Research Papers: Gas Turbines: Ceramics

J. Eng. Gas Turbines Power. 2011;133(6):061301-061301-7. doi:10.1115/1.4002469.

Plasma spray-physical vapor deposition (PS-PVD) is a low pressure plasma spray technology recently developed by Sulzer Metco AG (Switzerland) to deposit coatings out of the vapor phase. PS-PVD is developed on the basis of the well established low pressure plasma spraying technology. In comparison to conventional vacuum plasma spraying and low pressure plasma spraying, these new processes use a high energy plasma gun operated at a work pressure below 2 mbar. This leads to unconventional plasma jet characteristics, which can be used to obtain specific and unique coatings. An important new feature of PS-PVD is the possibility to deposit a coating not only by melting the feed stock material, which builds up a layer from liquid splats, but also by vaporizing the injected material. Therefore, the PS-PVD process fills the gap between the conventional PVD technologies and standard thermal spray processes. The possibility to vaporize feedstock material and to produce layers out of the vapor phase results in new and unique coating microstructures. The properties of such coatings are superior to those of thermal spray and electron beam-physical vapor deposition (EB-PVD) coatings. In contrast to EB-PVD, PS-PVD incorporates the vaporized coating material into a supersonic plasma plume. Due to the forced gas stream of the plasma jet, complex shaped parts like multi-airfoil turbine vanes can be coated with columnar thermal barrier coatings using PS-PVD. Even shadowed areas and areas, which are not in the line-of-sight to the coating source, can be coated homogeneously. This paper reports on the progress made by Sulzer Metco to develop a thermal spray process to produce coatings out of the vapor phase. Columnar thermal barrier coatings made of yttria stabilized zircona are optimized to serve in a turbine engine. This includes coating properties like strain tolerance and erosion resistance but also the coverage of multiple air foils.

Commentary by Dr. Valentin Fuster

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

J. Eng. Gas Turbines Power. 2011;133(6):061501-061501-9. doi:10.1115/1.4002273.

Atomizing liquids by injecting them into crossflows is a common approach in gas turbines and augmentors. Much of our current understanding of the processes resulting in atomization of the jets, the resulting jet penetration and spray drop size distribution have been obtained by performing laboratory experiments at ambient conditions. Yet, operating conditions under which jets in crossflows atomize can be far different from ambient. Hence, several dimensionless groups have been identified that are believed to determine jet penetration and resulting drop size distribution. These are usually the jet and crossflow Weber and Reynolds numbers and the momentum flux ratio. In this paper, we aim to answer the question of whether an additional dimensionless group, the liquid to gas density ratio must be matched. We perform detailed simulations of the primary atomization region using the refined level set grid (RLSG) method to track the motion of the liquid/gas phase interface. We employ a balanced force, interface projected curvature method to ensure high accuracy of the surface tension forces, use a multiscale approach to transfer broken off, small scale nearly spherical drops into a Lagrangian point particle description allowing for full two-way coupling and continued secondary atomization, and employ a dynamic Smagorinsky large eddy simulation (LES) approach in the single phase regions of the flow to describe turbulence. We present simulation results for a turbulent liquid jet (q=6.6, We=330, and Re=14,000) injected into a gaseous crossflow (Re=740,000) analyzed under ambient conditions (density ratio 816) experimentally by Brown and McDonnell [2006, “Near Field Behavior of a Liquid Jet in a Crossflow,” Proceedings of the ILASS Americas, 19th Annual Conference on Liquid Atomization and Spray Systems]. We compare simulation results obtained using a liquid to gas density ratio of 10 and 100. The results show that the increase in density ratio causes a noticeable increase in liquid core penetration with reduced bending and spreading in the transverse directions. The post-primary atomization spray penetrates further in both the jet and transverse direction. Results further show that the penetration correlations for the windward side trajectory commonly reported in the literature strongly depend on the value of threshold probability used to identify the leading edge. Correlations based on penetration of the jet’s liquid core center of mass, on the other hand, can provide a less ambiguous measure of jet penetration. Finally, the increase in density ratio results in a decrease in wavelength of the most dominant feature associated with a traveling wave along the jet as determined by proper orthogonal decomposition.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(6):061502-061502-8. doi:10.1115/1.4002275.

A recently developed nonlinear flame describing function (FDF) is used to analyze combustion instabilities in a system where the feeding manifold has a variable size and where the flame is confined by quartz tubes of variable length. Self-sustained combustion oscillations are observed when the geometry is changed. The regimes of oscillation are characterized at the limit cycle and also during the onset of oscillations. The theoretical predictions of the oscillation frequencies and levels are obtained using the FDF. This generalizes the concept of flame transfer function by including dependence on the frequency and level of oscillation. Predictions are compared with experimental results for two different lengths of the confinement tube. These results are, in turn, used to predict most of the experimentally observed phenomena and in particular, the correct oscillation levels and frequencies at limit cycles.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(6):061503-061503-9. doi:10.1115/1.4002296.

Increasing concerns about climate change have encouraged interest in zero-CO2 emission hydrocarbon combustion techniques. In one approach, nitrogen is removed from the combustion air and replaced with another diluent, typically carbon dioxide or steam. In this way, formation of nitrogen oxides is prevented and the exhaust stream can be separated into concentrated CO2 and water by a simple condensation process. The concentrated CO2 stream can then be sequestered or used for enhanced oil recovery. Burning fuels in an O2/CO2 diluent raises new combustion opportunities and challenges for both emissions and operability: this study focuses on the latter aspect. CH4/O2/CO2 flames have slower chemical kinetics than methane-air flames and as such, flame stability is more problematic as they are easier to blow off. This issue was investigated experimentally by characterizing the stability boundaries of a swirl stabilized combustor. Near stoichiometric CO2 and N2 diluted methane/oxygen flames were considered and compared with lean methane/air flames. Numerical modeling of chemical kinetics was also performed to analyze the dependence of laminar flame speeds and extinction strain rates upon dilution by different species and to develop correlations for blowoff boundaries. Finally, blowoff trends at high pressure were extrapolated from atmospheric pressure data to simulate conditions closer to those of gas turbines.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(6):061504-061504-10. doi:10.1115/1.4002319.

Computational fluid dynamics (CFD) has an important role in current research. While large eddy simulations (LES) are now common practice in academia, Reynolds-averaged Navier–Stokes (RANS) simulations are still very common in the industry. Using RANS allows faster simulations, however, the choice of the turbulence model has a bigger impact on the results. An important influence of the turbulence modeling is the description of turbulent mixing. Experience has shown that often inaccurate simulations of combustion processes originate from an inadequate description of the mixing field. A simple turbulent flow and mixing configuration of major theoretical and practical importance is the jet in crossflow (JIC). Due to its good fuel-air mixing capability over a small distance, JIC is favored by gas turbine manufacturers. As the design of the mixing process is the key to creating stable low NOx combustion systems, reliable predictive tools and detailed understanding of this basic system are still demanded. Therefore, the current study has re-investigated the JIC configuration under engine relevant conditions both experimentally and numerically using the most sophisticated tools available today. The combination of planar particle image velocimetry and laser induced fluorescence was used to measure the turbulent velocity and concentration fields as well as to determine the correlations of the Reynolds stress tensor uiuj¯ and the Reynolds flux vector uic¯. Boundary conditions were determined using laser Doppler velocimetry. The comparisons between the measurements and simulation using RANS and LES showed that the mean velocity field was well described using the SST turbulence model. However, the Reynolds stresses and more so, the Reynolds fluxes deviate substantially from the measured data. The systematic variation of the turbulent Schmidt number reveals the limited influence of this parameter indicating that the basic modeling is amiss. The results of the LES simulation using the standard Smagorinsky model were found to provide much better agreement with the experiments also in the description of turbulent mixing.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Heat Transfer

J. Eng. Gas Turbines Power. 2011;133(6):061901-061901-10. doi:10.1115/1.4002349.

Hot streaks can cause localized hot spots on the blade surfaces in a high pressure turbine, increasing the heat load locally and even leading to material loss in regions such as the rotor blade tip. This study explores numerically the effect of the hot streak’s clocking position at the stator inlet on the rotor blade heat load and on the tip in particular. The inlet boundary conditions are taken from the hot streak experiment conducted in the axial turbine facility “LISA” at ETH Zurich. Using a particle tracking tool, in conjunction with time resolved simulations, a detailed analysis of the migration pattern of the hot streak is performed and the underlying mechanisms are discussed. The effect of clocking the hot streak from midpitch to the stator pressure side and in the opposite direction is examined. By clocking this particular hot streak even 10% of the stator pitch toward the pressure side up to 24 K reduction in the rotor blade tip adiabatic wall temperatures could be achieved under realistic engine conditions. Finally, based on the observations made, the implications for an integrated combustor-turbine design strategy are discussed.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Oil and Gas Applications

J. Eng. Gas Turbines Power. 2011;133(6):062401-062401-8. doi:10.1115/1.4002470.

The Aero-Thermo-Mechanics (ATM) Department of ULB (Université Libre de Bruxelles) is developing an original system to pump and separate a two-phase flow. Many applications need to extract a certain phase of a multiphase flow: oil extraction, flow in nuclear pumps, flow in aircraft lubrication systems, pulp and paper processing, etc. The main objective of this study is to obtain a lightweight, compact, and efficient system that can both extract the gas of a two-phase flow and increase the pressure of the liquid phase. Prototypes with different designs were first tested at ULB on a specific test bench using water and air. The current prototype is a kind of axial-centrifugal pump. The axial part is used to separate the two phases of the flow and to collect, in the centrifugal part, the liquid phase only. The test results of the water-air prototypes have allowed to identify the key design and working parameters for efficient separation and pumping. A theoretical model has also been developed to describe the behavior of these prototypes. After successful tests with water-air mixtures, the technology has been implemented for a hot oil-air mixture. The tests with oil-air mixtures are performed on the aeroengine lubrication system test bench that the ATM Department developed and continues developing for other projects. At the same time, the flow field in the pump and separator system is being studied with commercial computational fluid dynamics software packages. Several two-phase flow models are considered for this particular application.

Commentary by Dr. Valentin Fuster

Research Papers: Gas Turbines: Structures and Dynamics

J. Eng. Gas Turbines Power. 2011;133(6):062501-062501-10. doi:10.1115/1.4001826.

Implementation of gas foil bearings (GFBs) into micro gas turbines requires careful thermal management with accurate measurements verifying model predictions. This two-part paper presents test data and analytical results for a test rotor and GFB system operating hot (157°C maximum rotor outer diameter (OD) temperature). Part I details the test rig and measurements of bearing temperatures and rotor dynamic motions obtained in a hollow rotor supported on a pair of second generation GFBs, each consisting of a single top foil (38.14 mm inner diameter) uncoated for high temperature operation and five bump strip support layers. An electric cartridge (maximum of 360°C) loosely installed inside the rotor (1.065 kg, 38.07 mm OD, and 4.8 mm thick) is a heat source warming the rotor-bearing system. While coasting down from 30 krpm to rest, large elapsed times (50–70 s) demonstrate rotor airborne operation, near friction free, and while traversing the system critical speed at 13krpm, the rotor peak motion amplitude decreases as the system temperature increases. In tests conducted at a fixed rotor speed of 30 krpm, while the shaft heats, a cooling gas stream of increasing strength is set to manage the temperatures in the bearings and rotor. The effect of the cooling flow, if turbulent in character, is most distinctive at the highest heater temperature. For operation at a lower heater temperature condition, however, the cooling flow stream demonstrates a very limited effectiveness. The measurements demonstrate the reliable performance of the rotor-GFB system when operating hot. The test results, along with full disclosure on the materials and geometry of the test bearings and rotor, serve to benchmark a predictive tool. A companion paper (Part II) compares the measured bearing temperatures and the rotor response amplitudes to predictions.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(6):062502-062502-8. doi:10.1115/1.4001827.

Implementation of gas foil bearings (GFBs) in microgas turbines relies on physics based computational models anchored to test data. This two-part paper presents test data and analytical results for a test rotor and GFB system operating hot. A companion paper (Part I) describes a test rotor-GFB system operating hot to 157°C rotor OD temperature, presents measurements of rotor dynamic response and temperatures in the bearings and rotor, and includes a cooling gas stream condition to manage the system temperatures. The second part briefs on a thermoelastohydrodynamic (TEHD) model for GFBs performance and presents predictions of the thermal energy transport and forced response, static and dynamic, in the tested gas foil bearing system. The model considers the heat flow from the rotor into the bearing cartridges and also the thermal expansion of the shaft and bearing cartridge and shaft centrifugal growth due to rotation. Predictions show that bearings’ ID temperatures increase linearly with rotor speed and shaft temperature. Large cooling flow rates, in excess of 100 l/min, reduce significantly the temperatures in the bearings and rotor. Predictions, agreeing well with recorded temperatures given in Part I, also reproduce the radial gradient of temperature between the hot shaft and the bearings ID, largest (37°C/mm) for the strongest cooling stream (150 l/min). The shaft thermal growth, more significant as the temperature grows, reduces the bearings operating clearances and also the minimum film thickness, in particular, at the highest rotor speed (30 krpm). A rotor finite element structural model and GFB force coefficients from the TEHD model are used to predict the test system critical speeds and damping ratios for operation at increasing shaft temperatures. In general, predictions of the rotor imbalance show good agreement with shaft motion measurements acquired during rotor speed coastdown tests. As the shaft temperature increases, the rotor peak motion amplitudes decrease and the system rigid-mode critical speed increases. The computational tool, benchmarked by the measurements, furthers the application of GFBs in high temperature oil-free rotating machinery.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(6):062503-062503-7. doi:10.1115/1.4002355.

Operating conditions in high speed mainshaft ball bearings applied in new aircraft propulsion systems require enhanced bearing designs and materials. Rotational speeds, loads, demands on higher thrust capability, and reliability have increased continuously over the last years. A consequence of these increasing operating conditions are increased bearing temperatures. A state of the art jet engine high speed ball bearing has been modified with an oil channel in the outer diameter of the bearing. This oil channel provides direct cooling of the outer ring. Rig testing under typical flight conditions has been performed to investigate the cooling efficiency of the outer ring oil channel. In this paper, the experimental results including bearing temperature distribution, power dissipation, and bearing oil pumping and the impact on oil mass and parasitic power loss reduction are presented.

Commentary by Dr. Valentin Fuster

Research Papers: Internal Combustion Engines

J. Eng. Gas Turbines Power. 2011;133(6):062801-062801-9. doi:10.1115/1.4002250.

An in-cylinder pressure-based control method is capable of improving engine performance, as well as reducing harmful emissions. However, this method is difficult to be implemented in a conventional engine management system due to the excessive data acquisition and long computation time. In this study, we propose a real-time indicated mean effective pressure (IMEP) estimation method using cylinder pressure in a common-rail direct injection diesel engine. In this method, difference pressure integral (DPI) was applied to the estimation. The DPI requires only 180 pressure data points during one engine cycle from top dead center to bottom dead center when pressure data are captured at every crank angle. Therefore, the IMEP can be estimated in real time. To further reduce the computational load, the IMEP was also estimated using DPI at 2 deg, 3 deg, and 4 deg crank angle resolutions. Furthermore, based on the estimated IMEP, we controlled IMEP using a radial basis function network and linear feedback controller. As a result of the study, successful estimation and control were demonstrated through engine experiments.

Commentary by Dr. Valentin Fuster

Research Papers: Power Engineering

J. Eng. Gas Turbines Power. 2011;133(6):063001-063001-12. doi:10.1115/1.4002251.

Thermodynamic optimization of power plants based on supercritical (SupC) and ultrasupercritical (USC) steam parameters is reported in this article. The objective is to compute the maximum attainable power plant efficiency in Indian climatic conditions using high ash (HA) indigenous coal. A unit size of 800 MWe presently under development in India is considered for energy and exergy analysis of power plants. Commercially established steam turbine parameters are used for the optimization of SupC power plant, whereas advanced steam turbine parameters currently under research and development are used for the optimization of USC power plant. The plant energy efficiency of the optimized SupC and USC power plant based on air-coal combustion (ACC) show considerable increases of 2.8 and 5.2% points, respectively compared with the current SupC ACC power plant (reference plant) being commissioned in India. The increases in plant exergy efficiency for the same power plants are 2.6 and 4.8% points and the corresponding CO2 reductions are about 6 and 11%, respectively. The maximum possible plant energy efficiency in Indian climatic conditions using HA Indian coal is about 42.7% (USC power plant). The effect of low ash coal on plant energy and exergy efficiencies compared with HA coal is also presented. Further, the effect of oxy-coal combustion (OCC) on the plant energy and exergy efficiencies compared with the ACC is studied for the double reheat SupC and USC power plants to account for the impact of CO2 capture. A significant reduction of 8.8 and 6.6% points in plant energy efficiency is observed for SupC and USC OCC power plants, respectively compared with the reference SupC ACC power plant.

Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(6):063002-063002-9. doi:10.1115/1.4002270.

A consolidated model for the turbine operation of centrifugal pumps comprising accurate prediction, optimum selection, and rigorous evaluation has been the primary need and the most challenging tasks for the industry to deal with. This paper introduces a consolidated model that is developed from experimental results of different pump shapes (20–80 rpm) and turbomachine fundamentals directly resulting in the parsimony feature of the model. The model also creates a new basis for dealing with uncertainties. The prediction model segment of the consolidated model requires only the pump shape and size as input parameters for bringing out the complete turbine characteristics. The selection model segment, on the other hand, requires the site head and flow as fixed input parameters and turbine speed as the control parameter to prescribe suitable pumps available in the market. The evaluation model segment compares the absolute turbine characteristics of the suitable pumps and recommends the most suitable pump for the given site. The model also includes an acceptance criterion that relates the deviation of the “pump as turbine” operating parameters with the site parameters, and it is very useful at the evaluation stage. The features of the consolidated model are illustrated with two case studies, which highlight the importance of evaluation in addition to the prediction and basic selection of pumps operating as turbines. In order to increase the accuracy and robustness of the model, the paper recommends an optimization routine stage on the existing model that comprises results of more pump shapes (obtained through field projects or extended laboratory work). The optimization procedure suggested would come a long way to provide a lasting solution for the search of a reliable pump as turbine model.

Commentary by Dr. Valentin Fuster

Technology Reviews

J. Eng. Gas Turbines Power. 2011;133(6):064001-064001-9. doi:10.1115/1.4000799.

It is already 10 years since the (European) High Temperature Reactor Technology Network (HTR-TN) launched a program for development of HTR technology, which expanded through three successive Euratom framework programs, with many projects in line with the network strategy. Widely relying in the beginning on the legacy of the former European HTR developments (DRAGON, AVR, THTR, etc.) that it contributed to safeguard, this program led to advances in HTR/VHTR technologies and produced significant results, which can contribute to the international cooperation through Euratom involvement in the Generation IV International Forum (GIF). the main achievements of the European program, performed in complement to efforts made in several European countries and other GIF partners, are presented: they concern the validation of computer codes (reactor physics, as well as system transient analysis from normal operation to air ingress accident and fuel performance in normal and accident conditions), materials (metallic materials for vessel, direct cycle turbines and intermediate heat exchanger, graphite, etc.), component development, fuel manufacturing and irradiation behavior, and specific HTR waste management (fuel and graphite). Key experiments have been performed or are still ongoing, like irradiation of graphite and of fuel material (PYCASSO experiment), high burn-up fuel PIE, safety test and isotopic analysis, IHX mock-up thermohydraulic test in helium atmosphere, air ingress experiment for a block type core, etc. Now HTR-TN partners consider that it is time for Europe to go a step forward toward industrial demonstration. In line with the orientations of the “Strategic Energy Technology Plan (SET-Plan)” recently issued by the European Commission that promotes a strategy for development of low-carbon energy technologies and mentions Generation IV nuclear systems as part of key technologies, HTR-TN proposes to launch a program for extending the contribution of nuclear energy to industrial process heat applications addressing (1) the development of a flexible HTR that can be coupled to many different process heat and cogeneration applications with very versatile requirements, (2) the development of coupling technologies for such coupling, (3) the possible adaptations of process heat applications required for nuclear coupling, and (4) the integration and optimization of the whole coupled system. As a preliminary step for this ambitious program, HTR-TN endeavors to create a strategic partnership between nuclear industry and R&D and process heat user industries.

Commentary by Dr. Valentin Fuster

Technical Briefs

J. Eng. Gas Turbines Power. 2011;133(6):064501-064501-3. doi:10.1115/1.4002252.

The abrasive water jet (AWJ), is to shoot the abrasive mixed with high-pressure water to the material for cutting, can cut most materials, such as metals and concretes in water with long stand-off means the length from the cutting head to the material for cutting. On the other hand, AWJ is required to reduce an amount of the abrasive because it becomes the waste. It is also difficult to monitor the cutting condition by any visual methods such as a TV camera in the water becoming cloudy by both used abrasive and cut metal grit. For solving these issues, some cutting tests were conducted and (1) it was possible to predict an optimal supply rate of abrasive by considering the conservation of momentum between the water jet and the abrasive. (2) It was also possible to judge whether the material could be cut successfully or not by detecting the change in the frequency characteristics of vibration or sound caused during the cutting process.

Topics: Cutting , Water
Commentary by Dr. Valentin Fuster
J. Eng. Gas Turbines Power. 2011;133(6):064502-064502-4. doi:10.1115/1.4002277.

The development of an efficient and reliable method of evaluating the performance of an air-cooled steam condenser (ACC) under windy conditions using computational fluid dynamics (CFD) is presented. A two-step modeling approach is employed as a result of computational limitations. The numerical ACC model developed in this study makes use of the pressure jump fan model, among other approximations, in an attempt to minimize the computational expense of the performance evaluation. The accuracy of the numerical model is verified through a comparison of the numerical results to test data collected during full-scale tests carried out on an operational ACC. Good correlation is achieved between the numerical results and test data. The effect of wind on ACC performance at El Dorado Power Plant (Nevada, USA) is investigated. It is found that reduced fan performance due to distorted flow at the inlet of the upstream fans is the primary contributor to the reduction in ACC performance associated with increased wind speed in this case. The model developed in this study has the potential to allow for the evaluation of large ACC installations and provides a reliable platform from which further investigations into improving ACC performance under windy conditions can be carried out.

Commentary by Dr. Valentin Fuster

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