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### Research Papers: Gas Turbines: Coal, Biomass, and Alternative Fuels

J. Eng. Gas Turbines Power. 2008;130(2):021401-021401-7. doi:10.1115/1.2799532.

This paper proposes a novel, multifunctional energy system (MES), in which hydrogen and electricity are cogenerated and about 90% of $CO2$ is removed. By integrating the methane/steam reforming reaction and combustion of coal, the natural gas and coal are utilized synthetically, and coal is burned to provide high-temperature thermal energy to the methane/steam reforming reaction. Afterwards, the resulting syngas enters a pressure swing adsorption (PSA) unit to separate about 70% of hydrogen, thereby significantly increasing the concentration of carbon dioxide from nearly 20% to 43% in the PSA tail gas. As a result, the overall efficiency of the new system becomes 63.2%. Compared to a conventional natural gas-based hydrogen plant and a coal-firing steam power plant without $CO2$ removal (the overall efficiency of the two systems is 63.0%), the energy penalty for $CO2$ removal in the new system is almost totally avoided. Based on the graphical exergy analysis, we propose that the integration of synthetic utilization of fossil fuel (natural gas and coal) and the $CO2$ removal process plays a significant role in zero energy penalty for $CO2$ removal and its liquefaction in the MES. The result obtained here provides a new approach for $CO2$ removal with zero or low thermal efficiency reduction (energy penalty) within an energy system.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

J. Eng. Gas Turbines Power. 2008;130(2):021501-021501-10. doi:10.1115/1.2795786.

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 NOx 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% CO2 . 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.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):021502-021502-9. doi:10.1115/1.2795787.

In this paper, the development of an eight-step global chemical kinetic mechanism for methane oxidation with nitric oxide formation in lean-premixed combustion at elevated pressures is described and applied. In particular, the mechanism has been developed for use in computational fluid dynamics and chemical reactor network simulations of combustion in lean-premixed gas turbine engines. Special attention is focused on the ability of the mechanism to predict $NOx$ and CO exhaust emissions. Applications of the eight-step mechanism are reported in the paper, all for high-pressure, lean-premixed, methane-air (or natural gas-air) combustion. The eight steps of the mechanism are as follows: (1) oxidation of the methane fuel to CO and $H2O$, (2) oxidation of the CO to $CO2$, (3) dissociation of the $CO2$ to CO, (4) flame-NO formation by the Zeldovich and nitrous oxide mechanisms, (5) flame-NO formation by the prompt and NNH mechanisms, (6) postflame-NO formation by equilibrium H-atom attack on equilibrium $N2O$, (7) postflame-NO formation by equilibrium O-atom attack on equilibrium $N2O$, and (8) postflame Zeldovich NO formation by equilibrium O-atom attack on $N2$.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):021503-021503-8. doi:10.1115/1.2799530.

It is well known that the process of vortex breakdown plays an important role in establishing the near-field aerodynamic characteristics of fuel injectors, influencing fuel/air mixing and flame stability. The precise nature of the vortex breakdown can take on several forms, which have been shown in previous papers to include both a precessing vortex core (PVC) and the appearance of multiple helical vortices formed in the swirl stream shear layer. The unsteady dynamics of these particular features can play an important role in combustion induced oscillations. The present paper reports an experimental investigation, using particle image velocimetry (PIV) and hot-wire anemometry, to document variations in the relative strength of PVC and helical vortex patterns as the configuration of a generic fuel injector is altered. Examples of geometric changes that have been investigated include: the combination of an annular swirl stream with and without a central jet; variation in geometric details of the swirler passage, e.g., alteration in the swirler entry slots to change swirl number, and variations in the area ratio of the swirler passage. The results show that these geometric variations can influence: the axial location of the origin of the helical vortices (from inside to outside the fuel injector), and the strength of the PVC. For example, in a configuration with no central jet (swirl number $S=0.72$), the helical vortex pattern was much less coherent, but the PVC was much stronger than when a central jet was present. These changes modify the magnitude of the turbulence energy in the fuel injector near field dramatically, and hence have an important influence on fuel air mixing patterns.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):021504-021504-6. doi:10.1115/1.2719258.

Two reduced reaction mechanisms were established that predict reliably for pressures up to about $20bar$ 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.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):021505-021505-15. doi:10.1115/1.2795758.

Aircraft emissions of trace sulfur and nitrogen oxides contribute to the generation of fine volatile particulate matter (PM). Resultant changes to ambient PM concentrations and radiative properties of the atmosphere may be important sources of aviation-related environmental impacts. This paper addresses engine design and operational impacts on aerosol precursor emissions of $SOx$ and $NOy$ species. Volatile PM formed from these species in the environment surrounding an aircraft is dependent on intraengine oxidation processes occurring both within and downstream of the combustor. This study examines the complex response of trace chemistry to the temporal and spatial evolution of temperature and pressure along this entire intraengine path after combustion through the aft combustor, turbine, and exhaust nozzle. Low-order and higher-fidelity tools are applied to model the interaction of chemical and fluid mechanical processes, identify important parameters, and assess uncertainties. The analysis suggests that intraengine processing is inefficient. For in-service engine types in the large commercial aviation fleet, mean conversion efficiency $(ε)$ is estimated to be 2.8–6.5% for sulfate precursors and 0.3–5.7% for nitrate precursors at the engine exit plane. These ranges reflect technological differences within the fleet, a variation in oxidative activity with operating mode, and modeling uncertainty stemming from variance in rate parameters and initial conditions. Assuming that sulfur-derived volatile PM is most likely, these results suggest emission indices of $0.06–0.13g∕kg$ fuel, assuming particles nucleated as $2H2SO4∙H2O$ for a fuel sulfur content of $500ppm$.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):021506-021506-11. doi:10.1115/1.2771564.

The emissions of liquid-fuel fired gas turbine engines are strongly affected by the fuel preparation process that includes atomization, evaporation, and mixing. In the present paper, the effects of fuel atomization and evaporation on emissions from an industrial gas turbine engine were investigated. In the engine studied, the fuel injector consists of a coaxial plain jet airblast atomizer and a premixer which consists of a cylindrical tube with four mixing holes and swirler slits. The goal of this device is to establish a fully vaporized, homogeneous fuel/air mixture for introduction into the combustion chamber and the reaction zone. In the present study, experiments were conducted at atmospheric pressure and room temperature as well as at actual engine conditions ($0.34MPa$, $740K$) both with and without the premixer. Measurements included visualization, droplet size, and velocity. By conducting tests with and without the premixing section, the effect of the mixing holes and swirler slit design on atomization and evaporation was isolated. The results were also compared with engine data and the relationship between premixer performance and emissions was evaluated. By comparing the results of tests over a range of pressures, the viability of two scaling methods was evaluated with the conclusion that spray angle correlates with fuel to atomizing air momentum ratio. For the injector studied, however, the conditions resulting in superior atomization and vaporization did not translate into superior emissions performance. This suggests that, while atomization and the evaporation of the fuel are important in the fuel preparation process, they are of secondary importance to the fuel/air mixing prior to, and in the early stages of the reaction in, governing emissions.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Controls, Diagnostics, and Instrumentation

J. Eng. Gas Turbines Power. 2008;130(2):021601-021601-9. doi:10.1115/1.2795761.

Prognostic health monitoring is an important element of condition-based maintenance and logistics support. The accuracy of prediction and the associated confidence in prediction greatly influence overall performance and subsequent actions either for maintenance or logistics support. Accuracy of prognosis is directly dependent on how closely one can capture the system and component interactions. Traditionally, such models assume a constant and univariate prognostic formulation—that is, components degrade at a constant rate and are independent of each other. Our objective in this paper is to model the degrading system as a collection of prognostic states (health vectors) that evolve continuously over time. The proposed model includes an age dependent deterioration distribution, component interactions, as well as effects of discrete events arising from line maintenance actions and/or abrupt faults. Mathematically, the proposed model can be summarized as a continuously evolving dynamic model, driven by non-Gaussian input and switches according to the discrete events in the system. We develop this model for aircraft auxiliary power units, but it can be generalized to other progressive deteriorating systems. The system identification and recursive state estimation scheme for the developed non-Gaussian model under a partially specified distribution framework has been deduced. The diagnostic/prognostic capabilities of our model and algorithms have been demonstrated using simulated and field data.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):021602-021602-11. doi:10.1115/1.2795776.

This paper presents a method for deriving requirements for the efficiency of diagnostic functions in distributed electronic turbofan engine control systems. Distributed engine control systems consist of sensor, actuator, and control unit nodes that exchange data over a communication network. The method is applicable to engine control systems that are partially redundant. Traditionally, turbofan engine control systems use dual channel solutions in which all units are duplicated. Our method is intended for analyzing the diagnostic requirements for systems in which a subset of the sensors and the actuators is nonredundant. Such systems rely on intelligent monitoring and analytical redundancy to detect and tolerate failures in the nonredundant units. These techniques cannot provide perfect diagnostic coverage and, hence, our method focuses on analyzing the impact of nonperfect diagnostic coverage on the reliability and safety of distributed engine control systems. The method is based on a probabilistic analysis that combines fault trees and Markov chains. The input parameters for these models include failure rates as well as several coverage factors that characterize the performance of the diagnostic functions. Since the use of intelligent monitoring can cause false alarms, i.e., an error is falsely indicated by a diagnostic function, the parameters also include a false alarm rate. The method was used to derive the diagnostic requirements for a hypothetical unmanned aerial vehicle engine control system. Given the requirement that an engine failure due to the control system is not allowed to occur more than ten times per million hours, the diagnostic functions in a node must achieve 99% error coverage for transient faults and 90–99% error coverage for permanent faults. The system-level diagnosis must achieve 90–95% detection coverage for node failures, which are not detected by the nodes themselves. These results are based on the assumption that transient faults are 100 times more frequent than permanent faults. It is important to have a method for deriving probabilistic requirements on diagnostic functions for engine control systems that rely on analytical redundancy as a means to reduce the hardware redundancy. The proposed method allows us to do this using an existing tool (FAULTTREE+ ) for safety and reliability analysis.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):021603-021603-9. doi:10.1115/1.2799525.

The time-dependent relative entropy field at the impeller exit of a centrifugal compressor is measured. This study is part of a broader effort to develop comprehensive measurement techniques that can be applied in the harsh environment of turbomachines. A miniature unsteady entropy probe (diameter of 1.8 mm) is designed and constructed in the present study. The unsteady entropy probe has two components: a one-sensor fast-response aerodynamic probe and a pair of thin-film gauges. The time-dependent total pressure and total temperature are measured from the fast-response aerodynamic probe and pair of thin-film gauges, respectively. The time-dependent relative entropy derived from these two measurements has a bandwidth of 40 kHz and an uncertainty of ±2 J/kg. The measurements show that for operating Condition A, $φ$=0.059 and $ψ$=0.478, the impeller exit flowfield is highly three dimensional. Adjacent to the shroud there are high levels of relative entropy and at the midspan there are low and moderate levels. Independent measurements made with a two-sensor aerodynamic probe show that the high velocity of the flow relative to the casing is responsible for the high relative entropy levels at the shroud. On the other hand, at the midspan, a loss free, jet flow region and a channel wake flow of moderate mixing characterize the flowfield. At both the shroud and midspan, there are strong circumferential variations in the relative entropy. These circumferential variations are much reduced when the centrifugal compressor is operated at operating Condition B, $φ$=0.0365 and $ψ$=0.54, near the onset of stall. In this condition, the impeller exit flowfield is less highly skewed; however, the time-averaged relative entropy is higher than at the operating Condition A. The relative entropy measurements with the unsteady entropy probe are thus complementary to other measurements, and more clearly document the losses in the centrifugal compressor.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):021604-021604-10. doi:10.1115/1.2799527.

A practical consideration for implementing a real-time on-board engine component performance tracking system is the development of high fidelity engine models capable of providing a reference level from which performance changes can be trended. Real-time engine models made their advent as state variable models in the mid-1980s, which utilized a piecewise linear model that granted a reasonable representation of the engine during steady state operation and mild transients. Increased processor speeds over the next decade allowed more complex models to be considered, that were a combination of linear and nonlinear physics-based elements. While the latter provided greater fidelity over both transient operation and the engine operational flight envelope, these models could be further improved to provide the high level of accuracy required for long-term performance tracking, as well as address the issue of engine-to-engine variation. Over time, these models may deviate enough from the actual engine being monitored, as a result of improvements made during an engine’s life cycle such as hardware modifications, bleed and stator vane schedule alterations, cooling flow adjustments, and the like, that the module performance estimations are inaccurate and often misleading. The process described in this paper will address these shortcomings while maintaining the execution speed required for real-time implementation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):021605-021605-7. doi:10.1115/1.2772637.

Kalman filters are widely used in the turbine engine community for health monitoring purpose. This algorithm gives a good estimate of the engine condition provided that the discrepancies between the model prediction and the measurements are zero-mean, white random variables. However, this assumption is not verified when instrumentation (sensor) faults occur. As a result, the identified health parameters tend to diverge from their actual values, which strongly deteriorates the diagnosis. The purpose of this contribution is to blend robustness against sensor faults into a tool for performance monitoring of jet engines. To this end, a robust estimation approach is considered and a sensor-fault detection and isolation module is derived. It relies on a quadratic program to estimate the sensor faults and is integrated easily with the original diagnosis tool. The improvements brought by this robust estimation approach are highlighted through a series of typical test cases that may be encountered on current turbine engines.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2008;130(2):021701-021701-8. doi:10.1115/1.2772636.

In recent years, civil aircraft projects are showing a continuous increase in the demand of onboard electrical power, both for the partial substitution of hydraulic or pneumatic controls and drives with electrical ones, and for the consumption of new auxiliary systems developed in response to flight safety and environmental control issues. Aiming to generate onboard power with low emissions and better efficiency, several manufacturers and research groups are considering the possibility to produce a relevant fraction of the electrical power required by the aircraft by a fuel cell system. The first step would be to replace the conventional auxiliary power unit (based on a small gas turbine) with a polymer electrolyte membrane (PEM) fuel cell type, which today is favored with respect to other fuel cell types; thanks to its higher power density and faster startup. The PEM fuel cell can be fed with a hydrogen rich gas coming from a fuel reformer, operating with the same jet fuel used by the aircraft, or relying on a dedicated hydrogen storage onboard. The cell requires also an air compression unit, where the temperature, pressure, and humidity of the air stream feeding the PEM unit during land and in-flight operation strongly influence the performance and the physical integrity of the fuel cell. In this work we consider different system architectures, where the air compression system may exploit an electrically driven compressor or a turbocharger unit. The compressor type and the system pressure level are optimized according to a fuel cell simulation model, which calculates the cell voltage and efficiency as a function of temperature and pressure, calibrated over the performances of real PEM cell components. The system performances are discussed under different operating conditions, covering ground operation, and intermediate and high altitude cruise conditions. The optimized configuration is selected, presenting energy balances and a complete thermodynamic analysis.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):021702-021702-9. doi:10.1115/1.2772638.

A dynamic solid oxide fuel cell (SOFC) model was integrated with other system components (i.e., reformer, anodic off-gas burner, anodic ejector) to build a system model that can simulate the time response of the anode side of an integrated $250kW$ pressurized SOFC hybrid system. After model description and data on previous validation work, this paper describes the results obtained for the dynamic analysis of the anodic loop, taking into account two different conditions for the fuel flow input: in the first case (I), the fuel flow follows with no delay the value provided by the control system, while in the second case (II), the flow is delayed by a volume between the regulating valve and the anode ejector, this being a more realistic case. The step analysis was used to obtain information about the time scales of the investigated phenomena: such characteristic times were successfully correlated to the results of the subsequent frequency analysis. This is expected to provide useful indications for designing robust anodic loop controllers. In the frequency analysis, most phase values remained in the $0–180deg$ range, thus showing the expected delay-dominated behavior in the anodic loop response to the input variations in the fuel and current. In Case I, a threshold frequency of $5Hz$ for the pressure and steam to carbon ratio and a threshold frequency of $31Hz$ for the anodic flow were obtained. In the more realistic Case II, natural gas pipe delay dominates, and a threshold frequency of $1.2Hz$ was identified, after which property oscillations start to decrease toward null values.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Electric Power

J. Eng. Gas Turbines Power. 2008;130(2):021801-021801-5. doi:10.1115/1.2795762.

This paper proposes a condition based maintenance policy for compressors of industrial gas turbine. Compressor blade fouling contributes a major performance loss in the operation of gas turbine. Water washing is usually done for recovery of the blade fouling problem. There exist two different washing methods, namely, online and offline washings. Many researchers suggested that performing a combined program of regular online washing plus periodic offline washing would give fruitful results with respect to economy. However, such studies are of empirical nature or have considered only deterministic treatment. Considering the rate of fouling as discrete state random process, we propose a condition based maintenance policy with periodic online washing and inspection directed offline washing. According to this policy, the compressor undergoes regular online washes for every $1∕λm$ operating hours, and also undergoes inspections at constant rate $λI$. If the observed condition at an inspection is worse than threshold deterioration state, then perform offline washing. Otherwise, continue with online washing. The proposed algorithm gives optimum schedules for both online washing and inspections considering minimization of total cost per operating hour as objective. It also gives optimum threshold deterioration level for performing offline washing. A comparison of the results for a hypothetical gas turbine compressor is presented as illustration.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Industrial & Cogeneration

J. Eng. Gas Turbines Power. 2008;130(2):022001-022001-7. doi:10.1115/1.2795771.

Following a detailed study of two of the mechanical precipitators in the air preheaters of a thermoelectric power plant, a large amount of ash that was deposited on one of the inlet conduits was observed, obstructing the incoming gas flow. A comparison of the available data for the two most recent hopper cleaning operations revealed that, on the one hand, the amount of ash collected by the clogged precipitator (A) was significantly less than that collected by the other (B) and, on the other hand, the temperature of the ash in the former was noticeably lower than in the latter. Prior to the cleaning of the conduits, a certain amount of damage was caused to the boiler dome, which meant that subsequent cleaning required the use of a hydrolazer, where it was noted that inlet pressures were very high. All of this indicated that the cause of the clogging was not physical. This paper provides a comprehensive analytical analysis that explains what happened, as well as resolving the situation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):022002-022002-9. doi:10.1115/1.2771570.

The paper presents the results of an investigation on inlet air cooling systems based on cool thermal storage, applied to combined cycle power plants. Such systems provide a significant increase of electric energy production in the peak hours; the charge of the cool thermal storage is performed instead during the night time. The inlet air cooling system also allows the plant to reduce power output dependence on ambient conditions. A $127MW$ combined cycle power plant operating in the Italian scenario is the object of this investigation. Two different technologies for cool thermal storage have been considered: ice harvester and stratified chilled water. To evaluate the performance of the combined cycle under different operating conditions, inlet cooling systems have been simulated with an in-house developed computational code. An economical analysis has been then performed. Different plant location sites have been considered, with the purpose to weigh up the influence of climatic conditions. Finally, a parametric analysis has been carried out in order to investigate how a variation of the thermal storage size affects the combined cycle performances and the investment profitability. It was found that both cool thermal storage technologies considered perform similarly in terms of gross extra production of energy. Despite this, the ice harvester shows higher parasitic load due to chillers consumptions. Warmer climates of the plant site resulted in a greater increase in the amount of operational hours than power output augmentation; investment profitability is different as well. Results of parametric analysis showed how important the size of inlet cooling storage may be for economical results.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Marine

J. Eng. Gas Turbines Power. 2008;130(2):022201-022201-6. doi:10.1115/1.2799526.

From the perspective of an overall entity we analyzed the performance obtainable from the adoption of an intercooled-cycle gas turbine under different typical cycle parameters of a gas turbine. On this basis, a study was conducted of the conversion of a high-power simple-cycle marine gas turbine (MGT-33) into a type of intercooled-cycle marine gas turbine. The precondition of the conversion is to keep the flow path and the majority of the structure of the original engine gas generator unchanged in order to inherit the reliability of the prototype machine. The results of the study indicate that after the adoption of an intercooled cycle under the precondition of performing minimum structure modifications and maintaining the compactness of the engine as a whole, there is still a significant enhancement of the gas turbine overall performance with its power output and efficiency being increased by about 34% and 4.1%, respectively, demonstrating the merits of the engineering conversion under discussion.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2008;130(2):022501-022501-11. doi:10.1115/1.2799524.

In this paper, robust maximum forced response reduction strategies based on a “large mistuning” concept are introduced, including both (i) random and (ii) deterministic approaches. An industrial bladed fan disk serves as an application example for a reliability assessment of the aforementioned strategies using two well-established tools for uncertainty analysis: (i) statistics and (ii) sensitivity and robustness. The feasibility and other practical aspects of implementing large mistuning as a means of preventing excessive forced response levels caused by random mistuning and ensuring the predictability of the response are discussed.

Topics: Disks , Blades , Robustness
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):022502-022502-11. doi:10.1115/1.2772633.

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 $104−106$ 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.

Topics: Friction , Dampers , Disks
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):022503-022503-9. doi:10.1115/1.2772634.

An efficient method is developed to calculate stochastic and uncertainty characteristics of forced response for nonlinear vibrations of bladed disks with friction and gap contact interfaces. Uncertainty ranges, statistical characteristics, and probability density functions for forced response levels are determined directly without any sampling procedure. The method uses approximations of the forced response level based on derived analytically and calculated extremely fast and accurately sensitivity coefficients of forced response with respect to friction contact interface parameters. The method effectiveness allows analysis of strongly nonlinear vibration of bladed disks using realistic large-scale finite element models. The method is implemented in a program code developed at Imperial College and numerical examples of application of the method for stochastic analysis of a realistic blisc with underplatform dampers are provided.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):022504-022504-6. doi:10.1115/1.2799529.

A method is presented for parameter identification of an annular gas seal on a flexible-rotor test rig. Dynamic loads are applied by magnetic bearings (MBs) that support the rotor. MB forces are measured using fiber-optic strain gauges that are bonded to the poles of the MBs. In addition to force and position measurements, a finite element rotor model is required for the identification algorithm. The FE rotor model matches free-free characteristics of the test rotor. The addition of smooth air sealed to the system introduces stiffness and damping terms for identification that are representative of reaction forces in turbomachines. Tests are performed to experimentally determine seal stiffness and damping coefficients for different running speeds and preswirl conditions. Stiffness and damping coefficients are determined using a frequency domain identification method. This method uses an iterative approach to minimize error between theoretical and experimental transfer functions. Test results produce seal coefficients with low uncertainties.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):022505-022505-5. doi:10.1115/1.2799531.

Improved rotordynamic stability is desired by end users, and centrifugal compressor manufacturers are expected to meet, if not exceed, this expectation. Compressor manufacturers are required to design and build machines that are rotordynamically stable on the test stand and in the field. Confidence has been established in predicting the excitation forces from seals and bearings, but impeller aerodynamic excitation forces continue to be a challenge. While much attention is paid to impellers from an aerodynamic performance point of view, more efforts are needed from a rotordynamic standpoint. A high-pressure, reinjection centrifugal compressor is analyzed in order to predict rotordynamic stability using the best available resources for seals and bearings. Impeller shroud forces are predicted using the bulk-flow model developed by Gupta and Childs (Gupta, M., and Childs, D., Proc. of ASME Turbo Expo 2000, Power for Land, Sea, and Air). Each impeller stage is analyzed and an attempt is made to improve the estimation of impeller aerodynamic excitation forces. Logarithmic decrement (log dec) predictions for the full rotor model consisting of all the stages and seals are compared to the full-load full-pressure test measured values using a magnetic bearing exciter. A good correlation is obtained between the measured test results and analytical predictions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):022506-022506-8. doi:10.1115/1.2772632.

The application of large-diameter bearing rings and the thereof inherited low stiffness make them susceptible to local distortions caused by their surrounding structures, which are often under heavy loads. The standard accepted design criteria for these bearings are based on the estimation of the internal load distribution of the bearing, under the assumption of rigid circular and flat supporting structures, that keep the bearing inner and outer races in circular, flat, i.e., not deformed shapes. However, in the presence of structural distortions, the element load distribution can be severely altered and cannot be predicted via the standard design criteria. Therefore, the application of large-diameter ball and roller bearing rings as the critical components in rotating machines becomes more of a design task than making a catalog selection. The analytical and finite element approach for fatigue life prediction of such a bearing application is presented. The undertaken approach and the results are illustrated based on the analysis and fatigue life simulation of the computed tomography scanner’s main rotor bearing. It has been demonstrated that flexibility of the rings can significantly reduce the fatigue life of the ball bearing.

Commentary by Dr. Valentin Fuster

### Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2008;130(2):022801-022801-9. doi:10.1115/1.2795764.

Experimental investigations were carried out to assess the use of hydrogen in a gasoline direct injection (GDI) engine. Injection of small amounts of hydrogen (up to 27% on energy basis) in the intake port creates a reactive homogeneous background for the direct injection of gasoline in the cylinder. In this way, it is possible to operate the engine with high exhaust gas recirculation (EGR) rates and, in certain conditions, to delay the ignition timing as compared to standard GDI operation, in order to reduce NOx and HC emissions to very low levels and possibly soot emissions. The results confirmed that high EGR rates can be achieved and NOx and HC emissions reduced, showed significant advantage in terms of combustion efficiency and gave unexpected results relative to the delaying of ignition, which only partly confirmed the expected behavior. A realistic application would make use of hydrogen-containing reformer gas produced on board the vehicle, but safety restrictions did not allow using carbon monoxide in the test facility. Thus, pure hydrogen was used for a best-case investigation. The expected difference in the use of the two gases is briefly discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):022802-022802-8. doi:10.1115/1.2795777.

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.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):022803-022803-13. doi:10.1115/1.2770486.

Many methodologies have been developed in the past for misfire detection purposes based on the analysis of the instantaneous engine speed. The missing combustion is usually detected, thanks to the sudden engine speed decrease that takes place after a misfire event. Misfire detection and, in particular, cylinder isolation are nevertheless still a challenging issue for engines with a high number of cylinders, for engine operating conditions at low load or high engine speed, and for multiple misfire events. When a misfire event takes place, a torsional vibration is excited and shows up in the instantaneous engine speed wave form. If a multiple misfire occurs, this torsional vibration is excited more than once in a very short time interval. The interaction between these successive vibrations can generate false alarms or misdetection, and an increased complexity when dealing with cylinder isolation. This paper presents the development of a powertrain torsional behavior model in order to identify the effects of a misfire event on the instantaneous engine speed signal. The identified wave form has then been used to filter out the torsional vibration effects in order to enlighten the missing combustions even in the case of multiple misfire events. The model response is also used to speed up the setup process for the detection algorithm employed, thus evaluating, before running specific experimental tests on a test bench facility, the values for the threshold and the optimal setup of the procedure. The proposed algorithm is developed in this paper for an SI L4 engine; its application to other engine configurations is possible, as is also discussed in this paper.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):022804-022804-12. doi:10.1115/1.2795770.

Accurate estimation of engine vibrations is essential in the design of new engines, engine mounts, and the vehicle frames to which they are attached. Mount force prediction has traditionally been simplified by assuming that the reciprocating dynamics of the engine can be decoupled from the three-dimensional motion of the block. The accuracy of the resulting one-way coupled models decreases as engine imbalance and cylinder-to-cylinder variations increase. Further, the form of the one-way coupled model must be assumed a priori, and there is no mechanism for generating an intermediate-complexity model if the one-way coupled model has insufficient fidelity. In this paper, a new dynamic system model decoupling algorithm is applied to a Detroit Diesel Series 60 in-line six-cylinder engine model to test one-way coupling assumptions and to automate generation of a proper model for mount force prediction. The algorithm, which identifies and removes unnecessary constraint equation terms, is reviewed with the aid of an illustrative example. A fully coupled, balanced rigid body model with no cylinder-to-cylinder variations is then constructed, from which $x$, $y$, and $z$ force components at the left-rear, right-rear, and front engine mounts are predicted. The decoupling algorithm is then applied to automatically generate a reduced model in which reciprocating dynamics and gross block motion are decoupled. The amplitudes of the varying components of the force time series are predicted to within 8%, with computation time reduced by 55%. The combustion pressure profile in one cylinder is then changed to represent a misfire that creates imbalance. The decoupled model generated by the algorithm is significantly more robust to imbalance than the traditional one-way coupled models in the literature; however, the vertical component of the front mount force is poorly predicted. Reapplication of the algorithm identifies constraint equation terms that must be reinstated. A new, nondecoupled model is generated that accurately predicts all mount components in the presence of the misfire, with computation time reduced by 39%. The algorithm can be easily reapplied, and a new model generated, whenever engine speed or individual cylinder parameters are changed.

Commentary by Dr. Valentin Fuster

### Research Papers: Power Engineering

J. Eng. Gas Turbines Power. 2008;130(2):023001-023001-8. doi:10.1115/1.2795757.

The fundamentals and thermodynamic analysis of high-temperature air combustion (HiTAC) technology is presented. The HiTAC is characterized by high temperature of combustion air having low oxygen concentration. This study provides a theoretical analysis of HiTAC process from the thermodynamic point of view. The results demonstrate the possibilities of reducing thermodynamic irreversibility of combustion by considering an oxygen-deficient combustion process that utilizes both gas and heat recirculations. HiTAC conditions reduce irreversibility. Furthermore, combustion with the use of oxygen (in place of air) is also analyzed. The results showed that a system, which utilizes oxygen as an oxidizer, results in higher first and second law efficiencies as compared to the case with air as the oxidizer. The entropy generation for an adiabatic combustion process is reduced by more than 60% due to the effect of either preheating or oxygen enrichment. This study is aimed at providing technical guidance to further improve efficiency of a combustion process, which shows very small temperature increases due to mild chemical reactions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):023002-023002-4. doi:10.1115/1.2795769.

In a large capacity tangentially fired boiler, the final reheater tubing sustained abnormal oxidation and localized excessive metal wastage in a short time of the unit operation. The root causes of the problem are identified by test data analysis. The test data indicated that the reheater tubing metal temperatures in the affected areas exceeded the recommended limit of the metal oxidation temperature due to higher than expected local gas temperatures and velocities. A soot-blower facing the overheated portion of the reheater leading tubes accelerated the process of metal wastage by periodically removing the oxide layer. The configuration of the boiler internals upstream of the reheater section is found to be the main cause of the localized overheating. Side-to-side gas flow/temperature stratification due to tangential firing contributed to a lesser degree to the problem. The results and conclusions presented in this paper should be a beneficial guide to the designer of large capacity boilers.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):023003-023003-8. doi:10.1115/1.2795779.

The Heat Exchange Institute (HEI) Standards for Steam Surface Condensers are used to design and predict the performance of condensers for power plant applications. Since their inception, the Standards have undergone numerous changes to incorporate technological advances and revisions to various factors based on testing and operating experiences. Admiralty and copper-nickel (CuNi) tubes were very popular until the 1970s. Subsequently, increasing concerns with the use of copper-based alloys in nuclear power plants as well as other factors led to specification and use of stainless steel (SS) and titanium. The first condenser designed with titanium tubes was put into service in 1977. In 1978, the HEI published the seventh edition of the HEI Standards for Steam Surface Condensers. The eighth edition was issued in 1984 followed by Addendum 1 in 1989. The ninth edition was issued in 1995 and Addendum 1 to the ninth edition was published in 2002. Notable differences between the ninth and seventh editions include higher circulating water inlet temperature correction factors below $70.0°F$; for Admiralty, higher tube material and gauge correction factors for tube wall gauge below 16 Birmingham wire gauge (BWG) and lower values above 20 BWG; for $90∕10$ CuNi and 304 SS, higher tube material and gauge correction factors for tube wall gauge between 12 BWG and 24 BWG; and for titanium, higher tube material and gauge correction factors for tube wall gauge above 18 BWG. Depending upon the tube diameter, material, wall gauge, and the correction factors used for a specific condenser application and its operating range, there could be substantial deviations in predicted condenser performance and associated impact on output. Using a case study, this paper examines the use of the correction factors from the seventh and ninth editions in power plant condenser performance predictions. It provides recommendations for developing proper benchmarks and for ensuring optimum condenser performance.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):023004-023004-7. doi:10.1115/1.2795781.

The last stage blades (LSBs) of low-pressure (LP) turbine power plants have been historically specified and designed on the basis of optimization studies by matching the turbine to the condenser/cooling system for a specified unit rating. LSB sizes for U.S. nuclear applications currently range from 38 in. to 52 in. for unit ratings of 600 MWe to 1200 MWe. LP turbine arrangements usually consist of two or three double-flow sections in parallel. Last stage end loadings (last stage mass flow divided by the last stage annulus area) vary from approximately $8000lb∕hsqftto14,000lb∕hsqft$, with corresponding unit loadings (electrical output in megawatts divided by last stage annulus area) of 1.1 $MWe∕sqftto2.1MWe∕sqft$. Several power plants have been upgrading/replacing their LP turbines. Considerations include efficiency, reliability, power uprates, operating license renewals (nuclear), aging, inspection, and maintenance. In some cases, LP turbine rotors are being replaced with new rotors, blading, and steam path. Others are replacing LP turbines with new and advanced designs incorporating improved technology, better materials, optimized steam paths, more efficient blading, longer LSB sizes, redesigned exhaust hoods, etc. Unlike the other stages in the LP turbine, the last stage performance is affected by both the upstream (load) and downstream (condenser) conditions. While the LP turbines are being upgraded or replaced, no major modifications or upgrades are being made to the condensers. To address vibration effects due to increased flows and velocities from power uprates, the condenser tubes may be staked. Circulating water pumps may or may not be upgraded depending upon the particular application. Consequently, while improvements in LP turbines lead to more efficient utilization of the available energy and higher output, the last stage performance may be out of synchronization with the existing condenser/cooling system. Undersized or oversized LSB sizes in relation to the unit rating and end loading may result in less than optimum performance depending upon the design and operating range of the condenser/cooling system. This paper examines the various factors that affect last stage performance of LP turbines. Using a case study, it discusses the relationships between the last stage, the unit rating, the end loading, and the operating range of the condenser/cooling system. It examines different last stage exhaust loss curves and provides recommendations for selection of LSB sizes for optimum performance.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):023005-023005-7. doi:10.1115/1.2771255.

This paper presents the thermodynamic and cost analysis of a coal-based zero-atmospheric emissions electric power plant. The approach involves an oxygen-blown coal gasification unit. The resulting synthetic gas (syngas) is combusted with oxygen in a gas generator to produce the working fluid for the turbines. The combustion produces a gas mixture composed almost entirely of steam and carbon dioxide. These gases drive multiple turbines to produce electricity. The turbine discharge gases pass to a condenser where water is captured. A stream of carbon dioxide then results that can be used for enhanced oil recovery or for sequestration. The term zero emission steam technology is used to describe this technology. We present the analysis of a $400MW$ electric power plant. The power plant has a net thermal efficiency of 39%. This efficiency is based on the lower heating value of the coal, and includes the energy necessary for coal gasification, air separation, and for carbon dioxide separation and sequestration. This paper also presents an analysis of the cost of electricity and the cost of conditioning carbon dioxide for sequestration. Electricity cost is compared for three different gasification processes (Texaco, Shell, and Koppers-Totzek) and two types of coals (Illinois 6 and Wyodak). COE ranges from $5.95¢∕kWhto6.15¢∕kWh$, indicating a 3.4% sensitivity to the gasification processes considered and the coal types used.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):023006-023006-7. doi:10.1115/1.2771567.

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.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2008;130(2):023007-023007-4. doi:10.1115/1.2772631.

When a turbine has combined high-pressure (HP) and intermediate-pressure (IP) sections, there is a steam flow path between the sections. In combined cycle steam turbines, this internal leakage flow rate needs to be determined for the steam turbine performance calculations. However, since the leakage is internal to the turbine, it cannot be measured directly. One method, which has been employed in determining the midpacking leakage flow rate, is the variation of initial and̸or reheat temperature method. It involves using the convergence of IP efficiency plots from multiple test runs to estimate the HP-IP leakage flow rate. Although this method has been employed successfully in large steam applications, it has generally not produced consistent results for combined cycle steam turbines. The lack of convergence for combined cycles may be due to the fact that some of the assumptions made in applying the method to large steam applications are not valid for combined cycle applications. Some of the assumptions, which need to be reviewed and modified for combined cycle application, are as follows: (a) constant IP efficiency for all test runs, (b) constant throttle flow during all test runs, (c) constant section pressure ratios for all test runs, and (d) no influence of external cooling or admission flows. This paper reviews the modifications to the traditional initial and/or reheat temperature variation method to make the midpacking leakage calculations more consistent for combined cycle applications. Some data have shown that incorporating these additional changes improves the convergence of midpacking leakage determination.

Commentary by Dr. Valentin Fuster

### Technical Briefs

J. Eng. Gas Turbines Power. 2008;130(2):024501-024501-4. doi:10.1115/1.2771565.

The thermodynamic performance of the combustion gas turbine trigeneration system has been studied based on first law as well as second law analysis. The effects of overall pressure ratio and process heat pressure on fuel utilization efficiency, electrical to thermal energy ratio, second law efficiency, and exergy destruction in each component are examined. Results for gas turbine cycle, cogeneration cycle, and trigeneration cycle are compared. Thermodynamic analysis indicates that maximum exergy is destroyed during the combustion and steam generation process, which represents over 80% of the total exergy destruction in the overall system. The first law efficiency, electrical to thermal energy ratio, and second law efficiency of trigeneration system, cogeneration system, and gas turbine cycle significantly varies with the change in overall pressure ratio but the change in process heat pressure shows small variations in these parameters. Results clearly show that performance evaluation of the trigeneration system based on first law analysis alone is not adequate and hence more meaningful evaluation must include second law analysis.

Commentary by Dr. Valentin Fuster