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# Accepted Manuscripts

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research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038081
The paper deals with the leakage and static stability behavior of a fully-partitioned pocket damper seal (FPDS) at high eccentricity ratios. The leakage flow rates, static fluid-induced response forces and static stiffness coefficients were solved for the FPDS at high eccentricity ratios, using the steady Reynolds-Averaged Navier-Stokes (RANS) solution approach. The calculations were performed at typical operating conditions including seven rotor eccentricity ratios up to 0.9 for four rotational speeds (0 rpm, 7 000 rpm, 11 000 rpm and 15 000 rpm) including the non-rotating condition, three pressure ratios (0.17, 0.35 and 0.50) including the choked exit flow condition, two inlet preswirl velocities (0 m/s, 60 m/s). An interesting observation stemming from these numerical results is that the FPDS has a positive direct stiffness as long as it operates at subsonic exit flow conditions, no matter the eccentricity ratio and rotational speed are high or low. For the choked exit condition, the FPDS shows negative direct stiffness at low eccentricity ratio and then crosses over to positive value at the crossover eccentricity ratio (0.5-0.7) following a trend indicative of a parabola. Therefore, the negative static direct stiffness is limited to the specific operating conditions: choked exit flow condition and low eccentricity ratio less than the crossover eccentricity ratio, where the pocket damper seal would be statically unstable.
TOPICS: Dampers, Leakage, Static stability, Stiffness, Flow (Dynamics), Fluids, Matter, Rotors, Reynolds-averaged Navier–Stokes equations, Leakage flows, Pressure
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038082
Carbon Capture and Storage could significantly reduce carbon dioxide (CO2) emissions. One of the major limitations of this technology is the energy penalty for the compression of CO2 to supercritical conditions. To reduce the power requirements supercritical carbon dioxide compressors must operate near saturation where phase change effects are important. Non-equilibrium condensation can occur at the leading edge of the compressor, causing performance and stability issues. The characterization of the fluid at these conditions is vital to enable advanced compressor designs at enhanced efficiency levels but the analysis is challenging due to the lack of data on fluid properties. In this paper we assess the behavior and nucleation characteristics of high-pressure subcooled CO2 during the expansion in a nozzle. The assessment is conducted with numerical calculations and corroborated by experimental measurements. The Wilson line is determined via optical measurements in the range of 41 and 82 bar. The state of the metastable fluid is characterized through pressure and density measurements, with the latter obtained in a first-of-its-kind laser interferometry setup. The inlet conditions of the nozzle are moved close to the critical point to allow for reduced margins to condensation. The analysis suggests that direct extrapolation using the Span and Wagner equation of state model yields results within 2% of the experimental data. The results are applied to define inlet conditions for a supercritical carbon dioxide compressor. Full-scale compressor experiments demonstrate that the reduced inlet temperature can decrease the shaft power input by 16%.
TOPICS: Condensation, Nozzles, Equilibrium (Physics), Supercritical carbon dioxide, Compressors, Carbon dioxide, Fluids, Lasers, Optical measurement, Interferometry, Density, Pressure, Stability, Temperature, Compression, Equations of state, Subcooling, Carbon capture and storage, Emissions, High pressure (Physics), Nucleation (Physics)
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038083
Spectral distributions of the sound pressure level (SPL) observed in a premixed, swirl stabilized combustion test rig are scrutinized. Spectral peaks in the SPL for stable as well as unstable cases are interpreted with the help of a novel criterion for the resonance frequencies of the intrinsic thermo-acoustic (ITA) feedback loop. This criterion takes into the account the flow inertia of the burner and indicates that in the limit of very large flow inertia, ITA resonance should appear at frequencies where the phase of the flame transfer function approaches $-\pi/2$. Conversely, in the limiting case of vanishing flow inertia, the new criterion agrees with previous results, which state that ITA modes may arise when the phase of the FTF is close to $-\pi$. Relying on the novel criterion, peaks in the SPL spectra are identified to correspond to either ITA or acoustic modes. Various combustor configurations are investigated over a range of operating conditions. It is found that in this particular combustor ITA modes are prevalent and dominate the unstable cases. Remarkably, the ITA frequencies change significantly with the bulk flow velocity and the position of the swirler, but are almost insensitive to changes in the length of the combustion chamber. These observations imply that the resonance frequencies of the ITA feedback loop are governed by convective time scales. A scaling rule for ITA frequencies that relies on a model for the overall convective flame time lag shows good consistency for all operating conditions considered in this study.
TOPICS: Acoustics, Combustion chambers, Flow (Dynamics), Inertia (Mechanics), Resonance, Feedback, Flames, Transfer functions, Sound pressure, Spectra (Spectroscopy), Combustion
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038084
In order to understand the reasons for the apparent benefits of using a flow-blurring (FB) atomizer in a combustion system, it is necessary to first examine fundamental spray characteristics under non-reacting conditions. Previous work on FB atomizers, however, has mostly involved only water and a relatively narrow range of parameters. In this study, a phase Doppler anemometry instrument was used to characterize FB atomizer sprays and determine the effects of varying surface tension and viscosity of the liquid. Operating at room pressure and temperature (i.e., a “cold spray”), droplet sizes and velocities were measured for water, a water/surfactant mixture (lower surface tension), a water/glycerol mixture (higher viscosity), and glycerol (much higher viscosity). For all of the tested fluids, with the exception of pure glycerol, the FB atomizer produced small droplets (below 50 µm) whose size did not vary significantly in the radial or axial direction, particularly above a characteristic distance from the atomizer exit. Results show that the spray is essentially unaffected by a 4.5x decrease in surface tension or a 7x increase in viscosity, and that Sauter mean diameter (SMD) only increased by approximately a factor of three when substituting glycerol (750x higher viscosity) for water. The results suggest that the FB atomizer can effectively atomize a wide range of liquids, making it a useful fuel-flexible atomizer for combustion applications.
TOPICS: Flow (Dynamics), Fluids, Sprays, Water, Viscosity, Surface tension, Drops, Combustion systems, Instrumentation, Pressure, Temperature, Combustion, Surface mount devices, Surfactants, Fuels
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038038
Owing to the increasing consumption of fossil fuels and emission of greenhouse gases, interests in highly efficient and low carbon emitting power systems are growing fast. Several research groups have been suggesting advanced systems based on fuel cells and have also been applying carbon capture and storage technology to satisfy the demand for clean energy. In this study, the performance of a hybrid system, which is a combination of a molten carbonate fuel cell (MCFC) with oxy-combustion carbon capture and an indirectly fired micro gas turbine (MGT) was predicted. A 2.5MW MCFC system for commercial applications was used as the reference system so that the results of the study could be applicable to practical situations. The ambient pressure type hybrid system was modeled by referring to the design parameters of an MGT under development. A semi-closed type design characterized by flow recirculation was adopted for this hybrid system. A part of the recirculating gas is converted into liquefied carbon dioxide and captured for storage at the carbon separation unit. Almost 100% carbon dioxide capture is possible. In these systems, the output power of the fuel cell is larger than in the normal hybrid system without carbon capture because the partial pressure of carbon dioxide increases. The increased cell power partially compensates for the power loss due to the carbon capture and MGT power reduction. The dependence of net system efficiency of the oxy-hybrid on compressor pressure ratio is marginal, especially beyond an optimal value.
TOPICS: Combustion, Molten carbonate fuel cells, Performance evaluation, Carbon capture and storage, Micro gas turbines, Pressure, Carbon, Design, Fuel cells, Carbon dioxide, Fossil fuels, Gases, Compressors, Emissivity, Flow (Dynamics), Separation (Technology), Emissions, System efficiency, Renewable energy, Storage
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038039
The precessing vortex core (PVC) represents a helicalshaped coherent flow structure typically occurring in both reacting and non-reacting swirling flows. Until now the fundamental impact of the PVC on flame dynamics, thermoacoustic instabilities and pollutant emissions is still unclear. In order to identify and investigate these mechanisms, the PVC needs to be controlled effectively with a feedback control system. A previous study successfully applied feedback control in a generic swirling jet setup. The next step is to transfer this approach into a swirl-stabilized combustor, which poses big challenges on the actuator and sensor design and placement. In this paper, different actuator designs are investigated with the goal of controlling the PVC dynamics. The actuation strategy aims to force the flow near the origin of the instability - the so-called wavemaker. To monitor the PVC dynamics, arrays of pressure sensors are flush-mounted at the combustor inlet and the combustion chamber walls. The best sensor placement is evaluated with respect to the prediction of the PVC dynamics. Particle image velocimetry is used to evaluate the passive impact of the actuator shape on the mean flow field. The performance of each actautor design is evaluated from lock-in experiments showing excellent control authority for two out of seven actuators. All measurements are conducted at isothermal conditions in a prototype of a swirl-stabilized combustor.
TOPICS: Combustion chambers, Actuators, Vortices, Dynamics (Mechanics), Flow (Dynamics), Design, Feedback, Swirling flow, Pollution, Emissions, Flames, Sensor placement, Shapes, Engineering prototypes, Locks (Waterways), Sensors, Particulate matter, Pressure sensors
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038040
Slender turbine blades are susceptible to excitation. Resulting vibrations stress the blade's fixture to the rotor or stator. In this paper, high cycle fatigue at the edge of contact between blade and rotor/stator of such fixtures is investigated both experimentally and numerically. Plasticity in the contact zone and its effects on e. g. contact tractions, fatigue determinative quantities and fatigue itself are shown to be of considerable relevance. The accuracy of the finite element analysis is demonstrated by comparing the predicted utilizations and slip region widths with data gained from tests. For the evaluation of edge of contact fatigue, tests on simple notched specimens provide the limit data. Predictions on the utilization are made for the edge of contact of a dovetail set-up. Tests with this set-up provide the experimental fatigue limit to be compared to. The comparisons carried out show a good agreement between the experimental results and the plasticity-based calculations of the demonstrated approach.
TOPICS: Turbine blades, High cycle fatigue, Stators, Plasticity, Fatigue, Rotors, Vibration, Blades, Fatigue testing, Stress, Finite element analysis, Fatigue limit, Excitation
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038041
A change in the combustion concept of gas turbines from conventional isobaric to constant volume combustion (CVC), such as in pulse detonation combustion (PDC), promises a significant increase in gas turbine efficiency. Current research focuses on the realization of reliable PDC operation and its challenging integration into a gas turbine. The topic of pollutant emissions from such systems has so far received very little attention. Few rare studies indicate that the extreme combustion conditions in PDC systems can lead to high emissions of nitrogen oxides (NOx). Therefore, it is essential already at this stage of development to begin working on primary measures for NOx emissions reduction, if commercialization is to be feasible. The present study evaluates the potential of different primary methods for reducing NOx emissions produced during pulsed detonation combustion of hydrogen. The considered primary methods involve utilization of lean combustion mixtures or its dilution by steam injection or exhaust gas recirculation. The influence of such measures on the detonability of the combustion mixture has been evaluated based on detonation cell sizes modelled with detailed chemistry. For the mixtures and operating conditions featuring promising detonability, NOx formation in the detonation wave has been simulated by solving the one-dimensional reacting Euler equations. The study enables an insight into the potential and limitations of considered measures for NOx emissions reduction and lays the groundwork for optimized operation of pulse detonation combustion systems.
TOPICS: Combustion, Explosions, Nitrogen oxides, Emissions, Gas turbines, Chemistry, Hydrogen, Steam, Pollution, Waves, Combustion systems, Exhaust gas recirculation
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038042
The dynamic characteristics of foil bearings operating at high rotation speeds depend very much on the mechanical characteristics of the foil structure. The mechanical characteristics of the foil structure are analyzed either by using a simple approach obtained for an isolated bump modeled as a beam or with more elaborate ones taking into account the three-dimensional nature of the bumps and their mutual interactions. These two kinds of models give different foil structure stiffness, with lower values for the simplified model. However, the published experimental results of the foil bearing structure tend to validate the simplified model. The present paper explains the differences between the simplified and the elaborate models by taking into account the manufacturing errors of the foil structure. A three-dimensional model based on the non-linear theory of elasticity is developed. Three realistic manufacturing errors, bump height, bump length and radius of the bump foil are analyzed. Bump height and length vary following a normal distribution with a given standard deviation while the radius of the bump foil is given a waviness form with an imposed peak-to-peak amplitude. Three to five cases were calculated for each kind of error. Results show that only the manufacturing errors of the bump height affect the stiffness of the foil structure by diminishing its values. Height errors of 20 µm standard deviation may induce a 40-50% reduction of the stiffness of the foil structure, i.e. in the range of the predictions of the simplified model.
TOPICS: Manufacturing, Numerical analysis, Errors, Stiffness, Foil bearings, Mechanical properties, Rotation, Elasticity, Three-dimensional models, Gaussian distribution
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038043
Literature shows that direct lubrication TPJBs exhibit unexpected shaft vibrations with a broadband low frequency range, albeit small in amplitude. Published industrial practice demonstrates the inlet lubrication type, a reduced supply flow rate causing film starvation, and the bearing discharge conditions (evacuated or end sealed) affect the onset, gravity, and persistency of the sub synchronous (SSV) hash motions. The paper presents a physical model to predict the performance of TPJBs with flow conditions ranging from over flooded to extreme starvation. Lubricant starvation occurs first on an unloaded pad, thus producing a (beneficial) reduction in drag power. As the supplied flowrate decreases further, fluid starvation moves towards the loaded pads and affects the film temperature and power loss, increases the journal eccentricity, and modifies the dynamic force coefficients of each tilting pad and thus the whole bearing. For a point mass rotor supported on a TPJB, the analysis produces eigenvalues and frequency response functions (FRFs). Predictions of rotordynamic performance follow for two TPJBs. Under increasingly poor lubricant flow conditions, the damping ratio for the rotor-bearing low frequency (SSV) modes decreases, thus producing an increase in the amplitude of the FRFs.A reduction in lubricant flow only exacerbates the phenomenon; starvation reaches the loaded pad to eventually cause the onset of low frequency (SSV) instability. The bearing analyzed showed similar behavior on a test bench. The predictions thus show a direct correlation between lubricant flow starvation and the onset of SSV.
TOPICS: Flow (Dynamics), Frequency response, Journal bearings, Lubricants, Bearings, Rotors, Lubrication, Temperature, Fluids, Drag (Fluid dynamics), Vibration, Eigenvalues, Damping, Gravity (Force)
Review Article
David A. Shifler
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038037
It has been conjectured that if sulfur in fuel is removed, engine materials will cease to experience attack from hot corrosion, since this sulfur has been viewed as the primary cause of hot corrosion and sulfidation. Historically, hot corrosion has been defined as an accelerated degradation process that generally involves deposition of corrosive species (e.g., sulfates) from the surrounding environment (e.g., combustion gas) onto the surface of hot components, resulting in destruction of the protective oxide scale. Most papers in the literature, since the 1970s, consider sodium sulfate salt as the single specie contributing to hot corrosion. Recent Navy standards for Navy F-76 and similar fuels have dropped the sulfur content down to 15 parts per million (ppm). Most observers believe that the removal of sulfur will end hot corrosion events in the Fleet. However, the deposit chemistry influencing hot corrosion is known to be much more complex consisting of multiple sulfates and silicates. Sulfur species may still enter the combustion chamber via ship's air intake, which may include seawater entrained in the air. In addition to sodium sulfate, seawater contains magnesium, calcium and potassium salts, and atmospheric contaminants that may contribute to hot corrosion. This paper will cover some of the revised understanding of hot corrosion and consider other possible contaminants that could further complicate a full understanding of hot corrosion.
TOPICS: Corrosion, Sulfur, Seawater, Sodium, Navy, Fuels, Engines, Combustion gases, Combustion chambers, Potassium, Chemistry, Magnesium (Metal)
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038036
This paper presents the experimental investigation of pulsation-amplitude-dependent flame dynamics associated with transverse thermoacoustic oscillations at screech level frequencies in a generic gas turbine combustor. Specifically, the flame behavior at different pulsation amplitude levels is assessed and interpreted. Spatial dynamics of the flame are measured by imaging the OH* chemiluminescence signal synchronously to the dynamic pressure at the combustor's face plate. First, linear thermoacoustic stability states, modal dynamics, and flame-acoustic phase relations are evaluated. It is found that the unstable acoustic modes converge into a predominantly rotating character with the mean flow swirl. Furthermore, the flame modulation is observed to be in phase with the acoustic pressure at all oscillation amplitude levels. Second, distributed flame dynamics are investigated by visualizing the mean and oscillating heat release distribution at different pulsation amplitudes. The observed flame dynamics are then compared against numerical evaluations of respective amplitude-dependent thermoacoustic growth rates, which are computed using analytical models in the fashion of a non-compact flame-describing function. While results show a non-linear contribution for the individual growth rates, superposition of flame deformation and displacements balances out to a constant flame driving. This latter observation contradicts the state-of-the-art perception of root-causes for limit-cycle oscillations in thermoacoustic gas turbine systems, for which the heat release saturates with increasing amplitudes. Consequently, the systematic analysis of amplitude-dependent flame modulation shows alternative paths to the explanation of mechanisms that might cause thermoacoustic saturation in high frequency systems.
TOPICS: Oscillations, Dynamics (Mechanics), Combustion chambers, Gas turbines, Flames, Heat, Acoustics, Chemiluminescence, Sound pressure, Pressure, Stability, Flow (Dynamics), Deformation, Signals, Imaging, Limit cycles
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038023
Experiments to investigate the effect of varying jet hole diameter and jet spacing on heat transfer and pressure loss from jet array impingement on a curved target surface are reported. The jet plate configurations studied have varying hole diameters and geometric spacing for spatial tuning of the heat transfer behaviour. The configuration also includes a straight section downstream of the curved section, where the effect on heat transfer and pressure loss is also investigated. A steady-state measurement technique utilizing temperature sensitive paint (TSP) was used on the target surface to obtain local heat transfer coefficients. Pressure taps placed on the sidewall and jet plate of the channel were used to evaluate the flow distribution in the impingement channel. First row jet Reynolds numbers ranging from 50,000 to 160,000 are reported. Further tests were performed to evaluate several modifications to the impingement array. These involve blocking several downstream rows of jets, measuring the subsequent shifts in the pressure and heat transfer data, and then applying different turbulator designs in an attempt to recover the loss in the heat transfer while retaining favorable pressure loss. It was found that by using W shaped turbulators, the downstream surface average Nusselt number increases up to ~13% as compared with a smooth case using the same amount of coolant .The results suggest that by properly combining impingement and turbulators (in the post impingement section), higher heat transfer, lower flow rate, and lower pressure drop are simultaneously obtained, thus providing an optimal scenario.
TOPICS: Pressure, Flow (Dynamics), Temperature, Heat transfer, Reynolds number, Coolants, Jets, Pressure drop, Steady state, Heat transfer coefficients
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038024
Turbochargers reduce fuel consumption and CO2 emissions from heavy-duty internal combustion engines by enabling downsizing and downspeeding through greater power density. This in turn raises turbine expansion ratio levels, leading to air systems with multiple stages and a need for interconnecting ducting, all subject to tight packaging constraints. This paper considers the aerodynamic optimization of the exhaust side of a two-stage air system for a Caterpillar 4.4-litre heavy-duty diesel engine, focusing on the high pressure turbine wheel and interstage duct. Using current production designs as a baseline, a genetic algorithm-based aerodynamic optimization process was carried out separately for the wheel and duct components in order to minimize the computational effort required to evaluate seven key operating points. While efficiency was a clear choice of cost function for turbine wheel optimization, different objectives were explored for interstage duct optimization to assess their impact. Optimization designs depended strongly on the engine operating point and so each case was evaluated at every other engine operating point, to determine the most appropriate designs to carry forward. Prototypes of the best compromise high pressure turbine wheel and interstage duct designs were manufactured and tested against baseline designs to validate CFD predictions. The best performing high pressure turbine design was predicted to show an efficiency improvement of 2.15 percentage points, for on-design operation. Meanwhile, the optimized interstage duct contributed a 0.2 and 0.5 percentage-point efficiency increase for the high and low pressure turbines, respectively.
TOPICS: High pressure (Physics), Optimization, Turbines, Diesel engines, Ducts, Wheels, Engines, Design, Internal combustion engines, Turbochargers, Engineering prototypes, Algorithms, Computational fluid dynamics, Fuel consumption, Emissions, Packaging, Power density, Exhaust systems, Carbon dioxide, Pressure
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038025
In this work a single sector lean burn model combustor operating in pilot only mode has been investigated using both experiments and computations with the main objective of analyzing the flame structure and soot formation at conditions relevant to aero-engine applications. Numerical simulations were performed using the Large Eddy Simulation (LES) approach and the Conditional Moment Closure (CMC) combustion model with detailed chemistry and a two-equation model for soot. The rig investigated in this work, called Big Optical Single Sector (BOSS) rig, allows to test real scale lean burn injectors. Experiments, performed at elevated pressure and temperature, corresponding to engine conditions at part load, include OH-PLIF and PDA and have been complemented with new LII measurements for soot location, allowing a comprehensive analysis of the primary combustion region and to further assess and validate the LES/CMC approach to capture the flame behaviour at engine conditions. It is shown that the LES/CMC approach is able to predict the main characteristics of the flame with a good agreement with the experiment in terms of flame shape, spray characteristics and soot location. Finite-rate chemistry effects appear to be important in the region very close to the injector exit leading to the lift-off of the flame. Low levels of soot are observed immediately downstream of the injector exit. Further downstream, the strong production of soot precursors together with high soot surface growth rates lead to high values of soot volume fraction in locations consistent with the experiment.
TOPICS: Computer simulation, Ceramic matrix composites, Combustion chambers, Flames, Soot, Ejectors, Engines, Combustion, Chemistry, Computation, Shapes, Aircraft engines, Large eddy simulation, Stress, Pressure, Temperature, Sprays
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038026
The reduction of noise emissions of modern aero engines represents a key concept to meet the requirements of the future air traffic. This requires an improvement in the understanding of combustion noise and its sources, as well as the development of accurate predictive tools. This is the major goal of the current study where the LOTAN network solver and a hybrid CFD/CAA approach are applied on a generic premixed and pressurized combustor to evaluate their capabilities for combustion noise predictions. LOTAN solves the linearized Euler equations (LEE) whereas the hybrid approach consists of RANS mean flow and frequency-domain simulations based on linearized Navier-Stokes equations (LNSE). Both solvers are fed in turn by three different combustion noise source terms which are obtained from the application of a statistical noise model on the RANS simulations and a postprocessing of an incompressible and compressible LES. In this way the influence of the source model and acoustic solver is identified. The numerical results are compared with experimental data. In general good agreement with the experiment is found for both the LOTAN and LNSE solvers. The LES source models deliver better results than the statistical noise model with respect to the amplitude and shape of the heat release spectrum. Beyond this it is demonstrated that the phase relation of the source term does not affect the noise spectrum. Finally, a second simulation based on the inhomogeneous Helmholtz equation indicates the minor importance of the aerodynamic mean flow on the broadband noise spectrum.
TOPICS: Combustion, Combustion chambers, Noise (Sound), Network models, Simulation, Reynolds-averaged Navier–Stokes equations, Flow (Dynamics), Heat, Acoustics, Navier-Stokes equations, Computational fluid dynamics, Shapes, Aircraft engines, Emissions, Air traffic control
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038021
Engine downsizing is a modern solution for the reduction of CO2 emissions from internal combustion engines. This technology has been gaining increasing attention from industry. In order to enable a downsized engine to operate properly at low speed conditions, it is essential to have a compressor stage with very good surge margin. The variable inlet guide vanes are a solution to this problem. By adjusting the setting angles of VIGVs, it is possible to shift the compressor map towards the smaller flow rates. However, this would also undermine the stage efficiency, require extra space for installing the IGVs, and add costs. The best solution is therefore to improve the design of impeller blade itself to attain high aerodynamic performances and wide operating ranges. This paper reports a recent study of using inverse design method for the redesign of a compressor stage used in an electric supercharger. The main requirements were to substantially increase the stable operating range of the compressor in order to meet the demands of the downsized engine. The 3D inverse design method was used to optimize the impeller geometry and achieve higher efficiency and stable operating range. The predicted performance map shows great advantages when compared with the existing design. To validate the CFD results, this new compressor stage has also been prototyped and tested. It will be shown that the CFD predictions have very good agreement with experiments and the redesigned compressor stage has improved the pressure ratio, aerodynamic efficiency, choke and surge margins considerably.
TOPICS: Superchargers, Engines, Compressors, Impellers, Surges, Computational fluid dynamics, Design, Design methodology, Internal combustion engines, Blades, Carbon dioxide, Geometry, Emissions, Inlet guide vanes, Pressure, Flow (Dynamics)
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038015
Light-end fuels have recently garnered interest as potential fuel for advanced compression ignition engines. This next generation of engines must meet increasingly stringent efficiency and emissions standards while minimizing cost, and novel fuels can assist in meeting these objectives. In this work, a 1-D heavy-duty engine model was validated with measured data and then used to generate boundary conditions for the detailed chemical kinetic simulation corresponding to various combustion modes and operating points. Using these boundary conditions, homogeneous simulations were conducted for 242 fuels with research octane number (RON) from 40 to 100 and sensitivity (S) from 0 to 12. Combustion phasing (CA50) was most dependent on RON and less dependent on S under all conditions. Both RON and S had a greater effect on combustion phasing under partially-premixed compression ignition (PPCI) conditions (19.3°) than under mixing-controlled combustion (MCC) conditions (5.8°). The effect of RON and S were also greatest for the lowest reactivity (RON>90) fuels and under low-load conditions. The results for CA50 reflect the relative ignition delay for the various fuels at the start-of-injection (SOI) temperature. At higher SOI temperatures (>950K), CA50 was found to be less dependent on fuel sensitivity due to the convergence of ignition delay behavior of different fuels in the high-temperature region. This work provides a first look at quantifying the effect of light-end fuel chemistry on advanced CI engine combustion across the entire light-end fuel reactivity space.
TOPICS: Combustion, Fuels, Engines, Simulation, Boundary-value problems, Silicon-on-insulator, Diesel engines, Temperature, Ignition delay, Ignition, Emissions, High temperature, Chemistry, Compression, Stress
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4038016
The increasing amount of renewable energy sources drives the necessity of flexible conventional power plants to compensate for power supply fluctuations. Gas turbines in combined cycle power plants (CCPPs) adjust power output quickly but a sudden increase of CO and UHC emissions limit their turn-down ratio. To extend the turn-down ratio, part of the fuel can be processed to syngas, which exerts a higher reactivity. An autothermal on-board syngas generator in combination with two different burner concepts for natural gas and syngas mixtures are presented here. A mixture of natural gas, water vapor and air reacts catalytically in an autothermal reactor test rig to syngas. At atmospheric pressure, the fuel processor generates syngas with a hydrogen content of 30 vol-% and a temperature of 800 K within a residence time of 200 ms. One concept for the combustion of natural gas/syngas mixtures comprises a generic swirl stage with a central lance injector for the syngas. The second concept includes a central swirl stage with a ring of jets. Both concepts are analyszed by OH*-chemiluminescence, lean blow out (LBO) limit and gaseous emissions. The central lance concept with syngas injection exhibits an LBO adiabatic flame temperature that is 150 K lower than in premixed natural gas operation. For the second concept an extension of 200 K with low CO emission levels can be reached. This study shows that autothermal on-board syngas generation is efficient in terms of turn-down ratio extension and CO burn-out.
TOPICS: Stress, Syngas, Natural gas, Emissions, Fuels, Combined cycle power stations, Temperature, Water vapor, Combustion, Atmospheric pressure, Fluctuations (Physics), Jets, Ejectors, Gas turbines, Flames, Generators, Hydrogen, Renewable energy sources, Chemiluminescence, Power stations
research-article
J. Eng. Gas Turbines Power   doi: 10.1115/1.4037960
This article focuses on Combustion Instabilities (CI) driven by entropy fluctuations which is of great importance in practical devices. A simplified geometry is introduced. It keeps the essential features of an aeronautical combustion chamber (swirler, dilution holes, outlet nozzle) while it is simplified sufficiently to ease the analysis (rectangular vane, one row of holes of the same diameter, no diffuser at the inlet of the chamber, circular nozzle at the outlet). A Large Eddy Simulation (LES) is carried out on this geometry and the limit cycle of a strong CI involving the convection of an entropy spot is obtained. The behavior of the instability is analyzed using phenomenological description and classical signal analysis. One shows that the system can be better described by considering two reacting zones: a rich mainly premixed flame is located downstream of the swirler and an overall lean diffusion flame is stabilized next to the dilution holes. In a second step, Dynamic Mode Decomposition (DMD) is used to visualize, analyze and model the complex phasing between the different processes affecting the reacting zones. Using these data, a 0D modeling of the premixed flame and of the diffusion flame is proposed. These models provides an extended understanding of the combustion process in an aeronautical combustor and could be used or adapted to address mixed acoustic-entropy CI in an acoustic code.
TOPICS: Combustion, Acoustics, Entropy, Modeling, Combustion chambers, Large eddy simulation, Diffusion flames, Nozzles, Flames, Geometry, Signals, Fluctuations (Physics), Limit cycles, Diffusers, Convection