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### Research Papers

J. Eng. Gas Turbines Power. 2018;141(3):031001-031001-8. doi:10.1115/1.4041516.

Homogeneous charge compression ignition (HCCI) is a feasible combustion mode meeting future stringent emissions regulations, and has high efficiency and low NOX and particle emissions. As the narrow working condition range is the main challenge limiting the industrialization of HCCI, combustion mode switching between SI and HCCI is necessary when employing HCCI in mass production engines. Based on a modified production gasoline direct injection (GDI) engine equipped with dual UniValve system (a fully continuously variable valvetrain system), SI/HCCI mode switching under low load condition is investigated. According to the results, combustion mode switching from SI to HCCI is more complicated than from HCCI to SI. As HCCI requires strict boundary conditions for reliable and repeatable fuel auto-ignition, abnormal combustion easily appears in transition cycle, especially when combustion switches from SI to HCCI. Timing control strategies can optimize the combustion of transition cycles. With the optimization of timing control, the mode switching from SI to HCCI can be completed with only two transition cycles of late combustion, and abnormal combustion can be avoided during the mode switching from HCCI to SI. Under the low load condition, the indicated efficiency reaches 39% and specific NOX emissions drop down to around 1 mg/L/s when the combustion mode is switched to HCCI mode. Compared to SI mode, the indicated efficiency is increased by 10% and the specific NOX emissions are reduced by around 85%.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031002-031002-9. doi:10.1115/1.4041521.

Requirements for the start-up operations of gas turbine combined cycle (GTCC) power plants have become more diverse and now include such items as reduced start-up time, life consumption, and fuel gas consumption. In this paper, an optimization method is developed to solve these multi-objective problems. The method obtains optimized start-up curves by iterating the search for the optimal combination of the start-up parameter values and the evaluation of multiple objective functions. The start-up curves generated by this method were found to converge near the Pareto-front representing the best trade-off between the fuel gas consumption of the gas turbine (GT) and thermal stress in the steam turbine (ST) rotor which are defined as the objective functions. To demonstrate the effectiveness of the developed method, field tests were performed in a commercial power plant. As a result, the fuel gas consumption of HOT start-up was reduced by 22.8% compared with the past operation data. From this result, the developed method was shown to be capable of optimizing the start-up process for GTCC power plants.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031003-031003-11. doi:10.1115/1.4041275.

Temperature and composition spots in a turbulent flow are detected and time-resolved using laser-induced thermal grating spectroscopy (LITGS). A 355 nm wavelength particle image velocimetry laser is operated at 0.5–1 kHz to generate the thermal grating using biacetyl as an absorber in trace amounts. In an open laminar jet, a feasibility study shows that small (≃ 3%) fluctuations in the mean flow properties are well captured with LITGS. However, corrections of the mean flow properties by the presence of the trace biacetyl are necessary to properly capture the fluctuations. The actual density and temperature variation in the flow are determined using a calibration procedure validated using a laminar jet flow. Finally, traveling entropy and composition spots are directly measured at different locations along a quartz tube, obtaining good agreement with expected values. This study demonstrates that LITGS can be used as a technique to obtain instantaneous, unsteady temperature and density variations in a combustion chamber, requiring only limited optical access.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031004-031004-7. doi:10.1115/1.4041309.

Several factors are being investigated that affect the performance of thermal barrier coatings (TBC) for use in land-based gas turbines where coatings are mainly thermally sprayed. This study examined high velocity oxygen fuel (HVOF), air plasma-sprayed (APS), and vacuum plasma-sprayed (VPS) MCrAlYHfSi bond coatings with APS YSZ top coatings at 900–1100 °C. For superalloy 247 substrates and VPS coatings tested in 1 h cycles at 1100 °C, removing 0.6 wt %Si had no effect on average lifetime in 1 h cycles at 1100 °C, but adding 0.3%Ti had a negative effect. Rod specimens were coated with APS, HVOF, and HVOF with an outer APS layer bond coating and tested in 100 h cycles in air + 10%H2O at 1100 °C. With an HVOF bond coating, initial results indicate that 12.5 mm diameter rod specimens have much shorter 100 h cycle lifetimes than disk specimens. Much longer lifetimes were obtained when the bond coating had an inner HVOF layer and outer APS layer.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031005-031005-9. doi:10.1115/1.4041149.

When modeling a droplet impingement, it is reasonable to assume a surface is flat when the radius of curvature of the surface is significantly larger than the droplet radius. In other contexts where water droplet erosion (WDE) has been investigated, the typical droplet size has either been sufficiently small, or the radius of curvature of the surface sufficiently large, that it has been sensible to make this assumption. The equations describing the kinematics of an impinging water droplet on a flat surface were reformulated for a curved surface. The results suggest the relatively similar radii of curvature, of the leading-edge of a fan blade and the impinging water droplet, will significantly affect the application of the initial high-pressures, along with the onset of lateral outflow jetting. Jetting is predicted to commence substantially sooner and not in unison along the contact periphery, leading to an asymmetric flow stage. This is likely to have significant implications for the WDE that occurs, and thus, the engineering approaches to minimize the WDE of fan blades.

Topics: Drops , Modeling , Blades , Waves
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031006-031006-10. doi:10.1115/1.4041279.

The air handling system for large diesel/gas engines such as those used on locomotive, marine, and power generation applications require turbochargers with a high reliability and with turbomachinery capable to adjust to different operating conditions and transient requirements. The usage of variable geometry turbocharging (VGT) provides flexibility to the air handling system but adds complexity, cost and reduces the reliability of the turbocharger in exchange for improved engine performance and transient response. For this reason, it was desirable to explore designs that could provide the variability required by the air handling system, without the efficiency penalty of a conventional waste gate and with as little added complexity as possible. The current work describes a new low-cost variable geometry turbine design to address these requirements. The new tandem nozzle concept proposed is applicable to both axial and radial turbines and has been designed using conventional one-dimensional models and two- three-dimensional computational fluid dynamics (CFD) methods. The concept has furthermore been validated experimentally on two different test rigs. In order to avoid the long lead times of procuring castings, the nozzle for the axial turbine was manufactured using new additive manufacturing techniques. Both the axial turbine and the radial turbine designs showed that the concept is capable to achieve a mass flow variability of more than 15% and provide a robust and cost-effective alternative to conventional VGT designs by significantly reducing the number of moveable parts.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031007-031007-6. doi:10.1115/1.4041515.

In the present paper, an approach for scaling the aerodynamics of advanced seals is presented. Modern advanced seals, such as a self-adaptive gas lubricated face seal, comprise elements that are commonly used in turbomachinery sealing. These are labyrinth seals and mechanical face seals. Parameters influencing the aerodynamical and mechanical behavior of each seals type are known. However, a combined methodology to scale the aerodynamics of the self-adaptive seal which consists of more than one element has not yet been published. The proposed methodology is applied to a model self-adaptive seal, and numerical simulations are performed to prove the validity of the approach. The new methodology ensures the transferability of experimental results at lab scale to engine conditions. Since the new approach allows scaling of self-adaptive seal tests, a new unique test rig will be designed accordingly.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031008-031008-7. doi:10.1115/1.4041244.

This paper presents the design and development of a noncontacting dry-gas mechanical seal for high performance automotive turbocharger applications. Turbochargers are increasingly being incorporated into high performance automobile engines to improve fuel efficiency, enhance energy recovery, and increase horsepower as compared with similar sized naturally aspirated engines. Minimizing the wear rate of tribological surfaces in the turbomachinery is critical to maximizing the reliability and durability of the turbocharger. A dry-gas seal for turbochargers and related technologies with 2–4 cm shafts has been developed. The seal provides a complete barrier between the bearing oil and compressor flow path and is capable of reverse pressure and high speed. The seal performance was evaluated for speeds between 60,000 and 80,000 rpm, pressure differentials between −0.8 (reverse pressure) to 6 bar, and temperatures between 20 and 200 °C. Structural and thermal response of the seal components to the operating conditions are analyzed using finite element methods and the tribological behavior of the seal rings are analyzed using computational fluid dynamics. The design is experimentally validated in a seal test stand. This novel approach reduces turbocharger blowby and shows no measurable wear when compared with piston ring seals.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031009-031009-9. doi:10.1115/1.4041138.

The detailed characterization of the thermal boundary layer under periodic fluctuations is vital to improve the performance of cooled turbine airfoils, as well as to assess noise thermal and structural fatigue. In the present contribution, we performed detailed unsteady Reynolds-averaged Navier–Stokes (URANS) simulations to investigate wall heat flux response to periodic flow velocity fluctuations over a flat plate. We also investigated the boundary layer response to sudden flow acceleration including periodic flow perturbations, caused by inlet total pressure variations. During a flow acceleration phase, the boundary layer is first stretched, resulting in an increase of the wall shear stress. Later on, due to the viscous diffusion, the low momentum flow adjusts to the new free stream conditions. The behavior of the boundary layer at low frequency is similar to the response to an individual deceleration followed by one acceleration. However, at higher frequencies, the mean flow topology is completely altered. One would expect that higher acceleration rates would cause a further stretching of the boundary layer that should cause even greater wall shear stresses and heat fluxes. However, we observed the opposite; the amplitude of the skin friction coefficient is abated, while the peak level is a full order of magnitude smaller than at low frequency.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031010-031010-9. doi:10.1115/1.4041027.

Sequential combustion constitutes a major technological step-change for gas turbines applications. This design provides higher operational flexibility, lower emissions, and higher efficiency compared to today's conventional architectures. Like any constant pressure combustion system, sequential combustors can undergo thermoacoustic instabilities. These instabilities potentially lead to high-amplitude acoustic limit cycles, which shorten the engine components' lifetime, and therefore, reduce their reliability and availability. In the case of a sequential system, the two flames are mutually coupled via acoustic and entropy waves. This additional interstages interaction markedly complicates the already challenging problem of thermoacoustic instabilities. As a result, new and unexplored system dynamics are possible. In this work, experimental data from our generic sequential combustor are presented. The system exhibits many different distinctive dynamics, as a function of the operation parameters and of the combustor arrangement. This paper investigates a particular bifurcation, where two thermoacoustic modes synchronize their self-sustained oscillations over a range of operating conditions. A low-order model of this thermoacoustic bifurcation is proposed. This consists of two coupled stochastically driven nonlinear oscillators and is able to reproduce the peculiar dynamics associated with this synchronization phenomenon. The model aids in understanding what the physical mechanisms that play a key role in the unsteady combustor physics are. In particular, it highlights the role of entropy waves, which are a significant driver of thermoacoustic instabilities in this sequential setup. This research helps to lay the foundations for understanding the thermoacoustic instabilities in sequential combustion systems.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031011-031011-10. doi:10.1115/1.4041242.

Many laboratory-scale combustors are equipped with viewing windows to allow for characterization of the reactive flow. Additionally, pressure housing is used in this configuration to study confined pressurized flames. Since the flame characteristics are influenced by heat losses, the prediction of wall temperature fields becomes increasingly necessary to account for conjugate heat transfer (CHT) in simulations of reactive flows. For configurations similar to this one, the pressure housing makes the use of such computations difficult in the whole system. It is, therefore, more appropriate to model the external heat transfer beyond the first set of quartz windows. The present study deals with the derivation of such a model, which accounts for convective heat transfer from quartz windows external face cooling system, free convection on the quartz windows 2, quartz windows radiative properties, radiative transfer inside the pressure housing, and heat conduction through the quartz window. The presence of semi-transparent viewing windows demands additional care in describing its effects in combustor heat transfers. Because this presence is not an issue in industrial-scale combustors with opaque enclosures, it remains hitherto unaddressed in laboratory-scale combustors. After validating the model for the selected setup, the sensitivity of several modeling choices is computed. This enables a simpler expression of the external heat transfer model that can be easily implemented in coupled simulations.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031012-031012-7. doi:10.1115/1.4041145.

The life-limiting behavior of an N720/alumina oxide/oxide ceramic matrix composite (CMC) was assessed in tension in air at 1200 °C for unimpacted and impacted specimens. CMC targets were subjected to ballistic impact at ambient temperature with an impact velocity of 250 m/s under a full support configuration. Subsequent postimpact ultimate tensile strength was determined as a function of test rate in order to determine the susceptibility to delayed failure or slow crack growth (SCG). Unimpacted and impacted specimens exhibited a significant dependency of ultimate tensile strength on test rate such that the ultimate tensile strength decreased with decreasing test rate. Damage was characterized using X-ray computed tomography (CT) and scanning electron microscopy (SEM). A phenomenological life prediction model was developed in order to predict life from one loading condition (constant stress-rate loading) to another (constant stress loading). The model was verified in part via a theoretical preloading analysis.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031013-031013-10. doi:10.1115/1.4041385.

Ceramic matrix composite (CMC) have higher temperature durability and lower density property compared to nickel-based super-alloys which so far have been widely applied to hot section components of aero-engines/gas turbines. One of promising CMC systems, SiC–SiC CMC is able to sustain its mechanical property at higher temperature, though it inherently needs environmental barrier coating (EBC) to avoid oxidation. There are several requirements for EBC. One of such critical requirements is its resistance to particle erosion, whereas this subject has not been well investigated in the past. The present work presents the results of a combined experimental and numerical research to evaluate the erosion characteristics of CMC + EBC material developed by IHI. First, experiments were carried out in an erosion test facility using 50 μm diameter silica as erosion media under typical engine conditions with velocity of 225 m/s, temperature of 1311 K, and impingement angles of 30, 60, and 80 deg. The data displayed brittle erosion mode in that the erosion rate increased with impact angles. Also, it was verified that a typical erosion model, Neilson–Gilchrist model, can reproduce the experimental behavior fairly well if its model constants were properly determined. The numerical method solving particle-laden flow was then applied with the tuned erosion model to compute three dimensional flow field, particle trajectories, and erosion profile around a generic turbine airfoil to assess the erosion characteristics of the proposed CMC + EBC material when applied to airfoil. The trajectories indicated that the particles primarily impacted the airfoil leading edge and the pressure surface. Surface erosion patterns were predicted based on the calculated trajectories and the experimentally based erosion characteristics.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031014-031014-7. doi:10.1115/1.4041205.

This numerical study deals with a premixed ethylene–air jet at 300 K injected into a hot vitiated crossflow at 1500 K and atmospheric pressure. The reactive jet in crossflow (RJICF) was simulated with compressible 3D large eddy simulations (LES) with an analytically reduced chemistry (ARC) mechanism and the dynamic thickened flame (DTF) model. ARC enables simulations of mixed combustion modes, such as autoignition and flame propagation, that are both present in this RJICF. 0D and 1D simulations provide a comparison with excellent agreement between ARC and detailed chemistry in terms of autoignition time and laminar flame speed. The effect of the DTF model on autoignition was investigated for varying species compositions and mesh sizes. Comparisons between LES and experiments are in good agreement for average velocity distributions and jet trajectories; LES remarkably capture experimentally observed flame dynamics. An analysis of the simulated RJICF shows that the leeward propagating flame has a stable flame root close to the jet exit. The lifted windward flame, on the contrary, is anchored in an intermittent fashion due to autoignition flame stabilization. The windward flame base convects downstream and is “brought back” by autoignition alternately. These autoignition events occur close to a thin layer that is associated with radical build-up and that stretches down to the jet exit.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031015-031015-11. doi:10.1115/1.4041451.

Experimentally measured results are presented for different experimental conditions for a test plate with double wall cooling, composed of full-coverage effusion-cooling on the hot side of the plate, and cross-flow cooling on the cold side of the plate. The results presented are different from those from past investigations, because of the addition of a significant mainstream pressure gradient. Main stream flow is provided along a passage with a contraction ratio of 4, given by the ratio upstream flow area, to downstream flow area. With this arrangement, local blowing ratio decreases significantly with streamwise development along the test section, for every value of initial blowing ratio considered, where this initial value is determined at the most upstream row of effusion holes. Experimental data are given for a sparse effusion hole array. The experimental results are provided for mainstream Reynolds numbers of 92,400–96,600, and from 128,400 to 135,000, and initial blowing ratios of 3.3–3.6, 4.4, 5.2, 6.1–6.3, and 7.3–7.4. Results illustrate the effects of blowing ratio for the hot side and the cold side of the effusion plate. Of particular interest are values of line-averaged film cooling effectiveness and line-averaged heat transfer coefficient, which are generally different for contraction ratio of 4, compared to a contraction ratio of 1, because of different amounts and concentrations of effusion coolant near the test surface. In regard to cold-side measurements on the crossflow side of the effusion plate, line-averaged Nusselt numbers for contraction ratio 4 are often less than values for contraction ratio 1, when compared at the same main flow Reynolds number, initial blowing ratio, and streamwise location.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031016-031016-8. doi:10.1115/1.4040691.

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

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031017-031017-10. doi:10.1115/1.4041255.

Industrial compressors suffer from strong aerodynamic instability that arises when low ranges of flow rate are achieved; this instability is called surge. This phenomenon creates strong vibrations and forces acting on the compressor and system components due to the fact that it produces variable time-averaged mass flow and pressure. Therefore, surge is dangerous not only for aerodynamic structures but also for mechanical parts. Surge is usually prevented in industrial plants by means of anti-surge systems, which act as soon as surge occurs; however, some rapid transients or system upsets can lead the compressor to surge anyway. Despite the fact that surge can be classified as mild, classic, or deep, depending on the amplitudes and frequency of the fluctuations, operators are used to simply referring to surge, without making a distinction between the three main classes. This is one of the reasons why, when surge occurs in industrial plants, it is a common practice to stop the machine to perform inspections and check if any damage occurred. Obviously, this implies maintenance costs and time, during which the machine does not operate. On the other hand, not all surge events are dangerous in terms of damage, and they can be tolerated by the mechanical structures of the compressor; thus, in these cases, inspections would not be required. Unfortunately, a method for establishing the potential damage of a surge event is not available in literature. In order to fill this gap, this paper proposes a final formulation of a surge severity index, which was only preliminarily formulated by the authors in a previous work. The preliminary form of this coefficient demonstrated some limitations, which are overcome in this paper. The surge severity index derives from an energy-force based analysis. The coefficient demonstration is carried out in this paper by means of (i) the application of the Buckingham's Pi-theorem, and (ii) a careful analysis of the causative and restorative factors of surge. Finally, some simple practical evaluations are shown by means of a sensitivity analysis, using simulation results of an existing model, to effectively further highlight the consistency of this coefficient for industry.

Topics: Surges , Damage , Compressors
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031018-031018-7. doi:10.1115/1.4041252.

Some cracks were detected on the fir-tree root of turbine blade in an in-service aero-engine, and the aluminized coating was considered to be the main cause of these cracks. To study the effect of aluminized coating on fatigue life of turbine blade, the combined low and high cycle fatigue (CCF) tests are carried out at elevated temperature on both aluminized and untreated turbine blades. Probability analysis of test data is conducted and the result indicates that the median life is decreased by 62.2% due to the effect of the aluminized coating. Further study on the mechanism of crack initiation and propagation has been conducted based on fractography and cross section morphology analysis by using scanning electron microscope (SEM), and the results indicate: (1) The aluminized coating consists of two layers, of which the inner layer is considered to contain the σ phase and it reduces the resistance to fatigue of blade. (2) Many cavities are found in the inner layer of aluminized coating, which lead to the initiation of cracks and result in the reduction of crack initiation life. (3) The marker band widths of aluminized and untreated blade are very close, which indicated the aluminized coating may have no effect on the crack propagation life of the blade.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031019-031019-8. doi:10.1115/1.4041453.

Forced response is the main reason for high cycle fatigue in turbomachinery. Not all resonance points in the operating range can be avoided especially for low order excitation. For highly flexible carbon fiber reinforced polymer (CFRP) fans, an accurate calculation of vibration amplitudes is required. Forced response analyses were performed for blade row interaction and boundary layer ingestion (BLI). The resonance points considered were identified in the Campbell diagram. Forced response amplitudes were calculated using a modal approach and the results are compared to the widely used energy method. For the unsteady simulations, a time-based linearization of the unsteady Reynolds average Navier–Stokes equations were applied. If only the resonant mode was considered, the forced response amplitude from the modal approach was confirmed with the energy method. Thereby, forced response due to BLI showed higher vibration amplitudes than for blade row interaction. The impact of modes which are not in resonant to the total deformation were investigated by using the modal approach, which so far only considers one excitation order. A doubling of vibrational amplitude was shown in the case of blade row interaction for higher rotational speeds. The first and third modeshapes as well as modes with similar natural frequencies were identified as critical cases. The behavior in the vicinity of resonance shows high vibration amplitudes over a larger frequency range. This is also valid for high modes with many nodal diameters, which have a greater risk of critical strain.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031022-031022-10. doi:10.1115/1.4041518.

Impact of the diverging cup angle of a swirling injector on the flow pattern and stabilization of technically premixed flames is investigated both theoretically and experimentally with the help of OH* chemiluminescence, OH laser-induced fluorescence and particle image velocimetry (PIV) measurements. Recirculation enhancement with a lower position of the internal recirculation zone (IRZ) and a flame leading edge protruding further upstream in the swirled flow are observed as the injector nozzle cup angle is increased. A theoretical analysis is carried out to examine whether this could be explained by changes of the swirl level as the diffuser cup angle is varied. It is shown that pressure effects need in this case to be taken into account in the swirl number definition and expressions for changes of the swirl level through a diffuser are derived. It is demonstrated that changes of the swirl level including or not the pressure contribution to the axial momentum flux are not at the origin of the changes observed of the flow and flame patterns in the experiments. The swirl number without the pressure term, designated as pressure-less swirl, is then determined experimentally with laser Doppler velocimetry (LDV) measurements at the injector outlet for a set of diffusers with increasing quarl angles under nonreacting conditions and the values found corroborate the predictions. It is finally shown that the decline of axial velocity and the rise of adverse axial pressure gradient, both due to the cross section area change through the diffuser cup, are the dominant effects that control the leading edge position of the IRZ of the swirled flow. This is used to develop a model for the displacement of the recirculation bubble as the quarl angle varies that shows very good agreement with experiments.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031023-031023-11. doi:10.1115/1.4040850.

A method for parametric analysis of the stability loss boundary has been developed for periodic regimes of nonlinear forced vibrations for a first time. The method allows parametric frequency-domain calculations of the stability loss together with the vibration amplitudes and design parameter values corresponding to the stability boundaries. The tracing algorithm is applied to obtain the trajectories of stability loss points as functions of design parameters. The parametric stability loss is formulated for cases when (i) the design parameters characterize the properties of nonlinear contact interfaces (e.g., gap, contact stiffness, and friction coefficient); (ii) the design parameters describe linear components of the analyzed structure (e.g., parameters of geometric shape, material, natural frequencies, and modal damping); and (iii) these parameters describe the excitation loads (e.g., their level, distribution or frequency). An approach allowing the multiparametric analysis of stability boundaries is proposed. The method uses the multiharmonic representation of the periodic forced response and aimed at the analysis of realistic gas-turbine structures comprising thousands and millions degrees-of-freedom (DOF). The method can be used for the effective search of isolated branches of the nonlinear solutions and examples of detection and search of the isolated branches are given: for relatively small and for large-scale finite element (FE) models. The efficiency of the method for calculation of the stability boundaries and for the search of isolated branches is demonstrated on simple systems and on a large-scale model of a turbine blade.

Topics: Stability
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031024-031024-10. doi:10.1115/1.4041136.

Sensitivity to stretch and differential diffusion of chemical species are known to influence premixed flame propagation, even in the turbulent environment where mass diffusion can be greatly enhanced. In this context, it is convenient to characterize flames by their Lewis number (Le), a ratio of thermal-to-mass diffusion. The work reported in this paper describes a study of flame stabilization characteristics when Le is varied. The test data are comprised of $Le≪1$ (hydrogen), $Le≈1$ (methane), and $Le>1$ (propane) flames stabilized at various turbulence levels. The experiments were carried out in a hot exhaust opposed-flow turbulent flame rig (HOTFR), which consists of two axially opposed, symmetric jets. The stagnation plane between the two jets allows the aerodynamic stabilization of a flame and clearly identifies fuel influences on turbulent flames. Furthermore, high-speed particle image velocimetry (PIV), using oil droplet seeding, allowed simultaneous recordings of velocity (mean and rms) and flame surface position. These experiments, along with data processing tools developed through this study, illustrated that in the mixtures with $Le≪1$, turbulent flame speed increases considerably compared to the laminar flame speed due to differential diffusion effects, where higher burning rates compensate for the steepening average velocity gradient and keeps these flames almost stationary as bulk flow velocity increases. These experiments are suitable for validating the ability of turbulent combustion models to predict lifted, aerodynamically stabilized flames. In the final part of this paper, we model the three fuels at two turbulence intensities using the flamelet generated manifolds (FGM) model in a Reynolds-averaged Navier–Stokes (RANS) context. Computations reveal that the qualitative flame stabilization trends reproduce the effects of turbulence intensity; however, more accurate predictions are required to capture the influences of fuel variations and differential diffusion.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031025-031025-9. doi:10.1115/1.4040769.

It is well known that the last stage of a turbine and the subsequent diffuser should be viewed at and designed as a coupled system rather than as single standalone components. The turbine outlet flow imposes the inlet conditions to the diffuser, whereas the recovered dynamic pressure in the diffuser directly controls the turbine back pressure. With changing operating point, the turbine outflow can vary significantly. This results consequently in large variations of the diffuser performance. A major role in the coupled system of turbine and diffuser can be attributed to the tip leakage flow. While it is desirable to minimize the tip leakage with regard to the turbine, a higher leakage mass flow can often be beneficial for the diffuser performance. As there is currently a trend toward aggressive and hence shorter diffusers which are particularly prone to separation, the question arises where the optimum for this tradeoff problem lies. To investigate the performance in the coupled turbine/diffuser system, a generic last stage with shrouded rotor and axial exhaust diffuser has been designed. The components are representative for heavy duty stationary gas turbine applications. Results are presented for three different operating points representing part-load (PL), design-load (DL), and over-load (OL) condition. Three different seal gap widths are taken into account to control the leakage flow. The results indicate that an operating point-dependent optimum gap width can be found for the coupled system efficiency, whereas the maximum turbine performance is always achieved with a minimum gap width.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031026-031026-8. doi:10.1115/1.4041114.

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

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031027-031027-12. doi:10.1115/1.4041147.

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

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031028-031028-6. doi:10.1115/1.4040712.

The combustion of conventional fuels (diesel and Jet A-1) with 10–20% vol oxygenated biofuels (ethanol, 1-butanol, methyl octanoate, rapeseed oil methyl ester (RME), diethyl carbonate, tri(propylene glycol)methyl ether, i.e., CH3(OC3H6)3OH, and 2,5-dimethylfuran (2,5-DMF)) and a synthetic paraffinic kerosene (SPK) was studied. The experiments were performed using an atmospheric pressure laboratory premixed flame and a four-cylinder four-stroke diesel engine operating at 1500 rpm. Soot samples from kerosene blends were collected above a premixed flame for analysis. Polyaromatic hydrocarbons (PAHs) were extracted from the soot samples. After fractioning, they were analyzed by high-pressure liquid chromatography (HPLC) with UV and fluorescence detectors. C1 to C8 carbonyl compounds (CBCs) were collected at the diesel engine exhaust on 2,4-dinitrophenylhydrazine coated cartridges (DNPH) and analyzed by HPLC with UV detection. The data indicated that blending conventional fuels with biofuels has a significant impact on the emission of both CBCs and PAHs adsorbed on soot. The global concentration of 18 PAHs (1-methyl-naphthalene, 2-methyl-naphthalene, and the 16 U.S. priority EPA PAHs) on soot was considerably lowered using oxygenated fuels, except 2,5-DMF. Conversely, the total carbonyl emission increased by oxygenated biofuels blending. Among them, ethanol and 1-butanol were found to increase considerably the emissions of CBCs.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031029-031029-9. doi:10.1115/1.4040742.

Secondary flows through annular seals in pumps must be minimized to improve their mechanical efficiency. Annular seals, in particular balance piston seals, also produce rotordynamic force coefficients, which easily control the placement of rotor critical speeds and determine system stability. A uniform clearance annular seal produces a direct (centering) static stiffness as a result of the sudden entrance pressure drop at its inlet plane when the fluid flow accelerates from an upstream (stagnant) flow region into a narrow film land. This so-called Lomakin effect equates the entrance pressure drop to the dynamic flow head through an empirical entrance pressure loss coefficient. Most seal designs regard the inlet as a sharp edge or square corner. In actuality, a customary manufacturing process could produce a rounded corner at the seal inlet. Furthermore, after a long period of operation, a sharp corner may wear out into a round section. Notice that to this date, bulk-flow model (BFM) analyses rely on a hitherto unknown entrance pressure coefficient to deliver accurate predictions for seal force coefficients. This paper establishes the ground to quantify the influence of an inlet round corner on the performance of a water lubricated seal reproducing a configuration tested by Marquette et al. (1997). The smooth surface seal has clearance Cr = 0.11 mm, length L = 35 mm, and diameter D = 76 mm (L/D = 0.46). The test case considers design operation at 10.2 krpm and 6.9 MPa pressure drop. Computational fluid dynamics (CFD) simulations apply to a seal with either a sharp edge or an inlet section with curvature rc varying from ¼Cr to 5Cr. Note the largest radius (rc) is just 1.6% of the overall seal length L. Going from a sharp edge inlet plane to one with a small curvature rc = ¼Cr produces a ∼20% decrease on the inlet pressure loss coefficient (ξ). A further reduction occurs with a larger circular corner; ξ drops from 0.43 to 0.17. That is, the entrance pressure loss will be lesser in a seal with a curved inlet. This can occur easily if the inlet edge wears due to solid particles eroding the seal inlet section. Further CFD simulations show that operating conditions in rotor speed and pressure drop do not affect the inlet loss coefficient, while the inlet circumferential swirl velocity does. In addition, further CFD results for a shorter (half) length seal produce a very similar entrance loss coefficient, whereas an enlarged (double) clearance seal leads to an increase in the entrance pressure loss parameter as the inlet section becomes less round. CFD predictions for most rotordynamic coefficients are within 10% relative to published test data, except for the direct damping coefficient C. For the seal with a rounded edge (rc = 5 Cr) at the inlet plane, both the direct stiffness K and direct damping C decrease about 10% compared against the coefficients for the seal with a sharp inlet edge. The other force coefficients, namely cross-coupled stiffness and added mass, are unaffected by the inlet edge geometry. The same result holds for seal leakage, as expected. A BFM incorporates the CFD derived entrance pressure loss coefficients and produces rotordynamic coefficients for the same operating conditions. The CFD and BFM predictions are in good agreement, though there is still ∼10% discrepancy for the direct stiffnesses delivered by the two methods. In the end, the analysis of the CFD results quantifies the pressure loss coefficient as a function of the inlet geometry for ready use in engineering BFM tools.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031030-031030-9. doi:10.1115/1.4040844.

This work presents a very detailed techno-economic analysis of the technology, made up of two complementary models. A performance model implemented in Thermoflex environment is used to explore alternative integration layouts in order to enable the simultaneous operation on solar and fossil energy. Then, a detailed cost analysis calculates the capital and operation costs of the plant from the engineering, procurement and construction standpoints. These two models are then combined in annual simulations to obtain the final levelized cost of electricity (LCoE) from which a solid conclusion about the true potential of solar gas turbines can be ascertained. A sensitivity analysis with respect to the main boundary conditions is also provided. The results confirm that LCoE in the order of 14 c€/kWh can be obtained when running the plant during sun hours (daily operation), yielding almost 70% annual solar share and for a fuel cost of 8 €/MBTU. In a higher fuel cost scenario (12 €/MBTU), this cost rises to almost 17 c€/kWh whereas it decreases to 10.5 c€/kWh if fuel costs are 4 €/MBTU. The different sensitivity analyses performed highlight the very strong regional effect on LCoE, not only for direct normal irradiance (DNI) but also for the largely variable local labor costs.

Commentary by Dr. Valentin Fuster

### Research Papers: Research Papers

J. Eng. Gas Turbines Power. 2018;141(3):031020-031020-10. doi:10.1115/1.4041146.

The wind industry needs reliable and accurate airfoil polars to properly predict wind turbine performance, especially during the initial design phase. Medium- and low-fidelity simulations directly depend on the accuracy of the airfoil data and even more so if, e.g., dynamic effects are modeled. This becomes crucial if the blades of a turbine operate under stalled conditions for a significant part of the turbine's lifetime. In addition, the design process of vertical axis wind turbines needs data across the full range of angles of attack between 0 and 180 deg. Lift, drag, and surface pressure distributions of a NACA 0021 airfoil equipped with surface pressure taps were investigated based on time-resolved pressure measurements. The present study discusses full range static polars and several dynamic sinusoidal pitching configurations covering two Reynolds numbers Re = 140k and 180k, and different incidence ranges: near stall, poststall, and deep stall. Various bistable flow phenomena are discussed based on high frequency measurements revealing large lift-fluctuations in the post and deep stall regime that exceed the maximum lift of the static polars and are not captured by averaged measurements. Detailed surface pressure distributions are discussed to provide further insight into the flow conditions and pressure development during dynamic motion. The experimental data provided within the present paper are dedicated to the scientific community for calibration and reference purposes, which in the future may lead to higher accuracy in performance predictions during the design process of wind turbines.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):031021-031021-13. doi:10.1115/1.4041206.

This paper presents volumetric velocimetry (VV) measurements for a jet in crossflow that is representative of film cooling. VV employs particle tracking to nonintrusively extract all three components of velocity in a three-dimensional volume. This is its first use in a film-cooling context. The primary research objective was to develop this novel measurement technique for turbomachinery applications, while collecting a high-quality data set that can improve the understanding of the flow structure of the cooling jet. A new facility was designed and manufactured for this study with emphasis on optical access and controlled boundary conditions. For a range of momentum flux ratios from 0.65 to 6.5, the measurements clearly show the penetration of the cooling jet into the freestream, the formation of kidney-shaped vortices, and entrainment of main flow into the jet. The results are compared to published studies using different experimental techniques, with good agreement. Further quantitative analysis of the location of the kidney vortices demonstrates their lift off from the wall and increasing lateral separation with increasing momentum flux ratio. The lateral divergence correlates very well with the self-induced velocity created by the wall–vortex interaction. Circulation measurements quantify the initial roll up and decay of the kidney vortices and show that the point of maximum circulation moves downstream with increasing momentum flux ratio. The potential for nonintrusive VV measurements in turbomachinery flow has been clearly demonstrated.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Ceramics

J. Eng. Gas Turbines Power. 2018;141(3):031301-031301-10. doi:10.1115/1.4041271.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Coal, Biomass, and Alternative Fuels

J. Eng. Gas Turbines Power. 2018;141(3):031401-031401-9. doi:10.1115/1.4041312.

Increased public concerns and stricter regulatory frameworks promote the role of bioliquids (liquid fuel for energy purposes other than for transport, including electricity and heating and cooling, produced from biomass). This is a driving force for development and employment of micro-gas turbines (MGTs) due to their ability to combust bioliquids with less favorable properties in a decentralized manner. Gas turbines are characterized by relatively high combustion efficiency at relatively low concentrations of harmful emissions, relatively high effective efficiency and durability when utilizing a common portfolio of gas turbine approved fuels. It is thus desired to preserve these advantages of gas turbines also while burning bioliquids and further relying on their scalability that is crucial to efficient support of decentralized energy production at small scales. To support these objectives, MGT technology needs to allow for utilization of bioliquids with much wider spectrum of physical and chemical properties compared to common commercially available MGTs in a single MGT-based plant. In this view, the present study is providing the first thorough overview of challenges and solutions encountered by researchers across the wide area of bioliquids in MGTs.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2018;141(3):031501-031501-13. doi:10.1115/1.4040815.

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

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Microturbines and Small Turbomachinery

J. Eng. Gas Turbines Power. 2018;141(3):032301-032301-11. doi:10.1115/1.4041109.

Exhaust gases from an internal combustion engine (ICE) contain approximately 30% of the total energy released from combustion of the fuel. In order to improve fuel economy and reduce emissions, there are a number of technologies available to recover some of the otherwise wasted energy. The inverted Brayton cycle (IBC) is one such technology. The purpose of this study is to conduct a parametric experimental investigation of the IBC. The hot air from a turbocharger test facility is used. The system is sized to operate using the exhaust gases produced by a 2 l turbocharged engine at motorway cruise conditions. A number of parameters are investigated that impact the performance of the system such as turbine inlet temperature, system pressure drop, and compressor inlet temperature. The results confirm that the output power is strongly affected by the turbine inlet temperature and system pressure drop. The study also highlights the packaging and performance advantages of using an additively manufactured heat exchanger to reject the excess heat. Due to rotordynamic issues, the speed of the system was limited to 80,000 rpm rather than the target 120,000 rpm. However, the results show that the system can generate a specific work of up to 17 kJ/kg at 80,000 rpm. At full speed, it is estimated that the system can develop approximately 47 kJ/kg, which represents a thermal efficiency of approximately 5%.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Oil and Gas Applications

J. Eng. Gas Turbines Power. 2018;141(3):032401-032401-12. doi:10.1115/1.4041249.

Over recent decades, the variability and high costs of the traditional gas turbine fuels (e.g., natural gas) have pushed operators to consider low-grade fuels for running heavy-duty frames. Synfuels, obtained from coal, petroleum, or biomass gasification, could represent valid alternatives in this sense. Although these alternatives match the reduction of costs and, in the case of biomass sources, would potentially provide a CO2 emission benefit (reduction of the CO2 capture and sequestration costs), these low-grade fuels have a higher content of contaminants. Synfuels are filtered before the combustor stage, but the contaminants are not removed completely. This fact leads to a considerable amount of deposition on the nozzle vanes due to the high temperature value. In addition to this, the continuous demand for increasing gas turbine efficiency determines a higher combustor outlet temperature. Current advanced gas turbine engines operate at a turbine inlet temperature (TIT) of (1400–1500) °C, which is high enough to melt a high proportion of the contaminants introduced by low-grade fuels. Particle deposition can increase surface roughness, modify the airfoil shape, and clog the coolant passages. At the same time, land-based power units experience compressor fouling, due to the air contaminants able to pass through the filtration barriers. Hot sections and compressor fouling work together to determine performance degradation. This paper proposes an analysis of the contaminant deposition on hot gas turbine sections based on machine nameplate data. Hot section and compressor fouling are estimated using a fouling susceptibility criterion. The combination of gas turbine net power, efficiency, and TIT with different types of synfuel contaminants highlights how each gas turbine is subjected to particle deposition. The simulation of particle deposition on 100 gas turbines ranging from 1.2 MW to 420 MW was conducted following the fouling susceptibility criterion. Using a simplified particle deposition calculation based on TIT and contaminant viscosity estimation, the analysis shows how the correlation between type of contaminant and gas turbine performance plays a key role. The results allow the choice of the best heavy-duty frame as a function of the fuel. Low-efficiency frames (characterized by lower values of TIT) show the best compromise in order to reduce the effects of particle deposition in the presence of high-temperature melting contaminants. A high-efficiency frame is suitable when the contaminants are characterized by a low-melting point thanks to their lower fuel consumption.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2018;141(3):032501-032501-8. doi:10.1115/1.4041161.

A mandrel-free hot-spinning was developed as a near-net-shape titanium alloy plate forming technique. In this work, a Ti–6Al–4V alloy conical product with a wall angle of 34 deg and 170 mm height was formed from a large size Ti–6Al–4V plate (890–920 mm diameter × 30 mm thick). The product was characterized metallurgically and mechanical properties were measured, and the shape of formed products was investigated. It was found that the mandrel-free hot-spinning is able to form a large size titanium alloy plate. Material properties including tensile strength and microstructure of the formed products satisfied the material specifications. Fatigue stress of the formed product was higher than that of the typical Ti–6Al–4V material. For a further improvement, a forming method using preformed material was proposed and which was successfully conducted with two types of preforms and found to be effective in the forming process.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):032502-032502-10. doi:10.1115/1.4041129.

A novel test facility has been designed and setup for the investigation of the influence of stationary temperature, and thus thermally induced stress gradients with respect to the damage evolution of cooled gas turbine components. Thermally induced stress gradients differ from geometrically induced stress gradients. From the point of view of stress mechanics, they are independent from external loads. From the perspective of material mechanics, their impact on service life is influenced by locally different material properties and strength. However, the impact of thermally induced stress gradients on the cyclic life of high loaded, cooled components is not precisely known. In order to increase knowledge surrounding these mechanisms, a research project was launched. To achieve high temperature gradients and extended mechanical stress gradients, large heat fluxes are required. The authors developed a test bench with a unique radiant heating to achieve very high heat fluxes of $q˙$ ≥ 1.6 MW/m2 on cylindrical specimen. Special emphasis has been placed on homogenous temperature and loading conditions in order to achieve valid test results comparable to standard low-cycle or thermo-mechanical fatigue tests. Different test concepts of the literature were reviewed and the superior performance of the new test rig concept was demonstrated. The austenitic stainless steel 316 L was chosen as the model material for commissioning and validation of the test facility. The investigation of thermally induced stress gradients and, based on this analysis, low-cycle fatigue (LCF) tests with superimposed temperature gradients were conducted. Linear elastic finite element studies were performed to calculate the local stress–strain field and the service life of the test specimens. The test results show a considerable influence of the temperature gradient on the LCF life of the investigated material. Both the temperature variation over the specimen wall and thermally induced stresses (TIS) are stated to be the main drivers for the change in LCF life. The test results increase the understanding of fatigue damage mechanisms under local unsteady conditions and can serve as a basis for improved lifetime calculation methods.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):032503-032503-11. doi:10.1115/1.4041270.

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

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):032504-032504-16. doi:10.1115/1.4041313.

The modern compressor operation is challenged by the liquid presence in wet gas operating conditions. The liquid phase may affect the compressor stability by partially flooding the internal annular gas seals and inducing subsynchronous vibration (SSV). To improve the annular seal behavior and increase the rotor stability, high-precision results of leakage flow rates and rotordynamic force coefficients are needed for annular gas seals in wet gas conditions. In order to better understand the leakage and rotordynamic characteristics of the annular gas seal in wet gas conditions, a 3D transient CFD-based perturbation method was proposed for computations of leakage flow rates and rotordynamic force coefficients of annular gas seals with liquid phase in main gas phase, based on inhomogeneous Eulerian-Eulerian multiphase flow model, mesh deformation technique, and the multifrequency rotor whirling orbit model. Numerical results of frequency-dependent rotordynamic force coefficients and leakage flow rates were presented and compared for three types of noncontact annular gas seals, which include a smooth plain annular seal (SPAS), a labyrinth (LABY) seal, and a fully partitioned pocket damper seal (FPDS). These three seals were designed to have the identical rotor diameter, sealing clearance, and axial length. The accuracy and the availability of the present transient CFD numerical method were demonstrated with the experiment data of leakage flow rates and frequency-dependent rotordynamic force coefficients of the smooth plain seal with four inlet liquid volume fractions (LVFs) of 0%, 2%, 5%, and 8%. Steady and transient numerical simulations were conducted at inlet air pressure of 62.1 bar, pressure ratio of 0.5, rotational speed of 15,000 rpm, and inlet preswirl ratio of 0.3 for four inlet LVFs varying from 0% to 8% and 14 subsynchronous and synchronous whirling frequencies up to 280 Hz. The numerical results show that the inlet liquid phase has a significant influence on the leakage and rotordynamic coefficients for all three types of annular gas seals. The mixture leakage flow rate increases with the increasing inlet LVF, combining the decreasing gas-phase and linearly increasing liquid-phase leakage flow rates. The smooth plain seal leaks the most gas phase and liquid phase, followed by the pocket damper seal (PDS) and then the labyrinth seal. Increasing inlet LVF significantly decreases the direct stiffness and slightly increases the effective damping of the smooth plain seal. The labyrinth seal possesses evident negative direct stiffness and shows a noticeable decreasing effective damping with the increasing inlet LVF at the subsynchronous frequency range. Increasing inlet LVF obviously increases all the force coefficients of the pocket damper seal including the positive effective damping. From a rotordynamic viewpoint, the FPDS possesses a better liquid tolerant capability and so is a better sealing scheme for the balance piston seals and center seals of the centrifugal compressor in wet gas operating condition.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):032505-032505-13. doi:10.1115/1.4041280.

Accuracy when assessing mistuned forced response analyses is still a major concern. Since a fully coupled analysis is still very computational expensive, several simplifications and reduced-order models (ROMs) are carried out. The use of a reduction method, the assumptions and simplifications, generate different uncertainties that challenge the accuracy of the results. Experimental data are needed for validation and also to understand the propagation of these uncertainties. This paper shows a detailed mistuned forced response analysis of a compressor blisk. The blisk belongs to the Purdue Three-Stage (P3S) Compressor Research Facility. Two different stator–rotor–stator configurations of 38 and 44 upstream stator vanes are taken into consideration. Several loading conditions are analyzed at three different speed lines. A ROM known as subset nominal mode (SNM), has been used for all the analyses. This reduction takes as a basis a set of modes within a selected frequency spectrum. It can consider a complete family of modes to study the disk–blade modal interaction. A detailed comparison between the predicted and measured results has been performed, showing a good agreement for the high loading (HL) conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2018;141(3):032506-032506-12. doi:10.1115/1.4041314.

This effort develops a surrogate modeling approach for predicting the effects of manufacturing variations on performance and unsteady loading of a transonic turbine. Computational fluid dynamics (CFD) results from a set of 105 as-manufactured turbine blade geometries are used to train and validate the surrogate models. Blade geometry variation is characterized with point clouds gathered from a structured light, optical measurement system and as-measured CFD grids are generated through mesh morphing of the nominal design grid data. Principal component analysis (PCA) of the measured airfoil geometry variations is used to create a reduced basis of independent surrogate model parameters. It is shown that the surrogate model typically captures between 60% and 80% of the CFD predicted variance. Three new approaches are introduced to improve surrogate effectiveness. First, a zonal PCA approach is defined which investigates surrogate accuracy when limiting analysis to key regions of the airfoil. Second, a training point reduction strategy is proposed that is based on the kd tree nearest neighbor search algorithm and reduces the required training points up to 38% while only having a small impact on accuracy. Finally, an alternate reduction approach uses k-means clustering to effectively select training points and reduces the required training points up to 66% with a small impact on accuracy.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2018;141(3):032601-032601-18. doi:10.1115/1.4041282.

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

Commentary by Dr. Valentin Fuster