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### Research Papers

J. Eng. Gas Turbines Power. 2018;141(1):011001-011001-9. doi:10.1115/1.4040686.

This work is motivated by the thermoacoustic instability challenges associated with ultra-low emissions gas turbine (GT) combustors. It demonstrates the first use of high-speed dual-plane orthogonally-polarized stereoscopic-particle image velocimetry (PIV) and synchronized OH planar laser-induced fluorescence in a premixed swirling flame. We use this technique to explore the effects of combustion and longitudinal acoustic forcing on the time- and phase-averaged flow field—particularly focusing on the behavior of the Reynolds stress in the presence of harmonic forcing. We observe significant differences between ensemble-averaged and time-averaged Reynolds stress. This implies that the large-scale motions are nonergodic, due to coherent oscillations in Reynolds stress associated with the convection of periodic vortical structures. This result has important implications on hydrodynamic stability models and reduced-order computational fluid dynamics simulations, which do show the importance of turbulent transport on the problem, but do not capture these coherent oscillations in their models.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011002-011002-11. doi:10.1115/1.4040685.

The aerothermal performance of the low-pressure turbine in unmanned aerial vehicles is significantly abated at high altitude, due to boundary layer separation. Different flow control strategies have been proposed to prevent boundary layer separation, such as dielectric barrier discharges (DBD) and synthetic jets. However, the optimization of the control approach requires a better characterization of the separated regions at transient conditions. The present investigation analyzes the behavior of separated flows, reporting the inception and separation length, allowing the development of efficient flow control methods under nontemporally uniform inlet conditions. The development of separated flows was investigated with numerical simulations including Unsteady Reynolds average Navier–Stokes (URANS) and large Eddy simulations (LES). The present research was performed on a wall-mounted hump, which imposes a pressure gradient representative of the suction side of low pressure turbines. Through sudden flow accelerations, we looked into the dynamic response of the shear layer detachment as it is modulated by the mean flow evolution. Similarly, we studied the behavior of the recirculation bubble under periodic disturbances imposed at various frequencies ranging from 10 to 500 Hz, at which the Reynolds number oscillates between 40,000 and 180,000. As a first step into the flow control, we added a slot to allow flow injection and ingestion upstream of the separation inception. Exploring the behavior of the separated region at different conditions, we defined the envelope for its periodic actuation. We found that by matching the actuator frequency with the frequency response of the separated region, the performance of the actuation is boosted.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011003-011003-8. doi:10.1115/1.4040689.

Extension of gas fuel flexibility of a current production SGT-400 industrial gas turbine combustor system is reported in this paper. A SGT-400 engine with hybrid combustion system configuration to meet a customer's specific requirements was string tested. This engine was tested with the gas turbine package driver unit and the gas compressor-driven unit to operate on and switch between three different fuels with temperature-corrected Wobbe index (TCWI) varying between 45 MJ/m3, 38 MJ/m3, and 30 MJ/m3. The alteration of fuel heating value was achieved by injection or withdrawal of N2 into or from the fuel system. The results show that the engine can maintain stable operation on and switching between these three different fuels with fast changeover rate of the heating value greater than 10% per minute without shutdown or change in load condition. High-pressure rig tests were carried out to demonstrate the capabilities of the combustion system at engine operating conditions across a wide range of ambient conditions. Variations of the fuel heating value, with Wobbe index (WI) of 30 MJ/Sm3, 33 MJ/Sm3, 35 MJ/Sm3, and 45 MJ/Sm3 (natural gas, NG) at standard conditions, were achieved by blending NG with CO2 as diluent. Emissions, combustion dynamics, fuel pressure, and flashback monitoring via measurement of burner metal temperatures, were the main parameters used to evaluate the impact of fuel flexibility on combustor performance. Test results show that NOx emissions decrease as the fuel heating value is reduced. Also note that a decreasing fuel heating value leads to a requirement to increase the fuel supply pressure. Effect of fuel heating value on combustion was investigated, and the reduction in adiabatic flame temperature and laminar flame speed was observed for lower heating value fuels. The successful development program has increased the capability of the SGT-400 standard production dry low emissions (DLE) burner configuration to operate with a range of fuels covering a WI corrected to the normal conditions from 30 MJ/N·m3 to 49 MJ/N·m3. The tests results obtained on the Siemens SGT-400 combustion system provide significant experience for industrial gas turbine burner design for fuel flexibility.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011004-011004-8. doi:10.1115/1.4040740.

Bladed disks are subjected to different types of excitations, which cannot, in any, case be described in a deterministic manner. Fuzzy factors, such as slightly varying airflow or density fluctuation, can lead to an uncertain excitation in terms of amplitude and frequency, which has to be described by random variables. The computation of frictionally damped blades under random excitation becomes highly complex due to the presence of nonlinearities. Only a few publications are dedicated to this particular problem. Most of these deal with systems of only one or two degrees-of-freedom (DOFs) and use computational expensive methods, like finite element method or finite differences method (FDM), to solve the determining differential equation. The stochastic stationary response of a mechanical system is characterized by the joint probability density function (JPDF), which is driven by the Fokker–Planck equation (FPE). Exact stationary solutions of the FPE only exist for a few classes of mechanical systems. This paper presents the application of a semi-analytical Galerkin-type method to a frictionally damped bladed disk under influence of Gaussian white noise (GWN) excitation in order to calculate its stationary response. One of the main difficulties is the selection of a proper initial approximate solution, which is applicable as a weighting function. Comparing the presented results with those from the FDM, Monte–Carlo simulation (MCS) as well as analytical solutions proves the applicability of the methodology.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011005-011005-11. doi:10.1115/1.4040823.

In pursuit of flexibility improvements, General Electric has developed a product to warm-keep high/intermediate pressure steam turbines using hot air. In order to optimize the warm-keeping operation and to gain knowledge about the dominant heat transfer phenomena and flow structures, detailed numerical investigations are required. For the sake of the investigation of the warm-keeping process as found in the presented research, single and multistage numerical turbine models were developed. Furthermore, an innovative calculation approach called the equalized timescales method (ET) was applied for the modeling of unsteady conjugate heat transfer (CHT). In the course of the research, the setup of the ET approach has been additionally investigated. Using the ET method, the mass flow rate and the rotational speed were varied to generate a database of warm-keeping operating points. The main goal of this work is to provide a comprehensive knowledge of the flow field and heat transfer in a wide range of turbine warm-keeping operations and to characterize the flow patterns observed at these operating points. For varying values of flow coefficient and angle of incidence, the secondary flow phenomena change from well-known vortex systems occurring in design operation to effects typical for windage, like patterns of alternating vortices and strong backflows. Furthermore, the identified flow patterns have been compared to vortex systems described in cited literature and summarized in the so-called blade vortex diagram. The analysis of heat transfer in turbine warm-keeping operation is additionally provided.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011006-011006-9. doi:10.1115/1.4040687.

The dynamic modeling of energy systems can be used for different purposes, obtaining important information both for the design phase and control system strategies, increasing the confidence during experimental phase. Such analysis in dynamic conditions is generally performed considering fixed values for both geometrical and operational parameters such as volumes, orifices, but also initial temperatures, pressure. However, such characteristics are often subject to uncertainty, either because they are not known accurately or because they may depend on the operating conditions at the beginning of the relevant transient. With focus on a gas turbine fuel cell hybrid system (HS), compressor surge may or may not occur during transients, depending on the aforementioned cycle characteristics; hence, compressor surge events are affected by uncertainty. In this paper, a stochastic analysis was performed taking into account an emergency shut-down (ESD) in a fuel cell gas turbine HS, modeled with TRANSEO, a deterministic tool for the dynamic simulations. The aim of the paper is to identify the main parameters that impact on compressor surge margin. The stochastic analysis was performed through the response sensitivity analysis (RSA) method, a sensitivity-based approximation approach that overcomes the computational burden of sampling methods. The results show that the minimum surge margin occurs in two different ranges of rotational speed: a high-speed range and a low-speed range. The temperature and geometrical characteristics of the pressure vessel, where the fuel cell is installed, are the two main parameters that affect the surge margin during an emergency shut down.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011007-011007-10. doi:10.1115/1.4040743.

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 toward the adjacent transition ducts, so that neighboring 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 synchronized 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.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011008-011008-8. doi:10.1115/1.4040715.

The effect of intentional mistuning has been analyzed for an axial turbocharger blisk with the objective of limiting the forced response due to low engine order excitation (LEO). The idea behind the approach was to increase the aerodynamic damping for the most critical fundamental mode in a way that a safe operation is ensured without severely losing aerodynamic performance. Apart from alternate mistuning, a more effective mistuning pattern is investigated, which has been derived by means of optimization employing genetic algorithms. In order to keep the manufacturing effort as small as possible, only two blade different geometries have been allowed, which means that an integer optimization problem has been formulated. Two blisk prototypes have been manufactured for purpose of demonstrating the benefit of the intentional mistuning pattern identified in this way: A first one with and a second one without employing intentional mistuning. The real mistuning of the prototypes has been experimentally identified. It is shown that the benefit regarding the forced response reduction is retained in spite of the negative impact of unavoidable additional mistuning due to the manufacturing process. Independently, further analyzes have been focused on the robustness of the solution by considering increasing random structural mistuning and aerodynamic mistuning as well. The latter one has been modeled by means of varying aerodynamic influence coefficients (AIC) as part of Monte Carlo simulations. Reduced order models have been employed for these purposes.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011009-011009-12. doi:10.1115/1.4040688.

As well known, the stability assessment of turbomachines is strongly related to internal sealing components. For instance, labyrinth seals are widely used in compressors, steam, and gas turbines and pumps to control the clearance leakage between rotating and stationary parts, owing to their simplicity, reliability, and tolerance to large thermal and pressure variations. Labyrinth seals working principle consists of reducing the leakage by imposing tortuous passages to the fluid that are effective on dissipating the kinetic energy of the fluid from high-pressure regions to low-pressure regions. Conversely, labyrinth seals could lead to dynamics issues. Therefore, an accurate estimation of their dynamic behavior is very important. In this paper, the experimental results of a long-staggered labyrinth seal will be presented. The results in terms of rotordynamic coefficients and leakage will be discussed as well as the critical assessment of the experimental measurements. Eventually, the experimental data are compared to the numerical results obtained with the new bulk-flow model (BFM) introduced in this paper.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011010-011010-12. doi:10.1115/1.4040734.

The current work focuses on mission-based evaluation of a novel engine architecture arising from the conversion of a microturbojet to a microturbofan via introduction of a variable speed fan and bypass nozzle. The solution significantly improves maximum thrust by 260%, reduces fuel consumption by as much as 60% through maintaining the core independently running at its optimum, and enables a wider operational range, all the meanwhile preserving a simple single spool configuration. Particularly, the introduction of a variable-speed fan enables real-time optimization for both high-speed cruise and low-speed loitering. In order to characterize the performance of the adaptive cycle engine with increased number of controls (engine speed, gear ratio, bypass opening), a component map-based thermodynamic study is used to contrast it against other similar propulsion systems with incrementally reduced input variables. In the following, a shortest path-based optimization is conducted over the locally minimum fuel consumption operating points, based on a set of gradient driven connectivity constraints for changes in gear ratio and bypass nozzle area. The resultant state transition graphs provide global optimum for fuel consumption at a thrust range in a given altitude and Mach flight envelope. Then, the engine model is coupled to a flight mechanics solver supplied with a conceptual design for a representative multipurpose unmanned aerial vehicle (UAV). Finally, the associated mission benefits are demonstrated in surveillance and firefighting scenarios.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011011-011011-10. doi:10.1115/1.4040739.

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.

Topics: Blades , Stress , Design
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011012-011012-10. doi:10.1115/1.4040713.

In this paper, we propose a two-step methodology to evaluate the convective heat flux along the rotor casing using an engine-scalable approach based on discrete Green's functions . The first step consists in the use of an inverse heat transfer technique to retrieve the heat flux distribution on the shroud inner wall by measuring the temperature of the outside wall; the second step is the calculation of the convective heat flux at engine conditions, using the experimental heat flux and the Green functions engine-scalable technique. Inverse methodologies allow the determination of boundary conditions; in this case, the inner casing surface heat flux, based on measurements from outside of the system, which prevents aerothermal distortion caused by routing the instrumentation into the test article. The heat flux, retrieved from the inverse heat transfer methodology, is related to the rotor tip gap. Therefore, for a given geometry and tip gap, the pressure and temperature can also be retrieved. In this work, the digital filter method is applied in order to take advantage of the response of the temperature to heat flux pulses. The discrete Green's function approach employs a matrix to relate an arbitrary temperature distribution to a series of pulses of heat flux. In this procedure, the terms of the Green's function matrix are evaluated with the output of the inverse heat transfer method. Given that key dimensionless numbers are conserved, the Green's functions matrix can be extrapolated to engine-like conditions. A validation of the methodology is performed by imposing different arbitrary heat flux distributions, to finally demonstrate the scalability of the Green's function method to engine conditions. A detailed uncertainty analysis of the two-step routine is included based on the value of the pulse of heat flux, the temperature measurement uncertainty, the thermal properties of the material, and the physical properties of the rotor casing.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011013-011013-9. doi:10.1115/1.4040717.

Due to the growing share of volatile renewable power generation, conventional power plants with a high flexibility are required. This leads to high thermal stresses inside the heavy components which reduces the lifetime. To improve the ability for fast start-ups, information about the metal temperature inside the rotor and the casing are crucial. Thus, an efficient calculation approach is required which enables the prediction of the temperature distribution in a whole multistage steam turbine. Considerable improvements of the computing power and numerical simulation tools today allow detailed investigations of the heat transfer and the flow phenomena by conjugate-heat-transfer (CHT) simulations. However, these simulations are still restricted to smaller geometries mostly by the number of elements. This leads to coarser numerical meshes for larger geometries, and thus, to a reduced accuracy. A highly accurate three-dimensional-CHT simulation of a whole multistage steam turbine can only be conducted with huge computational expense. Therefore, a simplified calculation approach is required. Heat transfer correlations are a commonly used tool for the calculation of the heat exchange between fluid and solid. Heat transfer correlations for steam turbines have been developed in a multitude of investigations. However, these investigations were based on design or to some extent on part-load operations with steam as the working fluid. The present paper deals with the theoretical investigation of steam turbine warm-keeping operation with hot air. This operation is totally different from the conventional operation conditions, due to a different working fluid with low mass flow rates and a slow rotation. Based on quasi-steady transient multistage CHT simulations, an analytical heat transfer correlation has been developed, since the commonly known calculation approaches from the literature are not suitable for this case. The presented heat transfer correlations describe the convective heat transfer separately at vane and blade as well as the seal surfaces. The correlations are based on a CHT model of three repetitive steam turbine stages. The simulations show a similar behavior of the Nusselt-number in consecutive stages. Hence, the developed area related heat transfer correlations are independent of the position of the stage. Finally, the correlations are implemented into a solid body finite element model and compared to the fluid-dynamic simulations.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011014-011014-8. doi:10.1115/1.4040747.

Volatile renewable energy sources induce power supply fluctuations. These need to be compensated by flexible conventional power plants. Gas turbines in combined cycle power plants adjust the power output quickly but their turn-down ratio is limited by the slow reaction kinetics, which leads to CO and unburned hydrocarbon emissions. To extend the turn-down ratio, part of the fuel can be converted to syngas, which exhibits a higher reactivity. By an increasing fraction of syngas in the fuel, the reactivity of the mixture is increased and total fuel mass flow and the power output can be reduced. An autothermal on-board syngas generator in combination with two different burner concepts for natural gas (NG)/syngas mixtures was presented in a previous study (Baumgärtner, M. H., and Sattelmayer, T., 2017, “Low Load Operation Range Extension by Autothermal On-Board Syngas Generation,” ASME J. Eng. Gas Turbines Power, 140(4), p. 041505). The study at hand shows a mass-flow variation of the reforming process with mass flows, which allow for pure syngas combustion and further improvements of the two burner concepts which result in a more application-oriented operation. The first of the two burner concepts comprises a generic swirl stage with a central lance for syngas injection. Syngas is injected with swirl to avoid a negative impact on the total swirl intensity and nonswirled. The second concept includes a central swirl stage with an outer ring of jets. For this burner, syngas is injected in both stages to avoid NOx emissions from the swirl stage. Increased NOx emissions produced by NG combustion of the swirl pilot were reported in last year's paper. For both burners, combustion performance is analyzed by OH*-chemiluminescence and gaseous emissions. The lowest possible adiabatic flame temperature without a significant increase of CO emissions was 170–210 K lower for the syngas compared to low load pure NG combustion. This corresponds to a decrease of 15–20% in terms of thermal power.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011015-011015-9. doi:10.1115/1.4041070.

This paper describes the coupled flow and flame dynamics during blowoff and reattachment events in a combustor consisting of a linear array of five interacting nozzles using 10 kHz repetition-rate OH planar laser-induced fluorescence and stereoscopic particle image velocimetry (S-PIV). Steady operating conditions were studied at which the three central flames randomly blew-off and subsequently reattached to the bluff-bodies. Transition of the flame from one nozzle was rapidly followed by transition of the other nozzles, indicating cross-nozzle coupling. Blow-off transitions were preferentially initiated in one of the off-center nozzles, with the transition of subsequent nozzles occurring in a random order. Similarly, the center nozzle tended to be the last nozzle to reattach. The blow-off process of any individual nozzle was similar to that for a single bluff-body stabilized flame, though with cross-flame interactions providing additional means of restabilizing a partially extinguished flame. Subsequent to blowoff of the first nozzle, the other nozzles underwent similar blow-off processes. Flame reattachment was initiated by entrainment of a burning pocket into a recirculation zone, followed by transport to the bluff-body; the other nozzles subsequently underwent similar reattachment processes. Several forms of cross-nozzle interaction that can promote or prevent transition are identified. Furthermore, the velocity measurements indicated that blowoff or reattachment of the first nozzle during a multinozzle transition causes significant changes to the flow fields of the other nozzles. It is proposed that a single-nozzle transition redistributes the flow to the other nozzles in a manner that promotes their transition.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011016-011016-11. doi:10.1115/1.4040741.

In the present paper, the synergistic combination of intercooling with pulsed detonation combustion is analyzed concerning its contribution to NOx and CO2 emissions. CO2 is directly proportional to fuel burn and can, therefore, be reduced by improving specific fuel consumption (SFC) and reducing engine weight and nacelle drag. A model predicting NOx generation per unit of fuel for pulsed detonation combustors (PDCs), operating with jet-A fuel, is developed and integrated within Chalmers University's gas turbine simulation tool GESTPAN. The model is constructed using computational fluid dynamics (CFD) data obtained for different combustor inlet pressure, temperature, and equivalence ratio levels. The NOx model supports the quantification of the trade-off between CO2 and NOx emissions in a 2050 geared turbofan architecture incorporating intercooling and pulsed detonation combustion and operating at pressures and temperatures of interest in gas turbine technology for aero-engine civil applications.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011017-011017-11. doi:10.1115/1.4040750.

The purpose of this paper is to present a multidisciplinary predesign process and its application to three aero-engine models. First, a twin spool mixed flow turbofan engine model is created for validation purposes. The second and third engine models investigated comprise future engine concepts: a counter rotating open rotor (CROR) and an ultrahigh bypass turbofan. The turbofan used for validation is based on publicly available reference data from manufacturing and emission certification. At first, the identified interfaces and constraints of the entire predesign process are presented. An important factor of complexity in this highly iterative procedure is the intricate data flow, as well as the extensive amount of data transferred between all involved disciplines and among different fidelity levels applied within the design phases. To cope with the inherent complexity, data modeling techniques have been applied to explicitly determine required data structures of those complex systems. The resulting data model characterizing the components of a gas turbine and their relationships in the design process is presented in detail. Based on the data model, the entire engine predesign process is presented. Starting with the definition of a flight mission scenario and resulting top level engine requirements, thermodynamic engine performance models are developed. By means of these thermodynamic models, a detailed engine component predesign is conducted. The aerodynamic and structural design of the engine components are executed using a stepwise increase in level of detail and are continuously evaluated in context of the overall engine system.

Topics: Engines , Design
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011018-011018-11. doi:10.1115/1.4040851.

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.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011019-011019-9. doi:10.1115/1.4040848.

Erosion behavior of a large number of gas-turbine grade ceramic matrix composites (CMCs) was assessed using fine to medium grain garnet erodents at velocities of 200 and 300 m/s at ambient temperature. The CMCs used in the current work were comprised of nine different SiC/SiCs, one SiC/C, one C/SiC, one SiC/MAS, and one oxide/oxide. Erosion damage was quantified with respect to erosion rate and the damage morphology was assessed via scanning electron microscopy (SEM) and optical microscopy in conjunction with three-dimensional (3D) image mapping. The CMCs response to erosion appeared to be very complicated due to their architectural complexity, multiple material constituents, and presence of pores. Effects of architecture, material constituents, density, matrix hardness, and elastic modulus of the CMCs were taken into account and correlated to overall erosion behavior. The erosion of monolithic ceramics such as silicon carbide and silicon nitrides was also examined to gain a better understanding of the governing damage mechanisms for the CMC material systems used in this work.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011020-011020-8. doi:10.1115/1.4040745.

Nonintrusive polynomial chaos expansion (NIPCE) is used to quantify the impact of uncertainties in operating conditions on the flame transfer function (FTF) of a premixed laminar flame. NIPCE requires only a small number of system evaluations, so it can be applied in cases where a Monte Carlo simulation is unfeasible. We consider three uncertain operating parameters: inlet velocity, burner plate temperature, and equivalence ratio. The FTF is identified in terms of the finite impulse response (FIR) from computational fluid dynamics (CFD) simulations with broadband velocity excitation. NIPCE yields uncertainties in the FTF due to the uncertain operating conditions. For the chosen uncertain operating bounds, a second-order expansion is found to be sufficient to represent the resulting uncertainties in the FTF with good accuracy. The effect of each operating parameter on the FTF is studied using Sobol indices, i.e., a variance-based measure of sensitivity, which are computed from the NIPCE. It is observed that in the present case, uncertainties in the FIR as well as in the phase of the FTF are dominated by the equivalence-ratio uncertainty. For frequencies below 150 Hz, the uncertainty in the gain of the FTF is also attributable to the uncertainty in equivalence-ratio, but for higher frequencies, the uncertainties in velocity and temperature dominate. At last, we adopt the polynomial approximation of the output quantity, provided by the NIPCE method, for further uncertainty quantification (UQ) studies with modified input uncertainties.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):011021-011021-7. doi:10.1115/1.4040849.

Influenced by the growing share of Renewable Energies, higher flexibility and increased efficiency of fossil power plants as well as improved cost efficiency in production of turbine components are evident market trends. Daily cycling in turbine operations leads to advanced requirements for robust design especially of rotating parts. Low pressure (LP) steam turbine end-stage blades with larger exhaust areas are one lever to increase the efficiency of the turbine by reduction of exhaust losses and also to realize cost-efficient single flow exhaust applications. Consequently, blade steels with improved mechanical properties are required. The results of the development of a new high-strength precipitation-hardening (PH) steel for LP end-stage blade application with significantly enhanced material properties are reported. The paper covers the testing strategy applied and information on crucial material parameters like improved low cycle and high cycle fatigue (HCF) behavior while keeping good stress corrosion cracking (SCC) resistance and corrosion fatigue (CF) properties. Furthermore, first manufacturing experiences and validation results from a full-scale component test rig are presented.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2018;141(1):011501-011501-5. doi:10.1115/1.4040811.

This work used the established mathematic models of an intermediate-temperature solid oxide fuel cell (IT-SOFC) and gas turbine (GT) hybrid system fueled with wood chip gas to investigate the load performance and safe characteristic under off-design conditions. Three different operating modes (mode A: regulating the fuel proportionally, and the air is passively regulated. Mode B: regulating the fuel only. Mode C: simultaneously regulating the fuel and air) were chosen, and the component safety factors (such as fuel cell maximum temperature, compressor surge margin, carbon deposition in reformer) were considered. Results show that when the operation modes A and C are executed, the hybrid system output power can be safely changed from 41% to 104%, and 45% to 103%, respectively. When mode B is executed, the load adjustment range of hybrid system is from 20% to 134%, which is wider than that of two above operation modes. However, the safety characteristic in this case is very complicated. The system will suffer from two potential malfunctions caused by too lower temperature entering turbine and CH4/CO cracking in reforming reactor when it operates in low load conditions. When the system operates in the high load conditions exceeding 130% of relative power, the potential thermal cracking of fuel cell will be occurred.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2018;141(1):012501-012501-9. doi:10.1115/1.4040680.

Ingress is the leakage of hot mainstream gas through the rim-seal clearance into the wheel-space between the rotating turbine disk (the rotor) and the adjacent stationary casing (the stator). The high-pressure rotor is purged by a radial outflow of air from the high-pressure compressor, and this cooling air is also used to reduce the ingress. The engine designer needs to predict the stator and rotor temperatures as a function of cooling-flow rate. The sealing effectiveness determines how much air is needed to reduce or prevent ingress; although there are numerous theoretical and experimental papers on the effectiveness of different seal geometries, there are few papers on the effect of ingress on the temperature of the rotating disk. This is an unsolved problem of great practical importance: under high stress, a small increase in metal temperature can significantly reduce operating life. In this paper, conservation equations and control volumes are used to develop theoretical equations for the exchange of mass, concentration and enthalpy in an adiabatic rotor–stator system when ingress occurs. It is assumed that there are boundary layers on the rotor and stator, separated by an inviscid rotating core, and the fluid entrained from the core into the boundary layer on the rotor is recirculated into that on the stator. The superposed cooling flow protects the rotor surface from the adverse effects of hot-gas ingress, which increases the temperature of the fluid entrained into the rotor boundary layer. A theoretical model has been developed to predict the relationship between the sealing effectiveness on the stator and the adiabatic effectiveness on the rotor, including the effects of both ingress and frictional heating. The model involves the use of a nondimensional buffer parameter, $Ψ$, which is related to the relative amount of fluid entrained into the rotor boundary layer. The analysis shows that the cooling flow acts as a buffer, which attenuates the effect of hot gas ingress on the rotor, but frictional heating reduces the buffer effect. The theoretical effectiveness curves are in good agreement with experimental data obtained from a rotor–stator heat-transfer rig, and the results confirm that the buffer effect increases as the sealing effectiveness of the rim seals decreases. The analysis quantifies the increase in the adiabatic rotor temperature due to direct frictional heating, which is separate from the increase due to the combined effects of the ingress and the indirect frictional heating of the entrained fluid. These combined effects are reduced as $Ψ$ increases, and $Ψ$ = 1 at a critical flow rate above which there is no entrained fluid and consequently no indirect heating of the rotor. The model also challenges the conventional physical interpretation of ingress as, in general, not all the hot gas that enters the rim-seal clearance can penetrate into the wheel-space. The ingress manifests itself through a mixing of enthalpy, which can be exchanged even if no ingested fluid enters the wheel-space.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2018;141(1):012601-012601-13. doi:10.1115/1.4040683.

Following three decades of research in short duration facilities, Purdue University has developed an alternative turbine facility in view of the modern technology in computational fluid mechanics, structural analysis, manufacturing, heating, control, and electronics. The proposed turbine facility can operate continuously and also perform transients, suited for precise heat flux, efficiency, and optical measurement techniques to advance turbine aerothermo-structural engineering. The facility has two different test sections, linear and annular, to service both fundamental and applied research. The linear test section is completely transparent for optical imaging and spectroscopy, aimed at technology readiness levels (TRLs) of 1–2. The annular test section was designed with optical access to perform proof of concepts as well as validation of turbine component performance for relevant nondimensional parameters at TRLs of 3–4. The large mass flow rate (28 kg/s) combined with a minimum hub to tip ratio of 0.85 allows high spatial resolution. The Reynolds number (Re) extends from 60,000 to 3,000,000, based on the vane outlet flow properties with an axial chord of 0.06 m and a turning angle of 72 deg. The pressure ratio can be independently adjusted, enabling testing from low subsonic to Mach 3.2. This paper provides a detailed description of the sequential design methodology from zero-dimensional to three-dimensional (3D) unsteady analysis as well as of the measurement techniques available in this turbine facility.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(1):012602-012602-8. doi:10.1115/1.4040566.

Deteriorations of nozzle guide vanes (NGVs) and rotor blades of a steam turbine through a long-time operation usually decrease a thermal efficiency and a power output of the turbine. In this study, influences of blade deformations due to erosion are discussed. Experiments were carried out in order to validate numerical simulations using a commercial software ANSYS-cfx. The numerical results showed acceptable agreements with experimental results. Variation of flow characteristics in the first stage of the intermediate pressure steam turbine is examined using numerical simulations. Geometries of the NGVs and the rotor blades are measured using a 3D scanner during an overhaul. The old NGVs and the rotor blades, which were used in operation, were eroded through the operation. The erosion of the NGVs leaded to increase of the throat area of the nozzle. The numerical results showed that rotor inlet velocity through the old NGVs became smaller and the flow angle of attack to the rotor blade leading edge became smaller. Consequently, the rotor power decreased significantly. Influences of the flow angle of at the rotor inlet were examined by parametric calculations and results showed that the angle of attack was an important parameter to determine the rotor performance. In addition, the influence of the deformation of the rotor blade was examined. The results showed that the degradation of the rotor performance decreased in accordance with the decrease of blade surface area.

Commentary by Dr. Valentin Fuster