Showing papers in "Journal of Engineering for Gas Turbines and Power-transactions of The Asme in 2011"

Comparative Validation Study on Identification of Premixed Flame Transfer Function

Abstract: The flame transfer function (FTF) of a premixed swirl burner was identified from time series generated with CFD simulation of compressible, turbulent, reacting flow at non-adiabatic conditions. Results were validated against experimental data. For large eddy simulation (LES), the Dynamically Thickened Flame combustion model with one step kinetics was used. For unsteady simulation in a Reynolds-averaged Navier-Stokes framework (URANS), the Turbulent Flame Closure model was employed. The FTF identified from LES shows quantitative agreement with experiment for amplitude and phase, especially for frequencies below 200 Hz. At higher frequencies, the gain of the FTF is underpredicted. URANS results show good qualitative agreement, capturing the main features of the flame response. However, the maximum amplitude and the phase lag of the FTF are underpredicted. Using a low-order network model of the test rig, the impact of the discrepancies in predicted FTFs on frequencies and growth rates of the lowest order eigenmodes were assessed. Small differences in predicted FTFs were found to have a significant impact on stability limits. Stability behavior in agreement with experimental data was achieved only with the LES-based flame transfer function.Copyright © 2011 by ASME

Development of a 3 kW Microturbine for CHP Applications

Abstract: Combined heat and power (CHP) concepts for small-scale distributed power generation offer significant potential for saving energy and reducing CO 2 emissions. Microturbines are an interesting candidate for small CHP systems with advantages in terms of performance, size, noise, and costs. MTT is developing a 3 kW recuperated microturbine for micro CHP applications for large households and for truck combined APU-heating systems. To minimize costs, off-the-shelf automotive turbocharger technology has been used for the turbomachinery. During recent years, turbocharger turbomachinery performance and efficiencies have significantly increased, even for very small sizes. At the same time, efficient high-speed motor-generators have become available at relatively low prices. The development of a concept demonstrator started in May 2008. This program phase included a cycle analysis and component selection study around off-the-shelf turbomachinery, design of a custom combustor, recuperator and generator, and a test program. In this paper, results of the cycle definition, conceptual design and component matching study are presented. Next, the development of a detailed performance model is described and performance prediction results are given. Also, results of the test program and test analysis work are presented. Finally, from the conclusion of the demonstrator phase an outlook is given on the prototype design and performance, which will be the next phase of the development program.

Fouling Mechanisms in Axial Compressors

Abstract: Fouling of compressor blades is an important mechanism leading to performance deterioration in gas turbines over time. Fouling is caused by the adherence of particles to airfoils and annulus surfaces. Particles that cause fouling are typically smaller than 2 to 10 microns. Smoke, oil mists, carbon, and sea salts are common examples. Fouling can be controlled by appropriate air filtration systems, and can often be reversed to some degree by detergent washing of components. The adherence of particles is impacted by oil or water mists. The result is a build-up of material that causes increased surface roughness and to some degree changes the shape of the airfoil (if the material build up forms thicker layers of deposits). Fouling mechanisms are evaluated based on observed data, and a discussion on fouling susceptibility is provided. A particular emphasis will be on the capabilities of modern air filtration systems.Copyright © 2011 by Solar Turbines Inc.

A Finite Element Method for Three-Dimensional Analysis of Thermo-acoustic Combustion Instability

Abstract: A method for predicting the onset of acoustically driven combustion instabilities in gas turbine combustor is examined. The basic idea is that the governing equations of the acoustic waves can be coupled with a flame heat release model and solved in the frequency domain. The paper shows that a complex eigenvalue problem is obtained that can be solved numerically by implementing the governing equations in a finite element code. This procedure allows one to identify the frequencies at which thermo-acoustic instabilities are expected and the growth rate of the pressure oscillations, at the onset of instability, when the hypothesis of linear behavior of the acoustic waves can be applied. The method can be applied virtually to any three-dimensional geometry, provided the necessary computational resources that are, anyway, much less than those required by computational fluid dynamics methods proposed for analyzing the combustion chamber under instability condition. Furthermore, in comparison with the "lumped" approach that characterizes popular acoustics networks, the proposed method allows one for much more flexibility in defining the geometry of the combustion chamber. The paper shows that different types of heat release laws, for instance, heat release concentrated in a flame sheet, as well as distributed in a larger domain, can be adopted. Moreover, experimentally or numerically determined flame transfer functions, giving the response of heat release to acoustic velocity fluctuations, can be incorporated in the model. To establish proof of concept, the method is validated at the beginning against simple test cases taken from literature. Over the frequency range considered, frequencies and growth rates both of stable and unstable eigenmodes are accurately evaluated. Then the method is applied to a much more complex annular combustor geometry in order to evaluate frequencies and growth rates of the unstable modes and to show how the variation in the parameters of the heat release law can influence the transition to instability.

Data-Driven Fault Detection in Aircraft Engines With Noisy Sensor Measurements

Abstract: An inherent difficulty in sensor-data-driven fault detection is that the detection performance could be drastically reduced under sensor degradation (e.g., drift and noise). Complementary to traditional model-based techniques for fault detection, this paper proposes symbolic dynamic filtering by optimally partitioning the time series data of sensor observation. The objective here is to mask the effects of sensor noise level variation and magnify the system fault signatures. In this regard, the concepts of feature extraction and pattern classification are used for fault detection in aircraft gas turbine engines. The proposed methodology of data-driven fault detection is tested and validated on the Commercial Modular Aero-Propulsion System Simulation (C-MAPSS ) test-bed developed by NASA for noisy (i.e., increased variance) sensor signals.

Reactivity Controlled Compression Ignition Using Premixed Hydrated Ethanol and Direct Injection Diesel

Abstract: Previous research has shown that a Homogeneous Charge Compression Ignition (HCCI) engine with efficient heat recovery can operate on a 35 to 65% volumetric mixture of ethanol-in-water while achieving high brake thermal efficiency (∼39%) and very low NOx emissions [4]. The major advantage of utilizing hydrated ethanol as a fuel is that the net energy gain improves from 21 to 55% of the heating value of ethanol and its co-products, since significant energy must be expended to remove water during production. This is required because wet ethanol is not suitable for conventional combustion engines. For example, spark ignition engines demand the use of pure ethanol because the dilution caused by water reduces the flame speed, resulting in misfire and problems due to condensation. The present study uses numerical simulations to explore the use of wet ethanol for Reactivity Controlled Compression Ignition (RCCI) operation in a heavy duty diesel engine. RCCI uses in-cylinder blending of a low reactivity fuel with a high reactivity fuel and has demonstrated significant fuel efficiency and emissions benefits using a variety of fuels, including gasoline and diesel. Combustion timing is controlled by the local blended fuel reactivity (i.e. octane number), and the combustion duration can be controlled by establishing optimized gradients in fuel reactivity in the combustion chamber. In the present study, the low reactivity fuel was hydrated ethanol while the higher reactivity fuel was diesel. First, the effect of water on ethanol/water/diesel HCCI was investigated using GT-Power and single-zone CHEMKIN simulations. The results showed that the main impact of the water in the ethanol is to reduce the IVC temperature due to vaporization cooling. Next, multidimensional engine modeling was performed using the KIVA code at engine loads from 5 to 17 bar IMEP at 1300 rev/min with various grades of hydrated ethanol and a fixed diesel fraction of the total fuel. The results show that hydrated ethanol can be used in a RCCI engine with gross indicated thermal efficiencies up to 55% and very low emissions. A 70/30 ethanol/water mixture (by mass) was found to yield the best results across the entire load range without the need for EGR.Copyright © 2011 by ASME

Nonlinear Fault Diagnosis of Jet Engines by Using a Multiple Model-Based Approach

Abstract: In this paper, a nonlinear fault detection and isolation (FDI) scheme that is based on the concept of multiple model (MM) approach is proposed for jet engines. A modular and a hierarchical architecture is proposed which enables the detection and isolation of both single as well as concurrent permanent faults in the engine. A set of nonlinear models of the jet engine in which compressor and turbine maps are used for performance calculations corresponding to various operating modes of the engine (namely, healthy and different fault modes) is obtained. Using the multiple model approach the probabilities corresponding to the engine modes of operation are first generated. The current operating mode of the system is then detected based on evaluating the maximum probability criteria. The performance of our proposed multiple model FDI scheme is evaluated by implementing both the Extended Kalman Filter (EKF) and the Unscented Kalman Filter (UKF). Simulation results presented demonstrate the effectiveness of our proposed multiple model FDI algorithm for both structural and actuator faults in the jet engine.Copyright © 2011 by ASME

Understanding Ice Crystal Accretion and Shedding Phenomenon in Jet Engines Using a Rig Test

Abstract: The aviation industry has now connected a number of engine power-loss events to the ingestion of atmospheric ice crystals. Ice crystals are believed to penetrate to and eventually accrete on surfaces in the engine core where local air temperatures are warmer than freezing. Research aimed at understanding the accretion and shedding of ice crystals within the engine is being conducted industrywide. Although this specific icing condition is readily produced inside an operating engine, rig testing is the preferred research tool because it has the advantage of good visibility of the ice accretion process and easy access for video documentation. This paper presents one of the first efforts to simulate the warm air/cold ice conditions occurring inside the engine core using a test rig. The test section contains geometry simulating the transition duct between the low and high compressors in a typical jet engine and an airfoil simulating the engine strut connecting the inner and outer surfaces. Test results showed ice formed on the airfoil and other surfaces in the test section at air temperatures warmer than freezing. However, when both the air and surface temperatures were held below freezing, the injected ice did not melt and no ice accretion was observed. Ice only formed on the airfoil when mixed-phase conditions (liquid and ice) were produced, by introducing the ice into a warm airflow. This test concludes that a rig-level ice crystal icing test is feasible and capable of producing ice accretion in a simulated engine environment. As it was the first test of its kind, reporting of these preliminary test results are expected to benefit future experimenters.

Assessment of Future Aero-engine Designs With Intercooled and Intercooled Recuperated Cores

Abstract: Reduction in CO2 emissions is strongly linked with the improvement of engine specific fuel consumption, as well as the reduction in engine nacelle drag and weight. Conventional turbofan designs, however, that reduce CO2 emissions—such as increased overall pressure ratio designs—can increase the production of NOx emissions. In the present work, funded by the European Framework 6 collaborative project NEW Aero engine Core concepts (NEWAC), an aero-engine multidisciplinary design tool, Techno-economic, Environmental, and Risk Assessment for 2020 (TERA2020), has been utilized to study the potential benefits from introducing heat-exchanged cores in future turbofan engine designs. The tool comprises of various modules covering a wide range of disciplines: engine performance, engine aerodynamic and mechanical design, aircraft design and performance, emissions prediction and environmental impact, engine and airframe noise, as well as production, maintenance and direct operating costs. Fundamental performance differences between heat-exchanged cores and a conventional core are discussed and quantified. Cycle limitations imposed by mechanical considerations, operational limitations and emissions legislation are also discussed. The research work presented in this paper concludes with a full assessment at aircraft system level that reveals the significant potential performance benefits for the intercooled and intercooled recuperated cycles. An intercooled core can be designed for a significantly higher overall pressure ratio and with reduced cooling air requirements, providing a higher thermal efficiency than could otherwise be practically achieved with a conventional core. Variable geometry can be implemented to optimize the use of the intercooler for a given flight mission. An intercooled recuperated core can provide high thermal efficiency at low overall pressure ratio values and also benefit significantly from the introduction of a variable geometry low pressure turbine. The necessity of introducing novel lean-burn combustion technology to reduce NOx emissions at cruise as well as for the landing and take-off cycle, is demonstrated for both heat-exchanged cores and conventional designs. Significant benefits in terms of NOx reduction are predicted from the introduction of a variable geometry low pressure turbine in an intercooled core with lean-burn combustion technology.

Rotordynamic Force Coefficients for Three Types of Annular Gas Seals With Inlet Preswirl and High Differential Pressure Ratio

Abstract: The following paper presents and compares rotordynamic force coefficients for three types of non-contact annular gas seals, which include a labyrinth (LABY), honeycomb (HC), and a fully partitioned damper seal (FPDS). These three annular seals represent the typical seal types used in process gas centrifugal compressors at the balance piston location or center seal location to limit internal leakage and ensure a robust rotordynamic design. Tests were conducted on 170.6mm (6.716 in) diameter seals for rotor speeds up to 15kprm, inlet air pressure of 6.9 bar (100 psi), ambient back pressure, and with inlet gas preswirl. The three seals were designed to have the same nominal clearance and similar axial lengths. Testing was conducted on a controlled motion test rig possessing non-synchronous excitation capability up to 250Hz. Three different test methods were employed to give confidence in the rotordynamic coefficients, which include static force deflection tests, mechanical impedance tests, and dynamic cavity pressure tests. Results from experiments compare force coefficients for all seal configurations while paying special attention to the cross-over frequencies of the effective damping term. All seals possessed negative effective damping at lower excitation frequencies with inlet preswirl, where the straight-through FPDS possessed the lowest cross over frequency of 40Hz at 15krpm. The testing also revealed that the preswirl parameter had significantly more influence on effective damping levels and cross-over frequencies when compared to rotor speed.Copyright © 2011 by ASME

Effects of Contact Interface Parameters on Vibration of Turbine Bladed Disks With Underplatform Dampers

Abstract: The design of high cycle fatigue resistant bladed disks requires the ability to predict the expected damping of the structure in order to evaluate the dynamic behavior and ensure structural integrity. Highly sophisticated software codes are available today for this nonlinear analysis, but their correct use requires a good understanding of the correct model generation and the input parameters involved to ensure a reliable prediction of the blade behavior. The aim of the work described in this paper is to determine the suitability of current modeling approaches and to enhance the quality of the nonlinear modeling of turbine blades with underplatform dampers. This includes an investigation of a choice of the required input parameters, an evaluation of their best use in nonlinear friction analysis, and an assessment of the sensitivity of the response to variations in these parameters. Part of the problem is that the input parameters come with varying degrees of uncertainty because some are experimentally determined, others are derived from analysis, and a final set are often based on estimates from previous experience. In this investigation the model of a commercial turbine bladed disk with an underplatform damper is studied, and its first flap, first torsion, and first edgewise modes are considered for 6 EO and 36 EO excitation. The influence of different contact interface meshes on the results is investigated, together with several distributions of the static normal contact loads, to enhance the model setup and, hence, increase accuracy in the response predictions of the blade with an underplatform damper. A parametric analysis is carried out on the friction contact parameters and the correct setup of the nonlinear contact model to determine their influence on the dynamic response and to define the required accuracy of the input parameters

Determination of the Heat Release Distribution in Turbulent Flames by a Model Based Correction of OH* Chemiluminescence

Abstract: Imaging of OH* or CH* chemiluminescence with intensified cameras is often employed for the determination of heat release in premixed flames. Proportionality is commonly assumed, but in the turbulent case this assumption is not justified. Substantial deviations from proportionality are observed, which are due to turbulence-chemistry interactions. In this study a model based correction method is presented to obtain a better approximation of the spatially resolved heat release rate of lean turbulent flames from OH* measurements. The correction method uses a statistical strain rate model to account for the turbulence influence. The strain rate model is evaluated with time-resolved velocity measurements of the turbulent flow. Additionally, one-dimensional simulations of strained counterflow flames are performed to consider the non-linear effect of turbulence on chemi-luminescence intensities. A detailed reaction mechanism, which includes all relevant chemiluminescence reactions and deactivation processes, is used. The result of the simulations is a lookup table of the ratio between heat release rate and OH* intensity with strain rate as parameter. This lookup table is linked with the statistical strain rate model to obtain a correction factor which accounts for the non-linear relationships between OH* intensity, heat release rate, and strain rate. The factor is then used to correct measured OH* intensities to obtain the local heat release rate. The corrected intensities are compared to heat release distributions which are measured with an alternative method. For all investigated flames in the lean, partially premixed regime the corrected OH* intensities are in very good agreement with the heat release rate distributions of the flames.Copyright © 2011 by ASME

Remaining Technical Challenges and Future Plans for Oil-Free Turbomachinery

Abstract: The application of oil-free technologies (foil gas bearings, solid lubricants, and advanced analysis and predictive modeling tools) to advanced turbomachinery has been underway for several decades. During that time, full commercialization has occurred in aircraft air cycle machines, turbocompressors, cryocoolers, and ever-larger microturbines. Emerging products in the automotive sector (turbochargers and superchargers) indicate that a high volume serial production of foil bearings is imminent. The demonstration of foil bearings in auxiliary power units and select locations in propulsion gas turbines illustrates that such technology also has a place in these future systems. Foil bearing designs, predictive tools, and advanced solid lubricants that can satisfy anticipated requirements have been reported, but a major question remains regarding the scalability of foil bearings to ever-larger sizes to support heavier rotors. In this paper, the technological history, primary physics, engineering practicalities, and existing experimental and experiential database for scaling foil bearings are reviewed, and the major remaining technical challenges are identified.

Design and Performance Prediction of Hybrid Air Foil Thrust Bearings

Abstract: Air foil bearings (AFBs) have been recognized as the most promising for oil-free turbomachinery. However, the applications of AFBs to the relatively large turbomachinery have many technical challenges due to limited load capacity and wear during start/stops. A hybrid air foil bearing (HAFB), which combines the benefits of AFB and hydrostatic air bearing, was introduced earlier by the authors, and the experimental studies showed much larger load capacity at low speeds and much lesser friction torque during start/stop than hydrodynamic counterpart. The benefit of HAFB was recognized through the experimental studies, and the concept of hybrid operation was further developed to thrust air foil bearings. This paper presents novel design features of the hybrid air foil thrust bearing (HAFTB) with radially arranged bump foils and preformed Rayleigh step contour, and presents simulated static and dynamic characteristics of the HAFTB. A 2D thin plate equation in cylindrical coordinate was solved with the finite difference method for the prediction of the top foil deflection. Parametric studies were performed to evaluate the effect of various design parameters on the static and dynamic performances of HAFTB. At low speeds, a design with orifice located at the center of land region showed the highest load capacity, while a design with orifice located near the leading edge of land region showed the highest load capacity at high speeds. Direct and coupled bearing coefficients were also calculated for various operating conditions. The direct stiffness increases with supply pressure but the direct damping decreases with supply pressure. In addition, typical hardening effect of gas film accompanying increase of stiffness and decrease of damping was predicted in high frequency excitations.

Improved Multiple Point Nonlinear Genetic Algorithm Based Performance Adaptation Using Least Square Method

Abstract: At off-design conditions, engine performance model prediction accuracy depends largely on its component characteristic maps. With the absence of actual characteristic maps, performance adaptation needs to be done for good imitations of actual engine performance. A non-linear multiple point Genetic Algorithm based performance adaptation developed earlier by the authors using a set of non-linear scaling factor functions has been proven capable of making accurate performance prediction over a wide range of operating conditions. However, the success depends on searching the right range of scaling factor coefficients heuristically, in order to obtain optimum scaling factor functions. Such search ranges may be difficult to obtain and in many off-design adaption cases, it may be very time consuming due to the nature of trial and error process. In this paper, an improvement on the present adaptation method is presented using a Least Square method where the search range can be selected deterministically. In the new method, off-design adaptation is applied to individual off-design point first to obtain individual off-design point scaling factors. Then plots of the scaling factors against the off-design conditions are generated. Using the Least Square method, the relationship between each scaling factor and the off-design operating condition is generated. The regression coefficients are then used to determine the search range of the scaling factor coefficients before multiple off-design points performance adaptation is finally applied. The developed adaptation approach has been applied to a model single-spool turboshaft engine and demonstrated a simpler and faster way of obtaining the optimal scaling factor coefficients compared with the original off-design adaptation method.Copyright © 2011 by ASME

Methane Oxycombustion for Low CO2 Cycles: Blowoff Measurements and Analysis

Abstract: Increasing concerns about climate change have encouraged interest in zero-CO 2 emission hydrocarbon combustion techniques. In one approach, nitrogen is removed from the combustion air and replaced with another diluent, typically carbon dioxide or steam. In this way, formation of nitrogen oxides is prevented and the exhaust stream can be separated into concentrated CO 2 and water by a simple condensation process. The concentrated CO 2 stream can then be sequestered or used for enhanced oil recovery. Burning fuels in an O 2 /CO 2 diluent raises new combustion opportunities and challenges for both emissions and operability: this study focuses on the latter aspect. CH 4 /O 2 /CO 2 flames have slower chemical kinetics than methane-air flames and as such, flame stability is more problematic as they are easier to blow off. This issue was investigated experimentally by characterizing the stability boundaries of a swirl stabilized combustor. Near stoichiometric CO 2 and N 2 diluted methane/oxygen flames were considered and compared with lean methane/air flames. Numerical modeling of chemical kinetics was also performed to analyze the dependence of laminar flame speeds and extinction strain rates upon dilution by different species and to develop correlations for blowoff boundaries. Finally, blowoff trends at high pressure were extrapolated from atmospheric pressure data to simulate conditions closer to those of gas turbines.

Establishment of a High Quality Database for the Acoustic Modeling of Perforated Liners

Abstract: Perforated liners, especially in combination with a bias flow, are very effective sound absorbers. When appplied to gas turbine combustors, they can suppress thermo-acoustic instabilities and thus allow the application of new combustion concepts concerning higher efficiency and lower emissions. While the successful application of such a damping concept has been shown, it is still not possible to accurately predict the damping performance of a given configuration. This paper provides a comprehensive database of high quality experimental data. Variations of geometric, fluid mechanic, and acoustic parameters have been studied, including realistic engine configurations. The results demonstrate each parameter influence on the damping performance. A low order thermo-acoustic model is used to simulate the test configurations numerically. The model shows a good agreement with the measurements for a wide range of geometries and Strouhal and bias flow Mach numbers.

Experimental Investigation of Turbulent Boundary Layer Flashback Limits for Premixed Hydrogen-Air Flames Confined in Ducts

Abstract: The design of flashback-resistant premixed burners for hydrogen-rich fuels is strongly dependent on reliable turbulent boundary layer flashback limits, since this process can be the dominant failure type for mixtures with high burning velocities. So far, the flashback data published in literature is based on tube burner experiments with unconfined flames. However, this flame configuration may not be representative for the most critical design case, which is a flame being already present inside the duct geometry. In order to shed light on this potential misconception, boundary layer flashback limits have been measured for unconfined and confined flames in fully premixed hydrogen-air mixtures at atmospheric conditions. Two duct geometries were considered, a tube burner and a quasi-2D turbulent channel flow. Furthermore, two confined flame holding configurations were realized, a small backward-facing step inside the duct and a ceramic tile at high temperature, which was mounted flush with the duct wall. While the measured flashback limits for unconfined tube burner flames compare well with literature results, a confinement of the stable flame leads to a shift of the flashback limits towards higher critical velocity gradients, which are in good agreement between the tube burner and the quasi-2D channel setup. The underestimation of flashback propensity resulting from unconfined tube burner experiments emerges from the physical situation at the burner rim. Heat loss from the flame to the wall results in a quenching gap, which causes a radial leakage flow of fresh gases. This flow in turn tends to increase the quenching distance, since it constitutes an additional convective heat loss. On the one hand, the quenching gap reduces the local adverse pressure gradient on the boundary layer. On the other hand, the flame base is pushed outward, which deters the flame from entering the boundary layer region inside the duct. The flashback limits of confined flames stabilized at backward-facing steps followed this interpretation, and experiments with a flush ceramic flame holder constituted the upper limit of flashback propensity. It is concluded that the distribution of the flame backpressure and the flame position itself are key parameters for the determination of meaningful turbulent boundary layer flashback limits. For a conservative design path, the present results obtained from confined flames should be considered instead of unconfined tube burner values.Copyright © 2011 by ASME

Analysis of Flow Migration in an Ultra-Compact Combustor

Abstract: The Ultra Compact Combustor (UCC) has the potential to offer improved thrust-to-weight and overall efficiency in a turbojet engine. The thrust-to-weight improvement is due to a reduction in engine weight by shortening the combustor section through the use of the revolutionary UCC design. The improved efficiency is achieved by using an increased fuel-to-air mass ratio, and allowing the fuel to fully combust prior to exiting the UCC system. Furthermore, g-loaded combustion offers increased flame speeds that can lead to smaller combustion volumes. The circumferential combustion of the fuel in the UCC cavity results in hot gases present at the outside diameter of the core flow. This orientation creates an issue in that the flow from the circumferential cavity needs to migrate radially and blend with the core flow to present a uniform temperature distribution to the high-pressure turbine rotor. A computational fluid dynamics (CFD) analysis is presented for the flow patterns in the combustor section of a representative fighter-scale engine. The analysis included a study of secondary flows, cavity flow characteristics, shear layer interactions and mixing properties. An initial understanding of primary factors that impact the radial migration is presented. Computational comparisons were also made between an engine realistic condition and an ambient pressure rig environment.

Understanding Ignition Delay Effects With Pure Component Fuels in a Single-Cylinder Diesel Engine

Abstract: In order to better understand how future candidate diesel fuels may affect combustion characteristics in diesel engines, 21 pure component hydrocarbon fuels were tested in a single-cylinder diesel engine. These pure component fuels included normal alkanes (C6–C16), normal primary alkenes (C6–C18), isoalkanes, cycloalkanes/-enes, and aromatic species. In addition, seven fuel blends were tested, including commercial diesel fuel, U.S. Navy JP-5 aviation fuel, and five Fischer–Tropsch synthetic fuels. Ignition delay was used as a primary combustion metric for each fuel, and the ignition delay period was analyzed from the perspective of the physical delay period followed by the chemical delay period. While fuel properties could not strictly be varied independently of each other, several ignition delay correlations with respect to physical properties were suggested. In general, longer ignition delays were observed for component fuels with lower liquid fuel density, kinematic viscosity, and liquid-air surface tension. Longer ignition delay was also observed for component fuels with higher fuel volatility, as measured by boiling point and vapor pressure. Experimental data show two regimes of operation: For a carbon chain length of 12 or greater, there is little variation in ignition delay for the tested fuels. For shorter chain lengths, a fuel molecular structure is very important. Carbon chain length was used as a scaling variable with an empirical factor to collapse the ignition delay onto a single trend line. Companion detailed kinetic modeling was pursued on the lightest fuel species set (C6) since this fuel set possessed the greatest ignition delay differences. The kinetic model gives a chemical ignition delay time, which, together with the measured experimental ignition delay, suggests that the physical and chemical delay period have comparable importance. However, the calculated chemical delay periods capture the general variation in the overall ignition delay and could be used to predict the ignition delay of possible future synthetic diesel fuels.

Experimental and Numerical Study of NOx Formation From the Lean Premixed Combustion of CH4 Mixed With CO2 and N2

Abstract: This paper describes an experimental and numerical study of the emission of nitrogen oxides (NO x ) from the lean premixed (LPM) combustion of gaseous fuel alternatives to typical pipeline natural gas in a high intensity, single-jet, stirred reactor (JSR). In this study, CH 4 is mixed with varying levels CO 2 and N 2 . NO x measurements are taken at a nominal combustion temperature of 1800K, atmospheric pressure, and a reactor residence time of 3 ms. The experimental results show the following trends for NO x emissions as a function of fuel dilution: (1) more NO x is produced per kg of C H 4 consumed with the addition of a diluent, (2) the degree of increase in emission index is dependent on the chosen diluent; N 2 dilution increases NO x production more effectively than equivalent CO 2 dilution. Chemical kinetic modeling suggests that NO x production is less effective for the mixture diluted with CO 2 due to both a decrease in N 2 concentration and the ability of CO 2 to deplete the radicals taking part in NO x formation chemistry. In order to gain insight on flame structure within the JSR, three dimensional computational fluid dynamic (CFD) simulations are carried out for LPM CH 4 combustion. A global CH 4 combustion mechanism is used to model the chemistry. While it does not predict intermediate radicals, it does predict CH 4 and CO oxidation quite well. The CFD model illustrates the flow field, temperature variation, and flame structure within the JSR. A 3-element chemical reactor network (CRN), including detailed chemistry, is constricted using insight from spatial measurements of the reactor, the results of CFD simulations, and classical fluid dynamic correlations. GRI 3.0 is used in the CRN to model the NO x emissions for all fuel blends. The experimental and modeling results are in good agreement and suggest the underlying chemical kinetic reasons for the trends.

The Impact of Density Ratio on the Liquid Core Dynamics of a Turbulent Liquid Jet Injected Into a Crossflow

Abstract: Atomizing liquids by injecting them into crossflows is a common approach in gas turbines and augmentors. Much of our current understanding of the processes resulting in atomization of the jets, the resulting jet penetration and spray drop size distribution have been obtained by performing laboratory experiments at ambient conditions. Yet, operating conditions under which jets in crossflows atomize can be far different from ambient. Hence, several dimensionless groups have been identified that are believed to determine jet penetration and resulting drop size distribution. These are usually the jet and crossflow Weber and Reynolds numbers and the momentum flux ratio. In this paper, we aim to answer the question of whether an additional dimensionless group, the liquid to gas density ratio must be matched. We perform detailed simulations of the primary atomization region using the refined level set grid (RLSG) method to track the motion of the liquid/gas phase interface. We employ a balanced force, interface projected curvature method to ensure high accuracy of the surface tension forces, use a multiscale approach to transfer broken off, small scale nearly spherical drops into a Lagrangian point particle description allowing for full two-way coupling and continued secondary atomization, and employ a dynamic Smagorinsky large eddy simulation (LES) approach in the single phase regions of the flow to describe turbulence. We present simulation results for a turbulent liquid jet (q=6.6, We =330, and Re=14,000) injected into a gaseous crossflow (Re =740,000) analyzed under ambient conditions (density ratio 816) experimentally by Brown and McDonnell [2006, "Near Field Behavior of a Liquid Jet in a Crossflow, " Proceedings of the ILASS Americas, 19th Annual Conference on Liquid Atomization and Spray Systems]. We compare simulation results obtained using a liquid to gas density ratio of 10 and 100. The results show that the increase in density ratio causes a noticeable increase in liquid core penetration with reduced bending and spreading in the transverse directions. The post-primary atomization spray penetrates further in both the jet and transverse direction. Results further show that the penetration correlations for the windward side trajectory commonly reported in the literature strongly depend on the value of threshold probability used to identify the leading edge. Correlations based on penetration of the jet's liquid core center of mass, on the other hand, can provide a less ambiguous measure of jet penetration. Finally, the increase in density ratio results in a decrease in wavelength of the most dominant feature associated with a traveling wave along the jet as determined by proper orthogonal decomposition.

Model-Based Actuator Trajectories Optimization for a Diesel Engine Using a Direct Method

Abstract: This paper proposes a novel optimization method that allows a reduction in the pollutant emission of diesel engines during transient operation. The key idea is to synthesize optimal actuator commands using reliable models of the engine system and powerful numerical optimization methods. The engine model includes a mean-value engine model for the dynamics of the gas paths, including the turbocharger of the fuel injection, and of the torque generation. The pollutant formation is modeled using an extended quasi-static modeling approach. The optimization substantially changes the input signals, such that the engine model is enabled to extrapolate all relevant outputs beyond the regular operating area. A feedforward controller for the injected fuel mass is used to eliminate the nonlinear path constraints during the optimization. The model is validated using experimental data obtained on a transient engine test bench. A direct single shooting method is found to be most effective for the numerical optimization. The results show a significant potential for reducing the pollutant emissions during transient operation of the engine. The optimized input trajectories derived assist the design of sophisticated engine control systems.

A Novel Approach of Retrofitting a Combined Cycle With Post Combustion CO2 Capture

Abstract: Most state-of-the-art natural gas-fired combined cycle (NGCC) plants are triple-pressure reheat cycles with efficiencies close to 60%. However, with carbon capture and storage, the efficiency will be penalized by almost 10% units. To limit the energy consumption for a carbon capture NGCC plant, exhaust gas recirculation (EGR) is necessary. Utilizing EGR increases the CO2 content in the gas turbine exhaust while it reduces the flue gas flow to be treated in the capture plant. Nevertheless, due to EGR, the gas turbine will experience a different media with different properties compared with the design case. This study looks into how the turbomachinery reacts to EGR. The work also discusses the potential of further improvements by utilizing pressurized water rather than extraction steam as the heat source for the CO2 stripper. The results show that the required low-pressure level should be elevated to a point close to the intermediate-pressure to achieve optimum efficiency, hence, one pressure level can be omitted. The main tool used for this study is an in-house off-design model based on fully dimensionless groups programmed in the commercially available heat and mass balance program IPSEPRO. The model is based on a GE 109FB machine with a triple-pressure reheat steam cycle. [DOI: 10.1115/1.4001988] (Less)

Analysis of a Micro Gas Turbine Fed by Natural Gas and Synthesis Gas: Test Bench and Combustor CFD Analysis

Abstract: In recent years, the interest in the research on energy production systems fed by biofuels is increased. Gaseous fuels obtained through biomass conversion processes such as gasification, pyrolysis and pyrogasification are generally defined as synthesis gas. The use of synthesis gas in small-size energy systems, such as those used for distributed micro-cogeneration, has not yet reached a level of technological maturity that could allow a large market diffusion. For this reason, further analyses (both experimental and numerical) have to be carried out to allow these technologies to achieve performance and reliability typical of established technologies based on traditional fuels. In this paper, an experimental and numerical analysis of a combustor of a 100-kW Micro Gas Turbine fed by synthesis gas is presented. The work has been developed in the framework of a collaboration among the Department of Engineering of the University of Ferrara, the Istituto Motori CNR of Naples, and Turbec SpA of Cento (FE). The main features of the microturbine MGT Turbec T100, located at the Istituto Motori CNR of Naples, are firstly described. A decompression and distribution system allows to feed the MGT with gaseous fuels characterized by different compositions. Moreover, a system of remote monitoring and control together with a data transfer system have been developed in order to set the operative parameters of the machine for the current test. The results of the tests performed under different operating conditions are then presented. Subsequently, the paper presents the numerical analysis of a model of the MGT combustor. The combustor model is validated against manufacturer performance data and experimental data with respect to steady state performance, i.e. average outlet temperature, emission levels, pressure drops. Then, a syngas, composed by different ratios of hydrogen, carbon monoxide, methane, carbon dioxide and water, is simulated and the results analyzed.Copyright © 2011 by ASME

Thermal Modeling of a Solar Steam Turbine With a Focus on Start-Up Time Reduction

Abstract: Steam turbines in solar thermal power plants experience a much greater number of starts than those operating in baseload plants. In order to preserve the lifetime of the turbine while still allowin ...

Experimental Analysis of Confined Jet Flames by Laser Measurement Techniques

Abstract: An experimental analysis of confined premixed turbulent methane/air and hydrogen/air jet flames is presented. A generic lab scale burner for high-velocity preheated jets equipped with an optical combustion chamber was designed and set up. The size and operating conditions were configured to enable flame stabilization by recirculation of hot combustion products. The geometry of the rectangular confinement and an off-center positioning of the jet nozzle were chosen to resemble one burner nozzle of a FLOX ® -based combustor. The off-center jet arrangement caused the formation of a pronounced lateral recirculation zone similar to the one in previously investigated FLOX ® -combustors (Luckerath et al., 2007. "FLOX ® Combustion at High Pressure with Different Fuel Compositions," ASME J. Eng. Gas Turbines Power, 130(1), pp. 011505; Lammel et al., 2010. "FLOX ® Combustion at High Power Density and High Flame Temperatures," ASME 1. Eng. Gas Turbines Power, 132(12), p. 121503ff). The analysis was accomplished by different laser measurement techniques. Flame structures were visualized by OH * chemiluminescence imaging and planar laser-induced fluorescence of the OH radical. Laser Raman scattering was used to determine concentrations of the major species and the temperature. Velocity fields were measured with particle image velocimetry. Results of measurements in two confined jet flames are shown. The mixing of fresh gas with recirculating combustion products and the stabilization of the methane flame are discussed in detail. The presented findings deliver important information for the understanding of confined jet flames operated with different fuels. The obtained data sets can be used for the validation of numerical simulations as well.

Assessment of Residual Mass Estimation Methods for Cylinder Pressure Heat Release Analysis of HCCI Engines With Negative Valve Overlap

Abstract: Increased residual levels in Homogeneous Charge Compression Ignition (HCCI) engines employing valve strategies such as recompression or negative valve overlap (NVO) imply that accurate estimation of residual gas fraction (RGF) is critical for cylinder pressure heat release analysis. The objective of the present work was to evaluate three residual estimation methods and assess their suitability under naturally aspirated and boosted HCCI operating conditions: i) the Simple State Equation method employs the Ideal Gas Law at exhaust valve closing (EVC); ii) the Mirsky method assumes isentropic exhaust process; and iii) the Fitzgerald method models in-cylinder temperature from exhaust valve opening (EVO) to EVC by accounting for heat loss during the exhaust process and uses measured exhaust temperature for calibration. Simulations with a calibrated and validated “virtual engine” were performed for representative HCCI operating conditions of engine speed, fuel-air equivalence ratio, NVO and intake pressure (boosting). The State Equation method always overestimated RGF by more than 10%. The Mirsky method was most robust, with average errors between 3–5%. The Fitzgerald method performed consistently better, ranging from no error to 5%, where increased boosting caused the largest discrepancies. A sensitivity study was also performed and determined that the Mirsky method was most robust to possible pressure and temperature measurement errors.© 2011 ASME