averaged Mach number of 2 at the nozzle outlet. In a first step, a supersonic turbine able to handle steady Mach 2 inflow was designed based on a method of characteristics solver and total pressure loss was assessed. Afterward, unsteady simulations of eight stator passages exposed to periodic oblique shocks were performed. Total pressure loss was evaluated for several oblique shock frequencies and amplitudes. The unsteady stator outlet profile was extracted and used as inlet condition for the unsteady rotor simulations. Finally, a full stage unsteady simulation was performed to characterize the flow field across the entire turbine stage. Power extraction, airfoil base pressure, and total pressure losses were assessed, which enabled the estimation of the loss mechanisms in supersonic turbine exposed to large unsteady inlet conditions.

T63 turbine response to rotating detonation combustor exhaust flow

Abstract: This paper describes testing an axial turbine response when driven by a Rotating Detonation Combustor (RDC). A T63 (C20-250) gas turbine is modified by replacing the combustor with a RDC. The stator vanes of the T63 are heavily instrumented for measurement of flow enthalpy and pressure. The engine is run at multiple power levels with the stock combustor using JetA and hydrogen fuel. The engine is then modified to have an open loop configuration, and is run with both the RDC and the stock combustor hardware with hydrogen fuel. Temperature pattern factor, flow unsteadiness, and turbine component efficiency are measured for all setups. High speed pressure transducers show substantially higher unsteadiness generated by the RDC than the conventional combustor. RDC turbine component efficiencies are compared to the conventional combustor. Results suggest that RDC unsteadiness does not significantly impact turbine efficiency.

A Reduced Order Model for Nonlinear Dynamics of Mistuned Bladed Disks With Shroud Friction Contacts

Abstract: A new reduced order modeling technique for nonlinear vibration analysis of mistuned bladed disks with shrouds is presented. It has been shown in the literature that the loss of cyclic symmetry properties which is known as mistuning could considerably increase the response level, localize the vibration around few number of blades and finally bring high cyclic fatigue. The developed reduction technique employs two component mode synthesis methods, namely, the Craig-Bampton (CB) method followed by a modal synthesis based on loaded interface modeshapes (Benfield and Hruda). In the new formulation the fundamental sector is divided into blade and disk components. The CB method is applied to the blade, where nodes lying on shroud contact surfaces and blade-disk interfaces are retained as master nodes, while modal reductions is performed on the disk sector with loaded interfaces. The use of loaded interface component modes allows removing the blade-disk interface nodes from the set of master nodes retained in the reduced model. The result is a much more reduced order model with no need to apply any secondary reduction. In the paper it is shown that the reduced order model of the mistuned bladed disk can be obtained with only single-sector calculation, so that the full finite element model of the entire bladed disk is not necessary. Furthermore, with the described approach it is possible to introduce the blade frequency mistuning directly into the reduced model. In this way, reduction is performed only once in case of multiple analyses, necessary for statistical characterization of the nonlinear response of the system. The nonlinear forced response is computed using the harmonic balance method (HBM) and alternating frequency/time domain (AFT) approach. Friction contacts are introduced into the FE model using a 3D contact element. Numerical simulations revealed the accuracy, efficiency and reliability of the new developed technique for nonlinear vibration analysis of mistuned bladed disks with shroud friction contacts.

Effects of Asymmetry on Thermoacoustic Modes in Annular Combustors: A Higher-Order Perturbation Study

Abstract: Copyright © 2018 ASME Gas-turbine combustion chambers typically consist of nominally identical sectors arranged in a rotationally symmetric pattern. However, in practice the geometry is not perfectly symmetric. This may be due to design decisions, such as placing dampers in an azimuthally non-uniform fashion, or to uncertainties in the design parameters, which break the rotational symmetry of the combustion chamber. The question is whether these deviations from symmetry have impact to the thermoacoustic-stability calculation. The paper addresses this question by proposing a fast adjoint-based perturbation method. This method can be integrated into numerical frameworks that are industrial standard such as lumped-network models, Helmholtz- and linearized Euler-equations. The thermoacoustic stability of asymmetric combustion chambers is investigated by perturbing rotationally symmetric combustor models. The approach proposed in this paper is applied to a realistic three-dimensional combustion chamber model with an experimentally measured flame transfer function, which is solved with a Helmholtz solver. Results for modes of zeroth, first, and second azimuthal mode order are presented and compared to exact solutions of the problem. A focus of the discussion is set on the loss of mode-degeneracy due to symmetry breaking and the capability of the perturbation theory to accurately predict it. In particular, an “inclination rule” that explains the behavior of degenerate eigenvalues at first order is proven.

Effects of Nonlinear Modal Interactions on the Thermoacoustic Stability of Annular Combustors

Abstract: In this theoretical and numerical analysis, a low-order network model is used to investigate a thermoacoustic system with discrete rotational symmetry. Its geometry resembles that of the MICCA combustor (Laboratoire EM2C, CentraleSupelec); the flame describing function (FDF) employed in the analysis is that of a single-burner configuration and is taken from experimental data reported in the literature. We show how most of the dynamical features observed in the MICCA experiment, including the so-called slanted mode, can be predicted within this framework, when the interaction between a longitudinal and an azimuthal thermoacoustic mode is considered. We show how these solutions relate to the symmetries contained in the equations that model the system. We also discuss how considering situations in which two modes are linearly unstable compromises the applicability of stability criteria available in the literature.

Compressible large eddy simulation of a Francis turbine during speed-no-load: Rotor stator interaction and inception of a vortical flow

Abstract: This work investigates the unsteady pressure fluctuations and inception of vortical flow in a hydraulic turbine during speed-no-load conditions. At speed-no-load (SNL), the available hydraulic energy dissipates to the blades without producing an effective torque. This results in high-amplitude pressure loading and fatigue development, which take a toll on a machine's operating life. The focus of the present study is to experimentally measure and numerically characterize time-dependent pressure amplitudes in the vaneless space, runner and draft tube of a model Francis turbine. To this end, ten pressure sensors, including four miniature sensors mounted in the runner, were integrated into a turbine. The numerical model consists of the entire turbine including Labyrinth seals. Compressible flow was considered for the numerical study to account for the effect of flow compressibility and the reflection of pressure waves. The results clearly showed that the vortical flow in the blade passages induces high-amplitude stochastic fluctuations. A distinct flow pattern in the turbine runner was found. The flow near the blade suction side close to the crown was more chaotic and reversible (pumping), whereas the flow on the blade pressure side close to the band was accelerating (turbine) and directed toward the outlet. Flow separation from the blade leading edge created a vortical flow, which broke up into four parts as it traveled further downstream and created high-energy turbulent eddies. The source of reversible flow was found at the draft tube elbow, where the flow in the center core region moves toward the runner cone. The vortical region located at the inner radius of the elbow gives momentum to the wall-attached flow and is pushed toward the outlet, whereas the flow at the outer radius is pushed toward the runner. The cycle repeats at a frequency of 22.3 Hz, which is four times the runner rotational speed.

Centrifugal compressor design for near-critical point applications

Abstract: The supercritical CO2 (sCO2) Brayton cycle has been attracting much attention to produce the electricity power, chiefly due to its higher thermal efficiency with the relatively lower temperature at the turbine inlet compared to other common energy conversion cycles. Centrifugal compressor operating conditions in the supercritical Brayton cycle are commonly set in vicinity of the critical point, owing to smaller compressibility factor and eventually lower compressor work. This paper investigates and compares different centrifugal compressor design methodologies in close proximity to the critical point and suggests the most accurate design procedure based on the findings. An in-house mean-line design code, which is based on the individual enthalpy loss models, is compared to stage efficiency correlation design methods. Moreover, modifications are introduced to the skin friction loss calculation to establish an accurate one-dimensional design methodology. Moreover, compressor performance is compared to the experimental measurements.

Vibration based condition monitoring of wind turbine gearboxes based on cyclostationary analysis

Abstract: Wind industry experiences a tremendous growth during the last few decades. As of the end of 2016, the worldwide total installed electricity generation capacity from wind power amounted to 486,790 MW, presenting an increase of 12.5% compared to the previous year. Nowadays wind turbine manufacturers tend to adopt new business models proposing total health monitoring services and solutions, using regular inspections or even embedding sensors and health monitoring systems within each unit. Regularly planned or permanent monitoring ensures a continuous power generation and reduces maintenance costs, prompting specific actions when necessary. The core of wind turbine drivetrain is usually a complicated planetary gearbox. One of the main gearbox components which are commonly responsible for the machinery breakdowns are rolling element bearings. The failure signs of an early bearing damage are usually weak compared to other sources of excitation (e.g., gears). Focusing toward the accurate and early bearing fault detection, a plethora of signal processing methods have been proposed including spectral analysis, synchronous averaging and enveloping. Envelope analysis is based on the extraction of the envelope of the signal, after filtering around a frequency band excited by impacts due to the bearing faults. Kurtogram has been proposed and widely used as an automatic methodology for the selection of the filtering band, being on the other hand sensible in outliers. Recently, an emerging interest has been focused on modeling rotating machinery signals as cyclostationary, which is a particular class of nonstationary stochastic processes. Cyclic spectral correlation and cyclic spectral coherence (CSC) have been presented as powerful tools for condition monitoring of rolling element bearings, exploiting their cyclostationary behavior. In this work, a new diagnostic tool is introduced based on the integration of the cyclic spectral coherence (CSC) along a frequency band that contains the diagnostic information. A special procedure is proposed in order to automatically select the filtering band, maximizing the corresponding fault indicators. The effectiveness of the methodology is validated using the National Renewable Energy Laboratory (NREL) wind turbine gearbox vibration condition monitoring benchmarking dataset which includes various faults with different levels of diagnostic complexity.

Turbomachine Design for Supercritical Carbon Dioxide Within the sCO2-HeRo.EU Project

Abstract: The paper aims to give an overview over the keystones of design of the turbomachine for a supercritical CO2 (sCO2) Brayton cycle. The described turbomachine is developed as part of a demonstration cycle on a laboratory scale with a low through flow. Therefore, the turbomachine is small and operates at high rotational speed. To give an overview on the development, the paper is divided into two parts regarding the aerodynamic and mechanical design. The aerodynamic design includes a detailed description on the steps from choosing an appropriate rotational speed to the design of the compressor impeller. For setting the rotational speed, the expected high windage losses are evaluated considering the reachable efficiencies of the compressor. The final impeller design includes a description of the blading development together with the final geometry parameters and calculated performance. The mechanical analysis shows the important considerations for building a turbomachine with integrated design of the three major components: turbine, alternator, and compressor (TAC). It includes different manufacturing techniques of the impellers, the bearing strategy, the sealing components, and the cooling of the generator utilizing the compressor leakage. Concluding the final design of the TAC is shown and future work on the machine is introduced.

A critical analysis on low-order simulation models for darrieus vawts: How much do they pertain to the real flow?

Abstract: To improve the efficiency of Darrieus wind turbines, which still lacks from that of horizontal-axis rotors, computational fluid dynamics (CFD) techniques are now extensively applied, since they only provide a detailed and comprehensive flow representation. Their computational cost makes them, however, still prohibitive for routine application in the industrial context, which still makes large use of low-order simulation models like the blade element momentum (BEM) theory. These models have been shown to provide relatively accurate estimations of the overall turbine performance; conversely, the description of the flow field suffers from the strong approximations introduced in the modeling of the flow physics. In this study, the effectiveness of the simplified BEM approach was critically benchmarked against a comprehensive description of the flow field past the rotating blades coming from the combination of a two-dimensional (2D) unsteady CFD model and experimental wind tunnel tests; for both data sets, the overall performance and the wake characteristics on the midplane of a small-scale H-shaped Darrieus turbine were available. Upon examination of the flow field, the validity of the ubiquitous use of induction factors is discussed, together with the resulting velocity profiles upstream and downstream the rotor. Particular attention is paid on the actual flow conditions (i.e., incidence angle and relative speed) experienced by the airfoils in motion at different azimuthal angles, for which a new procedure for the postprocessing of CFD data is here proposed. Based on this model, the actual lift and drag coefficients produced by the airfoils in motion are analyzed and discussed, with particular focus on dynamic stall. The analysis highlights the main critical issues and flaws of the low-order BEM approach, but also sheds new light on the physical reasons why the overall performance prediction of these models is often acceptable for a first-design analysis.

Aeromechanical Response of a Distortion Tolerant Boundary Layer Ingesting Fan

Abstract: Boundary layer ingestion (BLI) is a propulsion technology being investigated at NASA by the Advanced Aircraft Transportation Technology (AATT) Program to facilitate a substantial reduction in aircraft fuel burn. In an attempt to experimentally demonstrate an increase in the propulsive efficiency of a BLI engine, a first-of-its-kind subscale high-bypass ratio 22″ titanium fan, designed to structurally withstand significant unsteady pressure loading caused by a heavily distorted axial air inflow, was built and then tested in the transonic section of the GRC 8′ × 6′ supersonic wind tunnel. The vibratory responses of a subset of fan blades were measured using strain gages placed in four different blade pressure side surface locations. Response highlights include a significant response of the blade's first resonance to engine order excitation below idle as the fan was spooled up and down. The fan fluttered at the design speed under off operating line, low flow conditions. This paper presents the blade vibration response characteristics over the operating range of the fan and compares them to predicted behaviors. It also provides an assessment of this distortion-tolerant fan's (DTF) ability to withstand the harsh dynamic BLI environment over an entire design life of billions of load cycles at design speed.

Flame and Spray Dynamics During the Light-Round Process in an Annular System Equipped With Multiple Swirl Spray Injectors

Abstract: A successful ignition in an annular multi-injector combustor follows a sequence of steps The first injector is ignited; two arch-shaped flame branches nearly perpendicular to the combustor backplane form; they propagate, igniting each injection unit; they merge In this paper, characterization of the propagation phase is performed in an annular combus- tor with spray flames fed with liquid n-hepane The velocity and the direction of the arch- like flame branch are investigated Near the backplane, the flame is moving in a purely azi- muthal direction Higher up in the chamber, it is also moving in the axial direction due to the volumetric expansion of the burnt gases Time-resolved particle image velocimetry (PIV) measurements are used to investigate the evaporating fuel droplets dynamics A new result is that, during the light-round, the incoming flame front pushes the fuel droplets in the azimuthal direction well before its leading point This leads to a decrease in the local droplet concentration and local mixture composition over not yet lit injectors For the first time, the behavior of an individual injector ignited by the passing flame front is examined The swirling flame structure formed by each injection unit evolves in time From the igni- tion of an individual injector to the stabilization of its flame in its final shape, approxi- mately 50ms elapse After the passage of the traveling flame, the newly ignited flame flashbacks into the injector during a few milliseconds, for example, 5 ms for the conditions that are tested This could be detrimental to the service life of the unit Then, the flame exits from the injection unit, and its external branch detaches under the action of cooled burnt gases in the outer recirculation zone (ORZ)

Influence of Geometric Design Parameters onto Vibratory Response and HCF Safety for Turbine Blades with Friction Damper

Abstract: Among the major concerns for high aspect-ratio, turbine blades are forced and self-excited (flutter) vibrations, which can cause failure by high-cycle fatigue (HCF). The introduction of friction damping in turbine blades, such as by coupling of adjacent blades via under platform dampers, can lead to a significant reduction of resonance amplitudes at critical operational conditions. In this paper, the influence of basic geometric blade design parameters onto the damped system response will be investigated to link design parameters with functional parameters like damper normal load, frequently used in nonlinear dynamic analysis. The shape of a simplified turbine blade is parameterized along with the under platform damper configuration. The airfoil is explicitly included into the parameterization in order to account for changes in blade mode shapes. For evaluation of the damped system response, a reduced-order model for nonlinear friction damping is included into an automated three-dimensional (3D) finite element analysis (FEA) tool-chain. Based on a design of experiments approach, the design space will be sampled and surrogate models will be trained on the received dataset. Subsequently, the mean and interaction effects of the geometric design parameters onto the resonance amplitude and safety against HCF will be outlined. The HCF safety is found to be affected by strong secondary effects onto static and resonant vibratory stress levels. Applying an evolutionary optimization algorithm, it is shown that the optimum blade design with respect to minimum vibratory response can differ significantly from a blade designed toward maximum HCF safety.

The Effect of Stratification Ratio on the Macrostructure of Stratified Swirl Flames: Experimental and Numerical Study

Abstract: The present paper reports experimental and numerical analyses of the macrostructures featured by a stratified swirling flame for varying stratification ratio (SR). The studies are performed with the Beihang Axial Swirler Independently Stratified (BASIS) burner, a novel double-swirled full-scale burner developed at Beihang University. Experimentally, it is found that depending on the ratio between the equivalence ratios of the methane–air mixtures from the two swirlers, the flame stabilizes with three different shapes: attached V-flame, attached stratified flame, and lifted flame. In order to better understand the mechanisms leading to the three macrostructures, large eddy simulations (LES) are performed via the open-source computational fluid dynamics (CFD) software OpenFOAM using the incompressible solver ReactingFoam. Changing SR, simulation results show good agreement with experimentally observed time-averaged flame shapes, demonstrating that the incompressible LES are able to fully characterize the different flame behaviors observed in stratified burners. When the LES account for heat loss from walls, they better capture the experimentally observed flame quenching in the outer shear layer (OSL). Finally, insights into the flame dynamics are provided by analyzing probes located near the two separate streams.

Gas Turbine Fouling Tests: Review, Critical Analysis and Particle Impact Behavior Map

Abstract: Fouling affects gas turbine operation, and airborne or fuel contaminants, under certain conditions, become very likely to adhere to surfaces if impact takes place. Particle sticking implies the change in shape in terms of roughness of the impinged surface. The consequences of these deposits could be dramatic: these effects can shut an aircraft engine down or derate a land-based power unit. This occurrence may happen due to the reduction of the compressor flow rate and the turbine capacity, caused by a variation in the HPT nozzle throat area (geometric blockage due to the thickness of the deposited layer and the aerodynamic blockage due to the increased roughness, and in turn boundary layer). Several methods to quantify particle sticking have been proposed in literature so far, and the experimental data used for their validation vary in a wide range of materials and conditions. The experimental analyzes have been supported by (and have given inspiration to) increasingly realistic mathematical models. Experimental tests have been carried out on (i) a full scale gas turbine unit, (ii) wind tunnel testing or hot gas facilities using stationary cascades, able to reproduce the same conditions of gas turbine nozzle operation and finally, (iii) wind tunnel testing or hot gas facilities using a coupon as the target. In this review, the whole variety of experimental tests performed are gathered and classified according to composition, size, temperature, and particle impact velocity. Using particle viscosity and sticking prediction models, over seventy (70) tests are compared with each other and with the model previsions providing a useful starting point for a comprehensive critical analysis. Due to the variety of test conditions, the related results are difficult to be pieced together due to differences in particle material and properties. The historical data of particle deposition obtained over thirty (30) years are classified using particle kinetic energy and the ratio between particle temperature and its softening temperature. Qualitative thresholds for the distinction between particle deposition, surface erosion, and particle break-up, based on particle properties and impact conditions, are identified. The outcome of this paper can be used for further development of sticking models or as a starting point for new insight into the problem.

Numerical Assessment of Reduced Order Modeling Techniques for Dynamic Analysis of Jointed Structures With Contact Nonlinearities

Abstract: This work presents an assessment of classical and state of the art reduced order modeling (ROM) techniques to enhance the computational efficiency for dynamic analysis of jointed structures with local contact nonlinearities. These ROM methods include classical free interface method (Rubin method, MacNeal method), fixed interface method Craig-Bampton (CB), Dual Craig-Bampton (DCB) method and also recently developed joint interface mode (JIM) and trial vector derivative (TVD) approaches. A finite element (FE) jointed beam model is considered as the test case taking into account two different setups: one with a linearized spring joint and the other with a nonlinear macroslip contact friction joint. Using these ROM techniques, the accuracy of dynamic behaviors and their computational expense are compared separately. We also studied the effect of excitation levels, joint region size, and number of modes on the performance of these ROM methods.

Wave rotor design method with three steps including experimental validation

Abstract: This paper adapted and extended the preliminary two-step wave rotor design method with another step of experimental validation so that it became a self-validating wave rotor design method with three steps. First, the analytic design based on unsteady pressure wave models was elucidated and adapted to a design function. It was quick and convenient for a first prediction of the wave rotor. Second, the computational fluid dynamics (CFD) simulation was adapted so that it helped to adjust the first prediction. It provided detailed information of the wave rotor inner flow. Thirdly, an experimental method was proposed to complement the validation of the wave rotor design. This experimental method realized tracing the pressure waves and the flows in the wave rotor with measurement on pressure and temperature distributions. The critical point of the experiment is that the essential flow characteristics in the rotor were reflected by the measurements in the static ports. In all, the three steps compensated for each other in a global design procedure, and formed an applicable design method for generic cases.

Flow inhomogeneities in a realistic aeronautical gas-turbine combustor: formation, evolution and indirect noise

Abstract: Indirect noise generated by the acceleration of combustion inhomogeneities is an important aspect in the design of aeroengines because of its impact on the overall noise emitted by an aircraft and the possible contribution to combustion instabilities. In this study, a realistic rich-quench-lean combustor is numerically investigated, with the objective of quantitatively analyzing the formation and evolution of flow inhomogeneities and determine the level of indirect combustion noise in the nozzle guide vane (NGV). Both entropy and compositional noise are calculated in this work. A high-fidelity numerical simulation of the combustion chamber, based on the Large-Eddy Simulation (LES) approach with the Conditional Moment Closure (CMC) combustion model, is performed. The contribution of the different air streams to the formation of flow inhomogeneity is identified and separated through seven dedicated passive scalars. This pins down the individual contributions of the air streams to combustion inhomogeneity at the combustor’s exit. LES-CMC results are then used to determine the acoustic sources to feed an NGV aeroacoustic model, which outputs the noise generated by entropy and compositional inhomogeneities. Results show that non-negligible fluctuations of temperature and composition reach the combustor’s exit. Combustion inhomogeneities originate both from finite-rate chemistry effects and incomplete mixing. In particular, the role of mixing with dilution and liner air flows on the level of com-bustion inhomogeneities at the combustor’s exit is highlighted. The species that most contribute to indirect noise are identified and the transfer functions of a realistic NGV are computed. The noise level indicates that indirect noise generated by temperature fluctuations is larger that the indirect noise generated by compositional inhomogeneities, although the latter is not negligible and is expected to become louder in supersonic nozzles. It is also shown that relatively small fluctuations of the local flame structure can lead to significant variations of the nozzle transfer function, whose gain increases with the Mach number. This highlights the necessity of an on-line solution of the local flame structure, which is performed in this paper by CMC, for an accurate prediction of the level of compositional noise. This study opens new possibilities for the identification, separation and calculation of the sources of indirect combustion noise in realistic aeronautical gas turbines.

A Feasibility Study of Controllable Gas Foil Bearings With Piezoelectric Materials via Rotordynamic Model Predictions

Abstract: This study presents a new concept of gas bearings consisting of controllable gas foil bearings (C-GFBs) in which piezoelectric actuators are applied to the GFBs. The C-GFB consists of a laminated top foil, bump foil, and piezo stacks and can simply change the bearing shape or film thickness locally and globally by varying the thickness of the piezo stacks by adjusting the input voltage. The working method of C-GFBs is as follows. First, the bearing clearance is adjusted by changing the overall piezo stack thickness (clearance control). Second, the bearing preload (preload control) is modulated by the thickness expansion of several piezo stacks. Bearing lubrication performance is predicted for four cases of C-GFBs that controlled to have different bearing clearances and preloads. The piezo stack control generates meaningful differences in the fluid-film thickness and pressure. Clearance control has a great effect on the dynamic force coefficients, but preload control slightly increases the dynamic stiffness and damping coefficients. Furthermore, the rotordynamic prediction of a rotor supported on two journal C-GFBs is conducted. As a result, both control mode for C-GFB is found to have a positive effect on rotordynamic performance in terms of synchronous motions. Finally, the C-GFB is controlled to have a small bearing clearance and large preload at critical speeds to make it possible to stably pass through the critical speeds. Consequently, it turns out that the C-GFB can generate meaningful performance change in terms of bearing lubrication and ro-tordynamic performances by controlling only the input voltage of the piezo stacks. In addition, the C-GFB can be used to form various shapes to meet the operation conditions of an applied system.

Theoretical and Experimental Analyses of Directly Lubricated Tilting-Pad Journal Bearings With Leading Edge Groove

Abstract: Flooded lubrication of tilting-pad journal bearings provides safe and robust operation for many applications due to a completely filled gap at the leading edge of each pad. Direct lubrication by leading edge grooves (LEG) located on the pads represents an alternative to restrictive end seals to ensure these conditions at the entrance to the convergent lubricant film. A theoretical model is presented that describes the specific influences of LEG design on operating characteristics. First, in contrast to conventional tilting-pad journal bearing designs, the LEG is a self-contained lube oil pocket, which is generally connected to an outer annular oil supply channel. Consequently, each LEG can feature a specific speed and load-dependent effective pocket pressure, which influences the pad tilting angle. Second, the thermal inlet mixing model must consider the specific flow conditions depending on the main flow direction within the film as well as the one between outer annular channel and pocket. The novel LEG model is integrated into a comprehensive bearing code and validated with test from a high performance test rig for a four tilting-pad bearing in load between pivot orientations. Within the investigated operating range good agreement between theoretical and experimental data is achieved if all boundary conditions are accurately considered. Additionally, the impact of single simplifications within the model is studied and evaluated. Finally, the test data are compared to results from the same test bearing with modified lubricant oil supply conditions in order to identify specific properties of LEG design.

Evaluating Optimization Strategies for Engine Simulations Using Machine Learning Emulators

Abstract: This work evaluates different optimization algorithms for computational fluid dynamics (CFD) simulations of engine combustion. Due to the computational expense of CFD simulations, emulators built with machine learning algorithms were used as surrogates for the optimizers. Two types of emulators were used: a Gaussian process (GP) and a weighted variety of machine learning methods called SuperLearner (SL). The emulators were trained using a dataset of 2048 CFD simulations that were run concurrently on a supercomputer. The design of experiments (DOE) for the CFD runs was obtained by perturbing nine input parameters using a Monte-Carlo method. The CFD simulations were of a heavy duty engine running with a low octane gasoline-like fuel at a partially premixed compression ignition mode. Ten optimization algorithms were tested, including types typically used in research applications. Each optimizer was allowed 800 function evaluations and was randomly tested 100 times. The optimizers were evaluated for the median, minimum, and maximum merits obtained in the 100 attempts. Some optimizers required more sequential evaluations, thereby resulting in longer wall clock times to reach an optimum. The best performing optimization methods were particle swarm optimization (PSO), differential evolution (DE), GENOUD (an evolutionary algorithm), and micro-genetic algorithm (GA). These methods found a high median optimum as well as a reasonable minimum optimum of the 100 trials. Moreover, all of these methods were able to operate with less than 100 successive iterations, which reduced the wall clock time required in practice. Two methods were found to be effective but required a much larger number of successive iterations: the DIRECT and MALSCHAINS algorithms. A random search method that completed in a single iteration performed poorly in finding optimum designs but was included to illustrate the limitation of highly concurrent search methods. The last three methods, Nelder–Mead, bound optimization by quadratic approximation (BOBYQA), and constrained optimization by linear approximation (COBYLA), did not perform as well.