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### TECHNICAL PAPERS: Gas Turbines: Controls, Diagnostics & Instrumentation

J. Eng. Gas Turbines Power. 2004;128(1):49-56. doi:10.1115/1.1995771.

This paper presents the development of an integrated fault diagnostics model for identifying shifts in component performance and sensor faults using the Genetic Algorithm and Artificial Neural Network. The diagnostics model operates in two distinct stages. The first stage uses response surfaces for computing objective functions to increase the exploration potential of the search space while easing the computational burden. The second stage uses the concept of a hybrid diagnostics model in which a nested neural network is used with genetic algorithm to form a hybrid diagnostics model. The nested neural network functions as a pre-processor or filter to reduce the number of fault classes to be explored by the genetic algorithm based diagnostics model. The hybrid model improves the accuracy, reliability, and consistency of the results obtained. In addition significant improvements in the total run time have also been observed. The advanced cycle Intercooled Recuperated WR21 engine has been used as the test engine for implementing the diagnostics model.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(1):57-63. doi:10.1115/1.1924535.

A variety of methods can be used for the diagnosis of faults in gas path components of gas turbines. Problems that are common for diagnostic method implementation are the choice of measured quantities, choice of health parameters, and choice of operating conditions for data retrieval. The present paper introduces some general principles for evaluation of the effectiveness of different diagnostic schemes. They encompass criteria proposed in past publications, while they offer additional possibilities for assessment of diagnostic effectiveness in various situations. The method is based on the evaluation of the behavior of linear systems, which are a good approximation of the nonlinear ones for small deviations and employs the concept of system condition number to formulate criteria. The determination of limits for this number for establishing system condition criteria and quantification of observability is examined, on the basis of uncertainty propagation. Sample problems evaluated are: maximizing effectiveness of individual component identification from a multiplicity of available measurements, selection of individual operating points for multipoint applications.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(1):64-72. doi:10.1115/1.1924536.

A method for solving the gas path analysis problem of jet engine diagnostics based on a probabilistic approach is presented. The method is materialized through the use of a Bayesian Belief Network (BBN). Building a BBN for gas turbine performance fault diagnosis requires information of a stochastic nature expressing the probability of whether a series of events occurred or not. This information can be extracted by a deterministic model and does not depend on hard to find flight data of different faulty operations of the engine. The diagnostic problem and the overall diagnostic procedure are first described. A detailed description of the way the diagnostic procedure is set-up, with focus on building the BBN from an engine performance model, follows. The case of a turbofan engine is used to evaluate the effectiveness of the method. Several simulated and benchmark fault case scenarios have been considered for this reason. The examined cases demonstrate that the proposed BBN-based diagnostic method composes a powerful tool. This work also shows that building a diagnostic tool, based on information provided by an engine performance model, is feasible and can be efficient as well.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(1):73-80. doi:10.1115/1.2056534.

A life management system was developed for hot components of large industrial gas turbines, in the form of a software tool for predicting component lives under typical operational transients (normal and also abnormal) and steady-state periods. The method utilizes results of previous thermo-mechanical finite element and finite volume fluid mechanics analyses. The basic idea of this method is using data from structural and aero-thermal analyses (pressures and temperatures), blade life theory, and material properties as an input to algorithms, and using operational and historical data to validate the predicted damage amounts. The software developed in this project, of general applicability to all GT models, has been implemented with reference to the geometries, materials, and service conditions of a Fiat-Westinghouse model.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2005;128(1):203-212. doi:10.1115/1.1805551.

To achieve high power and high efficiency in a hydrogen-fueled engine for all load conditions, the dual-injection hydrogen-fueled engine, which can derive the advantages of both high efficiency from external mixture hydrogen engine and high power from direct cylinder injection was developed. For verifying the feasibility of the above engine, a high-pressure hydrogen injector of ball-valve type and actuated by a solenoid was developed. A systematic experimental study was conducted by using a modified single-cylinder dual-injection hydrogen-fueled engine, which was equipped with both an intake injector and high-pressure in-cylinder injector. The results showed that (i) the developed high pressure hydrogen injector with a solenoid actuator had good gas tightness and fine control performance, (ii) the transient injection region, in which injection methods are changed from external fuel injection to direct-cylinder injection, ranged from 59 to 74% of the load, and (iii) the dual-injection hydrogen-fueled engine had the maximum torque of direct-cylinder fuel injection and the maximum efficiency of external fuel mixture hydrogen engines.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(1):213-218. doi:10.1115/1.1915428.

The Advanced (injection) Low Pilot Ignited Natural Gas (ALPING) engine is proposed as an alternative to diesel and conventional dual fuel engines. Experimental results from full load operation at a constant speed of $1700rev∕min$ are presented in this paper. The potential of the ALPING engine is realized in reduced $NOx$ emissions (to less than $0.2g∕kWh$) accompanied by fuel conversion efficiencies comparable to straight diesel operation. Some problems at advanced injection timings are recognized in high unburned hydrocarbon (HC) emissions $(25g∕kWh)$ and poor engine stability reflected by high $COVIMEP$ (about 6%). This paper focuses on the combustion aspects of low pilot ignited natural gas engines with particular emphasis on advanced injection timings (45°–60° BTDC). Ignition phasing at advanced injection timings ($∼60°$ BTDC), and combustion phasing at retarded injection timings ($∼15°$ BTDC) are recognized as important combustion parameters that profoundly impact the combustion process, HC emissions, and the stability of engine operation.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;128(1):219-229. doi:10.1115/1.1995768.

This paper describes an experimental study of heat transfer in a reciprocating planar curved tube that simulates a cooling passage in piston. The coupled inertial, centrifugal, and reciprocating forces in the reciprocating curved tube interact with buoyancy to exhibit a synergistic effect on heat transfer. For the present experimental conditions, the local Nusselt numbers in the reciprocating curved tube are in the range of 0.6–1.15 times of static tube levels. Without buoyancy interaction, the coupled reciprocating and centrifugal force effect causes the heat transfer to be initially reduced from the static level but recovered when the reciprocating force is further increased. Heat transfer improvement and impediment could be superimposed by the location-dependent buoyancy effect. The empirical heat transfer correlation has been developed to permit the evaluation of the individual and interactive effects of inertial, centrifugal, and reciprocating forces with and without buoyancy interaction on local heat transfer in a reciprocating planar curved tube.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(1):230-236. doi:10.1115/1.2055987.

Hydrogen is well recognized as a suitable fuel for spark-ignition engine applications that has many unique attractive features and limitations. It is a fuel that can continue potentially to meet the ever-increasingly stringent regulations for exhaust and greenhouse gas emissions. The application of hydrogen as an engine fuel has been tried over many decades by numerous investigators with varying degrees of success. However, the performance data reported often tend not to display consistent agreement between the various investigators, mainly because of the wide differences in engine type, size, operating conditions used, and the differing criteria employed to judge whether knock is taking place or not. With the ever-increasing interest in hydrogen as an engine fuel, there is a need to be able to model extensively various features of the performance of spark ignition (S.I.) hydrogen engines so as to investigate and compare reliably the performance of widely different engines under a wide variety of operating conditions. In the paper we employ a quasidimensional two-zone model for the operation of S.I. engines when fueled with hydrogen. In this approach, the engine combustion chamber at any instant of time during combustion is considered to be divided into two temporally varying zones: a burned zone and an unburned zone. The model incorporates a detailed chemical kinetic model scheme of 30 reaction steps and 12 species, to simulate the oxidation reactions of hydrogen in air. A knock prediction model, developed previously for S.I. methane-hydrogen fueled engine applications was extended to consider operation on hydrogen. The effects of changes in operating conditions, including a very wide range of variations in the equivalence ratio on the onset of knock and its intensity, combustion duration, power, efficiency, and operational limits were investigated. The results of this predictive approach were shown to validate well against the corresponding experimental results, obtained mostly in a variable compression ratio CFR engine. On this basis, the effects of changes in some of the key operational engine variables, such as compression ratio, intake temperature, and spark timing are presented and discussed. Some guidelines for superior knock-free operation of engines on hydrogen are also made.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Microturbines & Small Turbomachinery

J. Eng. Gas Turbines Power. 2004;128(1):190-202. doi:10.1115/1.1924484.

Significant performance enhancement of microturbines is predicted by implementing various wave-rotor-topping cycles. Five different advantageous cases are considered for implementation of a four-port wave rotor into two given baseline engines. In these thermodynamic analyses, the compressor and turbine pressure ratios and the turbine inlet temperatures are varied, according to the anticipated design objectives of the cases. Advantages and disadvantages are discussed. Comparison between the theoretic performance of wave-rotor-topped and baseline engines shows a performance enhancement up to 34%. General design maps are generated for the small gas turbines, showing the design space and optima for baseline and topped engines. Also, the impact of ambient temperature on the performance of both baseline and topped engines is investigated. It is shown that the wave-rotor-topped engines are less prone to performance degradation under hot-weather conditions than the baseline engines.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Fuels & Combustion Technology

J. Eng. Gas Turbines Power. 2004;128(1):1-7. doi:10.1115/1.2032449.

Buoyant and nonbuoyant shapes of methane flames issued from a 2:1 aspect ratio elliptic tube burner were measured. Nonbuoyant conditions were obtained in the KC-135 microgravity research aircraft operated by NASA’s Johnson Space Center. A mathematical model based on the extended Burke-Schumann flame theory is developed to predict the flame length of an elliptic burner. The model utilizes Roper’s theoretical method for circular burners and extends the analysis for elliptic burners. The predicted flame length using the theoretical model agrees well with experimental measurements. In general for the elliptic burner the nonbuoyant flames are longer than the buoyant flames. However, measured lengths of both buoyant and nonbuoyant flame lengths change proportionally with the volumetric fuel flow rate and support the $L$ vs $Q$ correlation. The maximum flame width measured at buoyant and nonbuoyant conditions also show a proportional relation with the volumetric fuel flow rate. Normalized buoyant and nonbuoyant flame lengths of the elliptic burner correlate $(L∕d∝Re)$ with the jet exit Reynolds number and exhibit a higher slope compared to a circular burner. Normalized flame width data show a power correlation $(w∕d=cFrn)$ with the jet exit Froude number.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(1):8-12. doi:10.1115/1.2056528.

Addition of steam to a flame has important implications in the combustion process. The dissociation of the added steam (e.g., $H2O↔H+OH$, etc.) is one of the effects that contribute to the production of radical species, such as OH, H, and O, in the flame. In order to distinctly visualize two types of OH radicals produced from the fuel-air combustion reaction and that from the dissociation reaction with the added steam, we have developed a new method for planar laser-induced fluorescence spectroscopy in combination with isotope shift (herein called IS/PLIF spectroscopy). This technique has been applied to examine a methane-oxygen-nitrogen premixed flame. Two-dimensional fluorescence intensity distributions of OH radicals in the flames were monitored under three different conditions. They include without steam addition, with $H2O$ steam addition, and with $D2O$ steam addition. From the experimental data obtained under the three conditions, the distinction between the two types of OH radicals could be obtained. The results showed that steam addition reduced the total concentration of OH produced from the combustion and dissociation reactions and that the dissociation reaction of the added steam contributed to the production of OH. Furthermore, the results indicated that the percentage decrease in OH from fuel-air combustion reactions due to the temperature decrease effect with steam addition was almost independent of the equivalence ratio during combustion. In contrast, the percentage increase in OH produced from dissociation reaction with the steam depended on the equivalence ratio.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Aircraft Engine

J. Eng. Gas Turbines Power. 2004;128(1):13-19. doi:10.1115/1.1995770.

For gas turbine engine performance analysis, a variety of simulation tools is available. In order to minimize model development and software maintenance costs, generic gas turbine system simulation tools are required for new modeling tasks. Many modeling aspects remain engine specific however and still require large implementation efforts. One of those aspects is adaptive modeling. Therefore, an adaptive modeling functionality has been developed that can be implemented in a generic component-based gas turbine environment. A single component in a system modeling environment is able to turn any new or existing model into an adaptive model without extra coding. The concept has been demonstrated in the GSP gas turbine modeling environment. An object-oriented architecture allows automatic addition of the necessary equations for the adaptation to measurement values. Using the adaptive modeling component, the user can preconfigure the adaptive model and quickly optimize gas path diagnostics capability using experimentation with field data. The resulting adaptive model can be used by maintenance engineers for diagnostics. In this paper the integration of the adaptive modeling function into a system modeling environment is described. Results of a case study on a large turbofan engine application are presented.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(1):20-28. doi:10.1115/1.2031247.

Modern engine operation is guided by the aim to broaden the operating range and to increase the stage loading allowing the stage count to be reduced. This is possible by active stability control measures to extend the available stable operating range. Different strategies of an active control system, such as air injection and air recirculation have been applied. While in the past results have been published mainly regarding the stability enhancement of compressor rigs or single-spool engines, this experimental study focuses on both the stability as well as the operating range extension of a twin-spool turbofan engine as an example of a real engine application on an aircraft. The objective of this investigation is the analysis of the engine behavior with active stabilization compared to unsupported operation. For this purpose, high-frequency pressure signals are used and analyzed to investigate the effects of air injection with respect to the instability onset progress and the development of any instabilities, such as rotating stall and surge in the low-pressure compression (LPC) system. These Kulite signals are fed to a control system. Its amplified output signals control fast acting direct-drive valves circumferentially distributed ahead of the LPC. For the application of air injection described in the paper, the air is delivered by an external source. The control system responsible for air injection is a real-time system which directly reacts on marked instabilities and their precursors. It allows the LPC System to recover from fully developed rotating stall by asymmetric air injection based on the pressure signals. Additionally, a delayed appearance of instabilities can be provoked by the system. Air injection guided by this control system resulted in a reduction of the required amount of air compared to constant air injection. Also, disturbances travelling at rotor speed can be detected, damped, and eliminated by this control system with a modulation of the injected air in such a way that the injection maximum travels around the ten injection positions.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Combustion and Fuels

J. Eng. Gas Turbines Power. 2004;128(1):29-39. doi:10.1115/1.1924720.

Lean premixing prevaporizing (LPP) burners represent a promising solution for low-emission combustion in aeroengines. Since lean premixed combustion suffers from pressure and heat release fluctuations that can be triggered by unsteady large-scale flow structures, a deep knowledge of flow structures formation mechanisms in complex swirling flows is a necessary step in suppressing combustion instabilities. The present paper describes a detailed investigation of the unsteady aerodynamics of a large-scale model of a double swirler aeroengine LPP burner at isothermal conditions. A three-dimensional (3D) laser Doppler velocimeter and an ensemble-averaging technique have been employed to obtain a detailed time-resolved description of the periodically perturbed flow field at the mixing duct exit and associated Reynolds stress and vorticity distributions. Results show a swirling annular jet with an extended region of reverse flow near to the axis. The flow is dominated by a strong periodic perturbation, which occurs in all the three components of velocity. Radial velocity fluctuations cause important periodic displacement of the jet and the inner separated region in the meridional plane. The flow, as expected, is highly turbulent. The periodic stress components have the same order of magnitude of the Reynolds stress components. As a consequence the flow-mixing process is highly enhanced. Turbulence acts on a large spectrum of fluctuation frequencies, whereas the large-scale motion influences the whole flow field in an ordered way that can be dangerous for stability in reactive conditions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(1):40-48. doi:10.1115/1.2032450.

The objective of this study was to assess the accuracy of the large-eddy simulation (LES) methodology, with a simple combustion closure based on equilibrium chemistry, for simulating turbulent reacting flows behind a bluff body flameholder. Specifically, the variation in recirculation zone length with change in equivalence ratio was calculated and compared to experimental measurements. It was found that the present LES modeling approach can reproduce this variation accurately. However, it understated the recirculation zone length at the stoichiometric condition. The approach was assessed at the lean blow out condition to evaluate its behavior at the lean limit and to analyze the physics of combustion instability.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2004;128(1):163-172. doi:10.1115/1.1924432.

A computational preliminary design tool has been developed to help simulate drop-related processes that take place in an oil sump of a gas turbine engine accounting for drop motion, deformation, breakup, and drop∕wall interactions including wall film impact and potential splashing. Aerodynamic interactions with the gas phase are considered using an exact solution of the Navier–Stokes equations to approximate the annular gas flow. Detailed results for the baseline case that attempts to replicate the conditions found in a typical oil sump of a turbofan engine are presented. In addition, the results of more general parametric studies utilizing a simplified geometry that investigated the effects of changing various parameters are discussed.

Topics: Drops
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;128(1):173-177. doi:10.1115/1.1924483.

A nonlinear continuum damage mechanics model is proposed to assess the high temperature creep life of a steam turbine rotor, in which the effect of mean stress is taken into account and the damage is accumulated nonlinearly. The model is applied to a 300 MW steam turbine under hot start operation. The results are compared with those from the linear accumulation theory that is dominant in the creep life assessment of steam turbine rotors at present. The comparison results show that the nonlinear continuum damage mechanics model describes the accumulation and development of damage better than the linear accumulation theory.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2005;128(1):178-182. doi:10.1115/1.1995769.

Trends in aircraft engine design have caused an increase in mechanical stress requirements for rolling bearings. Consequently, a high amount of heat is rejected, which results in high oil scavenge temperatures. An RB199 turbofan bearing and its associated chamber were modified to carry out a survey aiming to reduce power losses in bearing chambers. The test bearing was a 124 mm PCD ball bearing with a split inner ring employing under-race lubrication by two individual jets. The survey was carried out in two parts. In the first part, the investigations were focused on the impact on the power losses in the bearing chamber of the operating parameters, such as oil flow, oil temperature, sealing air flow, bearing chamber pressure, and shaft speed. In the second part, the investigations focused on the reduction of the dwell time of the air and oil mixture in the bearing compartment and its impact on the power losses. In this part, porous screens were introduced around the bearing. These screens would aid the oil to flow out of the compartment and reduce droplet-droplet interactions as well as droplet-bearing chamber wall interactions. The performance of the screens was evaluated by torque measurements. A high-speed camera was used to visualize the flow in the chamber. Considerable reduction in power loss was achieved. This work is part of the European Research programme GROWTH ATOS (Advanced Transmission and Oil Systems).

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2003;128(1):183-189. doi:10.1115/1.2031227.

This paper presents an adjoint analysis for three-dimensional unsteady viscous flows aimed at the calculation of linear worksum sensitivities involved in turbomachinery forced response predictions. The worksum values are normally obtained from linear harmonic flow calculations but can also be computed using the solution to the adjoint of the linear harmonic flow equations. The adjoint method has a clear advantage over the linear approach if used within a rotor forced vibration minimization procedure which requires the structural response to a large number of different flow excitation sources characterized by a unique frequency and interblade phase angle. Whereas the linear approach requires a number of linear flow calculations at least equal to the number of excitation sources, the adjoint method reduces this cost to a single adjoint solution for each structural mode of rotor response. A practical example is given to illustrate the dramatic computational saving associated with the adjoint approach.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Cycle Innovations

J. Eng. Gas Turbines Power. 2004;128(1):81-91. doi:10.1115/1.2031228.

A novel liquefied natural gas (LNG) fueled power plant is proposed, which has virtually zero $CO2$ and other emissions and a high efficiency. Natural gas is fired in highly enriched oxygen and recycled $CO2$ flue gas. The plant operates in a quasi-combined cycle mode with a supercritical $CO2$ Rankine type cycle and a $CO2$ Brayton cycle, interconnected by the heat transfer process in the recuperation system. By coupling with the LNG evaporation system as the cycle cold sink, the cycle condensation process can be achieved at a temperature much lower than ambient, and high-pressure liquid $CO2$ ready for disposal can be withdrawn from the cycle without consuming additional power. Good use of the coldness exergy and internal exergy recovery produced a net energy and exergy efficiencies of a base-case cycle over 65% and 50%, respectively, which can be increased up to 68% and 54% when reheat is used. Cycle variants incorporating reheat, intercooling, and reheat$+$intercooling, as well as no use of LNG coldness, are also defined and analyzed for comparison. The approximate heat transfer area needed for the different cycle variants is also computed. Besides electricity and condensed $CO2$, the byproducts of the plant are $H2O$, liquid $N2$ and Ar.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(1):92-96. doi:10.1115/1.2032431.

In order to estimate the precise performance of the existing gas turbine engine, the component maps with more realistic performance characteristics are needed. Because the component maps are the engine manufacturer’s propriety obtained from very expensive experimental tests, they are not provided to the customers, generally. Therefore, because the engineers, who are working the performance simulation, have been mostly relying on component maps scaled from the similar existing maps, the accuracy of the performance analysis using the scaled maps may be relatively lower than that using the real component maps. Therefore, a component map generation method using experimental data and the genetic algorithms are newly proposed in this study. The engine test unit to be used for map generation has a free power turbine type small turboshaft engine. In order to generate the performance map for compressor of this engine, after obtaining engine performance data through experimental tests, and then the third order equations, which have relationships with the mass flow function, the pressure ratio, and the isentropic efficiency as to the engine rotational speed, were derived by using the genetic algorithms. A steady-state performance analysis was performed with the generated maps of the compressor by the commercial gas turbine performance analysis program GASTURB (Kurzke, 2001). In order to verify the proposed scheme, the experimental data for verification were compared with performance analysis results using traditional scaled component maps and performance analysis results using a generated compressor map by genetic algorithms (GAs). In comparison, it was found that the analysis results using the generated map by GAs were well agreed with experimental data. Therefore, it was confirmed that the component maps can be generated from the experimental data by using GAs and it may be considered that the more realistic component maps can be obtained if more various conditions and accurate sensors would be used.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(1):97-102. doi:10.1115/1.2032432.

Background: The injection of water droplets into industrial gas turbines is now commonplace and is central to several proposed advanced cycles. These cycles benefit from the subsequent reduction in compressor work, the increase in turbine work, and (in the case of recuperated cycles) reduction in compressor delivery temperature, which all act to increase the efficiency and power output. An investigation is presented here into the effect such water droplets will have on the operating point and flow characteristics of an aeroderivative gas turbine cycle. Method of Approach: The paper first describes the development of a computer program to study the effects of water injection in multispool industrial gas turbines. The program can operate in two modes: the first uses pre-determined nondimensional wet compressor maps to match the components and is instructive and fast but limited in scope; the second uses the compressor geometries as input and calculates the wet compressor operating conditions as and when required. As a result, it is more computationally demanding, but can cope with a wider range of circumstances. In both cases the compressor characteristics are calculated from a mean-line analysis using suitable loss, deviation and blockage models, coupled with Lagrangian-style droplet evaporation calculations. The program has been applied to a three-spool machine to address issues such as the effects of water injection on power output and overall efficiency, and the off-design nature of the compressor operation. Results: Preliminary results calculated on this basis show similar trends to predictions for single-shaft machines, namely that air mass flow rates and pressure ratios are increased by water injection, and that early stages of the compressor are shifted towards choke and rear stages towards stall. The LP compressor in particular operates at severely off-design conditions. Conclusions: The predicted overall performance of the three-spool machine shows a substantial power boost and a marginal increase in thermal efficiency.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Industrial and Cogeneration

J. Eng. Gas Turbines Power. 2004;128(1):135-143. doi:10.1115/1.1926313.

High fogging is a power augmentation device where water is sprayed upstream of the compressor inlet with higher mass flow than that which would be needed to saturate the intake air. The main focus of this paper is on applications of high fogging on the ALSTOM gas turbine engines of the family GT24/GT26. Engine operation and performance are illustrated based on test results obtained from four different engines that have meanwhile accumulated more than 12,000 operating hours (OH) in commercial operation with ALSTOM’s AL Fog® high fogging system. The effect of internal cooling (water evaporation inside the compressor) is investigated considering actual compressor boundaries matched within the complete engine. Changes in the secondary air system (SAS) and corresponding movement of the engine operating line have been taken into account. Power output gain as high as 7.1% was experimentally demonstrated for injected water mass fraction $(f=mH2O∕mair)$ equal to 1% and considering internal cooling effects only. Higher figures can be obtained for operation at low ambient relative humidity and partial evaporation upstream of the compressor inlet.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Manufacturing, Materials, and Metallurgy

J. Eng. Gas Turbines Power. 2004;128(1):144-152. doi:10.1115/1.1924534.

The hot corrosion resistance of lanthanum zirconate and $8wt.%$ yttria-stabilized zirconia coatings produced by thermal spraying for use as thermal barriers on industrial gas turbines or in aerospace applications was evaluated. The two ceramic oxide coatings were exposed for various periods of time at temperatures up to $1000°C$ to vanadium- and sulfur-containing compounds, species often produced during the combustion of typical fuels used in these applications. Changes in the coatings were studied using a scanning electron microscope to observe the microstructure and x-ray diffraction techniques to analyze the phase composition. The results showed different behaviors for the two materials: the zirconia-based coating being rapidly degraded by the vanadium compounds and resistant to attack by the sulfur materials while the lanthanum zirconate was less damaged by exposure to vanadia but severely attacked in the presence of sulfur-containing species.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Heat Transfer

J. Eng. Gas Turbines Power. 2004;128(1):103-110. doi:10.1115/1.1924485.

Increasing the efficiency of modern jet engines does not only imply to the mainstream but also to the secondary air and oil system. Within the oil system the bearing chamber is one of the most challenging components. Oil films on the chamber walls are generated from oil droplets, ligaments, or film fragments, which emerge from bearings, seal plates and shafts, and enter the bearing chamber with an angular momentum. Furthermore, shear forces at its surface, gravity forces, and the design of scavenge and vent ports strongly impact the behavior of the liquid film. The present paper focuses on the experimental determination of the film thickness in various geometries of bearing chambers for a wide range of engine relevant conditions. Therefore, each configuration was equipped with five capacitive probes positioned at different circumferential locations. Two analytical approaches are used for a comprehensive discussion of the complex film flow.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(1):111-117. doi:10.1115/1.1924719.

The work presented forms part of an ongoing investigation, focusing on modeling the motion of a wall oil film present in a bearing chamber and comparison to existing experimental data. The film is generated through the impingement of oil droplets shed from a roller bearing. Momentum resulting from the impact of oil droplets, interfacial shear from the airflow, and gravity cause the film to migrate around the chamber. Oil and air exit the chamber at scavenge and vent ports. A previously reported numerical approach to the simulation of steady-state two-phase flow in a bearing chamber, which includes in-house submodels for droplet-film interaction and oil film motion, has been extended. This paper includes the addition of boundary conditions for the vent and scavenge together with a comparison to experimental results obtained from ITS, University of Karlsruhe. The solution is found to be sensitive to the choice of boundary conditions applied to the vent and scavenge.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(1):118-127. doi:10.1115/1.1925648.

Previous heat transfer experiments showed that significant differences in the flow and heat transfer characteristics can occur in models of aircraft gas-turbine, high-compressor drums. Experiments with heated disks and colder flow show large-scale instabilities that cause mixing between the cooling flow and the flow in the trapped cavities. The general result of this mixing is relatively high heat flux on the disks. Other heat transfer experiments, simulating the aircraft take-off condition with cold disks and hotter coolant, show decreased heat transfer due to the stabilizing effects of positive radial density gradients. A stability analysis for inviscid, variable-density flow was developed to quantify the effects of axial velocity, tangential velocity, and density profiles in the bore region of the disk cavities on the stabilizing or destabilizing characteristics of the flow. The criteria from the stability analysis were used to evaluate the axial velocity and density profile conditions required to stabilize three tangential-velocity profiles, obtained from previous experiments and analyses. The results from the parametric study showed that for Rossby numbers, the ratio of axial velocity to disk bore velocity, less than 0.1, the flow can be stabilized with ratios of cavity density to coolant density of less than 1.1. However, for Rossby numbers greater than 1, the flow in the bore region is unlikely to be stabilized with a positive radial density gradient. For Rossby numbers between 0.1 and 1.0, the flow stability is a more complex relationship between the velocity and density profiles. Results from the analysis can be used to guide the correlation of experimental heat transfer data for design systems.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2004;128(1):128-134. doi:10.1115/1.2032451.

Experimental measurements were made in a rotating-cavity rig with an axial throughflow of cooling air at the center of the cavity, simulating the conditions that occur between corotating compressor disks of a gas-turbine engine. One of the disks in the rig was heated, and the other rotating surfaces were quasi-adiabatic; the temperature difference between the heated disk and the cooling air was between 40 and 100°C. Tests were conducted for axial Reynolds numbers, $Rez$, of the cooling air between $1.4×103$ and $5×104$, and for rotational Reynolds numbers, $Reϕ$, between $4×105$ and $3.2×106$. Velocity measurements inside the rotating cavity were made using laser Doppler anemometry, and temperatures and heat flux measurements on the heated disk were made using thermocouples and fluxmeters. The velocity measurements were consistent with a three-dimensional, unsteady, buoyancy-induced flow in which there was a multicell structure comprising one, two, or three pairs of cyclonic and anticyclonic vortices. The core of fluid between the boundary layers on the disks rotated at a slower speed than the disks, as found by other experimenters. At the smaller values of $Rez$, the radial distribution and magnitude of the local Nusselt numbers, Nu, were consistent with buoyancy-induced flow. At the larger values of $Rez$, the distribution of Nu changed, and its magnitude increased, suggesting the dominance of the axial throughflow.

Commentary by Dr. Valentin Fuster

### TECHNICAL PAPERS: Gas Turbines: Marine

J. Eng. Gas Turbines Power. 2004;128(1):153-162. doi:10.1115/1.1926315.

A managed program to review engine failures and take necessary preventative measures has been in place successfully in the Royal Navy since the introduction of gas turbines into service in the 1970s. One of the more prominent failure mechanisms with the Tyne RM1C and Spey SM1A engines has been the degradation of main line bearings accounting for 25% of all engines rejected. Historically, since the first recorded incident in March 1987, the failures pointed to poor performance of the bearings themselves. However, maintenance studies and recent analysis indicates that a vast proportion have occurred through previously unidentified chloride corrosion as a result of contamination of the lubricating oil system with salt water from the seawater lubricating oil cooler (SWLO cooler). Despite joint ownership of both engine variants with the Royal Netherlands Navy, there was no clear evidence until about five years ago to suggest why tube perforation was occurring. Indeed, the fact that failures have only occurred in Royal Navy service is an interesting twist to the problem. This paper summarizes the phenomenon of SWLO cooler corrosion caused by Microbial Induced Corrosion (principally Sulphate Reducing Bacteria—SRB). It highlights the conditions in which SRB occurs along with demonstrated prevention in Royal Navy gas turbine service through the combined efforts of maintenance and development of a new titanium tubestack. The fault finding and remedial recovery experience may well be of interest to operators of marine gas turbines, both naval and commercial, who use tube type heat exchangers, especially when operating or undertaking work in estuarial waters and nontidal basins or when undertaking littoral duties. This is a practical view of the issue from an operators perspective and while utilizing a wealth of research and technical data available on the subject, it relates the issues at hand to the particular corrosion problem and is not intended as an introduction into organic chemistry.

Commentary by Dr. Valentin Fuster