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

J. Eng. Gas Turbines Power. 2010;133(1):011601-011601-8. doi:10.1115/1.4001942.

This paper describes the development and initial application studies for an active combustion pattern factor controller (APFC) for gas turbines. The system is based around the use of a novel silicon carbide optical ultraviolet dual diode flame temperature sensor (FTS) developed by General Electric Co. The APFC system determines combustion flame temperatures, validates the values, and integrates an assessment of signal and combustion hardware health to determine how to trim the fuel flow to individual fuel nozzles. Key aspects of the system include the following: determination of each flame’s bulk temperature using the FTS, assessment of the reliability of the flame temperature data and physical combustion hardware health through analysis of the high-frequency output of the sensor, validation of the flame temperature signal using a data-driven approach, fusion of sensor “health indices” into the APFC to alter the trim control signal based on the health (or “believability”) of each sensor and fuel nozzle/combustor, fault-tolerant peak/valley detection and control module that selects individual fuel valves to target for reducing pattern factor while simultaneously balancing the overall fuel flow. The authors demonstrated feasibility of the approach by performing simulations using a quasi-2D T700 turbine engine model. Tests were run on the simulated platform with no faults, simulated sensor faults, and on a system with underlying combustion hardware issues. The final APFC system would be applicable for aviation, naval, and land-based commercial gas turbines, and can be used in closed-loop control or adapted as an open-loop advisory/diagnostic system.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;133(1):011602-011602-8. doi:10.1115/1.4001983.

A high temperature resistant fiber-optical microphone (FOM) was developed and successfully applied in a combustion chamber at a thermal power of 8.4 kW to measure thermo-acoustic oscillations at a frequency of 85 Hz and a sound pressure level of 154 dB. The sensor head temperature was estimated to $∼1000 K$. The core of the optical setup used for the FOM is a Fabry–Perot interferometer. To create an acoustical sensor based on this type of interferometer, a new method of generation and postprocessing of the interference signal was developed. The simple replaceability of the sensor membrane reduces the requirements concerning the sensor handling compared with conventional condenser microphones and allows the adaptation of the sensor sensitivity to its application case changing the membrane stiffness.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;133(1):011603-011603-10. doi:10.1115/1.4001824.

In order to advance the technology for measurements in higher temperature flows, a novel miniature (diameter 2.5 mm) fast response probe that can be applied in flows with temperatures of up to 533 K $(500°F)$ has been developed. The primary elements of the probe are two piezoresistive pressure transducers that are used to measure the unsteady pressure and unsteady velocity field, as well as the steady temperature. Additional temperature and strain gauge sensors are embedded in the shaft to allow a much higher degree of robustness in the use of this probe. The additional temperature sensor in the shaft is used to monitor and correct the heat flux through the probe shaft, facilitating thermal management of the probe. The strain gauge sensor is used to monitor and control probe shaft vibration. Entirely new packaging technology had to be developed to make possible the use of this probe at such high temperatures. Extensive calibration and thermal cycling of the probe used to bind the accuracy and the robustness of the probe. This novel probe is applied in the one-and-1/2-stage, unshrouded axial turbine at ETH Zurich; this turbine configuration is representative of a high work aero-engine. The flow conditioning stretch upstream of the first stator is equipped with a recently designed hot streak generator. Several parameters of the hot streak, including temperature, radial and circumferential position, and shape and size can be independently controlled. The interactions between the hot streak and the secondary flow present a perfect scenario to verify the probe’s capability to measure under real engine conditions. Therefore, measurements with the novel probe have been made in order to prove the principle and to detail the interaction effects with blade row pressure gradients and secondary flows.

Commentary by Dr. Valentin Fuster

### Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2010;133(1):012801-012801-7. doi:10.1115/1.4001071.

Intake tuning is a widely recognized method for optimizing the performance of a naturally aspirated engine for motorsports applications. Wave resonance and Helmholtz theories are useful for predicting the impact of intake runner length on engine performance. However, there is very little information in the literature regarding the effects of intake plenum volume. The goal of this study was to determine the effects of intake plenum volume on steady state and transient engine performance for a restricted naturally aspirated engine for Formula Society of Automotive Engineers (FSAE) vehicle use. Testing was conducted on a four cylinder 600 cc motorcycle engine fitted with a 20 mm restrictor in compliance with FSAE competition rules. Plenum sizes were varied from 2 to 10 times engine displacement (1.2–6.0 l) and engine speeds were varied from 3000 rpm to 12,500 rpm. Performance metrics including volumetric efficiency, torque, and power were recorded at steady state conditions. Experimental results showed that engine performance increased modestly as plenum volume was increased from 2 to 8 times engine displacement (4.8 l). Increasing plenum volume beyond 4.8 l resulted in significant improvement in performance parameters. Overall, peak power was shown to increase from 54 kW to 63 kW over the range of plenums tested. Additionally, transient engine performance was evaluated using extremely fast (60 ms) throttle opening times for the full range of plenum sizes tested. In-cylinder pressure was used to calculate cycle-resolved gross indicated mean effective pressure (IMEPg) development during these transients. Interestingly, the cases with the largest plenum sizes only took 1 to 2 extra cycles (30–60 ms) to achieve maximum IMEPg levels when compared with the smaller volumes. In fact, the differences were so minor that it would be doubtful that a driver would notice the lag. Additional metrics included time for the plenums to fill and an analysis of manifold absolute pressure and peak in-cylinder pressure development during and after the throttle transient. Plenums below 4.8 l completely filled even before the transient was completed.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;133(1):012802-012802-9. doi:10.1115/1.4001073.

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

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2010;133(1):012501-012501-8. doi:10.1115/1.4001830.

Rotating machines that include geared shaft-trains can be affected by a dynamic behavior that is influenced by coupling effects between lateral and torsional vibrations. These latter vibrations are rarely measured by means of permanent probes in industrial machines; therefore, these phenomena can be often pointed out only by detecting abnormal harmonic components in the frequency spectrum of the lateral vibrations measured during common monitoring activities. Then, the diagnosis of the occurrence of coupling effects between torsional and lateral vibrations can be supported by the results provided by model-based techniques. In this paper, the results of the analysis of the dynamic behavior of a small power unit that was affected by abnormal lateral vibrations caused by the excitation of torsional normal modes of the geared shaft-train are shown and discussed.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2010;133(1):012001-012001-13. doi:10.1115/1.4001828.

Gas turbine inlet fog/overspray cooling is considered as a simple and effective method to increase power output. To help understand the water mist transport in the compressor flow passage, this study conducts a 3D computational simulation of wet compression in a single rotor-stator compressor stage using the commercial code FLUENT . A sliding mesh scheme is used to simulate the stator-rotor interaction in a rotating frame. Eulerian–Lagrangian method is used to calculate the continuous phase and track the discrete (droplet) phase. Models to simulate droplet breakup and coalescence are incorporated to take into consideration the effect of local acceleration and deceleration on water droplet dynamics. Analysis on the droplet history (trajectory and size) with stochastic tracking is employed to interpret the mechanism of droplet dynamics under the influence of local turbulence, acceleration, diffusion, and body forces. A liquid-droplet erosion model is included. The sensitivity of the turbulence models on the results is conducted by employing six different turbulence models and four different time constants. The result shows that the local thermal equilibrium is not always achieved due to short residence time and high value of latent heat of water. Local pressure gradients in both the rotor and stator flow passages drive up the droplet slip velocity during compression. The erosion model predicts that the most eroded area occurs in the leading edge and one spot of the trailing edge of the rotor suction side.

Commentary by Dr. Valentin Fuster

### Research Papers: Nuclear Power

J. Eng. Gas Turbines Power. 2010;133(1):012901-012901-7. doi:10.1115/1.4000897.

Flow accelerated corrosion (FAC) is a serious form of degradation in primary heat transport piping system (PHTS) of the nuclear reactor. Pipes transporting hot coolant from the reactor to steam generators are particularly vulnerable to FAC degradation, such as tight radius pipe bends with high flow velocity. FAC is a life limiting factor, as excessive degradation can result in the loss of structural integrity of the pipe. To prevent this, engineering codes and regulations have specified minimum wall thickness requirements to ensure fitness for service of the piping system. Nuclear utilities have implemented periodic wall thickness inspection programs and carried out replacement of pipes prior to reaching an unsafe state. To optimize the life-cycle management of PHTS, accurate prediction of time of replacement or “end of life” of pipe sections is important. Since FAC is a time-dependent process of uncertain nature, this paper presents two probabilistic models for predicting the end of life. This paper illustrates that the modeling assumptions have a significant impact on the predicted number of replacements and life-cycle management of the nuclear piping system. A practical case study is presented using wall thickness inspection data collected from Canadian nuclear plants.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;133(1):012902-012902-7. doi:10.1115/1.4001061.

The three groups of parameters that affect flow-accelerated corrosion (FAC) are the flow conditions, water chemistry, and materials. Nuclear power plant (NPP) data and laboratory tests confirm that, under alkaline water chemistry, there is a close relationship between local flow conditions and FAC rates in the piping components. The knowledge of the local flow effects can be useful for developing targeted inspection plans for piping components and predicting the location of the highest FAC rate for a given piping component. A similar evaluation applies also to the FAC in heat transfer equipments such as heat exchangers and steam generators. The objective of this paper is to examine the role of the flow and mass transfer in bends under alkaline FAC conditions. Bends experience increased FAC rates compared with straight pipes, and are the most common components in piping systems. This study presents numerical simulations of the mass transfer of ferrous ions and experimental results of the FAC rate in bends. It also shows correlations for mass transfer coefficients in bends and reviews the most important flow parameters affecting the mass transfer coefficient. The role of bend geometry and, in particular, the short and long radii, surface roughness, wall shear stress, and local turbulence, is discussed. Computational fluid dynamics calculations and plant artifact measurements for short- and long-radius bends are presented. The effect of the close proximity of the two bends on the FAC rate is also examined based on CANDU (CANDU is a registered trademark of the Atomic Energy of Canada Limited) NPP inspection data and compared with literature data.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2010;133(1):011701-011701-10. doi:10.1115/1.4001982.

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

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;133(1):011702-011702-7. doi:10.1115/1.4001822.

This article illustrates aspects of heat recovery steam generator (HRSG) design when employing process integration in an integrated reforming combined cycle (IRCC) with precombustion $CO2$ capture. Specifically, the contribution of this paper is to show how heat integration in a precombustion $CO2$ capture plant impacts the selection of HRSG design. The purpose of such a plant is to generate power with very low $CO2$ emissions, typically below 100 g $CO2/net$ kWh electricity. This should be compared with a state-of-the-art natural gas combined cycle (NGCC) plant with $CO2$ emissions around 380 g $CO2/net$ kWh electricity. The design of the HRSG for the IRCC process was far from standard because of the significant amount of steam production from the heat generated by the autothermal reforming process. This externally generated steam was transferred to the HRSG superheaters and used in a steam turbine. For an NGCC plant, a triple-pressure reheat steam cycle would yield the highest net plant efficiency. However, when generating a significant amount of high-pressure steam external to the HRSG, the picture changed. The complexity of selecting an HRSG design increased when also considering that steam can be superheated and low-pressure and intermediate-pressure steam can be generated in the process heat exchangers. For the concepts studied, it was also of importance to maintain a high net plant efficiency when operating on natural gas. Therefore, the selection of HRSG design had to be a compromise between NGCC and IRCC operating modes.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;133(1):011703-011703-7. doi:10.1115/1.4001988.

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

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2010;133(1):011501-011501-7. doi:10.1115/1.4001978.

An experiment was conducted to characterize a superheated fuel jet (Jet-A) injected into an unheated crossflow of air. The liquid phase of the fuel jet was characterized with high speed imaging and phase Doppler interferometry. The transition from a shear-atomized to a flash-atomized spray at a fuel temperature of 513 K $(465°F)$ was observed at an ambient pressure of 1 atm, which is consistent with the bubble and dew point curves predicted for JP-8. The explosive breakup that was seen in the flash-atomized spray produced submicron droplets with a high radial momentum. This unique behavior makes superheated fuels an attractive design feature for fuel preparation devices that can employ flash boiling to enhance fuel atomization and mixing in a compact volume.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;133(1):011502-011502-8. doi:10.1115/1.4001981.

Autoignition delay times of mixtures of alkanes and natural gas were studied experimentally in a high pressure and intermediate temperature turbulent flow reactor. Measurements were made at pressures between 7 atm and 15 atm and temperatures from 785 K to 935 K. The blends include binary and ternary mixtures of methane, ethane, and propane along with various natural gas blends. Based on these data, the effect of higher hydrocarbons on the ignition delay time of natural gas type fuels at actual gas turbine engine conditions has been quantified. While the addition of higher hydrocarbons in quantities of up to 30% was found to reduce the ignition delay by up to a factor of 4, the delay times were still found to be greater than 60 ms in all cases, which is well above the residence times of most engine premixers. The data were used to develop simple Arrhenius type correlations as a function of temperature, pressure, and fuel composition for design use.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;133(1):011503-011503-7. doi:10.1115/1.4001985.

Premixed combustion of hydrogen-rich mixtures involves the risk of flame flashback through wall boundary layers. For laminar flow conditions, the flashback mechanism is well understood and is usually correlated by a critical velocity gradient at the wall. Turbulent transport inside the boundary layer considerably increases the flashback propensity. Only tube burner setups were investigated in the past, and thus turbulent flashback limits were only derived for a fully developed Blasius wall friction profile. For turbulent flows, details of the flame propagation in proximity to the wall remain unclear. This paper presents results from a new experimental combustion rig, apt for detailed optical investigations of flame flashbacks in a turbulent wall boundary layer developing on a flat plate and being subject to an adjustable pressure gradient. Turbulent flashback limits are derived from the observed flame position inside the measurement section. The fuels investigated cover mixtures of methane, hydrogen, and air at various mixing ratios. The associated wall friction distributions are determined by Reynolds-averaged Navier-Stokes (RANS) computations of the flow inside the measurement section with fully resolved boundary layers. Consequently, the interaction between flame back pressure and incoming flow is not taken into account explicitly, in accordance with the evaluation procedure used for tube burner experiments. The results are compared with literature values, and the critical gradient concept is reviewed in light of the new data.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;133(1):011504-011504-7. doi:10.1115/1.4001987.

A methodology is proposed to determine on the fly the acoustic impedances at the boundaries of a combustor under operation and without the need for an external forcing device. The methodology is applied hereby to obtain the reflection coefficients of the air and fuel feeding lines supplying a swirled combustor operating in lean conditions and featuring an unstable regime. It is based on a three-microphone technique and uses the combustion roar noise inside the chamber to get estimates of the boundary impedances in the low frequency limit. Conditions under which this method yields reliable results for the reflection coefficient are examined, and a criterion based on the coherence between microphone signals is proposed to determine the frequency bandwidth for impedance reconstruction. The method also takes into account the flow Mach number in the supplying pipes, which can be non-negligible because of their reduced diameter. The technique is validated against data obtained in a dedicated impedance measurement setup equipped with a high-efficiency loudspeaker.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;133(1):011505-011505-8. doi:10.1115/1.4002004.

Transverse dilution jets are widely used in combustion systems. The current research provides a detailed study of the primary jets of a realistic annular combustion chamber sector. The combustor sector comprises an aerodynamic diffuser, inlet cowl, combustion dome, primary dilution jets, secondary dilution jets, and cooling strips to provide convective cooling to the liner. The chamber contracts toward the end to fit the turbine nozzle ring. 2D PIV is employed at an atmospheric pressure drop of 4% (isothermal) to delineate the flow field characteristics. The laser is introduced to the sector through the exit flange. The interaction between the primary jets and the swirling flow as well as the sensitivity of the primary jets to perturbations is discussed. The perturbation study includes: effect of partially blocking the jets, one at a time, the effect of blocking the convective cooling holes, placed underneath the primary jets and shooting perpendicular to it. In addition, the effect of reducing the size of the primary jets as well as off-centering the primary jets is explained. Moreover, PIV is employed to study the flow field with and without fuel injection at four different fuel flow rates. The results show that the flow field is very sensitive to perturbations. The cooling air interacts with the primary jet and influences the flow field although the momentum ratio has a 100:1 order of magnitude. The results also show that the big primary jets dictate the flow field in the primary zone as well as the secondary zone. However, relatively smaller jets mainly influence the primary combustion zone because most of the jet is recirculated back to the CRZ. Also, the jet penetration is reduced with 25% and 11.5% corresponding to a 77% and 62% reduction of the jet’s area, respectively. The study indicates the presence of a critical jet diameter beyond which the dilution jets have minimum impact on the secondary region. The jet off-centering shows significant effect on the flow field though it is in the order of 0.4 mm. The fuel injection is also shown to influence the flow field as well as the primary jets angle. High fuel flow rate is shown to have very strong impact on the flow field and thus results in a strong distortion of both the primary and secondary zones. The results provide useful methods to be used in the flow field structure control. Most of the effects shown are attributed to the difference in jet opposition. Hence, the results are applicable to reacting flow.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2010;133(1):011506-011506-13. doi:10.1115/1.4000606.

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

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Electric Power

J. Eng. Gas Turbines Power. 2010;133(1):011801-011801-13. doi:10.1115/1.4001829.

A natural gas fired gas turbine combined cycle power plant is the most efficient option for fossil fuel based electric power generation that is commercially available. Trade publications report that currently available technology is rated near 60% thermal efficiency. Research and development efforts are in place targeting even higher efficiencies in the next two decades. In the face of diminishing natural resources and increasing carbon dioxide emissions, leading to greenhouse gas effect and global warming, these efforts are even more critical today than in the last century. The main performance driver in a combined cycle power plant is the gas turbine. The basic thermodynamics of the gas turbine, described by the well-known Brayton cycle, dictates that the key design parameters that determine the gas turbine performance are the cycle pressure ratio and maximum cycle temperature at the turbine inlet. While performance calculations for an ideal gas turbine are straightforward with compact mathematical formulations, detailed engineering analysis of real machines with turbine hot gas path cooling requires complex models. Such models, requisite for detailed engineering design work, involve highly empirical heat transfer formulations embedded in a complex system of equations that are amenable only to numerical solutions. A cooled turbine modeling system incorporating all pertinent physical phenomena into compact formulations is developed and presented in this paper. The model is fully physics-based and amenable to simple spreadsheet calculations while illustrating the basic principles with sufficient accuracy and extreme qualitative rigor. This model is valuable not only as a teaching and training tool, it is also suitable to preliminary gas turbine combined cycle design calculations in narrowing down the field of feasible design options.

Commentary by Dr. Valentin Fuster

### Research Papers: Gas Turbines: Microturbines and Small Turbomachinery

J. Eng. Gas Turbines Power. 2010;133(1):012301-012301-10. doi:10.1115/1.4001990.

An optimization study of trapezoidal surface texturing in slider micro-bearings, via computational fluid dynamics (CFD), is presented. The bearings are modeled as micro-channels, consisting of a moving and a stationary wall. The moving wall (rotor) is assumed smooth, while part of the stationary wall (stator) exhibits periodic dimples of trapezoidal form. The extent of the textured part of the stator and the dimple geometry are defined parametrically; thus, a wide range of texturing configurations is considered. Flow simulations are based on the numerical solution of the Navier–Stokes equations for incompressible isothermal flow. To optimize the bearing performance, an optimization problem is formulated and solved by coupling the CFD code with an optimization tool based on genetic algorithms and local search methods. Here, the design variables define the bearing geometry, while load carrying capacity is the objective function to be maximized. Optimized texturing geometries are obtained for the case of parallel bearings for several numbers of dimples, illustrating significant load carrying capacity levels. Further, these optimized texturing patterns are applied to converging bearings for different convergence ratio values; the results demonstrate that, for small and moderate convergence ratios, a substantial increase in load carrying capacity, in comparison to smooth bearings, is obtained. Finally, an optimization study performed at a high convergence ratio shows that, in comparison to the parallel slider, the optimal texturing geometry is substantially different, and that performance improvement over smooth bearings is possible even for steep sliders.

Commentary by Dr. Valentin Fuster