Showing papers in "Journal of Engineering for Gas Turbines and Power-transactions of The Asme in 2008"

A Comparison of Ethanol and Butanol as Oxygenates Using a Direct-Injection, Spark-Ignition (DISI) Engine

Abstract: This study was designed to evaluate a ‘what-if’ scenario in terms of using butanol as an oxygenate, in place of ethanol in an engine calibrated for gasoline operation. No changes to the stock engine calibration were performed for this study. Combustion analysis, efficiency and emissions of pure gasoline, 10% ethanol and 10% butanol blends in a modern, direct-injection four-cylinder, spark ignition engine were analyzed. Data was taken at engine speeds of 1000 RPM up to 4000 RPM with load varying from 0 Nm (idle) to 150 Nm. Relatively minor differences existed between the three fuels for the combustion characteristics such as heat release rate, 50% mass fraction burned, and coefficient of variation of indicated mean effective pressure at low and medium engine loads. However at high engine loads the reduced knock resistance of the butanol blend forced the engine control unit to retard the ignition timing substantially, compared to the gasoline baseline and even more pronounced compared to the ethanol blend. Brake specific volumetric fuel consumption which represented a normalized volumetric fuel flow rate, was lowest for the gasoline baseline fuel, due to the higher energy density. The 10% butanol blend had a lower volumetric fuel consumption compared to the ethanol blend, as expected based on energy density differences. Results showed little difference in regulated emissions between 10% ethanol and 10% butanol. The ethanol blend produced the highest peak specific NOx due to the high octane rating of ethanol and effective anti-knock characteristics. Overall, the ability of butanol to perform equally as well as ethanol from an emissions and combustion standpoint, with a decrease in fuel consumption, initially appears promising. Further experiments are planned to explore the full operating range of the engine and the potential benefits of higher blend ratios of butanol.Copyright © 2008 by UChicago Argonne LLC, Operator of Argonne National Laboratory

The Fuel Mix Limits and Efficiency of a Stoichiometric, Ammonia, and Gasoline Dual Fueled Spark Ignition Engine

Abstract: An overall stoichiometric mixture of air, gaseous ammonia, and gasoline was metered into a single cylinder, variable compression ratio, supercharged cooperative fuel research (CFR) engine at varying ratios of gasoline to ammonia. The engine was operated such that the combustion was knock-free with minimal roughness for all loads ranging from idle up to a maximum load in the supercharge regime. For a given load, speed, and compression ratio, there was a range of ratios of gasoline to ammonia for which knockfree, smooth firing was obtained. This range was investigated at its rough limit and also at its maximum brake torque (MBT) knock limit. If too much ammonia was used, then the engine fired with an excessive roughness. If too much gasoline was used, then knock-free combustion could not be obtained while the maximum brake torque spark timing was maintained. Stoichiometric operation on gasoline alone is also presented, for comparison. It was found that a significant fraction of the gasoline used in spark ignition engines could be replaced with ammonia. Operation on about 100% gasoline was required at idle. However, a fuel mix comprising 70% ammonia/30% gasoline on an energy basis could be used at normally aspirated, wide open throttle. Even greater ammonia to gasoline ratios were permitted for supercharged operation. The use of ammonia with gasoline allowed knock-free operation with MBT spark timing at higher compression ratios and higher loads than could be obtained with the use of gasoline alone. DOI: 10.1115/1.2898837

Effect of Flame Structure on the Flame Transfer Function in a Premixed Gas Turbine Combustor

Abstract: The flame transfer function in a premixed gas turbine combustor is experimentally determined. The fuel (natural gas) is premixed with air upstream of a choked inlet to combustor. Therefore, the input to the flame transfer function is the imposed velocity fluctuations of the fuel/air mixture without equivalence ratio fluctuations. The inlet-velocity fluctuations are achieved by a variable-speed siren over the forcing frequency of 75–280 Hz and measured using a hot-wire-anemometer at the inlet to the combustor. The output function (heat release) is determined using chemiluminescence measurement from the whole flame. Flame images are recorded to understand how the flame structure plays a role in the global heat release response of flame to the inlet-velocity perturbation. The results show that the gain and phase of the flame transfer function depend on flame structure as well as the frequency and magnitude of inlet-velocity modulation and can be generalized in terms of the relative length scale of flame to convection length scale of inlet-velocity perturbation, which is represented by a Strouhal number. Non-linear flame response is characterized by a periodic vortex shedding from shear layer and the non-linearity occurs at lower magnitude of inlet-velocity fluctuation as the modulation frequency increases. However, for a given modulation frequency, the flame structure does not affect the magnitude of inlet-velocity fluctuation at which the non-linearity starts.Copyright © 2008 by ASME

Compliant hybrid gas journal bearing using integral wire mesh dampers

Abstract: A compliant hybrid gas journal bearing includes compliant hybrid bearing pads having a hydrostatic recess and a capillary restrictor for providing a flow of pressurized gas to the bearing The bearing also includes an inner rim adjacent the bearing pads, an outer rim and a damper bridge between the inner and outer rims The damper bridge has an axial length that is less than an axial length of the bearing pads and the outer rim to form a damper cavity on each side of the damper bridge An integral wire mesh damper is situated within the damper cavity on each side of the damper bridge Integral centering springs are located between the inner and outer rims to provide radial and rotational compliance to the bearing pads The oil-free bearing design addresses the low damping and low load capacity characteristics that are inherent in present day compliant air foil bearing designs, while retaining the compliance to changes in rotor geometry

Asymptotic Description of Maximum Mistuning Amplification of Bladed Disk Forced Response

Abstract: The problem of determining the maximum forced response vibration amplification that can be produced just by the addition of a small mistuning to a perfectly cyclical bladed disk still remains not completely clear. In this paper we apply a recently introduced perturbation methodology, the Asymptotic Mistuning Model (AMM), to determine which are the key ingredients of this amplification process, and to evaluate the maximum mistuning amplification factor that a given modal family with a particular distribution of tuned frequencies can exhibit. A more accurate upper bound for the maximum forced response amplification of a mistuned bladed disk is obtained from this description, and the results of the AMM are validated numerically using a simple mass-spring model.Copyright © 2008 by ASME

Ignition and Flame Speed Kinetics of Two Natural Gas Blends With High Levels of Heavier Hydrocarbons

Abstract: High-pressure experiments and chemical kinetics modeling were performed to generate a database and a chemical kinetic model that can characterize the combustion chemistry of methane-based fuel blends containing significant levels of heavy hydrocarbons (up to 37.5% by volume). Ignition delay times were measured in two different shock tubes and in a rapid compression machine at pressures up to 34 atm and temperatures from 740 to 1660 K. Laminar flame speeds were also measured at pressures up to 4 atm using a high-pressure vessel with optical access. Two different fuel blends containing ethane, propane, n-butane, and n-pentane added to methane were studied at equivalence ratios varying from lean (0.3) to rich (2.0). This paper represents the most comprehensive set of experimental ignition and laminar flame speed data available in the open literature for CH4 /C2 H6 /C3 H8 /C4 H10 /C5 H12 fuel blends with significant levels of C2+ hydrocarbons. Using these data, a detailed chemical kinetics model, based on current and recent work by the authors, was compiled and refined. The predictions of the model are very good over the entire range of ignition delay times, considering the fact that the data set is so thorough. Nonetheless, some improvements to the model can still be made with respect to ignition times at the lowest temperatures and for the laminar flame speeds at pressures above 1 atm and rich conditions.Copyright © 2008 by ASME

A Time-Domain Network Model for Nonlinear Thermoacoustic Oscillations

Abstract: Lean premixed prevaporised (LPP) combustion can reduce NOx emissions from gas turbines, but often leads to combustion instability. Acoustic waves produce fluctuations in heat release, for instance by perturbing the fuel-air ratio. These heat fluctuations will in turn generate more acoustic waves and in some situations linear oscillations grow into large amplitude self-sustained oscillations. The resulting limit cycles can cause structural damage. Thermoacoustic oscillations will have a low amplitude initially. Thus linear models can describe the initial growth and hence give stability predictions. An unstable linear mode will grow in amplitude until nonlinear effects become sufficiently important to achieve a limit cycle. While the frequency of the linear mode can often provide a good approximation to that of the resulting limit cycle, linear theories give no prediction of its resulting amplitude. In previous work, we developed a low-order frequency-domain method to model thermoacoustic limit cycles in LPP combustors. This was based on a ‘describing function’ approach and is only applicable when there is a dominant mode and the main nonlinearity is in the combustion response to flow perturbations. In this paper that method is extended into the time domain. The main advantage of the time-domain approach is that limit-cycle stability, the influence of harmonics, and the interaction between different modes can be simulated. In LPP combustion, fluctuations in the inlet fuel-air ratio have been shown to be the dominant cause of unsteady combustion: these occur because velocity perturbations in the premix ducts cause a time-varying fuel-air ratio, which then convects downstream. If the velocity perturbation becomes comparable to the mean flow, there will be an amplitude-dependent effect on the equivalence ratio fluctuations entering the combustor and hence on the rate of heat release. Since the Mach number is low, the velocity perturbation can be comparable to the mean flow, with even reverse flow occurring, while the disturbances are still acoustically linear in that the pressure perturbation is still much smaller than the mean. Hence while the combustion response to flow velocity and equivalence ratio fluctuations must be modelled nonlinearly, the flow perturbations generated as a result of the unsteady combustion can be treated as linear. In developing a time-domain network model for nonlinear thermoacoustic oscillations an initial frequency-domain calculation is performed. The linear network model, LOTAN, is used to categorise the combustor geometry by finding the transfer function for the response of flow perturbations (at the fuel injectors, say) to heat-release oscillations. This transfer function is then converted into the time domain through an inverse Fourier transform to obtain the Green’s function, which thus relates unsteady flow to heat release at previous times. By combining this with a nonlinear flame model (relating heat release to unsteady flow at previous times) a complete time-domain solution can be found by stepping forward in time. If an unstable mode is present, its amplitude will initially grow exponentially (in accordance with linear theory) until saturation effects in the flame model become significant, and eventually a stable limit cycle will be attained. The time-domain approach enables determination of the limit-cycle. In addition, the influence of harmonics and the interaction and exchange of energy between different modes can be simulated. These effects are investigated for longitudinal and circumferential instabilities in an example combustor system and results are compared to frequency-domain limit-cycle predictions.Copyright © 2008 by ASME

Spatiotemporal Characterization of a Conical Swirler Flow Field Under Strong Forcing

Abstract: In this study, a spatio-temporal characterization of forced and unforced flows of a conical swirler is done based on Particle Image Velocimetry (PIV) and Laser Doppler Anemometry (LDA). The measurements are performed at a Reynolds number of 33,000 and a swirl number of 0.71. Axisymmetric forcing is applied to approximate the effects of thermoacoustic instabilities on the flow field at the burner inlet and outlet. The actuation frequencies are set at the natural flow frequency (Strouhal number Stf ≈ 0.92) and two higher frequencies (Stf ≈ 1.3 and 1.55) that are not harmonically related. Phase locked and phase averaged measurement are used as a first step to visualize the coherent flow structures. Secondly, Proper Orthogonal Decomposition (POD) is applied to the PIV data to characterize the effect of the actuation on the fluctuating flow. Measurements indicate a typical natural flow instability of helical nature in the unforced case. The associated induced pressure and flow oscillations travel upstream to the swirler inlet where generally fuel is injected. This observation is of critical importance with respect to the stability of the combustion. Harmonic actuation at different frequencies and amplitudes affects the mean-field profile most at the outlet, while the coherent velocity fluctuations are strongly influenced at both inlet and outlet. On one hand, the dominant helical mode is replaced by an axisymmetric vortex ring if the flow is forced at the natural flow frequency. On the other hand, the natural flow frequency prevails at the outlet under forcing at higher frequencies and POD analysis indicates that the helical structure is still present. The presented results give new insight into the flow dynamics of a swirling flow burner under strong forcing.Copyright © 2008 by ASME

Effects of Temperature and Particle Size on Deposition in Land Based Turbines

Abstract: Four series of tests were performed in an accelerated deposition test facility to study the independent effects of particle size, gas temperature, and metal temperature on ash deposits from two candidate power turbine synfuels (coal and petcoke). The facility matches the gas temperature and velocity of modern first stage high pressure turbine vanes while accelerating the deposition process. Particle size was found to have a significant effect on capture efficiency with larger particles causing significant thermal barrier coating (TBC) spallation durin ga4h accelerated test. In the second series of tests, particle deposition rate was found to decrease with decreasing gas temperature. The threshold gas temperature for deposition was approximately 960°C. In the third and fourth test series, impingement cooling was applied to the back side of the target coupon to simulate internal vane cooling. Capture efficiency was reduced with increasing mass flow of coolant air; however, at low levels of cooling, the deposits attached more tenaciously to the TBC layer. Postexposure analyses of the third test series (scanning electron microscopy and X-ray spectroscopy) show decreasing TBC damage with increased cooling levels. DOI: 10.1115/1.2903901

Performance Trends of an Air-Cooled Steam Condenser Under Windy Conditions

Abstract: Air-cooled steam condensers (ACSCs) are increasingly employed to reject heat in modern power plants. Unfortunately, these cooling systems become less effective under windy conditions and when ambient temperatures are high. A better understanding of the fundamental air flow patterns about and through such ACSCs is essential if their performance is to be improved under these conditions. The present numerical study models the air flow field about and through a particular ACSC. The performance of the fans is modeled with the aid of a novel numerical approach known as the “actuator disc model.” Distorted air flow patterns that significantly reduce fan performance in certain areas and recirculatory flows that entrain hot plume air are found to be the reasons for poor ACSC performance. It is found that the reduction in fan performance is the main reason for the poor ACSC performance while recirculation of hot plume air only reduces performance by a small amount.

Laboratory Studies of the Flow Field Characteristics of Low-Swirl Injectors for Adaptation to Fuel-Flexible Turbines

Abstract: The low-swirl injector (LSI) is a simple and cost-effective lean premixed combustion method for natural-gas turbines to achieve ultralow emissions (<5 ppm NO x and CO) without invoking tight control of mixture stoichiometry, elaborate active tip cooling, or costly materials and catalysts. To gain an understanding of how this flame stabilization mechanism remains robust throughout a large range of Reynolds numbers, laboratory experiments were performed to characterize the flowfield of natural-gas flames at simulated partial load conditions. Also studied was a flame using simulated landfill gas of 50% natural gas and 50% CO 2 . Using particle image velocimetry, the nonreacting and reacting flowfields were measured at five bulk flow velocities. The results show that the LSI flowfield exhibits similarity features. From the velocity data, an analytical expression for the flame position as function of the flowfield characteristics and turbulent flame speed has been deduced. It shows that the similarity feature coupled with a linear dependency of the turbulent flame speed with bulk flow velocity enables the flame to remain relatively stationary throughout the load range. This expression can be the basis for an analytical model for designing LSIs that operate on alternate gaseous fuels such as slower burning biomass gases or faster burning coal-based syngases.

Explicit Finite Element Models of Friction Dampers in Forced Response Analysis of Bladed Disks

Abstract: A generic method for analysis of nonlinear forced response for bladed disks with friction dampers of different designs has been developed The method uses explicit finite element modeling of dampers, which allows accurate description of flexibility and, for the first time, dynamic properties of dampers of different designs in multiharmonic analysis of bladed disks Large-scale finite element damper and bladed disk models containing 10(4) - 10(6) degrees of freedom can be used These models, together with detailed description of contact interactions over contact interface areas, allow for any level of refinement required for modeling of elastic damper bodies and for modeling of friction contact interactions Numerical studies of realistic bladed disks have been performed with three different types of underplatform dampers: (i) a "cottage-roof" (also called "wedge") damper (ii) seal wire damper and (iii) a strip damper Effects of contact interface parameters and excitation levels on damping properties of the dampers and forced response are extensively explored

Laboratory Investigations of Low-Swirl Injectors Operating With Syngases

Abstract: The low-swirl injector (LSI) is a lean premixed combustion technology that has the potential for adaptation to fuel-flexible gas turbines operating on a variety of fuels. The objective of this study is to gain a fundamental understanding of the effect of syngas on the LSI flame behavior, the emissions and the flowfield characteristics for its adaptation to the combustion turbines in IGCC clean coal power plants. The experiments were conducted in two facilities. Open laboratory flames generated by a full size (6.35 cm) LSI were used to investigate the lean-blow off limits, emissions, and the flowfield characteristics. Verification of syngas operation at elevated temperatures and pressures were performed with a reduced scale (2.54 cm) LSI in a small pressurized combustion channel. The results show that the basic LSI design is amenable to burning syngases with up to 60% H2 . Syngases with high H2 concentration have lower lean blow-off limits. From PIV measurements, the flowfield similarity behavior and the turbulent flame speeds of syngases flames are consistent with those observed in hydrocarbon and pure or diluted hydrogen flames. The NOx emissions from syngas flames show log-linear dependency on the adiabatic flame temperature and are comparable to those reported for the gaseous fuels reported previously. Successful firing of the reduced-scale LSI at 330 < T < 446° F and 8 atm verified the operability of this concept at gas turbine conditions.Copyright © 2008 by ASME

An Energy-Based Uniaxial Fatigue Life Prediction Method for Commonly Used Gas Turbine Engine Materials

Abstract: A new energy-based life prediction framework for calculation of axial and bending fatigue results at various stress ratios has been developed. The purpose of the life prediction framework is to assess the behavior of materials used in gas turbine engines, such as Titanium 6Al-4V (Ti 6AI-4V) and Aluminum 6061-T6 (Al 6061-T6). The work conducted to develop this energy-based framework consists of the following entities: (1) a new life prediction criterion for axial and bending fatigue at various stress ratios for Al 6061-T6, (2) the use of the previously developed improved uniaxial energy-based method to acquire fatigue life prior to endurance limit region (Scott-Emuakpor et al., 2007, "Development of an Improved High Cycle Fatigue Criterion, " ASME J. Eng. Gas Turbines Power, 129, pp. 162-169), (3) and the incorporation of a probabilistic energy-based fatigue life calculation scheme to the general uniaxial life criterion (the first entity of the framework), which is capable of constructing prediction intervals based on a specified percent confidence level. The precision of this work was verified by comparison between theoretical approximations and experimental results from recently acquired Al 606-T6 and Ti 6Al-4V data. The comparison shows very good agreement, thus validating the capability of the framework to produce accurate uniaxial fatigue life predictions for commonly used gas turbine engine materials.

Properties, Characteristics, and Combustion Performance of Sasol Fully Synthetic Jet Fuel

Abstract: In 1999, as the only inland petroleum refinery in South Africa was reaching capacity, Sasol gained approval of a semi-synthetic jet fuel (SSJF) for civil aviation to augment production and meet the growing demand for jet fuel at the airport in Johannesburg. Prior to this, all jet fuel had to be refined from petroleum sources. SSJF consists of up to 50% of an iso-paraffinic kerosene produced from coal using Fischer-Tropsch processes. The production of SSJF remains vulnerable to the production capacity of conventional jet fuel, however. To ensure supply, Sasol has proposed producing a fully synthetic jet fuel (FSJF) using synthetic kerosene streams that contain aromatics and satisfy all the property requirements of international specifications for jet fuel. Being fully synthetic, it was necessary to demonstrate that the fuel is “fit-for-purpose” as jet fuel, i.e., behaves like conventional jet fuel in all aspects of storage and handling as well as air worthiness and flight safety. Four sample blends were developed covering the practical range of production. Extensive tests on chemistry and physical properties and characteristics demonstrated that Sasol FSJF will be typical of conventional jet fuel. As a final demonstration, the engine manufacturers requested a series of engine and combustor tests to evaluate combustion characteristics, emissions, engine durability, and performance. The performance of the synthetic test fuel was typical of conventional jet fuel. This paper identifies the tests and presents the results demonstrating that Sasol fully synthetic jet fuel is fit-for-purpose as jet fuel for civilian aviation. Sasol FSJF is the first fully synthetic jet fuel approved for unrestricted use.Copyright © 2008 by Sasol Technology (Pty) Ltd

Oxidation of Natural Gas, Natural Gas/Syngas Mixtures, and Effect of Burnt Gas Recirculation: Experimental and Detailed Kinetic Modeling

Abstract: The oxidation of methane-based fuels was studied experimentally in a fused-silica jet-stirred reactor (JSR) operating at 1-10 atm, over the temperature range of 900-1450 K, from fuel-lean to fuel-rich conditions. Similar experiments were performed in the presence of carbon dioxide or syngas (CO/H 2 ). A previously proposed kinetic reaction mechanism updated for modeling the oxidation of hydrogen, CO, methane, methanol, formaldehyde, and natural gas over a wide range of conditions including JSR, flame, shock tube, and plug flow reactor was used. A detailed chemical kinetic modeling of the present experiments was performed yielding a good agreement between the modeling, the present data and literature burning velocities, and ignition data. Reaction path analyses were used to delineate the important reactions influencing the kinetic of oxidation of the fuels in the presence of variable amounts of CO 2 . The kinetic reaction scheme proposed helps understand the effect of the additives on the oxidation of methane.

Thermomechanical Fatigue Life Prediction of Cylinder Heads in Combustion Engines

Abstract: While the deformation and damage behavior of aluminum cylinder heads under complex thermal mechanical loading has been the subject of numerous studies in the past, cast iron cylinder heads have been in the focus of thermomechanical fatigue (TMF) only to a minor extent. In this paper, a feasible procedure is presented to set up material models and estimate service life of cast iron cylinder heads under variable thermomechanical loading conditions by the use of computer-aided engineering tools. In addition, the influence of thermal load and mechanical constraints on TMF life span is shown. A specimen model is used for parameter identification in material model setup and a cylinder head model is used for correlation with cracking phenomena. Investigation of different thermomechanical load influences is conducted on the cylinder head model. The principal strain and energy based fatigue criteria are used in assessment of TMF lifetime for the cast iron family and material specific evaluation procedures are pointed out. The results highlight the importance of exact definitions of the boundary conditions and underline the sensitivity of TMF lifespan of cast iron cylinder heads with respect to the defined boundary conditions. Considering this sensitivity, an approach conforming to the engine development requirements is proposed. It is shown that both the crack location and fatigue lifetime are predicted with high accuracy.

Nonlinear Numerical Prediction of Gas Foil Bearing Stability and Unbalanced Response

Abstract: One of the main interests of gas foil bearings lies in their superior rotordynamic characteristics compared to conventional bearings. A numerical investigation on the stability limit and on the unbalanced response of foil bearings is presented in this paper. The main difficulty in modeling the dynamic behavior of such bearings comes from the dry friction that occurs within the foil structure. Indeed, dry friction is highly non linear and is strongly influenced by the dynamic amplitude of the pressure field. To deal with these non linearities, a structural dynamic model has been developed in a previous work. This model considers the entire corrugated foil and the interactions between the bumps by describing the foil bearing structure as a multiple degrees of freedom system. It allows the determination of the dynamic friction forces at the top and at the bottom of the bumps by simple integration of ordinary differential equations. The dynamic displacements of the entire corrugated sheet are then easily obtained at each time step. The coupling between this structural model and a gas bearing prediction code is presented in this paper and allows performing full non linear analyses of a complete foil bearing. The bearing stability is the first investigated problem. The results show that the structural deflection enhances the stability of compliant surface bearings compared to rigid ones. Moreover, when friction is introduced, a new level of stability is reached revealing the importance of this dissipation mechanism. The second investigated problem is the unbalanced response of foil bearings. The shaft trajectories depict a non linear jump in the response of both rigid and foil bearings when the value of the unbalance increases. Again, it is evidenced that the foil bearing can support higher mass unbalance before this undesirable step occurs.Copyright © 2008 by ASME

A Comparison Between Combustion Phase Indicators for Optimal Spark Timing

Abstract: The closed-loop control of internal combustion engine spark timing may be accomplished by means of a combustion phase indicator, i.e., a parameter, derived from in-cylinder pressure analysis, whose variation is mainly referable to combustion phase shift and assumes a fixed reference value under optimal spark timing operation. The aim of the present work is a comparison between different combustion phase indicators, focusing on the performance attainable by a feedback spark timing control, which uses the indicator as pilot variable. An extensive experimental investigation has been carried out, verifying the relationship between indicators’ optimal values and the main engine running parameters: engine speed, load, and mixture strength. Moreover, assessment on the effect of the most common pressure measurement problems (which are mainly related to pressure referencing, sampling resolution, top dead center determination, and cycle-by-cycle variations) on the indicators’ values and on the performance attainable by the spark timing control is included. The results of the comparison point out two indicators as the most suitable: the location of pressure peak and the location of maximum heat release rate. The latter, not available in literature, has been introduced by the author as an alternative to the 50% of mass fraction burned. DOI: 10.1115/1.2939012

Static Performance Characteristics and Rotordynamic Coefficients for a Four-Pad Ball-in-Socket Tilting Pad Journal Bearing

Abstract: Static performance characteristics and rotordynamic coefficients were experimentally determined for a four-pad, spherical-seat, tilting-pad journal bearing in load-between-pad configuration. Measured static characteristics include journal static equilibrium position, estimated power loss, and trailing-edge pad temperatures. Rotordynamic coefficients were determined from curve fits of measured complex dynamic-stiffness coefficients as a functions of the excitation frequency. A frequency-independent [M]-[C]-[K] model did a good job of fitting the measurements. Test conditions included speeds from 4 to 12 krpm and unit loads from 0 to 1896 kPa (0 to 275 psi). The bearing uses cool inlet oil to decrease the pad operating temperatures and increase the bearing’s load and speed capacity. The bearing has a nominal diameter of 101.78 mm (4.0070 in). Measurements indicated significant bearing crush with a radial bearing clearance of 99.63 μm (3.92 mils) in the axis 45° counterclockwise from the loaded axis and 54.60 μm (2.15 mils) in the axis 45° clockwise from the loaded axis. The pad length is 101.60 mm (4.00 in), giving L/D = 1.00. The pad arc angle is 73°, and the pivot offset ratio is 65%. Testing was performed using a test rig described by Kaul [1], and rotordynamic coefficients were extracted using a procedure adapted from Childs and Hale [2]. A bulk-flow Navier-Stokes model was used for predictions, using adiabatic conditions for the fluid in the bearings. However, the model assumes constant nominal clearances at all pads, and an average clearance was used based on measured clearances. Measured static eccentricities and attitude angles were significantly lower than predicted. Attitude angles varied from 6° to 39° and decreased with load. Power loss was well-predicted, with a maximum value of 25 kW (34 hp). The maximum detected pad temperature was 71°C (160°C) while the temperature rise from inlet to exit was over-predicted by 8°C (14°F). Direct stiffness and damping coefficients were significantly over-predicted, but the addition of a simple pivot-stiffness in series with the measured stiffness and damping values vastly improved the agreement between theory and experiment. Direct added masses were negative to a higher degree for Myy (y load direction) at low speeds and increased with speed. With the exception of Myy at zero load, they became positive before reaching 8,000 rpm. Although significant cross-coupled stiffness terms were present, they always had the same sign, producing a whirl frequency ratio of zero and netting unconditional stability over all test conditions.© 2008 ASME

Flow Field and Combustion Characterization of Premixed Gas Turbine Flames by Planar Laser Techniques

Abstract: Lean premixed natural gas/air flames produced by an industrial gas turbine burner were analyzed using laser diagnostic methods. For this purpose, the burner was equipped with an optical combustion chamber and operated with preheated air at various thermal powers P, equivalence ratios Φ , and pressures up to p = 6 bar. For the visualization of the flame emissions OH* chemiluminescence imaging was applied. Absolute flow velocities were measured using particle image velocimetry (PIV), and the reaction zones as well as regions of burnt gas were characterized by planar laser induced fluorescence (PLIF) of OH. Using these techniques, the combustion behavior was characterized in detail. The mean flow field could be divided into different regimes: the inflow, a central and an outer recirculation zone, and the outgoing exhaust flow. Single-shot PIV images demonstrated that the instantaneous flow field was composed of small and medium sized vortices, mainly located along the shear layers. The chemiluminescence images reflected the regions of heat release. From the PLIF images it was seen that the primary reactions are located in the shear layers between the inflow and the recirculation zones and that the appearance of the reaction zones changed with flame parameters.Copyright © 2008 by ASME

Combustion Performance of Liquid Biofuels in a Swirl-Stabilized Burner

Abstract: Fuels produced from renewable sources offer an economically viable pathway to curtail emissions of greenhouse gases. Two such liquid fuels in common usage are biodiesel and ethanol derived from soybean, corn, or other food crops. In recent years, significant effort has been devoted to identify alternate feedstock sources and conversion techniques to diversify the biofuels portfolio. In this study, we have measured emissions from flames of diesel, biodiesel, emulsified bio-oil, and diesel-biodiesel blends. Experiments are conducted in an atmospheric pressure burner with an air-atomized injector and swirling primary air around it to replicate typical features of a gas turbine combustor. Experiments were conducted for fixed air and fuel flow rates, while the airflow split between the injector and the coflow swirler was varied. Results show a significant reduction in emissions as the fraction of total air fed into the atomizer is increased. Blue flames, reminiscent of premixed combustion, and low emissions of nitric oxides and carbon monoxide were observed for all fuel blends. In general, the emissions from biofuel flames were comparable or lower than those from diesel flames.

Experimental and Numerical Investigation of Rotating Bladed Disk Forced Response Using Underplatform Friction Dampers

Abstract: In this paper, we present a methodology and results from an experimental investigation of forced vibration response for a bladed disk with fitted underplatform "cottage-roof" friction dampers, together with the corresponding numerical predictions. A carefully designed and constructed rotating test rig is used to make precise measurements, which involve only the phenomena of interest. For this purpose, the measurement rig is operated under vacuum to eliminate aerodynamic effects on the rotating blisk and noncontact excitation and measurement techniques are employed so as not to modify the bladed disk dynamics. The experimental data measured are used for validation of multiharmonic balance-based prediction tools developed at the Imperial College. Predictions are carried out both with and without taking inherent mechanical mistuning into account, which is identified from measured data. Measured and predicted response curves are compared with each other and the degree of correlation is discussed

On Scaling Down Turbines to Millimeter Size

Abstract: This article discusses how to achieve maximum theoretical effectiveness when using a viscous flow turbine such as Tesla turbine.

Development of a Highly Reduced Mechanism for Iso-Octane HCCI Combustion With Targeted Search Algorithm

Abstract: This paper describes recent development of iso-octane skeletal and reduced mechanisms for speeding up numerical simulations of homogeneous charge compression ignition (HCCI) engines. A novel targeted search algorithm is developed to systematically screen species for quasisteady state (QSS) assumption in order to reduce the mechanism size while maintaining accuracy. This new approach is especially found useful when the chemical kinetics involve complex ignition pathways. Using the iso-octane mechanism developed by LLNL, a skeletal mechanism with 215 species (Skeletal-215) and a reduced mechanism with 63 non-QSS species (Reduced-63) were constructed. Evaluations of the performances of the Skeletal-215 and the Reduced-63 were extensively conducted for the operation regimes in HCCI engine applications. Both mechanisms are found satisfactory in predicting start of combustion and minor emission species.

Effect of Piston Friction on the Performance of SI Engine: A New Thermodynamic Approach

Abstract: This paper presents thermodynamic analysis of piston friction in spark-ignition internal combustion engines. The general effect of piston friction on engine performance was examined during cold starting and normal working conditions. Considerations were made using temperature-dependent specific heat model in order to make the analysis more realistic. A parametric study was performed covering wide range of dependent variables such as engine speed, taking into consideration piston friction combined with the variation of the specific heat with temperature, and heat loss from the cylinder. The results are presented for skirt friction only, and then for total piston friction (skirt and rings). The effect of oil viscosity is investigated over a wide range of engine speeds and oil temperatures. In general, it is found that oils with higher viscosities result in lower efficiency values. Using high viscosity oil can reduce the efficiency by more than 50% at cold oil temperatures. The efficiency maps for SAE 10, SAE 30, and SAE 50 are reported. The results of this model can be practically utilized to obtain optimized efficiency results either by selecting the optimum operating speed for a given oil type (viscosity) and temperature or by selecting the optimum oil type for a given operating speed and temperature. The effect of different piston ring configurations on the efficiency is also presented. Finally, the oil film thickness on the engine performance is studied in this paper.

Reduced Reaction Mechanisms for Methane and Syngas Combustion in Gas Turbines

Abstract: Two reduced reaction mechanisms were established that predict reliably for pressures up to about 20 bar the heat release for different syngas mixtures including initial concentrations of methane. The mechanisms were validated on the base of laminar flame speed data covering a wide range of preheat temperature, pressure, and fuel-air mixtures. Additionally, a global reduced mechanism for syngas, which comprises only two steps, was developed and validated, too. This global reduced and validated mechanism can be incorporated into CFD codes for modeling turbulent combustion in stationary gas turbines.

Hybrid Gas Bearings With Controlled Supply Pressure to Eliminate Rotor Vibrations While Crossing System Critical Speeds

Abstract: The continued support of the TAMU Turbomachinery Research Consortium is gratefully acknowledged. Thanks to KMC, Inc. for manufacturing the test bearings.