Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2016;139(3):031201-031201-9. doi:10.1115/1.4034498.

Motivated by the long-term target settings for research and innovation in Europe and in North America, initial investigations of parallel hybrid electric power plant systems have indicated significant in-flight fuel reduction potentials for short range air transport. Based on this topology, a special variant, namely the cycle-integrated parallel hybrid (CIPH), has been investigated. In this special configuration, electric motors supplied by batteries are powering an array of compressor stages of a power plant that are mechanically decoupled from the turbine section. The potentials with regard to in-flight fuel reduction and efficiency improvement of this concept are derived for a 12-ton-helicopter accommodating 19 passengers on a 450 nm mission. For the presented CIPH concept, the axial compressor section of a baseline turboshaft (TS) delivering a maximum shaft power of 3300 kW is electrified with the help of linear electric motors (LEMs). The highest potential for this arrangement was identified in part load for moderate degrees of power hybridization—the share between installed electric power and total power—of around 20%. The first assessment has revealed that this additional degrees-of-freedom allows to almost double the overall efficiency, compared to a conventional power with same technology time horizon, and a reduction in power-specific fuel consumption (PSFC) of roughly 45%. The range capability of a hybrid-powered helicopter has been decreased by more than 50%, mainly driven by the battery mass. However, a fuel burn, and thus, in-flight CO2 reduction of more than 40% against the reference at decreased mission range have been found.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):031202-031202-7. doi:10.1115/1.4034503.

Modern aircraft engine designs feature reduced clearances that may initiate structural contacts between rotating and static components. A numerical strategy dedicated to the simulation of such interactions is here enriched in order to account for time-dependent angular speeds. This contribution first details the evolution of the numerical strategy before validating the developments by comparing numerical results with experimental observations made on an industrial test bench. Further, numerical investigations allow to assess the sensitivity of the numerical results to acceleration and deceleration rates. The results, obtained with and without abradable coating, underline the fundamental nonlinear nature of the analyzed system. It is found that the lower acceleration rates favor the arisal of interaction phenomena, and that the amplitudes of vibration at a given angular speed are generally lower when the blade decelerates.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2016;139(3):031501-031501-12. doi:10.1115/1.4034446.

A lean partially premixed swirling combustor operated with synthesis gases is studied using large-eddy simulation (LES). The linear-eddy model (LEM) is employed to close the unresolved scalar fluxes with the nonunity Lewis number assumption. Several terms resulting from the LES filtering operation are not modeled but directly resolved considering their unique length and time scales, such as molecular diffusion, scalar mixing, and chemical reactions. First, the validation results on a well-established jet flame indicate a good level of correlation with the experimental data and allow a further analysis of syngas combustion on a practical combustor. Second, the effects of preferential diffusion on the characteristics of flow and combustion dynamics on a lean partially premixed swirling combustor are investigated. The obtained results are expected to provide useful information for the design and operation of gas turbine combustion systems using syngas fuels.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):031502-031502-7. doi:10.1115/1.4034447.

The reduction of full and part load emissions and the increase of the turndown ratio are important goals for gas turbine combustor development. Combustion techniques, which generate lower NOx emissions than unstaged premixed combustion in the full load range, and which have the potential of reducing minimum load while complying with emission legislation, are of high technical interest. Therefore, axial-staged combustion systems have been designed, either with or without expansion in a turbine stage between both stages. In its simpler form without intermediate expansion stage, a flow of hot combustion products is generated in the first stage of the premixed combustor, which interacts with the jets of premixed gas injected into the second stage. The level of NOx formation during combustion of the premixed jets in the hot cross flow determines the advantage of axially staged combustion regarding full load NOx emission reduction. Employing large-eddy simulation in openfoam, a tool has been developed, which allows to investigate staged combustion systems including not only temperature distribution but also NOx emissions under engine conditions. To be able to compute NOx formation correctly, the combustion process has to be captured with sufficient level of accuracy. This is achieved by the partially stirred reactor model. It is combined with a newly developed NOx model, which is a combination of a tabulation technique for the NOx source term based on mixture fraction and progress variable and a partial equilibrium approach. The NOx model is successfully validated with generic burner stabilized flame data and with measurements from a large-scale reacting jet in cross flow experiment. The new NOx model is finally used to compute a reacting jet in cross flow under engine conditions to investigate the NOx formation of staged combustion in detail. The comparison between the atmospheric and the pressurized configuration gives valuable insight in the NOx formation process. It can be shown that the NOx formation within a reacting jet in cross flow configuration is reduced and not only diluted.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):031503-031503-9. doi:10.1115/1.4034448.

The computational modeling of soot in aircraft engines is a formidable challenge, not only due to the multiscale interactions with the turbulent combustion process but the equally complex physical and chemical processes that drive the conversion of gas-phase fuel molecules into solid-phase particles. In particular, soot formation is highly sensitive to the gas-phase composition and temporal fluctuations in a turbulent background flow. In this work, a large-eddy simulation (LES) framework is used to study the soot formation in a model aircraft combustor with swirl-based fuel and air injection. Two different configurations are simulated: one with and one without secondary oxidation jets. Specific attention is paid to the LES numerical implementation such that the discrete solver minimizes the dissipation of kinetic energy. Simulation of the model combustor shows that the LES approach captures the two recirculation zones necessary for flame stabilization very accurately. Further, the model reasonably predicts the temperature profiles inside the combustor. The model also captures variation in soot volume fraction with global equivalence ratio. The structure of the soot field suggests that when secondary oxidation jets are present, the inner recirculation region becomes fuel lean, and soot generation is completely suppressed. Further, the soot field is highly intermittent suggesting that a very restrictive set of gas-phase conditions promotes soot generation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):031504-031504-8. doi:10.1115/1.4034449.

A significant challenge in improving the regeneration process of jet engines is the reduction of engine down-time during inspection. As such, early defect detection without engine disassembly will speed up the regeneration process. Defects in the engines hot-gas path (HGP) influence the density distribution of the flow and lead to irregularities in the density distribution of the exhaust jet which can be detected with the optical background-oriented Schlieren (BOS) method in a tomographic setup. The present paper proposes a combination of tomographic BOS measurements and supervised learning algorithms to develop a methodology for an automatic defect detection system. In the first step, the methodology is tested by analyzing the exhaust jet of a swirl burner array with a nonuniform fuel-supply of single burners with tomographic BOS measurements. The measurements are used to implement a support vector machine (SVM) pattern recognition algorithm. It is shown that the reconstruction quality of tomographic BOS measurements is high enough to be combined with pattern recognition algorithms. The results strengthen the hypothesis that it is possible to automatically detect defects in jet engines with tomographic BOS measurements and pattern recognition algorithms.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):031505-031505-12. doi:10.1115/1.4034499.

A novel parallelized, automated, and predictive imprint cooling model (PAPRICO) was developed for modeling of combustor liners using Reynolds-averaged Navier–Stokes (RANS). The methodology involves removing the film and effusion cooling jet geometry from the liner while retaining the cooling hole imprints on the liner. The PAPRICO can operate under two modalities, viz., two-sided and one-sided. For the two-sided PAPRICO model, the imprints are kept on the plenum and combustor sides of the liner. For the one-sided PAPRICO model, the imprints are retained only on the combustor side of the liner and there is no need for a plenum. The PAPRICO model neither needs a priori knowledge of the cooling flow rates through various combustor liner regions nor specific mesh partitioning. The imprint mass flow rate, momentum, enthalpy, turbulent kinetic energy, and eddy dissipation rate are included in the governing equations as volumetric source terms in cells adjacent to the liner on the combustor side. Additionally, the two-sided PAPRICO model includes corresponding volumetric sinks in cells adjacent to the liner on the plenum side. A referee combustor liner was simulated using PAPRICO under nonreacting flow conditions. The PAPRICO results were compared against predictions of nonreacting flow results of a resolved liner geometry, against a combustor liner with prescribed mass and enthalpy source terms (simplified liner) and against measurements. The results clearly conclude that PAPRICO can qualitatively and quantitatively emulate the local turbulent flow field with a reduced mesh size. The simplified liner fails to emulate the local turbulent flow field.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):031506-031506-9. doi:10.1115/1.4034235.

Low emission combustion is one of the most important requirements for industrial gas turbines. Siemens industrial gas turbines SGT-800 and SGT-700 use dry low emission (DLE) technology and are equipped with third generation of DLE burners. These burners demonstrate high-performance and reliable operation for the duration of their design lifetime. The design and shape of the burner tip is of great importance in order to achieve a good fuel/ air mixture and at the same time a resistance to the fatigue created by heat radiation input. This gives a requirement for a tip structure with delicate internal channels combined with thicker structure for load carrying and production reasons. It was found that the extension of the burner lifetime beyond the original design life could be accomplished by means of repair of the burner tip. Initially, the tip repair has been done by conventional methods— i.e., cutting off the tip and replacing it with a premanufactured one. Due to the sophisticated internal structure of the burner, the cuts have to be made fairly high upstream to avoid having the weld in the delicate channel area. Through the use of additive manufacturing (AM) technology, it has been possible to simplify the repair and only replace the damaged part of the tip. Special processes have been developed for AM repair procedure, including the following: machining off of the damaged and oxidized tip, positioning the sintered model on the burner face, sintering a new tip in place, quality assurance and inspection methods, powder handling, material qualification including bonding zone, development of methods for mechanical integrity calculation, and qualification of the whole repair process. This paper describes how we have developed and qualified SGT-800 and SGT-700 DLE burners repair with the help of additive manufacturing technology and our research work performed. In addition, this paper highlights the challenges we faced during design, materials qualification, and repair work shop set up.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):031507-031507-12. doi:10.1115/1.4034515.

Ceramic matrix composites (CMC) offer the potential of increased service temperatures and are thus an interesting alternative to conventional combustor alloys. Tubular combustor liner demonstrators made of an oxide/oxide CMC were developed for a lean combustor in a future aero-engine in the medium thrust range and tested at engine conditions. During the design, various aspects like protective coating, thermomechanical design, and development of a failure model for the CMC as well as design and test of an attachment system were taken into account. The tests of the two liners were conducted at conditions up to 80% take-off. A new protective coating was tested successfully with a coating thickness of up to t = 1 mm. Different inspection criteria were derived in order to detect crack initiation at an early stage for a validation of the failure model. With the help of detailed pre- and post-test computer tomography (CT) scans to account for the microstructure of the CMC, the findings of the failure model were in reasonable agreement with the test results.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):031508-031508-6. doi:10.1115/1.4034544.

This article is a report of experiments conducted in order to investigate the role of noise on thermoacoustic systems. In contrast to most studies in this direction, in the present work, the role of noise in the subthreshold region, prior to the (subcritical) Hopf bifurcation and the associated saddle-node bifurcation, is considered. In this regime, a thermoacoustic system is stable and does not undergo transition to self-excited thermoacoustic oscillations. However, the system can feature dynamics, which arise due to the proximity of the system to the approaching Hopf bifurcation, in response to noise. Experiments were performed on a model thermoacoustic system featuring a laminar flat flame. Noise was introduced in a controlled manner, and the effect of increasing levels of noise intensity was studied. Results presented here show that noise addition induces coherent oscillations. The induced coherence is observed to depend on the noise amplitude and the proximity to the Hopf bifurcation. Furthermore, this noise-induced behavior is characterized by a well-defined “resonance-like” response of the system: An optimum level of coherence is induced for an intermediate level of noise. These results can be of importance in practical thermoacoustic systems (e.g., combustors), which are inherently noisy due to factors such as flow turbulence and combustion noise.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):031509-031509-8. doi:10.1115/1.4034502.

The effects of jet fuel composition on ignition probability have been studied in a flowfield that is relevant to turbine engine combustors, but also fundamental and conducive to modeling. In the experiments, a spark kernel is ejected from a wall and propagates transversely into a crossflow. The kernel first encounters an air-only stream before transiting into a second, flammable (premixed) stream. The two streams have matched velocities, as verified by hot-wire measurements. The liquid fuels span a range of physical and chemical kinetic properties. To focus on their chemical differences, the fuels are prevaporized in a carrier air flow before being injected into the experimental facility. Ignition probabilities at atmospheric pressure and elevated crossflow temperature were determined from optical measurements of a large number of spark events, and high-speed imaging was used to characterize the kernel evolution. Eight fuel blends were tested experimentally; all exhibited increasing ignition probability as equivalence ratio increased, at least up to the maximum value studied (∼0.8). Statistically significant differences between fuels were measured that have some correlation with fuel properties. To elucidate these trends, the forced ignition process was also studied with a reduced-order numerical model of an entraining kernel. The simulations suggest ignition is successful if sufficient heat release occurs before entrainment of colder crossflow fluid quenches the exothermic oxidation reactions. As the kernel is initialized in air, it remains extremely lean during the initial entrainment of the fuel–air mixture; thus, richer crossflows lead to quicker and higher exothermicity.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):031510-031510-10. doi:10.1115/1.4034453.

A novel methodology for linear stability analysis of high-frequency thermoacoustic oscillations in gas turbine combustors is presented. The methodology is based on the linearized Euler equations (LEEs), which yield a high-fidelity description of acoustic wave propagation and damping in complex, nonuniform, reactive mean flow environments, such as encountered in gas turbine combustion chambers. Specifically, this work introduces three novelties to the community: (1) linear stability analysis on the basis of linearized Euler equations. (2) Explicit consideration of three-dimensional, acoustic oscillations at screech level frequencies, particularly the first-transversal mode. (3) Handling of noncompact flame coupling with LEE, that is, the spatially varying coupling dynamics between perturbation and unsteady flame response due to small acoustic wavelengths. Two different configurations of an experimental model combustor in terms of thermal power and mass flow rates are subject of the analysis. Linear flame driving is modeled by prescribing the unsteady heat release source term of the linearized Euler equations by local flame transfer functions, which are retrieved from first principles. The required steady-state flow field is numerically obtained via computational fluid dynamics (CFD), which is based on an extended flamelet-generated manifold (FGM) combustion model, taking into account heat transfer to the environment. The model is therefore highly suitable for such types of combustors. The configurations are simulated, and thermoacoustically characterized in terms of eigenfrequencies and growth rates associated with the first-transversal mode. The findings are validated against experimentally observed thermoacoustic stability characteristics. On the basis of the results, new insights into the acoustic field are discussed.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Heat Transfer

J. Eng. Gas Turbines Power. 2016;139(3):031901-031901-10. doi:10.1115/1.4034519.

Leaf seals are filament seals for use at static to rotating interfaces in the engine secondary air system. They offer reduced leakage rates and better off-design performance over conventional labyrinth seals. If compared with advanced brush seals, leaf seals are more compliant due to their lower stiffness and can withstand higher axial pressure differences. Although leaf seals can exhibit hydrodynamic air-riding, this is not always the case and seal–rotor contact can occur. As a result, friction between the leaf tips and the rotor causes heat generation and wear. To predict the diameter of the rotating shaft and the seal life, the shaft and seal interface temperature needs to be estimated. In the steady state, this is determined by the ratio of convective heat transfer through the seal to that through the shaft. To that end, the convective heat transfer characteristics of the flow over the shaft around the seal are required to build accurate thermal models. In this paper, the convective heat transfer coefficient (HTC) distribution in the close vicinity of a typical leaf seal is investigated in a new test facility. The experimental setup and test method are described in detail, and accuracy considerations are included. The methodology employed to derive HTC is explained with reference to an analogous computational fluid dynamics (CFD) model. The importance of the choice of an appropriate driving gas temperature is demonstrated. Experimental HTC maps are presented for a blow-down seal geometry operating over a range of engine representative pressure ratios. Insight is gained into the flow field characteristics and heat transfer around the seal.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):031902-031902-11. doi:10.1115/1.4034451.

Currently, fast-response aerodynamic probes are widely used for advanced experimental investigations in turbomachinery applications. The most common configuration is a virtual three-hole probe. This solution is a good compromise between probe dimension and accuracy. Several authors have attempted to extend the capabilities of these probes in terms of bandwidth and operating conditions. Even though differences exist between the solutions in the literature, all of the designs involve the positioning of a dynamic pressure sensor close to the measurement point. In general terms, the higher the frequency response, the more the sensor is exposed to the flow. This physical constraint puts a limit on the probe applicability since the measurement conditions have to comply with the maximum allowed operating conditions of the sensor. In other applications, when the conditions are particularly harsh and a direct measurement is not possible, a waveguide probe is commonly used to estimate the local pressure. In this device, the sensor is connected to the measurement point through a transmitting duct which guarantees that the sensor is operating in a less critical condition. Generally, the measurement is performed through a pressure tap and particular attention must be paid to the probe design in order to have an acceptable frequency response function. In this study, the authors conceived, developed, and tested a probe which combines the concept of a fast-response aerodynamic pressure probe with that of a waveguide probe. Such a device exploits the benefits of having the sensor far from the harsh conditions while maintaining the capability to perform an accurate flow measurement.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):031903-031903-10. doi:10.1115/1.4034513.

Today's power market asks for highly efficient turbines which can operate at a maximum flexibility, achieving a high lifetime and all of this on competitive product costs. In order to increase the plant cycle efficiency, during the past years, nominal steam temperatures and pressures have been continuously increased. To fulfill the lifetime requirements and still achieve the product cost requirements, accurate mechanical integrity based assessments on cyclic lifetime became more and more important. For this reason, precise boundary conditions in terms of local temperatures as well as heat transfer coefficients are essential. In order to gain such information and understand the flow physics behind them, more and more complex thermal modeling approaches are necessary, like computational fluid dynamics (CFD) or even conjugate heat transfer (CHT). A proper application of calculation rules and methods is crucial regarding the determination of thermal stresses, thermal expansion, lifetime, or creep. The aim is to exploit during turbine developments the limits of the designs with the selected materials. A huge effort especially in validation and understanding of those methodologies was done with detailed numerical investigations associated to extensive measurement studies at onsite turbines in operation. This paper focuses on the validation of numerical models based on CHT calculations against experimental data of a large intermediate pressure steam turbine module regarding the temperature distribution at the inner and outer casing for nominal load as well as transient shut-down.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):031904-031904-10. doi:10.1115/1.4034545.

Driven by the need for higher cycle efficiencies, overall pressure ratios for gas turbine engines continue to be pushed higher thereby resulting in increasing gas temperatures. Secondary air, bled from the compressor, is used to cool turbine components and seal the cavities between stages from the hot main gas path. This paper compares a range of purge flows and two different purge hole configurations for introducing the purge flow into the rim cavities. In addition, the mate face gap leakage between vanes is investigated. For this particular study, stationary vanes at engine-relevant Mach and Reynolds numbers were used with a static rim seal and rim cavity to remove rotational effects and isolate gas path effects. Sealing effectiveness measurements, deduced from the use of CO2 as a flow tracer, indicate that the effectiveness levels on the stator and rotor side of the cavity depend on the mass and momentum flux ratios of the purge jets relative to the swirl velocity. For a given purge flow rate, fewer purge holes resulted in better sealing than the case with a larger number of holes.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2016;139(3):032101-032101-7. doi:10.1115/1.4034539.

Since its introduction in 2003, alloy 718PlusTM spurred a lot of interest owing to its increased maximum service temperature over conventional Inconel 718 (704 °C versus 650 °C), good formability, and weldability together with its moderate cost. Understanding the high-temperature deformation characteristics and microstructural evolution is still of interest to many. It is known that the service performance and hot-flow behavior of this alloy are a strong function of the microstructure, particularly the grain size. To develop precise microstructure evolution models and foresee the final microstructure, it is important to understand how and under which forming conditions softening and precipitation processes occur concurrently. In this work, the softening behavior, its mechanisms, and the precipitation characteristics of 718PlusTM were investigated in two parallel studies. While cylindrical compression tests were employed to observe the hot-flow behavior, the precipitation behavior and other microstructural phenomena such as particle coarsening were tracked via hardness measurements. A precipitation–temperature–time (PTT) diagram was reported, and modeling of the flow curves via hyperbolic sine model was discussed in the light of the PTT behavior. Both “apparent” approach and “physically based” approach are implemented and two different sets of parameters were reported for the latter. Finally, recovery and recrystallization kinetics are described via Estrin–Mecking and Bergstrom, and Avrami kinetics, respectively.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Microturbines and Small Turbomachinery

J. Eng. Gas Turbines Power. 2016;139(3):032301-032301-10. doi:10.1115/1.4034359.

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

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2016;139(3):032501-032501-10. doi:10.1115/1.4034452.

While turbine rim sealing flows are an important aspect of turbomachinery design, affecting turbine aerodynamic performance and turbine disk temperatures, the present understanding and predictive capability for such flows is limited. The aim of the present study is to clarify the flow physics involved in rim sealing flows and to provide high-quality experimental data for use in evaluation of computational fluid dynamics (CFD) models. The seal considered is similar to a chute seal previously investigated by other workers, and the study focuses on the inherent unsteadiness of rim seal flows, rather than unsteadiness imposed by the rotating blades. Unsteady pressure measurements from radially and circumferentially distributed transducers are presented for flow in a rotor–stator disk cavity and the rim seal without imposed external flow. The test matrix covered ranges in rotational Reynolds number, Re, and nondimensional flow rate, Cw, of 2.2–3.0 × 106 and 0–3.5 × 103, respectively. Distinct frequencies are identified in the cavity flow, and detailed analysis of the pressure data associates these with large-scale flow structures rotating about the axis. This confirms the occurrence of such structures as predicted in previously published CFD studies and provides new data for detailed assessment of CFD models.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):032502-032502-9. doi:10.1115/1.4034500.

Sealing technology is a key feature to improve efficiency of steam turbines for both new power stations and modernization projects. One of the most powerful sealing alternatives for reducing parasitic leakages in the blade path of a turbine as well as in shaft sealing areas is the use of brush seals, which are also widely used in gas turbines and turbo compressors. The advantage of brush seals over other sealing concepts is based on the narrow gap that is formed between the brush seal bristle tips and the mating rotor surface together with its radial adaptivity. While the narrow gap between the bristle tips and the rotor leads to a strongly decreased flow through the seal compared with conventional turbomachinery seals, it is important to be aware of the tight gap that can be bridged by relative motion between the rotor and the brush seal, leading to a contact of the bristles and the rotor surface. Besides abrasive wear occurrence, the friction between the bristles and the rotor leads to heat generation which can be detrimental to turbine operation due to thermal effects, leading to rotor bending connected to increasing shaft vibrations. In order to investigate the frictional heat generation of brush seals, different investigation concepts have been introduced through the past years. To broaden the knowledge about frictional heat generation and to make it applicable for steam turbine applications, a new testing setup was designed for the steam test rig of the Institute of Jet Propulsion and Turbomachinery—TU Braunschweig, Germany, enabling temperature measurements in the rotor body under stationary and transient operation in steam by using rotor-integrated thermocouples. Within this paper, the development of the instrumented new rotor design and all relevant parts of the new testing setup is shown along with the testing ability by means of the validation of the test rig concept and the achieved measurement accuracy. First results prove that the new system can be used to investigate frictional heat generation of brush seals under conditions relevant for steam turbine shaft seals.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2016;139(3):032601-032601-7. doi:10.1115/1.4034501.

Fouling of compressor blades is an important mechanism leading to performance deterioration in gas turbines over time. Experimental and simulation data are available for the impact of specified amounts of fouling on the performance as well as the amount of foulants entering the engine for defined air filtration systems and ambient conditions. This study provides experimental data on the amount of foulants in the air that actually stick to a blade surface for different conditions. Quantitative results both indicate the amount of dust as well as the distribution of dust on the airfoil, for a dry airfoil, and also the airfoils that were wet from ingested water, in addition to, different types of oil. The retention patterns are correlated with the boundary layer shear stress. The tests show the higher dust retention from wet surfaces compared to dry surfaces. They also provide information about the behavior of the particles after they impact on the blade surface, showing for a certain amount of wet film thickness, the shear forces actually wash the dust downstream and off the airfoil. Further, the effect of particle agglomeration of particles to form larger clusters was observed, which would explain the disproportional impact of very small particles on boundary layer losses.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):032602-032602-9. doi:10.1115/1.4034540.

This paper deals with a numerical study aimed at the characterization of hot-gas ingestion through turbine rim seals. The numerical campaign focused on an experimental facility which models ingress through the rim seal into the upstream wheel-space of an axial-turbine stage. Single-clearance arrangements were considered in the form of axial- and radial-seal gap configurations. With the radial-seal clearance configuration, computational fluid dynamics (CFD) steady-state solutions were able to predict the system sealing effectiveness over a wide range of coolant mass flow rates reasonably well. The greater insight of flow field provided by the computations illustrates the thermal buffering effect when ingress occurs: For a given sealing flow rate, the effectiveness on the rotor was significantly higher than that on the stator due to the axial flow of hot gases from stator to rotor caused by pumping effects. The predicted effectiveness on the rotor was compared with a theoretical model for the thermal buffering effect showing good agreement. When the axial-seal clearance arrangement is considered, the agreement between CFD and experiments worsens; the variation of sealing effectiveness with coolant flow rate calculated by means of the simulations displays a distinct kink. It was found that the “kink phenomenon” can be ascribed to an overestimation of the egress spoiling effects due to turbulence modeling limitations. Despite some weaknesses in the numerical predictions, the paper shows that CFD can be used to characterize the sealing performance of axial- and radial-clearance turbine rim seals.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):032603-032603-7. doi:10.1115/1.4034541.

This paper summarizes the main results sorted out from a design of experiment (DoE) based on a validated computational fluid dynamics (CFD). Several tip recessed geometries applied to an unshrouded impeller were considered in conjunction with two tip clearance levels. The computations show that recessed tip geometries have positive effects when considering high-flow coefficient values, while in part-load conditions the gain is reduced. Starting from the results obtained when studying tip cavities, a single rim tip squealer geometry was then analyzed: the proposed geometry leads to performance improvements for all the tested conditions considered in this work.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(3):032604-032604-8. doi:10.1115/1.4034543.

The present paper analyzes the effect of passive flow control (PFC) with respect to the retrofitting on small horizontal axis wind turbines (sHAWTs). We conducted extensive wind tunnel studies on a high performance low Reynolds airfoil using different PFC elements, i.e., vortex generators (VGs) and Gurney flaps (GF). qblade, an open source blade element momentum (BEM) code, is used to study the retrofitting potential of a simulated small wind turbine. The turbine design is presented and discussed. The simulations include the data and polars gained from the experiments and give further insight into the effects of PFC on sHAWTs. Therefore, several different blades were simulated using several variations of VG positions. This paper discusses their influence on the turbine performance. The authors especially focus on the startup performance as well as achieving increased power output at lower wind speeds. The vortex generators reduce the risk of laminar separation and enhance the lift in some configurations by more than 40% at low Reynolds numbers.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2016;139(3):032801-032801-11. doi:10.1115/1.4034445.

Reactivity controlled compression ignition (RCCI) combustion in a light-duty multicylinder engine (MCE) over transient operating conditions using fast response exhaust unburned hydrocarbon (UHC1), nitric oxide (NO), and particulate matter (PM) measurement instruments was investigated. RCCI has demonstrated improvements in efficiency along with low NOx and PM emissions by utilizing in-cylinder fuel blending, generally using two fuels with different reactivity in order to optimize stratification. In the present work, a “single-fuel” approach for RCCI combustion using port-injected gasoline and direct-injected gasoline mixed with a small amount of the cetane improver 2-ethylhexyl nitrate (EHN) was studied with custom designed, compression ratio (CR) of 13.75:1, pistons under transient conditions. The EHN volume percentage in the mixture for the direct-injected fuel was set at 3%. In an experimental investigation, comparisons were made to transient RCCI combustion operation with gasoline/diesel. The experiments were performed over a step load change from 1 to 4 bar brake mean effective pressure (BMEP) at constant 1500 rev/min on a General Motors (GM) Z19DTH 1.9-L diesel engine. The transients were conducted by changing the accelerator pedal command to provide a desired torque output with a DRIVVEN engine control unit (ECU) that replaced the original Bosch ECU. All relevant engine parameters are adjusted accordingly, based on 2D-tables. Previous to the transient engine operation, four steady-state points were used to obtain performance and emission values. Engine calibration at these four points, as well as the interpolation of the intermediate points, allowed for smooth operation during the instantaneous step changes. Differences between the steady-state and transient results indicate the complexity of transient operation and show the need for additional controls to minimize undesirable effects. The steady-state points were calibrated by modifying the fuel injection strategy (actual start of injection (aSOI) timing, port-fuel injection (PFI) fraction, etc.), exhaust gas recirculation (EGR), and rail pressure in order to obtain predefined values for the crank-angle at 50% of total heat release (CA50). Furthermore, emission targets (HC1 < 1500 ppmC3, NO < 10 ppm, filter smoke number (FSN)<0.1 with a maximum pressure rise rate (MPRR) < 10 bar/deg) and noise level targets (<95 dB) for RCCI combustion were maintained during the calibration and mapping. The tests were performed with a closed-loop (CL) calibration by using a next-cycle (NC) controller to adjust the PFI ratio of each cycle in order to reach the steady-state CA50 values in the table. The results show that single-fuel RCCI operation can be achieved, but requires significant alteration of the operating conditions, and NOx emissions were significantly elevated for gasoline/gasoline–EHN operation. While combustion phasing could not be matched, UHC1 emissions were at a similar level as for gasoline/diesel combustion. It is expected that the implementation of different injection strategies and boosted operation, combined with use of higher compression ratio pistons in order to compensate for the lower reactivity direct injection (DI) fuel, could raise the potential for improved performance.

Commentary by Dr. Valentin Fuster

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