Research Papers: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2016;139(4):041201-041201-9. doi:10.1115/1.4034635.

Unsteady flow within the intake system of a hydrogen–air pulse detonation engine (PDE) has been analyzed using a quasi-one-dimensional (Q1D) computational fluid dynamic (CFD) code. The analysis provides insight into the unsteady nature of localized equivalence ratios and their effects on PDE performance. For this purpose, a code originally configured to model the PDE tube proper was modified to include a 6.1 m long intake with a single fuel injector located approximately 3.05 m upstream of the primary intake valve. The results show that constant fuel mass flow rate injection from the injector creates large local variations in equivalence ratio throughout the PDE within a cycle. The effect of fill fraction on the engine performance is better described with the presence of the inlet model. However, the effect of ignition delay is shown to be better predicted with a model without the inlet.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):041202-041202-10. doi:10.1115/1.4034356.

The effects of the aft rotor on the inter-rotor flow field of an open rotor propulsion rig (ORPR) were examined. A particle image velocimetry (PIV) dataset that was acquired phase locked to the front rotor position has been phase averaged based on the relative phase angle between the forward and aft rotors. The aft rotor phase was determined by feature tracking in raw PIV images through an image-processing algorithm. The effects of the aft rotor potential field on the inter-rotor flow were analyzed and shown to be in reasonably good agreement with computational fluid dynamics (CFD) simulations. The aft rotor position was shown to have a significant upstream effect, with implications for front rotor interaction noise. It was found that the aft rotor had no substantial effect on the position of the forward rotor tip vortex but did have a small effect on the circulation strength of the vortex when the rotors were highly loaded.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):041203-041203-12. doi:10.1115/1.4034600.

The main role of the intake is to provide a sufficient mass flow to the engine face and a sufficient flow homogeneity to the fan. Intake-fan interaction off design represents a critical issue in the design process because intake lines are set very early during the aircraft optimization. The offdesign operation of an aero-engine, strictly related to the intake flow field, can be mainly related to two different conditions. When the plane is in near ground position, vorticity can be ingested by the fan due to crosswind incidence. During the flight, distortions occur due to incidence. In these conditions, the windward lip is subjected to high acceleration followed by strong adverse pressure gradients, high streamline curvature, and cohabitation of incompressible and transonic flow around the lip. All these features increase the risk of lip stall in flight at incidence or in crosswind near ground operation and increase the level of forcing seen by the fan blades because of the interaction with nonuniform flow from the intake. This work deals with the study of two sources of distortions: ground vortex ingestion and flight at high incidence conditions. A test case representative of a current installation clearance from the ground has been investigated and the experimental data available in open literature validated the computational fluid dynamics (CFD) calculations. An intake, representative of a realistic civil aero-engine configuration flying at high incidence, has been investigated in powered and aspirated configurations. Distortion distributions have been characterized in terms of total loss distributions in space and in time. The beneficial effect of the presence of fan in terms of distortion control has been demonstrated. The mutual effect between fan and incoming distortion from the intake has been assessed in terms of modal force and distortion control. CFD has been validated by means of comparisons between numerical results and experimental data which have been provided. Waves predicted by CFD have been compared with an actuator disk approach prediction. The linear behavior of the lower disturbance frequency coming from distortion and the waves reflected by the fan has been demonstrated.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):041204-041204-10. doi:10.1115/1.4034701.

In this paper, we present an extensive numerical study on the interaction between the downstream fan and the flow separating over an intake under high incidence. The objectives of this investigation are twofold: (a) to gain qualitative insight into the mechanism of fan–intake interaction and (b) to quantitatively examine the effect of the proximity of the fan on the inlet distortion. The fan proximity is altered using the key design parameter, L/D, where D is the diameter of the intake, and L is the distance of the fan from the intake lip. Both steady and unsteady Reynolds-averaged numerical simulations (RANS) were carried out. For the steady calculations, a low-order fan model has been used, while a full 3D geometry has been used for the unsteady RANS. The numerical methodology is also thoroughly validated against the measurements for the intake-only and fan-only configurations on a high bypass ratio turbofan intake and fan, respectively. To systematically study the effect of fan on the intake separation and explore the design criteria, a simplified intake–fan configuration has been considered. In this fan–intake model, the proximity of the fan to the intake separation (L/D) can be conveniently altered without affecting other parameters. The key results indicate that, depending on L/D, the fan has either suppressed the level of the postseparation distortion or increased the separation-free operating range. At the lowest L/D (∼0.17), around a 5 deg increase in the separation-free angle of incidence was achieved. This delay in the separation-free angle of incidence decreased with increasing L/D. At the largest L/D (∼0.44), the fan was effective in suppressing the postseparation distortion rather than entirely eliminating the separation. Isentropic Mach number distribution over the intake lip for different L/D's revealed that the fan accelerates the flow near the casing upstream of the fan face, thereby decreasing the distortion level in the immediate vicinity. However, this acceleration effect decayed rapidly with increasing upstream distance from the fan-face.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2016;139(4):041401-041401-10. doi:10.1115/1.4034721.

Small-scale combined heat and power (CHP) plants present lower electric efficiency in comparison to large scale ones, and this is particularly true when biomass fuels are used. In most cases, the use of both heat and electricity to serve on-site energy demand is a key issue to achieve acceptable global energy efficiency and investment profitability. However, the heat demand follows a typical daily and seasonal pattern and is influenced by climatic conditions, in particular in the case of residential and tertiary end users. During low heat demand periods, a lot of heat produced by the CHP plant is discharged. In order to increase the electric conversion efficiency of small-scale micro-gas turbine for heat and power cogeneration, a bottoming organic Rankine cycle (ORC) system can be coupled to the cycle, however, this option reduces the temperature and the amount of cogenerated heat available to the thermal load. In this perspective, the paper presents the results of a thermo-economic analysis of small-scale CHP plants composed of a micro-gas turbine (MGT) and a bottoming ORC, serving a typical residential energy demand. For the topping cycle, three different configurations are examined: (1) a simple recuperative micro-gas turbine fueled by natural gas (NG); (2) a dual fuel externally fired gas turbine (EFGT) cycle, fueled by biomass and natural gas (50% share of energy input) (DF); and (3) an externally fired gas turbine (EFGT) with direct combustion of biomass (B). The bottoming ORC is a simple saturated cycle with regeneration and no superheating. The ORC cycle and the fluid selection are optimized on the basis of the available exhaust gas temperature at the turbine exit. The research assesses the influence of the thermal energy demand typology (residential demand with cold, mild, and hot climate conditions) and CHP plant operational strategies (baseload versus heat-driven versus electricity-driven operation mode) on the global energy efficiency and profitability of the following three configurations: (A) MGT with cogeneration; (B) MGT+ ORC without cogeneration; and (C) MGT+ORC with cogeneration. In all cases, a back-up boiler is assumed to match the heat demand of the load (fed by natural gas or biomass). The research explores the profitability of bottoming ORC in view of the following trade-offs: (i) lower energy conversion efficiency and higher investment cost of biomass input with respect to natural gas; (ii) higher efficiency but higher costs and reduced heat available for cogeneration with the bottoming ORC; and (iii) higher primary energy savings and revenues from feed-in tariff available for biomass electricity fed into the grid.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2016;139(4):041501-041501-8. doi:10.1115/1.4034542.

Lean premixed combustion promotes the occurrence of thermoacoustic phenomena in gas turbine combustors. One mechanism that contributes to the flame–acoustic interaction is entropy noise. Fluctuations of the equivalence ratio in the mixing section cause the generation of hot spots in the flame. These so-called entropy waves are convectively transported to the first stage of the turbine and generate acoustic waves that travel back to the flame; a thermoacoustic loop is closed. However, due to the lack of experimental tools, a detailed investigation of entropy waves in gas turbine combustion systems has not been possible up to now. This work presents an acoustic time-of-flight based temperature measurement method which allows the measurement of temperature fluctuations in the relevant frequency range. A narrow acoustic pulse is generated with an electric spark discharge close to the combustor wall. The acoustic response is measured at the same axial location with an array of microphones circumferentially distributed around the combustion chamber. The delay in the pulse arrival times corresponds to the line-integrated inverse speed of sound. For the measurement of entropy waves in an atmospheric combustion test rig, fuel is periodically injected into the mixing tube of a premixed combustor. The subsequently generated entropy waves are measured for different forcing frequencies of the fuel injection and for different mean flow velocities in the combustor. The amplitude decay and phase lag of the entropy waves adhere well to a Strouhal number scaling for different mean flow velocities.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):041502-041502-10. doi:10.1115/1.4034646.

This study investigates the influence of the fuel injection strategy on safety against flashback in a gas turbine model combustor with premixing of H2–air mixtures. The flashback propensity is quantified and the flashback mechanism is identified experimentally. The A2EV swirler concept exhibits a hollow, thick-walled conical structure with four tangential slots. Four fuel injector geometries were tested. One of them injects the fuel orthogonal to the air flow in the slots (jet-in-crossflow injector (JICI)). Three injector types introduce the fuel almost isokinetic to the air flow at the trailing edge of the swirler slots (trailing edge injector (TEI)). Velocity and mixing fields in mixing zone and combustion chamber in isothermal water flow were measured with high-speed particle image velocimetry (PIV) and high-speed laser-induced fluorescence (LIF). The flashback limit was determined under atmospheric pressure for three air mass flows and 673 K preheat temperature for H2–air mixtures. Flashback mechanism and trajectory of the flame tip during flashback were identified with two stereoscopically oriented intensified high-speed cameras observing the OH* radiation. We notice flashback in the core flow due to combustion-induced vortex breakdown (CIVB) and turbulent flame propagation (TFP) near the wall dependent on the injector type. The flashback resistance (FBR) defined as the ratio between a characteristic flow speed and a characteristic flame speed measures the direction of propagation of a turbulent flame in the flow field. Although CIVB cannot be predicted solely based on the FBR, its distribution gives evidence for CIVB-prone states. The fuel should be injected preferably isokinetic to the air flow along the entire trailing edge in order to reduce the RMS fluctuation of velocity and fuel concentration. The characteristic velocity in the entire cross section of the combustion chamber inlet should be at least twice the characteristic flame speed. The position of the stagnation point should be tuned to be located in the combustion chamber by adjusting the axial momentum. Those measures lead to safe operation with highly reactive fuels at high equivalence ratios.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):041503-041503-11. doi:10.1115/1.4034601.

Considerable research and development efforts are required to meet the targets of future gas turbine technologies in terms of performance, emissions, and operational flexibility. One of the recurring problems is the constructive coupling between flames and combustor's acoustics. These thermoacoustic interactions can cause high-amplitude dynamic pressure limit cycles, which reduce the lifetime of the hot gas path parts or in the worst-case scenario destroy these mechanical components as a result of a sudden catastrophic event. It is shown in this paper that the dynamics and the statistics of the acoustic signal envelope can be used to identify the linear growth rates hidden behind the observed pulsations, and the results are validated against numerical simulations. This is a major step forward and it will contribute to the development of future gas turbine combustors, because the knowledge of these linear growth rates is essential to develop robust active and passive systems to control these combustion instabilities.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):041504-041504-10. doi:10.1115/1.4034647.

Flashback (FB) and self-ignition in the premixing zone of typical gas turbine swirl combustors in lean premixed operation are immanent risks and can lead to damage and failure of components. Thus, steady combustion in the premixing zone must be avoided under all circumstances. This study experimentally investigates the flame holding propensity of fuel injectors in the swirler of a gas turbine model combustor with premixing of H2–natural gas (NG)–air mixtures under atmospheric pressure and proposes a model to predict the limit for safe operation. The A2EV swirler concept exhibits a hollow, thick walled conical structure with four tangential slots. Four fuel injector geometries were tested. One of them injects the fuel orthogonal to the air flow in the slots (jet-in-crossflow injector, JICI). Three injector types introduce the fuel almost isokinetic to the air flow at the trailing edge of the swirler slots (trailing edge injector, TEI). A cylindrical duct and a window in the swirler made of quartz glass allow the application of optical diagnostics (OH* chemiluminescence and planar laser induced fluorescence of the OH radical (OH-PLIF)) inside the swirler. The fuel–air mixture was ignited with a focused single laser pulse during steady operation. The position of ignition was located inside the swirler in proximity to a fuel injection hole. If the flame was washed out of the premixing zone not later than 4 s after the ignition, the operation point was defined as safe. Operation points were investigated at three air mass flows, three air ratios, two air preheat temperatures (573 K and 673 K), and 40 to 100 percent per volume hydrogen in the fuel composed of hydrogen and natural gas. The determined safety limit for atmospheric pressure yields a similarity rule based on a critical Damköhler number. Application of the proposed rule at conditions typical for gas turbines leads to these safety limits for the A2EV burner: With the TEIs, the swirler can safely operate with up to 80 percent per volume hydrogen content in the fuel at an air ratio of two. With the JIC injector, safe operation at stoichiometric conditions and 95 percent per volume hydrogen is possible.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):041505-041505-8. doi:10.1115/1.4034688.

A novel gas turbine combustor is investigated by means of a global flow analysis. Its main feature is the helical arrangement of the burners, which allows the utilization of the high angular momentum of the flow from compressor, so that the length of the flame tube and the number of NGV can be reduced. The concept was studied in Ariatabar et al. (2016, “Short Helical Combustor: Concept Study of an Innovative Gas Turbine Combustor With Angular Air Supply,” ASME J. Eng. Gas Turbines Power, 138(3), p. 031503) based on similarity considerations and a kinematic assessment of the simulated flow in various combustor models. For the best configuration found in the previous work, the exit mean flow angle was lower than the half of its initial value at the combustor inlet. The reason for this unwanted decay of the initial high angular momentum flux was not clear. In the present work, the underlying physics of the strong reduction of the mean flow angle is elucidated by analysis of the integral balance equation of angular momentum. It is shown that the flow in the vicinity of the burners is governed by inertial forces associated with an asymmetric pressure distribution on the sidewall and the combustor dome. The friction and turbulent mixing phenomena are found to have marginal effects on the flow pattern. To compare mean flow quantities of different combustor designs, a physically consistent averaging method is introduced, which can also be applied to a conventional combustor to assess different swirl configurations regarding the resulting flow pattern, mixing performance, and total pressure loss.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):041506-041506-8. doi:10.1115/1.4034649.

Because flashback is a key operability issue associated with low emission combustion of high hydrogen content fuels, design tools to predict flashback propensity are of interest. Such a design tool has been developed by the authors to predict boundary layer flashback using nondimensional parameters. The tool accounts for the thermal coupling between the flame and burner rim and was derived using detailed studies carried out in a test rig at elevated temperature and pressure. The present work evaluates the applicability of the model to a commercial 65 kW microturbine generator (MTG). Two sets of data are evaluated. One set is obtained using the combustor, removed from the engine, which has been configured to operate like it does in the engine but at atmospheric pressure and various preheat temperatures. The second set of data is from a combustor operated as it normally would in the commercial engine. In both configurations, studies are carried out with various amounts of hydrogen added to either natural gas or carbon monoxide. The previously developed model is able to capture the measured flashback tendencies in both configurations. In addition, the model is used to interpret flashback phenomena at high pressures and temperatures in the context of the engine conditions. An increase in pressure for a given preheat temperature and velocity reduces the equivalence ratio at which flashback occurs and increases the tip temperature due to lower quenching distance. The dependency of the flashback propensity on the injector tip temperature is enhanced with an increase in pressure. The variation of critical velocity gradient with equivalence ratio for a constant preheat temperature is more pronounced at higher pressures. In summary, the model developed using the high-pressure test rig is able to predict flashback tendencies for a commercial gas turbine engine and can thus serve as an effective design tool for identifying when flashback is likely to occur for a given geometry and condition.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):041507-041507-8. doi:10.1115/1.4034687.

It is generally accepted that combustion of hydrogen and natural gas mixtures will become more prevalent in the near future, to allow for a further penetration of renewables in the European power generation system. The current work aims at the demonstration of the advantages of steam dilution, when highly reactive combustible mixtures are used in a swirl-stabilized combustor. To this end, high-pressure experiments have been conducted with a generic swirl-stabilized combustor featuring axial air injection to increase flashback safety. The experiments have been conducted with two fuel mixtures, at various pressure levels up to 9 bar and at four levels of steam dilution up to 25% steam-to-air mass flow ratio. Natural gas has been used as a reference fuel, whereas a mixture of natural gas and hydrogen (10% hydrogen by mass) represented an upper limit of hydrogen concentration in a natural gas network with hydrogen enrichment. The results of the emissions measurements are presented along with a reactor network model. The latter is applied as a means to qualitatively understand the chemical processes responsible for the observed emissions and their trends with increasing pressure and steam injection.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):041508-041508-8. doi:10.1115/1.4034724.

The ever-increasing requirements on gas turbine efficiency and the simultaneous demand for reduced emissions, necessitate much more accurate calculations of the combustion process and combustor wall temperatures. Thermal history paints (THPs) is an innovative alternative to the established measurement techniques, but so far only a limited number of tests have been conducted under real engine conditions. A typical THP comprises oxide ceramic pigments and a water-based binder. The ceramic is synthesized to be amorphous and when heated it crystallizes, permanently changing the microstructure. The ceramic is doped with lanthanide ions to make it phosphorescent and as the structure of the material changes, so do the phosphorescent properties of the material. By measuring the phosphorescence, the maximum temperature of exposure can be determined, enabling postoperation measurements at ambient conditions. This paper describes a test in which THP was applied to an impingement-cooled front panel from a combustor of an industrial gas turbine. The panel was instrumented with a thermocouple (TC), and thermal paint was applied to the cold side of the impingement plate. The THP was applied to the hot-gas side of this plate for validation against the other measurement techniques and to evaluate its resilience against the reacting hot gas environment. The durability and temperature results of the three different measurement techniques are discussed. It is shown that the THP exhibited greater durability compared to the conventional thermal paint. Furthermore, the new technology provided detailed measurements indicating local temperature variations and global variations over the complete component.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):041509-041509-6. doi:10.1115/1.4034700.

We present an experimental study on the nonlinear dynamics of combustion instability in a lean premixed gas-turbine model combustor with a swirl-stabilized turbulent flame. Intermittent combustion oscillations switching irregularly back and forth between burst and pseudo-periodic oscillations exhibit the deterministic nature of chaos. This is clearly demonstrated by considering two nonlinear forecasting methods: an extended version (Gotoda et al., 2015, “Nonlinear Forecasting of the Generalized Kuramoto-Sivashinsky Equation,” Int. J. Bifurcation Chaos, 25, p. 1530015) of the Sugihara and May algorithm (Sugihara and May, 1990, “Nonlinear Forecasting as a Way of Distinguishing Chaos From Measurement Error in Time Series,” Nature, 344, pp. 734–741) as a local predictor, and a generalized radial basis function (GRBF) network as a global predictor (Gotoda et al., 2012, “Characterization of Complexities in Combustion Instability in a Lean Premixed Gas-Turbine Model Combustor,” Chaos, 22, p. 043128; Gotoda et al., 2016 (unpublished)). The former enables us to extract the short-term predictability and long-term unpredictability of chaos, while the latter can produce surrogate data to test for determinism by a free-running approach. The permutation entropy based on a symbolic sequence approach is estimated for the surrogate data to test for determinism and is also used as an online detector to prevent lean blowout.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):041510-041510-7. doi:10.1115/1.4034941.

An annular pulsed detonation combustor (PDC) basically consists of a number of detonation tubes which are firing in a predetermined sequence into a common downstream annular plenum. Fluctuating initial conditions and fluctuating environmental parameters strongly affect the detonation. Operating such a setup without misfiring is delicate. Misfiring of individual combustion tubes will significantly lower performance or even stop the engine. Hence, an operation of such an engine requires a misfiring detection. Here, a supervised data driven machine learning approach is used for the misfiring detection. The features used as inputs for the classifier are extracted from measurements incorporating physical knowledge about the given setup. To this end, a neural network is trained based on labeled data which is then used for classification purposes, i.e., misfiring detection. A surrogate, nonreacting experimental setup is considered in order to develop and test these methods.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Heat Transfer

J. Eng. Gas Turbines Power. 2016;139(4):041901-041901-10. doi:10.1115/1.4034636.

In the most evolved designs, it is common practice to expose engine components to main annulus air temperatures exceeding the thermal material limit in order to increase the overall performance and to minimize the engine-specific fuel consumption (SFC). To prevent overheating of the materials and thus the reduction of the component life, an internal flow system is required to cool the critical engine parts and to protect them. This paper shows a practical application and extension of the methodology developed during the five-year research program, main annulus gas path interaction (MAGPI). Extensive use was made of finite element analysis (FEA (solids)) and computational fluid dynamics (CFD (fluid)) modeling techniques to understand the thermomechanical behavior of a dedicated turbine stator well cavity rig, due to the interaction of cooling air supply with the main annulus. Previous work based on the same rig showed difficulties in matching predictions to thermocouple measurements near the rim seal gap. In this investigation, two different types of turbine stator well geometries were analyzed, where—in contrast to previous analyses—further use was made of the experimentally measured radial component displacements during hot running in the rig. The structural deflections were applied to the existing models to evaluate the impact inflow interactions and heat transfer. Additionally, to the already evaluated test cases without net ingestion, cases simulating engine deterioration with net ingestion were validated against the available test data, also taking into account cold and hot running seal clearances. 3D CFD simulations were conducted using the commercial solver fluent coupled to the in-house FEA tool SC03 to validate against available test data of the dedicated rig.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2016;139(4):042501-042501-9. doi:10.1115/1.4034650.

It is well-known that flutter vibrations of bladed disks can be saturated by dry friction. Previous theoretical investigations indicated that the steady-state, friction-damped flutter vibrations of tuned bladed disks are always dominated by a single traveling wave component, even if multiple traveling wave forms are unstable. This contrasts recent experimental investigations where multiple traveling wave forms were found to participate at steady state. In this paper, we demonstrate that this phenomenon can be explained by nonlinear frictional interblade coupling. To this end, we consider a simple phenomenological model of a bladed disk with frictional intersector coupling and two unstable traveling waves forms. Vibrations occur not only in the form of limit cycle oscillations (periodic) but also in the form of limit torus oscillations (quasi-periodic). It is shown how the limit state depends on the initial conditions, and that the occurrence of multiwave flutter depends on the proximity of the complex eigenvalues of the associated unstable waves. Finally, by computing the limit torus oscillation with a frequency-domain method, we lay the cornerstone for the systematic prediction of friction-saturated flutter vibrations of state-of-the-art bladed disk models.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):042502-042502-8. doi:10.1115/1.4034648.

Design and analysis of foil bearings involve consideration to various physical aspects such as fluid pressure, structural deformation, and heat generation due to viscous effects within the bearing. These complex physical interactions are mathematically governed by highly nonlinear partial differential equations. Therefore, foil bearing design involves detailed calculations of flow fields (velocities, pressures), support structure deflections (structural compliance), and heat transfer phenomena (viscous dissipation in the fluid, frictional heating, temperature profile, etc.). The computational effort in terms of time and hardware requirements make high level engineering analyses tedious which presents an opportunity for development of rule of thumb laws for design guidelines. Scaling laws for bearing clearance and support structure stiffness of radial foil bearings of various sizes are presented in this paper. The scaling laws are developed from first principles using the scale invariant Reynolds equation and support structure deflection equation. Power law relationships are established between the (1) radial clearance and bearing radius and (2) support structure stiffness and bearing radius. Simulation results of static and dynamic performance of various bearing sizes following the proposed scaling laws are presented.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):042503-042503-11. doi:10.1115/1.4034722.

Multiple-stage integrally geared compressors (IGCs) offer improved thermal efficiency and easier access for maintenance and overhaul than single-shaft centrifugal compressors. In an IGC, a main bull shaft drives pinion shafts, each having an impeller at its ends. The compression of process gas in the compressor stages induces axial loads along the pinion shafts that are transmitted via thrust collars (TCs) to the main bull gear (BG) shaft and balanced by a single thrust bearing. Manufacturing inaccuracies and a poor assembly process can lead to static angular misalignments of the TC and BG surfaces that affect the operating film thickness as well as the force and reaction moments of the lubricated mechanical element. In a follow-up to San Andrés et al. (2015, “On the Predicted Performance of Oil Lubricated Thrust Collars in Integrally Geared Compressors,” ASME J. Eng. Gas Turbines Power, 137(5), pp. 1–9), this paper presents an investigation of the performance of a single thrust collar configuration operating with increasing static angular misalignment of either the TC or BG. The flow model solves the Reynolds equation of hydrodynamic lubrication coupled to a thermal energy transport equation to determine the film pressure and bulk temperature fields, respectively. The model predicts performance parameters such as power loss and lubricant flow rate, and force and moment stiffness and damping coefficients. Predictions show that misaligning of either the thrust collar or bull gear alters the load-carrying area in the lubricated zone, shifts the pressure field with peak magnitudes doubling or more depending on the degree and direction of TC or BG misalignment. Static angular misalignment does not significantly affect the power loss, temperature rise, etc., but does have an effect on the dynamic coefficients (both axial and angular). Finally, a reduced complex dynamic stiffness matrix for the lubricated TC shows that some cross-coupled stiffness and moment coefficients are nonzero, indicating hydrodynamic coupling between axial and angular motions for the pinion and bull gear shafts. The coupling could affect the placement of the system natural frequencies and associated mode shapes as well as the system stability.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):042504-042504-9. doi:10.1115/1.4034702.

The secondary air system of a modern gas or steam turbine is configured to satisfy a number of requirements, such as to purge cavities and maintain a sufficient flow of cooling air to key engine components, for a minimum penalty on engine cycle efficiency and specific fuel consumption. Advanced sealing technologies, such as brush seals and leaf seals, are designed to maintain pressures in cavities adjacent to rotating shafts. They offer significant reductions in secondary air parasitic leakage flows over the legacy sealing technology, the labyrinth seal. The leaf seal comprises a series of stacked sheet elements which are inclined relative to the radial direction, offering increased axial rigidity, reduced radial stiffness, and good leakage performance. Investigations into leaf seal mechanical and flow performance have been conducted by previous researchers. However, limited understanding of the thermal behavior of contacting leaf seals under sustained shaft contact has led to the development of an analytical model in this study, which can be used to predict the power split between the leaf and rotor from predicted temperature rises during operation. This enables the effects of seal and rotor thermal growth and, therefore, implications on seal endurance and rotor mechanical integrity to be quantified. Consideration is given to the heat transfer coefficient in the leaf pack. A dimensional analysis of the leaf seal problem using the method of extended dimensions is presented, yielding the expected form of the relationship between seal frictional power generation, leakage mass flow rate, and rotor temperature rise. An analytical model is derived which is in agreement. Using the derived leaf temperature distribution formula, the theoretical leaf tip temperature rise and temperature distributions are computed over a range of mass flow rates and frictional heat values. Experimental data were collected in high-speed tests of a leaf seal prototype using the Engine Seal Test Facility at Oxford University. These data were used to populate the analytical model and collapsed well to confirm the expected linear relationship. In this form, the thermal characteristic can be used with predictions of mass flow rate and frictional power generated to estimate the leaf tip and rotor temperature rise in engine operation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):042505-042505-10. doi:10.1115/1.4034723.

In this paper, a quasi-three-dimensional fluid–structure model using computational fluid dynamics for the fluid phase is presented to study the elastohydrodynamic performance of foil thrust bearings for supercritical CO2 cycles. For the simulation of the gas flows within the thin gap, the computational fluid dynamics solver Eilmer is extended, and a new solver is developed to simulate the bump and top foil within foil thrust bearings. These two solvers are linked using a coupling algorithm that maps pressure and deflection at the fluid structure interface. Results are presented for ambient CO2 conditions varying between 0.1 and 4.0 MPa and 300 and 400 K. It is found that the centrifugal inertia force can play a significant impact on the performance of foil thrust bearings with the highly dense CO2 and that the centrifugal inertia forces create unusual radial velocity profiles. In the ramp region of the foil thrust bearings, they generate an additional inflow close to the rotor inner edge, resulting in a higher peak pressure. Contrary to the flat region, the inertia force creates a rapid mass loss through the bearing outer edge, which reduces pressure in this region. This different flow fields alter bearing performance compared to conventional air foil bearings. In addition, the effect of turbulence in load capacity and torque is investigated. This study provides new insight into the flow physics within foil bearings operating with dense gases and for the selection of optimal operating condition to suit CO2 foil bearings.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):042506-042506-9. doi:10.1115/1.4034918.

This work presents the theoretical and experimental rotordynamic evaluations of a rotor–air foil bearing (AFB) system supporting a large overhung mass for high-speed application. The proposed system highlights the compact design of a single shaft rotor configuration with turbomachine components arranged on one side of the bearing span. In this work, low-speed tests up to 45 krpm are performed to measure lift-off speed and to check bearing manufacturing quality. Rotordynamic performance at high speeds is evaluated both analytically and experimentally. In the analytical approach, simulated imbalance responses are studied using both rigid and flexible shaft models with bearing forces calculated from the transient Reynolds equation along with the rotor motion. The simulation predicts that the system experiences small synchronous rigid mode vibration at 20 krpm and bending mode at 200 krpm. A high-speed test rig is designed to experimentally evaluate the rotor–air foil bearing system. The high-speed tests are operated up to 160 krpm. The vibration spectrum indicates that the rotor–air foil bearing system operates under stable conditions. The experimental waterfall plots also show very small subsynchronous vibrations with frequency locked to the system natural frequency. Overall, this work demonstrates potential capability of the air foil bearings in supporting a shaft with a large overhung mass at high speed.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Turbomachinery

J. Eng. Gas Turbines Power. 2016;139(4):042601-042601-8. doi:10.1115/1.4034703.

Micro gas turbine (MGT) technology is evolving toward a large variety of novel applications, such as weak gas electrification, inverted Brayton cycles, and fuel cell hybrid cycles; however, many of these systems show very different dynamic behaviors compared to conventional MGTs. In addition, some applications impose more stringent requirements on transient maneuvers, e.g., to limit temperature and pressure gradients in a fuel cell hybrid cycle. Besides providing operational safety, optimizing system dynamics to meet the variable power demand of modern energy markets is also of increasing significance. Numerical cycle simulation programs are crucial tools to analyze these dynamics without endangering the machines, and to meet the challenges of automatic control design. For these tasks, complete cycle simulations of transient maneuvers lasting several minutes need to be calculated. Moreover, sensitivity analysis and optimization of dynamic properties like automatic control systems require many simulation runs. To perform these calculations in an acceptable timeframe, simplified component models based on lumped volume or one-dimensional discretization schemes are necessary. The accuracy of these models can be further improved by parameter identification, as most novel applications are modifications of well-known MGT systems and rely on proven, characterized components. This paper introduces a modular in-house simulation tool written in fortran to simulate the dynamic behavior of conventional and novel gas turbine cycles. Thermodynamics, gas composition, heat transfer to the casing and surroundings, shaft rotation and control system dynamics as well as mass and heat storage are simulated together to account for their interactions. While the presented models preserve a high level of detail, they also enable calculation speeds up to five times faster than real-time. The simulation tool is explained in detail, including a description of all component models, coupling of the elements and the ODE solver. Finally, validation results of the simulator based on measurement data from the DLR Turbec T100 recuperated MGT test rig are presented, including cold start-up and shutdown maneuvers.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):042602-042602-12. doi:10.1115/1.4034940.

Recent studies have demonstrated that, when rotating around an axis orthogonal to the flow direction, airfoils are virtually transformed into equivalent airfoils with a camber line defined by their arc of rotation. In these conditions, the symmetric airfoils commonly used for Darrieus blades actually behave like virtually cambered ones or, equivalently, rotors have to be manufactured with countercambered blades to ensure the attended performance. To complete these analyses, the present study first focuses the attention on the airfoils' aerodynamics during the startup of the rotors. It is shown that, contrary to conventional theories based on one-dimensional aerodynamic coefficients, symmetric airfoils exhibit a counterintuitive nonsymmetric starting torque over the revolution. Conversely, airfoils compensated for the virtual camber effect show a more symmetric distribution over the revolution. This behavior is due to the effect of the pitching moment, which is usually neglected in lumped parameters models. At very low revolution speeds, its contribution becomes significant due to the very high incidence angles experienced by the blades; the pitching moment is also nonsymmetric between the upwind and the downwind zone. For upwind azimuthal positions, the pitching moment reduces the overall torque output, while it changes sign in the downwind section, increasing the torque. The importance of accounting for the pitching moment contribution in the entire power curve is also discussed in relationship to the selection of the best blade–spoke connection (BSC) point, in order to maximize the performance and minimize the alternate stresses on the connection due to the pitching moment itself.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2016;139(4):042603-042603-10. doi:10.1115/1.4034919.

The accurate design, control, and monitoring of the running gaps between static and moving components are vital to preserve the mechanical integrity and ensure the correct functioning of any compact rotating machinery. Throughout engine service, the rotor tip clearance undergoes large variations due to installation tolerances or as the result of different thermal expansion rates of the blades, rotor disk, and casing during speed transients. Hence, active tip clearance control concepts and engine health-monitoring systems rely on precise real-time gap measurements. Moreover, this tip gap information is crucial for engine development programs to verify the mechanical and aerothermal designs and validate numerical predictions. This paper presents an overview of the critical design requirements for testing engine-representative blade tip flows in a rotating turbine facility. This paper specifically focuses on the challenges related with the design, verification, and monitoring of the running tip clearance during a turbine experiment. In the large-scale turbine facility of the von Karman Institute, a rainbow rotor was mounted for simultaneous aerothermal testing of multiple blade tip geometries. The tip shapes are a selection of high-performance squealer-like and contoured blade tip designs. On the rotor disk, the blades are arranged in seven sectors operating at different clearance levels from 0.5 up to 1.5% of the blade span. Prior to manufacturing, the blade geometry was modified to compensate for the radial deformation of the rotating assembly under centrifugal loads. A numerical procedure was implemented to minimize the residual unbalance of the rotor in rainbow configuration and to optimize the placement of every single airfoil within each sector. Subsequently, the rotor was balanced in situ to reduce the vibrations and satisfy the international standards for high balance quality. Three fast-response capacitive probes located at distinct circumferential locations around the rotor annulus measured the single-blade tip clearance in rotation. Additionally, the minimum running blade clearance is captured with wear gauges located at five axial positions along the blades chord. The capacitance probes are self-calibrated using a multitest strategy at several rotational speeds. The in situ calibration methodology and dedicated data reduction techniques allow the accurate measurement of the distance between the turbine casing and the local blade tip features (rims and cavities) for each rotating airfoil separately. General guidelines are given for the design and calibration of a tip clearance measurement system that meets the required measurement accuracy and resolution in function of the sensor uncertainty, nominal tip clearance levels, and tip seal geometry.

Commentary by Dr. Valentin Fuster

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