Showing papers in "Journal of Engineering for Gas Turbines and Power-transactions of The Asme in 2012"

Laminar Flame Speed and Ignition Delay Time Data for the Kinetic Modeling of Hydrogen and Syngas Fuel Blends

Abstract: Laminar flame speeds and ignition delay times have been measured for hydrogen and various compositions of H2/CO (syngas) at elevated pressures and elevated temperatures. Two constant-volume cylindrical vessels were used to visualize the spherical growth of the flame through the use of a schlieren optical setup to measure the laminar flame speed of the mixture. Hydrogen experiments were performed at initial pressures up to 10 atm and initial temperatures up to 443 K. A syngas composition of 50/50 by volume was chosen to demonstrate the effect of carbon monoxide on H2−O2 chemical kinetics at standard temperature and pressures up to 10 atm. All atmospheric mixtures were diluted with standard air, while all elevated-pressure experiments were diluted with a He:O2 of 7:1 to minimize instabilities. The laminar flame speed measurements of hydrogen and syngas are compared to available literature data over a wide range of equivalence ratios where good agreement can be seen with several data sets. Additionally, an improved chemical kinetics model is shown for all conditions within the current study. The model and the data presented herein agree well, which demonstrates the continual, improved accuracy of the chemical kinetic model.A high-pressure shock tube was used to measure ignition delay times for several baseline compositions of syngas at three pressures across a wide range of temperatures. The compositions of syngas (H2/CO) by volume presented in this study included 80/20, 50/50, 40/60, 20/80, and 10/90, all of which are compared to previously published ignition delay times from a hydrogen-oxygen mixture to demonstrate the effect of carbon monoxide addition. Generally, an increase in carbon monoxide increases the ignition delay time, but there does seem to be a pressure dependency. At low temperatures and pressures higher than about 12 atm, the ignition delay times appear to be indistinguishable with an increase in carbon monoxide. However, at high temperatures the relative composition of H2 and CO has a strong influence on ignition delay times. Model agreement is good across the range of the study, particularly at the elevated pressures. Also, an increase in carbon monoxide causes the activation energy of the mixture to decrease.© 2012 ASME

Performance Characteristics of an Operating Supercritical CO2 Brayton Cycle

Abstract: Supercritical CO2 (S-CO2) power cycles offer the potential for better overall plant economics due to their high power conversion efficiency over a moderate range of heat source temperatures, compact size, and potential use of standard materials in construction [1,2,3,4]. Sandia National Labs (Albuquerque, NM, US) and the US Department of Energy (DOE-NE) are in the process of constructing and operating a megawatt-scale supercritical CO2 split-flow recompression Brayton cycle with contractor Barber-Nichols Inc. [5] (Arvada, CO, US). This facility can be counted among the first and only S-CO2 power producing Brayton cycles anywhere in the world.The Sandia-DOE test-loop has recently concluded a phase of construction that has substantially upgraded the facility by installing additional heaters, a second recuperating printed circuit heat exchanger (PCHE), more waste heat removal capability, higher capacity load banks, higher temperature piping, and more capable scavenging pumps to reduce windage within the turbomachinery. With these additions, the loop has greatly increased its potential for electrical power generation — according to models, as much as 80 kWe per generator depending on loop configuration — and its ability to reach higher temperatures.To date, the loop has been primarily operated as a simple recuperated Brayton cycle, meaning a single turbine, single compressor, and undivided flow paths. In this configuration, the test facility has begun to realize its upgraded capacity by achieving new records in turbine inlet temperature (650°F/615K), shaft speed (52,000 rpm), pressure ratio (1.65), flow rate (2.7 kg/s), and electrical power generated (20kWe). Operation at higher speeds, flow rates, pressures and temperatures has allowed a more revealing look at the performance of essential power cycle components in a supercritical CO2 working fluid, including recuperation and waste heat rejection heat exchangers (PCHEs), turbines and compressors, bearings and seals, as well as auxiliary equipment. In this report, performance of these components to date will be detailed, including a discussion of expected operational limits as higher speeds and temperatures are approached.Copyright © 2012 by ASME

Jet-Surface Interaction Test: Far-Field Noise Results

Abstract: Many configurations proposed for the next generation of aircraft rely on the wing or other aircraft surfaces to shield the engine noise from the observers on the ground. However, the ability to predict the shielding effect and any new noise sources that arise from the high-speed jet flow interacting with a hard surface is currently limited. Furthermore, quality experimental data from jets with surfaces nearby suitable for developing and validating noise prediction methods are usually tied to a particular vehicle concept and, therefore, very complicated. The Jet/Surface Interaction Test was intended to supply a high quality set of data covering a wide range of surface geometries and positions and jet flows to researchers developing aircraft noise prediction tools. During phase one, the goal was to measure the noise of a jet near a simple planar surface while varying the surface length and location in order to: (1) validate noise prediction schemes when the surface is acting only as a jet noise shield and when the jet/surface interaction is creating additional noise, and (2) determine regions of interest for more detailed tests in phase two. To meet these phase one objectives, a flat plate was mounted on a two-axis traverse in two distinct configurations: (1) as a shield between the jet and the observer (microphone array) and (2) as a reflecting surface on the opposite side of the jet from the observer. The surface was moved through axial positions 2 ≤ xTE/Dj ≤ 20 (measured at the surface trailing edge, xTE, and normalized by the jet diameter, Dj) and radial positions 1 ≤ h/Dj ≤ 20. Far-field and phased array noise data were acquired at each combination of axial and radial surface location using two nozzles and at 8 different jet exit conditions across several flow regimes (subsonic cold, subsonic hot, underexpanded, ideally expanded, and overexpanded supersonic cold). The far-field noise results, discussed here, show where the surface shields some of the jet noise and, depending on the location of the surface and the observer, where scrubbing and trailing edge noise sources are created as a surface extends downstream and approaches the jet plume.

Bifurcations of Self-Excited Ducted Laminar Premixed Flames

Abstract: Bifurcation analysis is performed on experimental data obtained from a simple setup comprising ducted laminar premixed conical flames in order to investigate the features of nonlinear thermoacoustic oscillations. It is observed that as the bifurcation parameter is varied, the system undergoes a series of bifurcations leading to characteristically different nonlinear oscillations. Through the application of nonlinear time series analysis to pressure and flame (CH * chemiluminescence) intensity time traces, these oscillations are characterized as periodic, aperiodic, or chaotic oscillations, and subsequently the nature of the obtained bifurcations is explained based on dynamical systems theory. Nonlinear interaction between the flames and the acoustic modes of the duct is clearly reflected in the high speed flame images acquired simultaneously with pressure time series.

Numerical-experimental comparison in the simulation of rotor/stator interaction through blade-tip/abradable coating contact

Abstract: Higher aircraft energy efficiency may be achieved by minimizing the clearance between the rotating blade tips and respective surrounding casing. A common technical solution consists in the implementation of an abradable liner which improves both the operational safety and the efficiency of modern turbomachines. However, unexpected abradable wear removal mechanisms were recently observed in experimental set-ups as well as duringmaintenance procedures. Based on a numerical strategy previously developed, the present study introduces a numerical-experimental comparison of such occurrence. Attention is first paid to the review and analysis of existing experimental results. Good agreement with numerical predictions is then illustrated in terms of critical stress levels within the blade as well as final wear profiles of the abradable liner. Numerical results suggest an alteration of the abradablemechanical properties in order to explain the outbreak of a divergent interaction. New blade designs are also explored in this respect and it is found that the interaction phenomenon is highly sensitive to (1) the blade geometry, (2) the abradablematerial properties and (3) the distortion of the casing.

Computational Fluid Dynamics of a Radial Compressor Operating With Supercritical CO2

Abstract: The merit of using supercritical CO2 (scCO2) as the working fluid of a closed Brayton cycle gas turbine is now widely recognized, and the development of this technology is now actively pursued. scCO2 gas turbine power plants are an attractive option for solar, geothermal and nuclear energy conversion. Among the challenges which must be overcome in order to successfully bring the technology to the market, the efficiency of the compressor and turbine operating with the supercritical fluid should be increased as much as possible. High efficiency can be reached by means of sophisticated aerodynamic design, which, compared to other overall efficiency improvements, like cycle maximum pressure and temperature increase, or increase of recuperator effectiveness, does not require an increase in equipment cost, but only an additional effort in research and development.This paper reports a three-dimensional CFD study of a high-speed centrifugal compressor operating with CO2 in the thermodynamic region slightly above the vapor-liquid critical point. The investigated geometry is the compressor impeller tested in the Sandia scCO2 compression loop facility [1]. The fluid dynamic simulations are performed with a fully implicit parallel Reynolds-averaged Navier-Stokes code based on a finite volume formulation on arbitrary polyhedral mesh elements. The CFD code has been validated on test cases which are relevant for this study, see Ref. [2,3]. In order to account for the strongly nonlinear variation of the thermophysical properties of supercritical CO2, the CFD code is coupled with an extensive library for the computation of properties of fluids and mixtures [4]. Among the available models, the one based on reference equations of state for CO2 [5,6] has been selected, as implemented in one of the sub-libraries [7]. A specialized look-up table approach and a meshing technique suited for turbomachinery geometries are also among the novelties introduced in the developed methodology.A detailed evaluation of the CFD results highlights the challenges of numerical studies aimed at the simulation of technically relevant compressible flows occurring close to the liquid-vapor critical point. The data of the obtained flow field are used for a comparison with experiments performed at the Sandia scCO2 compression-loop facility.© 2012 ASME

Development and Testing of a Low NOX Hydrogen Combustion System for Heavy Duty Gas Turbines

Abstract: Interest in hydrogen as a primary fuel stream in heavy-duty gas turbine engines has increased as pre-combustion carbon capture and sequestration (CCS) has become a viable option for integrated gasification combined cycle (IGCC) power plants. The US Department of Energy has funded the Advanced IGCC/Hydrogen Gas Turbine Program since 2005 with an aggressive plant-level NOx target of 2 ppm @ 15% O2 for an advanced gas turbine cycle. Approaching this NOx level with highly-reactive hydrogen fuel at the conditions required is a formidable challenge that requires novel combustion technology. This study begins by measuring entitlement NOx emissions from perfectly-premixed combustion of the high-hydrogen fuels of interest.A new premixing fuel injector for high-hydrogen fuels was designed to balance reliable, flashback-free operation, reasonable pressure drop, and low emissions. The concept relies on distributed, small-scale jet-in-crossflow mixing that is a departure from traditional swirl-based premixing concepts. Single nozzle rig experiments were conducted at pressures of 10 atm and 17 atm, with air preheat temperatures of about 650K. With nitrogen-diluted hydrogen fuel, characteristic of carbon-free syngas, stable operation without flashback was conducted up to flame temperatures of approximately 1850K. In addition to the effects of operating pressure, the impact of minor constituents in the fuel — carbon monoxide, carbon dioxide, and methane — on flame holding in the premixer is presented.The new fuel injector concept has been incorporated into a full-scale, multi-nozzle combustor can with an energy conversion rate of more than 10 MW at F-class conditions. The full-can testing was conducted at full gas turbine conditions and various fuel compositions of hydrogen, natural gas, and nitrogen. This combustion system has accumulated over 100 hours of fired testing at full-load with hydrogen comprising over 90 percent of the reactants by volume. NOx emissions (ppm) have been measured in the single digits with hydrogen-nitrogen fuel at target gas turbine pressure and temperatures. Results of the testing show that small-scale fuel-air mixing can deliver a reliable, low-NOx solution to hydrogen combustion in advanced gas turbines.Copyright © 2012 by ASME

Crackle Noise in Heated Supersonic Jets

Abstract: Crackle noise from heated supersonic jets is characterized by the presence of strong positive pressure impulses resulting in a strongly skewed far-field pressure signal. These strong positive pressure impulses are associated with “N-shaped” waveforms involving a shock-like compression, and thus is very annoying to observers when it occurs. Unlike broadband shock-associated noise which dominates at upstream angles, crackle reaches a maximum at downstream angles associated with the peak jet noise directivity. Recent experiments [1] have shown that the addition of chevrons to the nozzle lip can significantly reduce crackle, especially in full-scale high-power tests. Because of these observations, it was conjectured that crackle is associated with coherent large scale flow structures produced by the baseline nozzle, and that the formation of these structures are interrupted by the presence of the chevrons, which leads to noise reduction. In particular, shocklets attached to large eddies are postulated as a possible aerodynamic mechanism for the formation of crackle. In this paper, we test this hypothesis through high-fidelity Large-Eddy Simulation (LES) of a hot supersonic jet of Mach number 1.56 and total temperature temperature ratio of 3.65. We use the LES solver “CharLES,” developed by Cascade Technologies, Inc., to capture the turbulent jet plume on fully-unstructured meshes.Copyright © 2012 by ASME

Analytical Analysis of Indirect Combustion Noise in Subcritical Nozzles

Abstract: This article revisits the problem of indirect combustion noise in nozzles of finite length. The analytical model proposed by Moase et al. (JFM 2007) for indirect combustion noise is red-erived and applied to subcritical nozzles having shapes of increasing complexity. This model is based on the equations formulated by Marble & Candel (JSV 1977) for which an explicit solution is obtained in the subsonic framework. The discretization of the nozzle into n elementary units of finite length implies the determination of 2n integration constants for which a set of linear equations is provided in this article. The analytical method is applied to configurations of increasing complexity. Analytical solutions are compared to numerical results obtained using SUNDAY (a 1D non linear Euler solver in temporal space) and CEDRE (3D Navier-Stokes flow solver). An excellent agreement is found for all configurations thereby showing that acceleration discontinuities at the boundaries between adjacent elements do not influence the actual acoustic transfer functions. The issue of nozzle compactness is addressed. It is found that in the subcritical domain, spectral results should be nondimensionalized using the flow-through-time of the entire nozzle. Doing so, transfer functions of nozzles of different lengths are successfully compared and a compactness criterion is proposed that writes Display Formulaω*∫0Ldζ/uζ<1 where L is the axial length of the nozzle. Finally, the EWG experimental setup of Bake et al. (JSV 2009) is considered. Analytical results are compared to the results reported by Howe (JFM 2010). Both models give similar trends and show the important role of the rising time of the fluctuating temperature front on the amplitude of the indirect acoustic emission. The experimental temperature profile and the impedance coefficients at the inlet and outlet provided by Bake et al. (JSV 2009) and Leyko et al. (JSV 2011) are introduced into the analytical formulation. Results show that the indirect combustion noise mechanism cannot be held responsible alone for the acoustic emission in the subcritical case.Copyright © 2012 by ASME

Ignition Delay Time and Laminar Flame Speed Calculations for Natural Gas/Hydrogen Blends at Elevated Pressures

Abstract: Applications of natural gas and hydrogen co-firing have received increased attention in the gas turbine market, which aims at higher flexibility due to concerns over the availability of fuels. While much work has been done in the development of a fuels database and corresponding chemical kinetics mechanism for natural gas mixtures, there are nonetheless few if any data for mixtures with high levels of hydrogen at conditions of interest to gas turbines. The focus of the present paper is on gas turbine engines with primary and secondary reaction zones as represented in the Alstom and Rolls Royce product portfolio. The present effort includes a parametric study, a gas turbine model study, and turbulent flame speed predictions. Using a highly optimized chemical kinetics mechanism, ignition delay times and laminar burning velocities were calculated for fuels from pure methane to pure hydrogen and with natural gas/hydrogen mixtures. A wide range of engine-relevant conditions were studied: pressures from 1 to 30 atm, flame temperatures from 1600 to 2200 K, primary combustor inlet temperature from 300 to 900 K, and secondary combustor inlet temperatures from 900 to 1400 K. Hydrogen addition was found to increase the reactivity of hydrocarbon fuels at all conditions by increasing the laminar flame speed and decreasing the ignition delay time. Predictions of turbulent flame speeds from the laminar flame speeds show that hydrogen addition affects the reactivity more when turbulence is considered. This combined effort of industrial and university partners brings together the know-how of applied, as well as experimental and theoretical disciplines.

A Numerical Study on the Turbulent Schmidt Numbers in a Jet in Crossflow

Abstract: This work presents a numerical study on the turbulent Schmidt numbers in jets in crossflow. This study contains two main parts. In the first part the problem of the proper choice of the turbulent Schmidt number in the Reynolds-Averaged Navier-Stokes (RANS) jet in crossflow mixing simulations is outlined. The results of RANS employing the shear-stress transport (SST) model of Menter and its curvature correction modification and different turbulent Schmidt number values are validated against experimental data. The dependence of the “optimal” value of the turbulent Schmidt number on the dynamic RANS model is studied. Furthermore a comparison is made with the large-eddy simulation (LES) results obtained using the WALE (Wall-Adapted Local Eddy Viscosity) model. The accuracy given by LES is superior in comparison to RANS results. This leads to the second part of the current study, in which the time-averaged mean and fluctuating velocity and scalar fields from LES are used for the evaluation of the turbulent viscosities, turbulent scalar diffusivities, and the turbulent Schmidt numbers in a jet in crossflow configuration. The values obtained from the LES data are compared with those given by the RANS modeling. The deviations are discussed and the possible ways for the RANS model improvements are outlined.© 2012 ASME

Influence of In-Plane Dynamics of Thin Compression Rings on Friction in Internal Combustion Engines

Abstract: The compression ring-bore conjunction accounts for significant frictional parasitic losses relative to its size. The prerequisite to improving the tribological performance of this contact is a fundamental understanding of ring dynamics within the prevailing transient nature of regime of lubrication. Studies reported thus far take into account ring-bore conformance, based on static fitment of the ring within an out-of-round bore, whose out-of-circularity is affected by manufacturing processes, surface treatment and assembly. The static fitment analyses presume quasi-static equilibrium between ring tension and gas pressure loading with generated conjunctional pressures. This is an implicit assumption of ring rigidity whilst in situ. The current analysis considers the global modal behaviour of the ring as an eigenvalue problem, thus including its dynamic in-plane behaviour in the tribological study of a mixed-hydrodynamic regime of lubrication. The results show that the contact transit time is shorter than that required for the ring to reach steady state condition. Hence, the conjunction is not only subject to transience on account of changing contact kinematics and varied combustion loading, but also subject to perpetual ring transient dynamics. This renders the ring-bore friction a more complex problem than usually assumed in idealised ring fitment analyses. An interesting finding of the analysis is increased ring-bore clearance at and in the vicinity of top dead centre, which reduces the ring sealing effect and suggests a possible increase in blow-by. The current analysis, integrating ring in-plane modal dynamics and mixed regime of lubrication includes salient features which are closer representation of practice, an approach which has not hitherto been reported in literature.

A Metal Mesh Foil Bearing and a Bump-Type Foil Bearing: Comparison of Performance for Two Similar Size Gas Bearings

Abstract: Gas bearings in oil-free microturbomachinery for gas process applications and power generation (< 400 kW) must be reliable and inexpensive, ensuring low drag power and thermal stability. Bump-type foil bearings (B-FBs) and overleaf-type foil bearings are in use in specialized applications, though their development-time (design and prototyping), exotic materials, and excessive manufacturing cost still prevent their widespread usage. Metal mesh foil bearings (MMFBs), on the other hand, are an inexpensive alternative that uses common materials and no restrictions on intellectual property. Laboratory testing shows that prototype MMFBs perform similarly as typical BFBs but offering significantly larger damping to dissipate mechanical energy due to rotor vibrations. This paper details a one-to-one comparison of the static and dynamic forced performance characteristics of a MMFB against a BFB of similar size and showcases the advantages and disadvantages of MMFBs. The bearings for comparison are a Generation I BFB and a MMFB, both with a slenderness ratio L/D = 1.04. Measurements of rotor lift-off speed and drag friction at start-up and airborne conditions were conducted for rotor speeds to 70 krpm and under identical specific loads (W/LD = 0.06 to 0.26 bar). Static load versus bearing elastic deflection tests evidence a typical hardening nonlinearity with mechanical hysteresis; the MMFB showing two to three times more material damping than the BFB. The MMFB exhibits larger drag torques during rotor start-up and shut-down tests though bearing lift-off happens at lower rotor speeds (∼15 krpm). As the rotor becomes airborne, both bearings offer very low drag friction coefficients, ∼0.03 for the MMFB and ∼0.04 for the BFB in the speed range 20–40 krpm. With the bearings floating on a journal spinning at 50 krpm, the MMFB dynamic direct force coefficients show little frequency dependency, while the BFB stiffness and damping increases with frequency (200–400 Hz). The BFB has a much larger stiffness and viscous damping coefficients than the MMFB. However, the MMFB material loss factor is at least twice as large as that in the BFB. The experiments show the MMFB, when compared to the BFB, has a lower drag power and earlier lift-off speed, and with dynamic force coefficients having a lesser dependency on whirl frequency excitation.© 2012 ASME

A Novel Damping Device for Broadband Attenuation of Low-Frequency Combustion Pulsations in Gas Turbines

Abstract: Damping of thermoacoustically induced pressure pulsations in combustion chambers is a major focus of gas turbine operation. Conventional Helmholtz resonators are an excellent means to attenuate thermoacoustic instabilities in gas turbines. Usually, however, the damping optimum is in a narrow frequency band at one operating condition. The work presented here deals with a modification of the basic Helmholtz resonator design over-coming this drawback. It consists of a damper body housing separated volumes that are connected to each other. Adequate adjustment of the governing parameters results in a broadband damping characteristic for low frequencies. In this way, changes in operating conditions and engine-to-engine variations involving shifts in the combustion pulsation frequency can conveniently be addressed. Genetic algorithms and optimization strategies are used to derive these parameters in a multi-dimensional parameter space. The novel damper concept is described in more detail and compared with cold-flow experiments. In order to validate the performance under realistic conditions, the new broadband dampers were implemented in a full-scale test engine. Pulsation amplitudes could be reduced by more than 80%. In addition, it is shown that due to sophisticated damper placement in the engine two unstable modes can be addressed simultaneously. Application of the damper concept allowed to considerably increase the engine operating regime and finally to reduce NOx emissions by 55%. Predictions obtained with the physics-based model excellently agree with experimental results for all tested damper geometries, bias flows, excitation amplitudes, and most important with the measurements in the engine.Copyright © 2012 by Alstom Technology Ltd.

Diesel Injector Coking: Optical-Chemical Analysis of Deposits and Influence on Injected Flow-Rate, Fuel Spray and Engine Performance

Abstract: The physical origin of injector coking in diesel engines has been clarified and the most critical design parameters and operating variables pertaining to the occurrence of the phenomenon have been identified. Fouling has been shown to be affected by many factors, such as injector temperature, nozzle configuration, hole diameter and conicity as well as fuel composition. Optical and scanning electron microscope (SEM) analyses have been conducted both inside and outside injectors of different type and four locations have been identified as the main deposition sites. Furthermore, different coking typologies, i.e., dry and wet coking, have been assessed and discussed. Energy Dispersive X-ray (EDX) spectroscopy images of the deposits on the spray hole walls have revealed that minute quantities of Zn catalyze the coking reactions to a great extent. Significant quantities of Zn have also been found in the injector deposits. An extensive experimental test campaign has been carried out at the engine test bench with different nozzle setups in order to evaluate performance deterioration after different ageing procedures. The effects of both the Zn concentration in the fuel and running time have been assessed separately on the fouling rate. Injection rate time histories have been acquired at the hydraulic test rig, under different working conditions, for both new and aged injectors. The experimental changes in the EVI profiles subsequent to fouling have been analyzed and related to the corresponding variations in engine power measured at the engine test bench. A previously developed combustion multi-zone diagnostic model has also been applied to gain a further insight into the cause and effect relationships between the experimental in-cylinder pressure time histories and engine-out emissions.

Evaluation of Design Performance of the Semi-Closed Oxy-Fuel Combustion Combined Cycle

Abstract: This study aims to present various design aspects and realizable performance of the natural gas fired semi-closed oxy-fuel combustion combined cycle (SCOC-CC). Design parameters of the cycle are set up on the basis of component technologies of today’s state-of-the-art gas turbines with a turbine inlet temperature between 1400°C and 1600°C. The most important part in the cycle analysis is the turbine cooling which affects the cycle performance considerably. A thermodynamic cooling model is introduced to predict the reasonable amount of turbine coolant to maintain the turbine blade temperature of the SCOC-CC at the levels of those of conventional gas turbines. Optimal pressure ratio ranges of the SCOC-CC for two different turbine inlet temperature levels are searched. The performance penalty due to the CO2 capture is examined. Also investigated are the influences of the purity of oxygen provided by the air separation unit on the cycle performance. A comparison with the conventional combined cycle adopting a post-combustion CO2 capture is carried out taking into account the relationship between performance and CO2 capture rate.© 2012 ASME

Advanced Exergoeconomic Analysis Applied to a Complex Energy Conversion System

Abstract: Exergy-based analyses are important tools for studying and evaluating energy conversion systems. Conventional exergy-based analyses provide us with important information on the design and operation of a system. However, further insight into the improvement potential of plant components and the overall plant, as well as into component interactions, is important when optimal operation is required. This necessity led to the development of advanced exergy-based analyses, in which the exergy destruction as well as the associated costs and environmental impacts are split into avoidable/unavoidable and endogenous/exogenous parts. Based on the avoidable exergy destruction, costs and environmental impacts potential and strategies for improvement are revealed. The objective of this paper is to demonstrate the application, the advantages, and the information obtained from an advanced exergoeconomic analysis by applying it to a complex plant, i.e., to a combined cycle power plant. The largest parts of the unavoidable cost rates are calculated for the components constituting the gas turbine system and the low-pressure steam turbine. The combustion chamber has the second highest avoidable investment cost and the highest avoidable cost of exergy destruction. In general, the investment cost of most of the components is unavoidable, with the exception of some heat exchangers. Similarly, most of the cost of exergy destruction is unavoidable, with the exception of the expander of the gas turbine system and the high-pressure and intermediate-pressure steam turbines. The advanced exergoeconomic analysis reveals high endogenous values, which suggest that improvement of the total plant can be achieved by improving the design of individual components, and lower exogenous values, which means that component interactions are in general of lower significance for this plant.

An Integrated Approach for Aircraft Engine Performance Estimation and Fault Diagnostics

Abstract: A Kalman filter-based approach for integrated on-line aircraft engine performance estimation and gas path fault diagnostics is presented. This technique is specifically designed for underdetermined estimation problems where there are more unknown system parameters representing deterioration and faults than available sensor measurements. A previously developed methodology is applied to optimally design a Kalman filter to estimate a vector of tuning parameters, appropriately sized to enable estimation. The estimated tuning parameters can then be transformed into a larger vector of health parameters representing system performance deterioration and fault effects. The results of this study show that basing fault isolation decisions solely on the estimated health parameter vector does not provide ideal results. Furthermore, expanding the number of the health parameters to address additional gas path faults causes a decrease in the estimation accuracy of those health parameters representative of turbomachinery performance deterioration. However, improved fault isolation performance is demonstrated through direct analysis of the estimated tuning parameters produced by the Kalman filter. This was found to provide equivalent or superior accuracy compared to the conventional fault isolation approach based on the analysis of sensed engine outputs, while simplifying online implementation requirements. Results from the application of these techniques to an aircraft engine simulation are presented and discussed.Copyright © 2012 by ASME

Understanding the Acoustic Oscillations Observed in the Injection Rate of a Common-Rail Direct Injection Diesel Injector

Abstract: Measuring the rate of injection of a common-rail injector is one of the first steps for diesel engine development. The injected quantity as a function of time is of prime interest for engine research and modeling activities, as it drives spray development and mixing, which, in current diesel engines, control combustion. On the other hand, the widely used long-tube method provides results that are neither straightforward nor fully understood. This study, performed on a 0.09-mm axially drilled single-hole nozzle, is part of the Engine Combustion Network (ECN) and aims at analyzing the acoustic oscillations observed in the rate of injection signal and measuring their impact on the real injection process and on the results recorded by the experimental devices. Several tests have been carried out for this study, including rate of injection and momentum, X-ray phase-contrast of the injector, and needle motion or injector displacement. The acoustic analysis revealed that these fluctuations found their origin in the sac of the injector and that they were the results of an interaction between the fluid in the chamber (generally gases) or in the nozzle sac and the liquid fuel to be injected. It has been observed that the relatively high oscillations recorded by the long-tube method were mainly caused by a displacement of the injector itself while injecting. In addition, the results showed that these acoustic features are also present in the spray, which means that the oscillations make it out of the injector, and that this temporal variation must be reflected in the actual rate of injection.

Numerical Modeling of Fluid-Induced Rotordynamic Forces in Seals With Large Aspect Ratios

Abstract: Traditional annular seal models are based on bulk flow theory While these methods are computationally efficient and can predict dynamic properties fairly well for short seals, they lack accuracy in cases of seals with complex geometry or with large aspect ratios (above 10) In this paper, the linearized rotordynamic coefficients for a seal with a large aspect ratio are calculated by means of a three-dimensional CFD analysis performed to predict the fluid-induced forces acting on the rotor For comparison, the dynamic coefficients were also calculated using two other codes: one developed on the bulk flow method and one based on finite difference method These two sets of dynamic coefficients were compared with those obtained from CFD Results show a reasonable correlation for the direct stiffness estimates, with largest value predicted by CFD In terms of cross-coupled stiffness, which is known to be directly related to cross-coupled forces that contribute to rotor instability, the CFD also predicts the highest value; however, a much larger discrepancy can be observed for this term (73% higher than the value predicted by the finite difference method and 79% higher than the bulk flow code prediction) One can see similar large differences in predictions in the estimates for damping and direct mass coefficients, where the highest values are predicted by the bulk flow method These large variations in damping and mass coefficients, and most importantly the large difference in the cross-coupled stiffness predictions, may be attributed to the large difference in seal geometry (ie, the large aspect ratio AR >10 of this seal model versus the short seal configuration the bulk flow code is usually calibrated for using an empirical friction factor) [DOI: 101115/14007341]

On the Effect of Thermal Energy Transport to the Performance of (Semi) Floating Ring Bearing Systems for Automotive Turbochargers

Abstract: Bearing systems in engine-oil lubricated turbochargers (TCs) must operate reliably over a wide range of shaft speeds and withstand severe axial and radial thermal gradients. An engineered thermal management of the energy flows into and out of the bearing system is paramount in order to ensure the component’s mechanical integrity and the robustness of the bearing system. The bearings, radial and thrust type, act both as a load bearing and low friction support with the lubricant carrying away a large fraction of the thermal energy generated by rotational drag and the heat flow disposed from a hot shaft. The paper introduces a thermohydrodynamic analysis for the prediction of the pressure and temperature fields in a (semi) floating ring bearing (S)FRB system. The analysis simultaneously solves the Reynolds equation with variable oil viscosity and the thermal energy transport equation in the inner and outer films of the bearing system. Flow conditions in both films are coupled to the temperature distribution and heat flow through the (semi) floating ring. Other constraints include calculating the fluid films’ forces reacting to the externally applied load and to determine the operating journal and ring eccentricities. The predictions of performance for a unique realistic (S)FRB configuration at typical TC operating conditions reveal a distinct knowledge: (a) the heat flow from the shaft into the inner film is overwhelming, in particular, at the inlet lubricant plane where the temperature difference with the cold oil is largest; (b) the inner film temperature quickly increases as soon as the (cold) lubricant enters the film and is due to the large amount of energy generated by shear drag and the heat transfer from the shaft; (c) a floating ring develops a significant radial temperature gradient; (d) at all shaft speeds, low and high, the thermal energy carried away by the lubricant streams is no less than 70% of the total energy input; the rest is conducted through the TC casing. To warrant this thermal energy distribution, enough lubricant flow must be supplied to the bearing system. The efficient computational model offers a distinct advantage over existing lumped parameters thermal models and there is no penalty in the execution time.

Prediction of Rotordynamic Coefficients for Short Labyrinth Gas Seals Using Computational Fluid Dynamics

Abstract: The analysis is presented for the computational fluid dynamics (CFD)-based modeling of short labyrinth gas seals. Seal leakage performance can be reliably predicted with CFD for a wide operating range and various sealing configurations. Prediction of seal influence on the rotordynamic stability, however, is a challenging task requiring relatively high computer processing power. A full 3D eccentric CFD model of a short staggered three-tooth-on-stator labyrinth seal is built in ANSYS CFX. An extensive grid independence study is carried out showing influence of the grid refinement on the stiffness coefficients. Three methods for the prediction of stiffness and damping coefficients as well as the effect of turbulence modeling, boundary conditions, and solver parameters are presented. The rest of the paper shows the results of a parameter variation (inlet pressure, preswirl, and shaft rotational speed) for two labyrinth seals with a tooth radial clearance of 0.5 mm and 0.27 mm, respectively. The latter was compared with experimental data in Pugachev and Deckner, 2010, textquotelefttextquoteleftAnalysis of the Experimental and CFD-Based Theoretical Methods for Studying Rotordynamic Characteristics of Labyrinth Gas Seals,textquoterighttextquoteright Proceedings of ASME Turbo Expo 2010, Paper No. GT2010-22058.

Hole-Pattern and Honeycomb Seal Rotordynamic Forces: Validation of CFD-Based Prediction Techniques

Abstract: This paper deals with modeling of hole-pattern and honeycomb seals. These are frequently used as balance piston seals in high pressure centrifugal compressor applications as they have the potential to facilitate superior rotordynamic damping characteristics while providing good leakage control. On the other hand it is also well-established that the rotordynamic performance of hole-pattern and honeycomb seals is very sensitive to convergence and divergence in the streamwise direction.The ISOTSEAL bulk-flow code has shown difficulties in predicting the rotordynamic coefficients for convergent seal geometries or in cases with negative preswirl. This has lead to increased interest in CFD-based analysis of seal dynamics. CFD-based models generally have less assumptions and are applicable for complex geometries or operating ranges not covered by bulk-flow codes.The CFD-based Instationary Perturbation Model (IPM) is utilized for the analysis of the hole-pattern and honeycomb seals. The rotordynamic forces are obtained by means of a time-dependent perturbation of the rotor position with respect to the stator. A sequence of perturbation frequencies is utilized to obtain the frequency dependence of the rotordynamic seal force coefficients.A strong effort has been put into validating the CFD-based perturbation modeling techniques against published experimental seal test data and the paper describes selected validation cases. A constant-clearance hole-pattern seal and a convergent honeycomb seal are analyzed and the results are compared to experimental results. The frequency dependence of the rotordynamic stiffness and damping characteristics of the seals is very well-captured for both types of seals.Finally the IPM method was applied to a convergent hole-pattern seal to investigate the effects of eccentricity on the rotordynamic coefficients. The results are consistent with available experimental data.Copyright © 2012 by ASME

Nonlinear Phenomena in Thermoacoustic Systems With Premixed Flames

Abstract: Nonlinear analysis of thermoacoustic instability is essential for the prediction of the frequencies, amplitudes, and stability of limit cycles. Limit cycles in thermoacoustic systems are reached when the energy input from driving processes and energy losses from damping processes balance each other over a cycle of the oscillation. In this paper, an integral relation for the rate of change of energy of a thermoacoustic system is derived. This relation is analogous to the well-known Rayleigh criterion in thermoacoustics, however, it can be used to calculate the amplitudes of limit cycles and their stability. The relation is applied to a thermoacoustic system of a ducted slot-stabilized 2-D premixed flame. The flame is modeled using a nonlinear kinematic model based on the G-equation, while the acoustics of planar waves in the tube are governed by linearized momentum and energy equations. Using open-loop forced simulations, the flame describing function (FDF) is calculated. The gain and phase information from the FDF is used with the integral relation to construct a cyclic integral rate of change of energy (CIRCE) diagram that indicates the amplitude and stability of limit cycles. This diagram is also used to identify the types of bifurcation the system exhibits and to find the minimum amplitude of excitation needed to reach a stable limit cycle from another linearly stable state for single-mode thermoacoustic systems. Furthermore, this diagram shows precisely how the choice of velocity model and the amplitude-dependence of the gain and the phase of the FDF influence the nonlinear dynamics of the system. Time domain simulations of the coupled thermoacoustic system are performed with a Galerkin discretization for acoustic pressure and velocity. Limit cycle calculations using a single mode, along with twenty modes, are compared against predictions from the CIRCE diagram. For the single mode system, the time domain calculations agree well with the frequency domain predictions. The heat release rate is highly nonlinear but, because there is only a single acoustic mode, this does not affect the limit cycle amplitude. For the twenty-mode system, however, the higher harmonics of the heat release rate and acoustic velocity interact, resulting in a larger limit cycle amplitude. Multimode simulations show that, in some situations, the contribution from higher harmonics to the nonlinear dynamics can be significant and must be considered for an accurate and comprehensive analysis of thermoacoustic systems. [DOI: 10.1115/1.4023305]

Quantifying Cyclic Variability in a Multicylinder HCCI Engine With High Residuals

Abstract: Cyclic variability (CV) in lean HCCI combustion at the limits of operation is a known phenomenon, and this work aims at investigating the dominant effects for the cycle evolution at these conditions in a multi-cylinder engine. Experiments are performed in a four-cylinder engine at the operating limits at late phasing of lean HCCI operation with negative valve overlap (nvo). A combustion analysis method that estimates the unburned fuel mass on a per-cycle basis is applied on both main combustion and the nvo period revealing and quantifying the dominant effects for the cycle evolution at high CV. The interpretation of the results and comparisons with data from a single-cylinder engine indicate that, at high CV, the evolution of combustion phasing is dominated by low-order deterministic couplings similar to the single-cylinder behavior. Variations, such as in air flow and wall temperature, between cylinders strongly influence the level of CV but the evolution of the combustion phasing is governed by the interactions between engine cycles of the individual cylinders.

Forward Blade Sweep Applied to Low-Speed Axial Fan Rotors of Controlled Vortex Design: An Overview

Abstract: An overview is given on the research maintained by the author about design aspects of three-dimensional blade passage flow in low-speed axial flow industrial fan rotors, affected by spanwise changing design blade circulation due to controlled vortex design (CVD), blade forward sweep (FSW), and their combination.It was pointed out that, comparing the CVD method to free vortex design, the fluid in the blade suction side boundary layer has increased inclination to migrate radially outward, increasing near-tip blockage and loss. It was concluded that the benefit of FSW, in terms of moderating loss near the tip, can be better utilized for rotors of CVD, in comparison to free vortex design.Compared to free vortex design, FSW applied to blades of CVD was found especially beneficial in loss reduction also away from the endwalls, via shortening the flow paths on the suction side — being anyway elongated by the radially outward flow due to CVD —, and thus, reducing the effect of wall skin friction. The necessity of correcting the swept blades was pointed out for matching with the prescribed CVD circulation distribution.Copyright © 2012 by ASME

Impact of Swirl Flow on the Cooling Performance of an Effusion Cooled Combustor Liner

Abstract: An experimental study on combustor liner cooling of modern direct lean injection (DLI) combustion chambers using coolant ejection from both effusion cooling holes and a starter film has been conducted The experimental setup consists of a generic scaled three sector planar rig in an open loop hot gas wind tunnel, which has been described earlier in Wurm et al [1] Experiments are performed without combustion Realistic engine conditions are achieved by applying engine-realistic Reynolds numbers, Mach numbers, and density ratios A Particle Image Velocimetry (PIV) measurement technique is employed, which has been adjusted to allow for high resolution near wall velocity measurements with and without coolant ejection As the main focus of the present study is a deeper understanding of the interaction of swirl flows and near wall cooling flows, wall pressure measurements are performed for the definition of local blowing ratios and to identify the impact on the local cooling performance For thermal investigations an infrared thermography measurement technique is employed that allows high resolution thermal studies on the effusion cooled liner surface The effects of different heat shield geometry on the flow field and performance of the cooling films are investigated in terms of near wall velocity distributions and film cooling effectiveness Two different heat shield configurations are investigated which differ in shape and inclination angle of the so called heat shield lip Operating conditions for the hot gas main flow are kept constant The pressure drop across the effusion cooled liner is varied between 1% and 3% of the total pressure Results show the impact of the swirled main flow on the stability of the starter film and on the effusion cooling performance Stagnation areas which could be identified by wall pressure measurements are confirmed by PIV measurements Thermal investigations reveal reduced cooling performance in the respective stagnation areasCopyright © 2012 by Rolls-Royce Deutschland Ltd & Co KG