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Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2013;135(5):051201-051201-11. doi:10.1115/1.4007753.

Aeroengine bearing chambers typically contain bearings, seals, shafts and static parts. Oil is introduced for lubrication and cooling and this creates a two phase flow environment that may contain droplets, mist, film, ligaments, froth or foam and liquid pools. Some regions of the chamber contain a highly rotating air flow such that there are zones where the flow is gravity dominated and zones where it is rotation dominated. The University of Nottingham Technology Centre in Gas Turbine Transmission Systems, is conducting an ongoing experimental program investigating liquid and gas flow behavior in a relevant highly rotating environment. Previously reported work by the UTC has investigated film thickness and residence volume within a simplified chamber consisting of outer cylindrical chamber, inner rotating shaft and cuboid off-take geometry (termed the generic deep sump). Recently, a more aeroengine relevant bearing chamber offtake geometry has been studied. This geometry is similar to one investigated at Purdue University and consists of a “sub-sump” region approached by curved surfaces linked to the bearing chamber. The test chamber consists of an outer, stationary cylinder with an inner rotating shaft. The rig runs at ambient pressure and the working fluid (water) is introduced either via a film generator on the chamber wall or through holes in the shaft. In addition to visual data (high speed and normal video), liquid residence volume within the chamber and film thickness were the two numerical comparators chosen. Data was obtained for a number of liquid supply rates, scavenge ratios and shaft rotation speeds. The data from the current model is compared to that from the earlier studies. The data shows that in contrast to the previously reported generic deep sump study, the residence volume of the curved wall deep sump (CWDS) designs is far less sensitive to shaft speed, liquid supply rate and scavenge ratio. The method of liquid supply only makes a significant difference at the lowest scavenge ratios. Residence volume data for the Nottingham CWDS is comparable, when appropriately scaled, to that for the Purdue design. The film thickness data shows that at the lower shaft speeds investigated the flow is gravity dominated whereas at higher shaft speeds shear dominates.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(5):051202-051202-7. doi:10.1115/1.4007867.

Crackle noise from heated supersonic jets is characterized by the presence of strong positive pressure impulses resulting in a strongly skewed far-field pressure signal. These strong positive pressure impulses are associated with N-shaped waveforms involving a shocklike compression and, thus, is very annoying to observers when it occurs. Unlike broadband shock-associated noise which dominates at upstream angles, crackle reaches a maximum at downstream angles associated with the peak jet noise directivity. Recent experiments (Martens et al., 2011, “The Effect of Chevrons on Crackle—Engine and Scale Model Results,” Proceedings of the ASME Turbo Expo, Paper No. GT2011-46417) have shown that the addition of chevrons to the nozzle lip can significantly reduce crackle, especially in full-scale high-power tests. Because of these observations, it was conjectured that crackle is associated with coherent large scale flow structures produced by the baseline nozzle and that the formation of these structures are interrupted by the presence of the chevrons, which leads to noise reduction. In particular, shocklets attached to large eddies are postulated as a possible aerodynamic mechanism for the formation of crackle. In this paper, we test this hypothesis through a high-fidelity large-eddy simulation (LES) of a hot supersonic jet of Mach number 1.56 and a total temperature ratio of 3.65. We use the LES solver CHARLES developed by Cascade Technologies, Inc., to capture the turbulent jet plume on fully-unstructured meshes.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Ceramics

J. Eng. Gas Turbines Power. 2013;135(5):051301-051301-9. doi:10.1115/1.4007978.

An integrated creep rupture strength degradation and water vapor degradation model for gas turbine oxide-based ceramic matrix composite (CMC) combustor liners was expanded with heat transfer computations to establish the maximum turbine rotor inlet temperature (TRIT) for gas turbines with 10:1 pressure ratio. Recession rates and average CMC operating temperatures were calculated for an existing baseline N720/A (N720/Al2O3) CMC combustor liner system with and without protective Al2O3 friable graded insulation (FGI) for 30,000-h liner service life. The potential for increasing TRIT by Y3Al5O12 (YAG) substitution for the fiber, matrix, and FGI constituents of the CMC system was explored, because of the known superior creep and water vapor degradation resistance of YAG compared to Al2O3. It was predicted that uncoated N720/A can be used as a combustor liner material up to a TRIT of ∼1200  °C, offering no TRIT advantage over a conventional metal + thermal barrier coating (TBC) combustor liner. A similar conclusion was previously reached for a SiC/SiC CMC liner with barium strontium aluminum silicate (BSAS)-type environmental barrier coating (EBC). The existing N720/A + Al2O3 FGI combustor liner system can be used at a maximum TRIT of ∼1350  °C, a TRIT increase over metal + TBC, and uncoated N720/A of ∼150  °C. Replacing the Al2O3 with YAG is predicted to increase the maximum allowable TRIT. Substitution of the fiber or matrix in N720/A increases TRIT by ∼100  °C. A YAG FGI improves the TRIT of the N720/A + Al2O3 FGI by ∼50  °C, enabling a TRIT of ∼1400 °C, similar to that predicted for SiC/SiC CMCs with protective rare earth monosilicate EBCs.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

J. Eng. Gas Turbines Power. 2013;135(5):051501-051501-9. doi:10.1115/1.4007752.

The prediction of the preswirl cooling air delivery and disk metal temperature are important for the cooling system performance and the rotor disk thermal stresses and life assessment. In this paper, standalone 3D steady and unsteady computation fluid dynamics (CFD), and coupled FE-CFD calculations are presented for prediction of these temperatures. CFD results are compared with previous measurements from a direct transfer preswirl test rig. The predicted cooling air temperatures agree well with the measurement, but the nozzle discharge coefficients are under predicted. Results from the coupled FE-CFD analyses are compared directly with thermocouple temperature measurements and with heat transfer coefficients on the rotor disk previously obtained from a rotor disk heat conduction solution. Considering the modeling limitations, the coupled approach predicted the solid metal temperatures well. Heat transfer coefficients on the rotor disk from CFD show some effect of the temperature variations on the heat transfer coefficients. Reasonable agreement is obtained with values deduced from the previous heat conduction solution.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(5):051502-051502-11. doi:10.1115/1.4007866.

The ultra-compact combustor (UCC) has the potential to offer improved thrust-to-weight and overall efficiency in a turbojet engine. The thrust-to-weight improvement is due to a reduction in engine weight by shortening the combustor section through the use of the revolutionary circumferential combustor design. The improved efficiency is achieved by using an increased fuel-to-air mass ratio and allowing the fuel to fully combust prior to exiting the UCC system. Furthermore, g-loaded combustion offers increased flame speeds that can lead to smaller combustion volumes. One of the issues with the UCC is that the circumferential combustion of the fuel results in hot gases present at the outside diameter of the core flow. These hot gases need to migrate radially from the circumferential cavity and blend with the core flow to present a uniform temperature distribution to the high-pressure turbine rotor. The current research focused on correlations to control the UCC cavity velocity, control the temperature profile throughout the UCC section, analyze the exhaust species exiting the combustor, and quantify pressure losses in the system. To achieve these goals, a computational fluid dynamics (CFD) analysis was used on a UCC geometry scaled to a representative fighter-scale engine. The analysis included a study of cavity to core flow interaction characteristics, a 5- and 12-species combustion model of liquid and gaseous fuel, and determination of species exiting the combustor. Computational comparisons were also made between an engine realistic condition and an ambient pressure rig environment.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(5):051503-051503-12. doi:10.1115/1.4022992.

Novel lean-burn combustor concepts were designed and evaluated for supersonic aircraft propulsion, with a focus on cruise NOx emissions. Premixing to lean conditions is especially challenging at supersonic cruise because combustor inlet temperatures are high and autoignition times are short. However, combustor pressure is significantly lower than at takeoff, so at cruise this allows heated jet fuel to be vaporized before injection as an aid to mixing. Two concepts—differentiated by swirler aerodynamics, swirler size, and staging method—were evaluated in the work reported here, both using injection of vaporized jet fuel. Computational fluid dynamics (CFD) calculations of mixing and combustion were used to design hardware for each concept. Injectors for each were fabricated using stereolithography (SLA) for cold-flow mixing tests, and using metal fabrication for subsequent combustion tests. Combustion test results show that emissions index for NOx (EINOx) < 5 was achieved for both concepts in single-sector tests at supersonic cruise combustor conditions.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Controls, Diagnostics, and Instrumentation

J. Eng. Gas Turbines Power. 2013;135(5):051601-051601-7. doi:10.1115/1.4007976.

For turbine engine performance monitoring purposes, system identification techniques are often used to adapt a turbine engine simulation model to some measurements performed while the engine is in service. Doing so, the simulation model is adapted through a set of so-called health parameters whose values are intended to represent a faithful image of the actual health condition of the engine. For the sake of low computational burden, the problem of random errors contaminating the measurements is often considered to be zero mean, white, and Gaussian random variables. However, when a sensor fault occurs, the measurement errors no longer satisfy the Gaussian assumption and the results given by the system identification rapidly become unreliable. The present contribution is dedicated to the development of a diagnosis tool based on a Kalman filter whose structure is slightly modified in order to accommodate sensor malfunctions. The benefit in terms of the diagnostic reliability of the resulting tool is illustrated on several sensor faults that may be encountered on a current turbofan layout.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Manufacturing, Materials, and Metallurgy

J. Eng. Gas Turbines Power. 2013;135(5):052101-052101-8. doi:10.1115/1.4007774.

Increased availability, reliability, and performance combined with reduced maintenance costs are key factors for the success of gas turbine users. This paper focuses on the reconditioning of film cooled single crystal (SX) components used in the GT24 and GT26 fleet and the latest enabling technologies. The general reconditioning strategy is based on a thorough analysis of the accumulated field experience with SX parts and a controlled, stepwise introduction of new techniques. Reconditioning processes have been developed for different damage scenarios for components. This would include the most technically challenging SX “heavy” scope reconditioning. This paper gives an overview about the reconditioning sequence for SX components and some of its key process steps. As an example, the crack braze repair process is described in detail and several novel SX welding techniques for crack repairs and blade tip and temperature controlled leading edge wall thickness restoration are shown. This covers different processes such as tungsten inert gas (TIG) welding or laser metal forming (LMF) of SX components. During the last few years, highly automated production solutions and innovative production tools have been implemented, which enable high capacity and consistently high quality of reconditioning. After their successful validation and a limited phase of monitored production, these techniques are applied on rotating and stationary SX turbine parts. Validation criteria and the experience gained during the first years of commercial production and operation in the field will be presented.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Microturbines and Small Turbomachinery

J. Eng. Gas Turbines Power. 2013;135(5):052301-052301-8. doi:10.1115/1.4007773.

A simulation method for load rejection with a 150 kW class radial inflow steam turbine system was proposed to determine over rotational speed at load rejection. Simulations were carried out for several parameters of valves which are operated in an emergency. In addition, load rejection tests were carried out to confirm the machine reliability and to obtain results for comparison with the simulation results. Simulation results show that operation delay times of the steam release and vacuum break valves greatly affect over rotational speed at load rejection. Load rejection tests were done for generator outputs from 69 kW to 113 kW. Maximum over rotational speed of 54,160 rpm was measured at the generator output of 113 kW. Over rotational speed calculated by the dynamic simulation has relatively good agreement with the result for the operation delay time of 0.21 s. If the operation delay time of the steam release valves are kept as 0.21 s at the load rejection for the rated load of 150 kW, the over rotational speed is suppressed within 55,200 rpm which is less than the allowed rotational speed of 56,100 rpm.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2013;135(5):052501-052501-8. doi:10.1115/1.4007783.

This paper deals with further development of modified modal domain analysis (MMDA), which is a breakthrough method in the reduced order modeling of a bladed rotor with geometric mistuning. The main focus of this paper is to show that deviations in mass and stiffness matrices due to mistuning, estimated by Taylor series expansions in terms of independent proper orthogonal decomposition variables representing geometric variations of blades, can be used for MMDA. This result has rendered Monte Carlo simulation of the response of a bladed rotor with geometric mistuning to be easy and computationally efficient.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2013;135(5):052601-052601-8. doi:10.1115/1.4022991.

There is a technical and economical need for a correction method to scale model test data, which fulfills five tasks: It should be (i) physically based, (ii) understandable and easy to apply, and (iii) universal, i.e., applicable to centrifugal as well as to axial machines of different specific speed. Moreover, the method should (iv) account for the aerodynamic quality of the machine and should (v) be reliable not only at peak efficiency, but also at off-design condition. Up to now, no method meets all five tasks. To fill that gap, a method developed at Technical University Darmstadt together with Forschungsvereinigung für Luft- und Trocknungstechnik e. V. (FLT) is introduced in this work. The method consists of three steps: Assuming the so-called master curve, scaling the efficiency itself and shifting the best efficiency point to a higher flow coefficient. For each step, a simple physical explanation is given. The validation of the method is done with test data of two axial fans with four different stagger angles and two centrifugal fans. In spite of its simplicity, the method shows a good agreement to test data compared with traditional and most recent scaling methods. A short overview about the advantages and disadvantages of compared methods and a conclusion is given at the end of this work.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Vehicular and Small Turbomachines

J. Eng. Gas Turbines Power. 2013;135(5):052701-052701-10. doi:10.1115/1.4007883.

This paper presents the feasibility study of an oil-free turbocharger (TC) supported on gas foil bearings (GFBs) via on-road tests of a 2-liter class diesel vehicle. The oil-free TC is constructed using a hollow rotor with a radial turbine at one end and a compressor impeller at the other end, a center housing with journal and thrust GFBs, and turbine and compressor casings. The oil-free TC reuses parts of a commercial variable geometry turbocharger, except for the rotor-bearing system. In a test rig driven by a diesel vehicle engine (EG), the rotordynamic performance of the oil-free TC is evaluated up to the rotor speed of 130 krpm, which is measured at the compressor end. The journal GFBs are modified to enhance the rotordynamic performance by inserting three metal shims between the bump-strip layers and bearing housing. The rotordynamic performance is also measured during on-road tests by replacing the original TC of the test diesel vehicle with the constructed oil-free TC. The journal GFBs have a relatively large bearing clearance and no metal shims to generate subsynchronous motions at low TC and EG speeds. During normal vehicle driving, the TC rotor motions show steady rotordynamic operations. The oil-free TC rotates at 25 krpm ∼ 50 krpm while the vehicle runs at 20 km/h ∼ 30 km/h on the road. Subsynchronous rotor motions initiate with a frequency of ∼100 Hz at the TC speed of ∼37 krpm. As expected, the TC rotor motion also shows multiple EG-induced harmonics. Upon external shocks, produced by driving the vehicle on road-bumps, the subsynchronous motions are only excited when the rotor rotates above the initiation speed of subsynchronous motion. The excitation is nondestructive because the vehicle suspension absorbs most of the external shock. Incidentally, the external shocks appear to have no influence on the synchronous motion and engine-induced harmonics of the TC rotor.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2013;135(5):052801-052801-7. doi:10.1115/1.4007963.

Active control turbocharger (ACT) has been proposed as a way to improve turbocharger performance under highly pulsating exhaust flows. This technique implies that the variable geometry mechanism in the turbine is used to optimize its position as a function of the instantaneous mass flow during the engine cycle. Tests presented in the literature showed promising results in a pulsating gas-stand. In this work, a modeling study has been conducted at different engine conditions aimed to quantify the gain in on-engine conditions and to develop a strategy to integrate the ACT system within the engine. Different ways of changing the displacement of the variable mechanism have been analyzed by means of a one-dimensional gas dynamic model. The simulations have been carried out at constant engine operating points defined by fixed air-to-fuel ratio for different mechanism displacement functions around an average position that guarantees the desired amount of intake air. The benefits in overall engine efficiency are lower to those predicted in the literature. It can be concluded that it is not possible to use the ACT system to optimize the turbine operating point and at the same time to control the engine operating point.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(5):052802-052802-8. doi:10.1115/1.4022990.

A simulation study was performed to evaluate the potential fuel economy benefits of integrating a dual-mode SI-HCCI engine into various vehicle architectures. The vehicle configurations that were considered include a conventional vehicle and a mild parallel hybrid electric vehicle. The two configurations were modeled and compared in detail for a given engine size (2.0 L) over the EPA UDDS (city) and highway cycles. The results show that the dual-mode engine in the conventional vehicle offers a modest gain in vehicle fuel economy of approximately 5–7%. The gains were modest because the baseline (the SI engine in the conventional vehicle) is relatively advanced with a six-speed automated manual transmission. The mild parallel hybrid with the SI engine achieved 32% better fuel economy than the conventional vehicle in the city, but only 6% on the highway. For the dual-mode engine in the mild parallel hybrid, a specific control strategy was used to manipulate engine operation in an attempt to minimize the number of engine mode transitions and maximize the time spent in HCCI. The parallel hybrid with the dual-mode engine and modified control strategy provides dramatic improvements of up to 48% for city driving, demonstrating that the addition of HCCI has a more significant impact with mild parallel hybrids than with conventional vehicles. Finally, a systematic study of engine sizing provides guidelines for selecting the best option for a given vehicle application.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2013;135(5):052803-052803-11. doi:10.1115/1.4023028.

The homogeneous charge compression ignition (HCCI) combustion process is highly reliant upon a favorable in-cylinder thermal environment in an engine, for a given fuel. Commercial fuels can differ considerably in composition and autoignition chemistry; hence, strategies intended to bring HCCI to market must account for this fuel variability. To this end, a test matrix consisting of eight gasoline fuels comprised of blends made solely from refinery streams were run in an experimental, single cylinder HCCI engine. All fuels contained 10% ethanol by volume and were representative of a cross section of fuels one would expect to find at gasoline pumps across the United States. The properties of the fuels were varied according to research octane number (RON), sensitivity (S = RON-MON), and volumetric content of aromatics and olefins. For each fuel, a sweep of load (mass of fuel injected per cycle) was conducted and the intake air temperature was adjusted in order to keep the crank angle of the 50% mass fraction burned point (CA50) constant. By analyzing the amount of temperature compensation required to maintain constant combustion phasing, it was possible to determine the sensitivity of HCCI to changes in load for various fuels. In addition, the deviation of fuel properties brought about variations in important engine performance metrics like specific fuel consumption. Though the injected energy content per cycle was matched at the baseline point across the test fuel matrix, thermodynamic differences resulted in a spread of specific fuel consumption for the fuels tested.

Commentary by Dr. Valentin Fuster

Research Papers: Nuclear Power

J. Eng. Gas Turbines Power. 2013;135(5):052901-052901-7. doi:10.1115/1.4007875.

The purpose of this study is to propose a dynamic heat transfer model for predicting transient heat recovery steam generator (HRSG) behaviors involving phase changes in heat exchanger tubes. The model deals with any combination of phase states by switching the equations for heat transfer coefficient, specific volume, and friction factor corresponding to their physical characteristics. The model also constrains the change of mass flow calculated by momentum balance to satisfy thermodynamic relationships which are neglected by conventional models. The simulation results show that the proposed model predicts the transient pressure drop, outlet mass flow changes, and the reduction in heat transfer coefficient caused by dryout during heating or evaporating processes. In addition, the model improves the accuracy of mass flow transients compared to those obtained by conventional models.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Eng. Gas Turbines Power. 2013;135(5):054501-054501-5. doi:10.1115/1.4007902.

This technical brief is focused on the research area of fault detection and diagnosis in a complex thermodynamical system: in this case, an axial flow compressor. Its main contribution is a new approach which combines a physical model and a multilayer perceptron (MLP) model using the best advantages of both types of modeling. Fault detection is carried out by an MLP model whose residuals against the real outputs of the system determine which observations could be considered abnormal. A physical model is used to generate different fault simulations by shifting physical parameters related to faults. After these simulations are performed, the different fault profiles obtained are collected within a fault dictionary. In order to identify and diagnose a fault, the anomalous residuals observed by the MLP model are compared with the fault profiles in the dictionary and a correlation that provides a hypothesis with respect to the causes of the fault is obtained. This methodology has been applied to axial compressor operational data obtained from a real power plant. A case study based on the successful diagnosis of compressor fouling is included in order to show the potential of the proposed method.

Commentary by Dr. Valentin Fuster

Design Innovation Papers

J. Eng. Gas Turbines Power. 2013;135(5):055001-055001-11. doi:10.1115/1.4007977.

Modern lean-burn combustors make use of high flow swirl to maintain flame stability. The swirling flow can persist downstream of the turbine first vane, changing the loading on the rotor, leading to a reduction in efficiency. This paper presents the results of an automatic optimization study carried out to mitigate the effect of high swirling flow on a high pressure turbine stage. A high-fidelity computational fluid dynamics (CFD)-based design optimization using a multipoint approximation (response surface) method is carried out to produce a new vane and a new rotor configuration with a significantly improved aerodynamic performance. It is demonstrated that the novel optimization methodology can cope well with a number of near equality constraints needed for a practical design.

Commentary by Dr. Valentin Fuster

Errata

J. Eng. Gas Turbines Power. 2013;135(5):057001-057001-1. doi:10.1115/1.4023018.
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An error was made in the first equation on page 2, just below the second paragraph. In the last term of the equation, the “Q” should have a “w” subscript. The corrected equation is

dQdθ=(CpPR)dVdθ+(CvVR)dPdθ-dQwdθ

Commentary by Dr. Valentin Fuster

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