Showing papers in "Journal of Engineering for Gas Turbines and Power-transactions of The Asme in 2009"

Detailed Numerical Simulations of the Primary Atomization of a Turbulent Liquid Jet in Crossflow

Abstract: This paper presents numerical simulation results of the primary atomization of a turbulent liquid jet injected into a gaseous crossflow. Simulations are performed using the balanced force refined level set grid method. The phase interface during the initial breakup phase is tracked by a level set method on a separate refined grid. A balanced force finite volume algorithm together with an interface projected curvature evaluation is used to ensure the stable and accurate treatment of surface tension forces even on small scales. Broken off, small scale nearly spherical drops are transferred into a Lagrangian point particle description allowing for full two-way coupling and continued secondary atomization. The numerical method is applied to the simulation of the primary atomization region of a turbulent liquid jet (q=6.6, We=330, Re=14,000) injected into a gaseous crossflow (Re=570,000), analyzed experimentally by Brown and McDonell (2006, “Near Field Behavior of a Liquid Jet in a Crossflow,” ILASS Americas, 19th Annual Conference on Liquid Atomization and Spray Systems). The simulations take the actual geometry of the injector into account. Grid converged simulation results of the jet penetration agree well with experimentally obtained correlations. Both column/bag breakup and shear/ligament breakup modes can be observed on the liquid jet. A grid refinement study shows that on the finest employed grids (flow solver 64 points per injector diameter, level set solver 128 points per injector diameter), grid converged drop sizes are achieved for drops as small as one-hundredth the size of the injector diameter.

Low NOx and Low Smoke Operation of a Diesel Engine Using Gasolinelike Fuels

Abstract: Much of the technology in advanced diesel engines, such as high injection pressures, is aimed at overcoming the short ignition delay of conventional diesel fuels to promote premixed combustion in order to reduce NOx and smoke. Previous work in a 2 l single-cylinder diesel engine with a compression ratio of 14 has demonstrated that gasoline fuel, because of its high ignition delay, is very beneficial for premixed compression-ignition compared with a conventional diesel fuel. We have now done similar studies in a smaller-0.537 l-single-cylinder diesel engine with a compression ratio of 15.8. The engine was run on three fuels of very different auto-ignition quality-a typical European diesel fuel with a cetane number (CN) of 56, a typical European gasoline of 95 RON and 85 MON with an estimated CN of 16 and another gasoline of 84 RON and 78 MON (estimated CN of 21). The previous results with gasoline were obtained only at 1200 rpm-here we compare the fuels also at 2000 rpm and 3000 rpm. At 1200 rpm, at low loads (similar to 4 bars indicated mean effective pressure (IMEP)) when smoke is negligible, NOx levels below 0.4 g/kWh can be easily attained with gasoline without using exhaust gas recirculation (EGR), while this is not possible with the 56 CN European diesel. At these loads, the maximum pressure-rise rate is also significantly lower for gasoline. At 2000 rpm, with 2 bars absolute intake pressure, NOx can be reduced below 0.4 g/kW h with negligible smoke (FSN < 0.1) with gasoline between 10 bars and 12 bars IMEP using sufficient EGR, while this is not possible with the diesel fuel. At 3000 rpm, with the intake pressure at 2.4 bars absolute, NOx of 0.4 g/kW h with negligible smoke was attainable with gasoline at 13 bars IMEP. Hydrocarbon and CO emissions are higher for gasoline and will require after-treatment. High peak heat release rates can be alleviated using multiple injections. Large amounts of gasoline, unlike diesel, can be injected very early in the cycle without causing heat release during the compression stroke and this enables the heat release profile to be shaped. [DOI: 10.1115/1.4000602] (Less)

Application of Exhaust Gas Recirculation in a DLN F-Class Combustion System for Postcombustion Carbon Capture

Abstract: This paper describes experimental work performed at General Electric, Global Research Center to evaluate the performance and understand the risks of using dry low NO x (DLN) technologies in exhaust gas recirculation (EGR) conditions. Exhaust gas recirculation is viewed as an enabling technology for increasing the CO 2 concentration of the flue gas while decreasing the volume of the postcombustion separation plant and therefore allowing a significant reduction in CO 2 capture cost. A research combustor was developed for exploring the performance of nozzles operating in low O 2 environment at representative pressures and temperatures. A series of experiments in a visually accessible test rig have been performed at gas turbine pressures and temperatures, in which inert gases such as N 2 /CO 2 were used to vitiate the fresh air to the levels determined by cycle models. Moreover, the paper discusses experimental work performed using a DLN nozzle used in GE's F-class heavy-duty gas turbines. Experimental results using a research combustor operating in a partially premixed mode include the effect of EGR on operability, efficiency, and emission performance under conditions of up to 40% EGR. Experiments performed in a fully premixed mode using a DLN single nozzle combustor revealed that further reductions in NO x could be achieved while at the same time still complying with CO emissions. While most existing studies concentrate on limitations related to the minimum oxygen concentration (MOC) at the combustor exit, we report the importance of CO 2 levels in the oxidizer. This limitation is as important as the MOC, and it varies with the pressure and firing temperatures.

Multiharmonic Forced Response Analysis of a Turbine Blading Coupled by Nonlinear Contact Forces

Abstract: The rotor blades of a low pressure (LP) steam turbine stage are subjected to high static and dynamic loads during operation. The static loads are mainly due to the centrifugal force and thermal strains, whereas the dynamic loads are caused by fluctuating gas forces resulting in forced vibrations of the blades. The forced vibrations can lead to high cycle fatigue (HCF) failures causing substantial damage and high maintenance effort. Thus, one of the main tasks in the design of LP steam turbine blading is the vibration amplitude reduction in order to avoid high dynamic stresses that could damage the blading. The vibration amplitudes of the blades in a LP steam turbine stage can be reduced significantly to a reasonable amount if adjacent blades are coupled by shroud contacts that reinforce the blading, see Fig. 1. Furthermore, in the case of blade vibrations, relative displacements between neighboring blades occur in the contacts and friction forces are generated that provide additional damping to the structure due to the energy dissipation caused by micro- and macroslip effects. Therefore, the coupling of the blades increases the overall mechanical damping. A three-dimensional structural dynamics model including an appropriate spatial contact model is necessary to predict the contact forces generated by the shroud contacts and to describe the vibrational behavior of the blading with sufficient accuracy. To compute the nonlinear forced vibrations of the coupled blading, the nonlinear equations of motion are solved in the frequency domain owing to the high computational efficiency of this approach. The transformation of the nonlinear equations of motion into the frequency domain can be carried out by representing the steady-state displacement in terms of its harmonic components. After that transformation, the nonlinear forced response is computed as a function of the excitation frequency in the frequency domain.Copyright © 2009 by ASME

Characterization of forced flame response of swirl-stabilized turbulent lean-premixed flames in a gas turbine combustor

Abstract: Flame transfer function measurements of turbulent premixed flames are made in a model lean-premixed, swirl-stabilized, gas turbine combustor. OH∗, CH∗, and CO2∗ chemiluminescence emissions are measured to determine heat release oscillation from a whole flame, and the two-microphone technique is used to measure inlet velocity fluctuation. 2D CH∗ chemiluminescence imaging is used to characterize the flame shape: the flame length (LCH∗ max) and flame angle (α). Using H2-natural gas composite fuels, XH2=0.00–0.60, a very short flame is obtained and hydrogen enrichment of natural gas is found to have a significant impact on the flame structure and flame attachment points. For a pure natural gas flame, the flames exhibit a “V” structure, whereas H2-enriched natural gas flames have an “M” structure. Results show that the gain of M flames is much smaller than that of V flames. Similar to results of analytic and experimental investigations on the flame transfer function of laminar premixed flames, it shows that the dynamics of a turbulent premixed flame is governed by three relevant parameters: the Strouhal number (St=LCH∗ max/Lconv), the flame length (LCH∗ max), and the flame angle (α). Two flames with the same flame shape exhibit very similar forced responses, regardless of their inlet flow conditions. This is significant because the forced flame response of a highly turbulent, practical gas turbine combustor can be quantitatively generalized using the nondimensional parameters, which collapse all relevant input conditions into the flame shape and the Strouhal number.

The Reheat Concept: The Proven Pathway to Ultralow Emissions and High Efficiency and Flexibility

Abstract: Reheat combustion has been proven now in over 80 units to be a robust and highly flexible gas turbine concept for power generation. This paper covers three key topics to explain the intrinsic advantage of reheat combustion to achieve ultralow emission levels. First, the fundamental kinetic and thermodynamic emission advantage of reheat combustion is discussed, analyzing in detail the emission levels of the first and second combustor stages, optimal firing temperatures for minimal emission levels, as well as benchmarking against single-stage combustion concepts. Second, the generic operational and fuel flexibility of the reheat system is emphasized, which is based on the presence of two fundamentally different flame stabilization mechanisms, namely, flame propagation in the first combustor stage and autoignition in the second combustor stage. This is shown using simple reasoning on generic kinetic models. Finally, the present fleet status is reported by highlighting the latest combustor hardware upgrade and its emission performance.

Losses in High Speed Permanent Magnet Machines Used in Microturbine Applications

Abstract: High speed permanent magnet (PM) machines are used in microturbine applications due to their compactness, robust construction, and high efficiency characteristics. These machines are integrated with the turbines and rotate at the same speeds. This paper discusses in detail the losses in high speed PM machines. A typical PM machine designed for microturbine application is presented with its detailed loss calculations. Various loss verification methods are also discussed.

Thermohydrodynamic Analysis of Bump Type Gas Foil Bearings: A Model Anchored to Test Data

Abstract: The paper introduces a thermohydrodynamic (THD) model for prediction of gas foil bearing (GFB) performance. The model includes thermal energy transport in the gas film region, and with cooling gas streams, inner or outer, as in typical rotor-GFBs systems. The analysis also accounts for material property changes and the bearing components’ expansion due to temperature rises and shaft centrifugal growth due to rotational speed. Gas inlet feed characteristics are thoroughly discussed in bearings whose top foil must detach, i.e., not allowing for subambient pressure. Thermal growths determine the actual bearing clearance needed for accurate prediction of GFB forced performance, static and dynamic. Model predictions are benchmarked against published measurements of (metal) temperatures in a GFB operating without a forced cooling gas flow. The tested foil bearing is proprietary; hence its geometry and material properties are largely unknown. Predictions are obtained for an assumed bearing configuration, with bump-foil geometry and materials taken from prior art and best known practices. The predicted film peak temperature occurs just downstream of the maximum gas pressure. The film temperature is higher at the bearing middle plane than at the foil edges, as the test results also show. The journal speed, rather than the applied static load, influences more the rise in film temperature and with a larger thermal gradient towards the bearing sides. In addition, as in the tests conducted at a constant rotor speed and even for the lowest static load, the gas film temperature increases rapidly due to the absence of a forced cooling air that could carry away the recirculation gas flow and thermal energy drawn by the spinning rotor, Predictions are in good agreement with the test data. A comparison of predicted static load parameters to those obtained from an isothermal condition shows the THD model producing a smaller journal eccentricity (larger minimum film thickness) and larger drag torque. A rise in gas temperature is tantamount to an increase in gas viscosity, hence the noted effect in the foil bearing forced performance.Copyright © 2009 by ASME

Measurement of Structural Stiffness and Damping Coefficients in a Metal Mesh Foil Bearing

Abstract: Engineered Metal Mesh Foil Bearings (MMFB) are a promising low cost bearing technology for oil-free microturbomachinery. In a MMFB, a ring shaped metal mesh (MM) provides a soft elastic support to a smooth arcuate foil wrapped around a rotating shaft. The paper details the construction of a MMFB and the static and dynamic load tests conducted on the bearing for estimation of its structural stiffness and equivalent viscous damping. The 28.00 mm diameter, 28.05 mm long bearing, with a metal mesh ring made of 0.3 mm Copper wire and compactness of 20%, is installed on a test shaft with a slight preload. Static load versus bearing deflection measurements display a cubic nonlinearity with large hysteresis. The bearing deflection varies linearly during loading, but nonlinearly during the unloading process. An electromagnetic shaker applies on the test bearing loads of controlled amplitude over a frequency range. In the frequency domain, the ratio of applied force to bearing deflection gives the bearing mechanical impedance, whose real part and imaginary part give the structural stiffness and damping coefficients, respectively. As with prior art published in the literature, the bearing stiffness decreases significantly with the amplitude of motion and shows a gradual increasing trend with frequency. The bearing equivalent viscous damping is inversely proportional to the excitation frequency and motion amplitude. Hence, it is best to describe the mechanical energy dissipation characteristics of the MMFB with a structural loss factor (material damping). The experimental results show a loss factor as high as 0.7 though dependent on the amplitude of motion. Empirically based formulas, originally developed for metal mesh rings, predict bearing structural stiffness and damping coefficients agreeing well with the experimentally estimated parameters. Note, however, that the metal mesh ring, after continuous operation and various dismantling and reassembly processes, showed significant creep or sag that resulted in a gradual decrease of its structural force coefficients.Copyright © 2009 by ASME

Degradation Effects on Industrial Gas Turbines

Abstract: This paper provides a discussion on how degradation develops and affects the performance of the gas turbine. Because the function of a gas turbine is the result of the fine-tuned cooperation of many different components, the emphasis of this paper is on the gas turbine and its components as a system. Therefore, the interaction of components is studied in detail. An engine model is subjected to various types of degradation, and the effect on operating parameters is studied. The focus is on three areas: How does component degradation impact the operating points of the engine compressor, how does component degradation impact full load and part load gas turbine performance characteristics, and how does component degradation impact measurable engine operating parameters. Experimental data are provided that supports the theoretical conclusion. Parameters that indicate levels of degradation are outlined, thus providing guidance for condition monitoring practice.

Forced Response Prediction of Constrained and Unconstrained Structures Coupled Through Frictional Contacts

Abstract: In this paper, a forced response prediction method for the analysis of constrained and unconstrained structures coupled through frictional contacts is presented. This type of frictional contact problem arises in vibration damping of turbine blades, in which dampers and blades constitute the unconstrained and constrained structures, respectively. The model of the unconstrained/free structure includes six rigid body modes and several elastic modes, the number of which depends on the excitation frequency. In other words, the motion of the free structure is not artificially constrained. When modeling the contact surfaces between the constrained and free structure, discrete contact points along with contact stiffnesses are distributed on the friction interfaces. At each contact point, contact stiffness is determined and employed in order to take into account the effects of higher frequency modes that are omitted in the dynamic analysis. Depending on the normal force acting on the contact interfaces, quasistatic contact analysis is initially employed to determine the contact area as well as the initial preload or gap at each contact point due to the normal load. A friction model is employed to determine the three-dimensional nonlinear contact forces, and the relationship between the contact forces and the relative motion is utilized by the harmonic balance method. As the relative motion is expressed as a modal superposition, the unknown variables, and thus the resulting nonlinear algebraic equations in the harmonic balance method, are in proportion to the number of modes employed. Therefore the number of contact points used is irrelevant. The developed method is applied to a bladed-disk system with wedge dampers where the dampers constitute the unconstrained structure, and the effects of normal load on the rigid body motion of the damper are investigated. It is shown that the effect of rotational motion is significant, particularly for the in-phase vibration modes. Moreover, the effect of partial slip in the forced response analysis and the effect of the number of harmonics employed by the harmonic balance method are examined. Finally, the prediction for a test case is compared with the test data to verify the developed method. DOI: 10.1115/1.2940356

Optimal Tuner Selection for Kalman Filter-Based Aircraft Engine Performance Estimation

Abstract: A linear point design methodology for minimizing the error in on-line Kalman filter-based aircraft engine performance estimation applications is presented. This technique specifically addresses the underdetermined estimation problem, where there are more unknown parameters than available sensor measurements. A systematic approach is applied to produce a model tuning parameter vector of appropriate dimension to enable estimation by a Kalman filter, while minimizing the estimation error in the parameters of interest. Tuning parameter selection is performed using a multi-variable iterative search routine which seeks to minimize the theoretical mean-squared estimation error. This paper derives theoretical Kalman filter estimation error bias and variance values at steady-state operating conditions, and presents the tuner selection routine applied to minimize these values. Results from the application of the technique to an aircraft engine simulation are presented and compared to the conventional approach of tuner selection. Experimental simulation results are found to be in agreement with theoretical predictions. The new methodology is shown to yield a significant improvement in on-line engine performance estimation accuracy

Characterization of Partially Premixed Combustion with Ethanol: EGR sweeps, Low and Maximum Loads

Abstract: Exhaust gas recirculation (EGR) sweeps were performed on ethanol partially premixed combustion (PPC) to show different emission and efficiency trends as compared with diesel PPC. The sweeps showed that when the EGR rate is increased, the efficiency does not diminish, HC trace is flat, and CO is low even with 45% of EGR. NOx exponentially decreases by increasing EGR while soot levels are nearly zero throughout the sweep. The EGR sweeps underlined that at high EGR levels, the pressure rise rate is a concern. To overcome this problem and keep high efficiency and low emissions, a sweep in the timing of the pilot injection and pilot-main ratio was done at similar to 16.5 bars gross IMEP. It was found that with a pilot-main ratio of 50: 50, and by placing the pilot at similar to 60 with 42% of EGR, NOx and soot are below EURO VI levels; the indicated efficiency is 47% and the maximum pressure rise rate is below 10 bar/CAD. Low load conditions were examined as well. It was found that by placing the start of injection at similar to 35 top dead center, the efficiency is maximized, on the other hand, when the injection is at similar to 25, the emissions are minimized, and the efficiency is only 1.64% lower than its optimum value. The idle test also showed that a certain amount of EGR is needed in order to minimize the pressure rise rate. (Less)

Ignition and Oxidation of 50/50 Butane Isomer Blends

Abstract: One of the alkanes found within gaseous fuel blends of interest to gas turbine applications is butane. There are two structural isomers of butane, normal butane and iso-butane, and the combustion characteristics of either isomer are not well known. Of particular interest to this work are mixtures of n-butane and iso-butane. A shock-tube experiment was performed to produce important ignition delay time data for these binary butane isomer mixtures which are not currently well studied, with emphasis on 50–50 blends of the two isomers. These data represent the most extensive shock-tube results to date for mixtures of n-butane and iso-butane. Ignition within the shock tube was determined from the sharp pressure rise measured at the endwall which is characteristic of such exothermic reactions. Both experimental and kinetics modeling results are presented for a wide range of stoichiometry (φ = 0.3–2.0), temperature (1056–1598 K), and pressure (1–21 atm). The results of this work serve as validation for the current chemical kinetics model. Correlations in the form of Arrhenius-type expressions are presented which agree well with both the experimental results and the kinetics modeling. The results of an ignition-delay-time sensitivity analysis are provided, and key reactions are identified. The data from this study are compared with the modeling results of 100% normal butane and 100% iso-butane. The 50/50 mixture of n-butane and iso-butane was shown to be more readily ignitable than 100% iso-butane but reacts slower than 100% n-butane only for the richer mixtures. There was little difference in ignition time between the lean mixtures.Copyright © 2009 by ASME

Combustion Performance of Biodiesel and Diesel-Vegetable Oil Blends in a Simulated Gas Turbine Burner

Abstract: Recent increases in fuel costs, concerns for global warming, and limited supplies of fossil fuels have prompted wide spread research on renewable liquid biofuels produced domestically from agricultural feedstock. In this study, two types of biodiesels and vegetable oil (VO) are investigated as potential fuels for gas turbines to generate power. Biodiesels produced from VO and animal fat were considered in this study. The problems of high viscosity and poor volatility of VO (soybean oil) were addressed by using diesel-VO blends with up to 30% VO by volume. Gas chromatography/mass spectrometry, thermogravimetric analysis, and density, kinematic viscosity, surface tension, and water content measurements were used to characterize the fuel properties. The combustion performance of different fuels was compared experimentally in an atmospheric pressure burner with an air-assist injector and swirling primary air around it. For different fuels, the effect of the atomizing airflow rate on Sauter mean diameter was determined from a correlation for air-assist atomizers. Profiles of nitric oxides (NO x ) and carbon monoxide (CO) emissions were obtained for different atomizing airflow rates, while the total airflow rate was kept constant. The results show that despite the compositional differences, the physical properties and emissions of the two biodiesel fuels are similar. Diesel-VO fuel blends resulted in slightly higher CO emissions compared with diesel, while the NO x emissions correlated well with the flame temperature. The results show that the CO and NO x emissions are determined mainly by fuel atomization and fuel/air mixing processes, and that the fuel composition effects are of secondary importance for fuels and operating conditions of the present study.

Investigation of High-Temperature Printed Circuit Heat Exchangers for Very High Temperature Reactors

Abstract: Very high-temperature reactors require high-temperature (900-950°C) and high-integrity heat exchangers with high effectiveness during normal and off-normal conditions. A class of compact heat exchangers, namely the printed circuit heat exchangers (PCHEs), made of high-temperature materials and found to have these above characteristics, are being increasingly pursued for heavy duty applications. A high-temperature helium test facility, primarily aimed at investigating the heat transfer and pressure drop characteristics of the PCHEs, was designed and is being built at Ohio State University. The test facility was designed to facilitate operation at temperatures and pressures up to 900°C and 3 MPa, respectively. Owing to the high operating conditions, a detailed investigation on various high-temperature materials was carried out to aid in the design of the test facility and the heat exchangers. The study showed that alloys 617 and 230 are the leading candidate materials for high-temperature heat exchangers. Two PCHEs, each having 10 hot plates and 10 cold plates, with 12 channels in each plate, were fabricated from alloy 617 plates and will be tested once the test facility is constructed. Simultaneously computational fluid dynamics calculations have been performed on a simplified PCHE model, and the results for three flow rate cases of 15, 40, and 80 kg/h at a system pressure of 3 MPa are discussed. In summary, this paper focuses on the study of the high-temperature materials, the design of the helium test facility, the design and fabrication of the PCHEs, and the computational modeling of a simplified PCHE model.

The Effects of Diesel Injector Needle Motion on Spray Structure

Abstract: The internal structure of diesel fuel injectors is known to have a significant impact on the steady-state fuel distribution within the spray. However, little experimental or computational work has been performed on the dynamics of fuel injectors. Recent studies have shown that it is possible to measure the three-dimensional geometry of the injector nozzle, and to track changes in that geometry as the needle opens and closes in real time. This has enabled the dynamics of the injector to be compared with the dynamics of the spray, and allows computational fluid dynamics (CFD) simulations to use realistic time-dependent flow passage geometries. In this study, X-ray phase-enhanced imaging has been used to perform time-resolved imaging of the needle seat area in several common-rail diesel injection nozzles. The fuel distributions of the sprays emitted by these injectors were also studied with fast X-ray radiography. Correlations between eccentric motions of the injector needle valve and oscillations in the fuel density as it emerges from the nozzle are examined. CFD modeling is used to interpret the effect of needle motion on fuel flow.

Design, Fabrication, and Performance of Foil Gas Thrust Bearings for Microturbomachinery Applications

Abstract: Amethodologyforthedesignandconstructionofsimplefoilthrust bearingsintendedfor parametricperformancetestingandlow marginal costs is presented. Features drawn from a reviewof the openliterature are discussed as they relate to bearingper-formance. The design of fixtures and tooling required to fab-ricate foil thrust bearings is presented, using conventional ma-chining processes where possible. A prototype bearing with di-mensionsdrawnfrom theliteratureis constructed,with allfabri-cationsteps described. Aload-deflectioncurvefor thebearingispresented to illustrate structural stiffness characteristics. Start-stop cycles are performed on the bearing at a temperature of425 ◦ C to demonstrate early-life wear patterns. A test of bearingload capacity demonstrates useful performance when comparedwith data obtained from the open literature. Introduction Foil gas bearings represent an enabling technology foradvanced oil-free turbomachinery systems. Operating athigh speeds and temperatures, these next-generation turboma-chines will present tribological challenges that conventional oil-lubricated rolling element bearings may be unable to meet, in-cluding shaft speeds well above three million DN and bearingtemperatures in excess of 400

Description of a Semi-Independent Time Discretization Methodology for a One-Dimensional Gas Dynamics Model

Abstract: Modeling has become an essential technique in design and opti- mization processes of internal combustion engines. As a conse- quence, the development of accurate modeling tools is, in this moment, an important research topic. In this paper, a gas- dynamics modeling tool is presented. The model is able to repro- duce the global behavior of complete engines. This paper empha- sizes an innovative feature: the independent time discretization of ducts. It is well known that 1D models solve the flow through the duct by means of finite difference methods in which a stability requirement limits the time step depending on the mesh size. Thus, the use of small ducts in some parts of the engine reduces the speed of the calculation. The model presented solves this limita- tion due to the independent calculation for each element. The different elements of the engine are calculated following their own stability criterion and a global manager of the model intercon- nects them. This new structure provides time saving of up to 50% depending on the engine configuration. DOI: 10.1115/1.2983015

Modeling Soot Formation Using Reduced Polycyclic Aromatic Hydrocarbon Chemistry in n-Heptane Lifted Flames With Application to Low Temperature Combustion

Abstract: A numerical study of in-cylinder soot formation and oxidation processes in n-heptane lifted flames using various soot inception species has been conducted. In a recent study by the authors, it was found that the soot formation and growth regions in lifted flames were not adequately represented by using acetylene alone as the soot inception species. Comparisons with a conceptual model and available experimental data suggested that the location of soot formation regions could be better represented if polycyclic aromatic hydrocarbon (PAH) species were considered as alternatives to acetylene for soot formation processes. Since the local temperatures are much lower under low temperature combustion conditions, it is believed that significant soot mass contribution can be attributed to PAH rather than to acetylene. To quantify and validate the above observations, a reduced n-heptane chemistry mechanism has been extended to include PAH species up to four fused aromatic rings (pyrene). The resulting chemistry mechanism was integrated into the multidimensional computational fluid dynamics code KIVA-CHEMKIN for modeling soot formation in lifted flames in a constant volume chamber. The investigation revealed that a simpler model that only considers up to phenanthrene (three fused rings) as the soot inception species has good possibilities for better soot location predictions. The present work highlights and illustrates the various research challenges toward accurate qualitative and quantitative predictions of the soot for new low emission combustion strategies for internal combustion engines.

Carbon Capture for Automobiles Using Internal Combustion Rankine Cycle Engines

Abstract: Internal combustion Rankine cycle (ICRC) power plants use oxy-fuel firing with recycled water in place of nitrogen to control combustion temperatures. High efficiency and specific power output can be achieved with this cycle, but importantly, the exhaust products are only CO 2 and water vapor: The CO 2 can be captured cheaply on condensation of the water vapor. Here we investigate the feasibility of using a reciprocating engine version of the ICRC cycle for automotive applications. The vehicle will carry its own supply of oxygen and store the captured CO 2 . On refueling with conventional gasoline, the CO 2 will be off-loaded and the oxygen supply replenished. Cycle performance is investigated on the basis of fuel-oxygen-water cycle calculations. Estimates are made for the system mass, volume, and cost and compared with other power plants for vehicles. It is found that high thermal efficiencies can be obtained and that huge increases in specific power output are achievable. The overall power-plant system mass and volume will be dominated by the requirements for oxygen and CO 2 storage. Even so, the performance of vehicles with ICRC power plants will be superior to those based on fuel cells and they will have much lower production costs. Operating costs arising from supply of oxygen and disposal of the CO 2 are expected to be around 20 c/l of gasoline consumed and about $25/tonne of carbon controlled. Over all, ICRC engines are found to be a potentially competitive option for the powering of motor vehicles in the forthcoming carbon-controlled energy market.

Lubricant-Derived Ash Properties and Their Effects on Diesel Particulate Filter Pressure-Drop Performance

Abstract: Diesel particulate filters (DPF) have seen widespread use in on- and off-road applications as an effective means for meeting increasingly stringent particle emissions regulations. Over time, incombustible material or ash, primarily derived from metallic additives in the engine lubricant, accumulates in the DPF. Ash accumulation leads to increased flow restriction and an associated increase in pressure drop across the particulate filter, negatively impacting engine performance and fuel economy, and eventually requiring periodic filter service or replacement. While the adverse effects of ash accumulation on DPF performance are well known, the underlying mechanisms controlling these effects are not. The results of this work show ash accumulation and distribution in the DPF as a dynamic process with each stage of ash accumulation altering the filter’s pressure drop response. Through a combined approach employing targeted experiments and comparison with the existing knowledge base, this work further demonstrates the significant effect ash deposits have on DPF pressure drop sensitivity to soot accumulation. Ash deposits reduce the available filtration area, resulting in locally elevated soot loads and higher exhaust gas velocities through the filter, altering the conditions under which the soot is deposited and ultimately control the filter’s pressure drop characteristics. In this study, a novel accelerated ash loading system was employed to generate the ash and load the DPFs under carefully-controlled exhaust conditions. The ash loading system was coupled to the exhaust of a Cummins ISB diesel engine, allowing for accelerated ash loading and DPF performance evaluation with realistic exhaust conditions. Following DPF performance evaluation, the filters were subjected to a detailed post-mortem analysis in which key ash properties were measured and quantified. The experimental results, coupled with the ash property measurements, provide additional insight into the underlying physical mechanisms controlling ash properties, ash/soot interactions, and their effects on DPF performance.© 2009 ASME

Passive Control of the Inlet Acoustic Boundary of a Swirled Burner at High Amplitude Combustion Instabilities

Abstract: Perforated panels placed upstream of the premixing tube of a turbulent swirled burner are investigated as a passive control solution for combustion instabilities. Perforated panels backed by a cavity are widely used as acoustic liners, mostly in the hot gas region of combustion chambers to reduce pure tone noises. This paper focuses on the use of this technology in the fresh reactants zone to control the inlet acoustic reflection coefficient of the burner and to stabilize the combustion. This method is shown to be particularly efficient because high acoustic fluxes issued from the combustion region are concentrated on a small surface area inside the premixer. Theoretical results are used to design two types of perforated plates featuring similar acoustic damping properties when submitted to low amplitude pressure fluctuations (linear regime). Their behaviors nonetheless largely differ when facing large pressure fluctuation levels (nonlinear regime) typical of those encountered during self-sustained combustion oscillations. Conjectures are given to explain these differences. These two plates are then used to clamp thermoacoustic oscillations. Significant damping is only observed for the plate featuring a robust response to increasing sound levels. While developed on a laboratory scale swirled combustor, this method is more general and may be adapted to more practical configurations.

Testing and Calibration Procedures for Mistuning Identification and Traveling Wave Excitation of Blisks

Abstract: In this work, an integrated testing and calibration procedure is presented for performing mistuning identification (ID) and traveling wave excitation (TWE) of one-piece bladed disks (blisks). The procedure yields accurate results while also being highly efficient and is comprised of three basic phases. First, selected modes from a tuned blisk finite element model are used to determine a minimal set of measurement degrees of freedom (and locations) that will work well for mistuning ID. Second, a testing procedure is presented that allows the mistuning to be identified from relatively few vibration response measurements. A numerical validation is used to investigate the convergence of the mistuning ID results to a prescribed mistuning pattern using the proposed approach and alternative testing strategies. Third, a method is derived to iteratively calibrate the excitation applied to each blade so that differences among the blade excitation magnitudes can be minimized for single blade excitation, and also the excitation phases can be accurately set to achieve the desired traveling wave excitation. The calibration algorithm uses the principle of reciprocity and involves solving a least squares problem to reduce the effects of measurement noise and uncertainty. Because the TWE calibration procedure re-uses data collected during the mistuning ID, the overall procedure is integrated and efficient.Copyright © 2009 by ASME

Multiobjective Robust Regulating and Protecting Control for Aeroengines

Abstract: The multiobjective regulating and protecting control method presented here will enable improved control of multiloop switching control of an aeroengine. The approach is based on switching control theory, the switching performance objectives and the strategy are given, and a family of H ∞ proportional-integral-derivative controllers was designed by using linear matrix inequality optimization algorithm. The simulation shows that using the switching control design method not only can improve the dynamic performance of the engine control system but also can guarantee the stability in some peculiar occasions.

Sensor Thermal Barrier Coatings: Remote In Situ Condition Monitoring of EB-PVD Coatings at Elevated Temperatures

Abstract: Thermal barrier coatings (TBCs) are used to reduce the actual working temperature of the high pressure turbine blade metal surface. Knowing the temperature of the surface of the TBC and at the interface between the bondcoat and the thermally grown oxide (TGO) under realistic conditions is highly desirable. As the major life-controlling factors for TBC systems are thermally activated, therefore linked with temperature, this would provide useful data for a better understanding of these phenomena and to assess the remaining lifetime of the TBC. This knowledge could also enable the design of advanced cooling strategies in the most efficient way using minimum amount of air. The integration of an on-line temperature detection system would enable the full potential of TBCs to be realized due to improved precision in temperature measurement and early warning of degradation. This, in turn, will increase fuel efficiency and reduce CO2 emissions. The concept of a thermal-sensing TBC was first introduced by Choy, Feist, and Heyes (1998, “Thermal Barrier Coating With Thermoluminescent Indicator Material Embedded Therein,” U.S. Patent U.S. 6974641 (B1)). The TBC is locally modified so it acts as a thermographic phosphor. Phosphors are an innovative way of remotely measuring temperatures and also other physical properties at different depths in the coating using photo stimulated phosphorescence (Allison and Gillies, 1997, “Remote Thermometry With Thermographic Phosphors: Instrumentation and Applications,” Rev. Sci. Instrum., 68(7), pp. 2615‐2650). In this study the temperature dependence of several rare earth doped EB-PVD coatings will be compared. Details of the measurements, the influence of aging, the composition, and the fabrication of the sensing TBC will be discussed in this paper. The coatings proved to be stable and have shown excellent luminescence properties. Temperature detection at ultrahigh temperatures above 1300°C is presented using new types of EB-PVD TBC ceramic compositions. Multilayer sensing TBCs will be presented, which enable the detection of temperatures below and on the surface of the TBC simultaneously. DOI: 10.1115/1.3077662

Application of Bayesian Forecasting to Change Detection and Prognosis of Gas Turbine Performance

Abstract: This paper presents a novel technique for automatic change detection of the performance of gas turbines. In addition to change detection the proposed technique has the ability to perform a prognosis of measurement values. The proposed technique is deemed to be new in the field of gas turbine monitoring and forms the basic building block of a patent pending filed by the authors [1]. The technique used is called Bayesian Forecasting and is applied to Dynamic Linear Models (DLMs). The idea of Bayesian Forecasting is based on Bayes’ Theorem, which enables the calculation of conditional probabilities. In combination with DLMs (which break down the chronological sequence of the observed parameter into mathematical components like value, gradient, etc.) Bayesian Forecasting can be used to calculate probability density functions prior to the next observation, so called forecast distributions. The change detection is carried out by comparing the current model with an alternative model which mean value is shifted by a prescribed offset. If the forecast distribution of the alternative model better fits the actual observation, a potential change is detected. To determine whether the respective observation is a single outlier or the first observation of a significant change, a special logic is developed. Studies have shown that a confident change detection is possible for a change height of only 1.5 times the standard deviation of the observed signal. In terms of prognostic abilities the proposed technique not only estimates the point of time of a potential limit exceedance of respective parameters, but also calculates confidence bounds as well as probability density and cumulative distribution functions for the prognosis.Copyright © 2009 by ASME

The Numerical and Experimental Characteristics of Multimode Dry-Friction Whip and Whirl

Abstract: The nature of dry-friction whip and whirl is investigated through experimental and numerical methods. A test rig was designed and constructed to demonstrate and record the character of multi-mode dry-friction whip and whirl. These tests examined steady state whip and whirl characteristics for a variety of rub materials and clearances. A simulation model was constructed using tapered Timoshenko beam finite elements to form multiple-degree-of-freedom rotor and stator models. These models were reduced by component mode synthesis to discard high-frequency modes while retaining physical coordinates at the rub location to model rotor-stator interaction using a nonlinear contact model with Coulomb friction. Simulations were performed for specific test cases and compared against experimental data; these comparisons are favorable. Experimental data analysis showed multiple whirl and whip regions, despite claims of previous investigators that these regions are predicted analytically but not produced in simulations or experiments. Spectral analysis illustrates the presence of harmonic sidebands that accompany the fundamental whirl solution. These sidebands are more evident in whip and can excite higher-frequency whirl solutions.Copyright © 2009 by ASME