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ASME Conference Presenter Attendance Policy and Archival Proceedings

2017;():V008T00A001. doi:10.1115/GT2017-NS8.
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This online compilation of papers from the ASME Turbo Expo 2017: Turbomachinery Technical Conference and Exposition (GT2017) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Microturbines, Turbochargers and Small Turbomachines

2017;():V008T26A001. doi:10.1115/GT2017-63069.

The study at hand analyzes the influence of aerodynamic mistuning and aerodynamic coupling on the vibration behavior of mistuned small radial turbine wheels. The aerodynamic mistuning is caused by angular non uniformity of the variable turbine guide vanes. Variable turbine guide vanes are state of the art in exhaust gas turbochargers for automotive diesel engines. Aerodynamic coupling describes the coupling of the turbine blades through the flow. It can influence the mistuned vibration behavior of the turbine wheel due to varying operation conditions, in which the turbine pressure ratio and the pressure distribution over the turbine wheel surface is changed.

It was analyzed whether the aerodynamic mistuning and aerodynamic coupling must be considered for small radial turbine wheel designs. The basis for this investigation were blade vibration measurements under standstill conditions with a laser vibrometer as well as blade vibration measurements during operation with a tip timing system. The mistuned turbine eigenforms were analyzed and compared at various ambient conditions using these measurement results.

By means of forced response calculations — unsteady 3D CFD and 3D FEA —, the influence of aerodynamic mistuning on the ideal tuned turbine was examined to be able to separate the aerodynamic mistuning from the mistuning of the structure. Furthermore, the superimposed effect of the aerodynamic mistuning and the mistuning of the structure on the turbine eigenforms and the amplitude amplification was analyzed using a mistuned 3D FE model and a population of samples with varying aerodynamic mistuning.

It was found, that the aerodynamic coupling and aerodynamic mistuning have a negligible effect on the mistuned vibration behavior for a small radial turbine with variable turbine guide vanes. These two parameters must not be considered when designing such a turbine wheel.

Topics: Turbines , Vibration , Wheels
Commentary by Dr. Valentin Fuster
2017;():V008T26A002. doi:10.1115/GT2017-63185.

To meet the challenging demands for high performance, affordable compliant foil bearings, a novel compliant support element has been developed. This recently patented, novel support element uses a multidimensional array of multiple, formed, cantilever “wing foil” tabs. The wing foil bearing has all the features required to achieve state of the art performance (Gen III for radial bearings). This paper describes two radial foil beings using the wing foil and the unique design features. Test data for a 31.75 mm diameter bearing operating in air and in steam up to 42 krpm are presented to demonstrate the performance of this bearing. It is shown to have low subsynchronous vibration and reasonable damping through rigid shaft critical speeds.

Topics: Design , Wings , Foil bearings
Commentary by Dr. Valentin Fuster
2017;():V008T26A003. doi:10.1115/GT2017-63195.

In the present study, the entire energy balance of a turbocharger is investigated applying an experimentally validated numerical approach with the intention of examining the heat transfer mechanism inside the turbo. The heat transfer results thus obtained are used amongst others to determine the diabatic effects on the turbine and compressor flow resulting in heat-transfer corrected performance maps. These maps are applied as matching data to 1D engine performance calculation and are utilized in the engine process simulation procedure with GT Power™. In detail, the numerical approach of the entire energy balance is based on a thermal network model (RC-resistance / capacity) where the 3D geometry of the turbocharger is subdivided into segments. These segments are defined as lumped mass elements of the thermal network. The entire energy balance of the modeled turbo is fulfilled by coupling the thermal network of the structure to the enthalpy flows of the turbine, compressor, oil circuit, and water coolant as well as the heat losses to ambient. The heat transfer between the structure and the enthalpy flows, respectively, is achieved by using heat transfer coefficients (HTC) performed in accordance with Nusselt-No. laws. Heat loss to ambient is expressed by natural convection and radiation. In general it would be possible to perform the energy balance of the turbo model in the steady state or transient regime. A time-governed finite volume calculation scheme is used for the solution algorithms. The code of the turbo heat transfer approach (THT) is written in Matlab™, something which facilitates flexible adjustments on the algorithm and good post-processing capabilities. Two routes are resorted to for validating the THT approach. Gas stand tests with instrumented turbochargers using thermocouples and pressure sensors are conducted in the first assignment for generating the essential experimental data. Segmentation of the 3D turbocharger geometry into discrete elements is accomplished in the second assignment by means of CAD technology and used for both, the setup of the THT thermal network model and in parallel for the generation of an AnsysCFX™ 3D CAE model. The same HTC thermal boundary conditions are applied to both models which is favorable in as far as it provides a one-to-one comparison of the heat flux and mean temperature in each segment of the two models, Matlab™ THT and CFX™. Ansys™ model heat flux and mean segment temperature results are validated by the measured experimental temperature data. The THT network model properties such as segment volume, areas, volume, and element distances are calibrated applying the results of the 3D CAD and CAE Ansys™ model. The results of the two numerical models are compared with each other, thus demonstrating the qualitative and quantitative level of agreement. The THT approach that has been developed is successfully applied to GT Power™ gasoline engine model. A thermal network model of that applied turbocharger was setup and validated by gas-stand and engine data obtained on an experimental basis with an instrumented turbo. Finally it was possible to demonstrate that the heat-transfer corrected turbocharger performance map data which was provided utilizing the THT model approach brings about a significant benefit to the determination process aimed at achieving a tailored turbocharger thermodynamic layout.

Commentary by Dr. Valentin Fuster
2017;():V008T26A004. doi:10.1115/GT2017-63360.

Current key technologies to meet the future emission standards for internal combustion engines are downsizing, down-speeding, and advanced charging concepts. While turbocharging already combines high specific rated engine power with low fuel consumption, there is still potential for optimization to achieve prospective demands for fuel efficiency with low emissions. Using engine exhaust energy, the turbine underlies pulsating flow conditions from high towards zero mass flow at almost constant blade speed. The average turbine efficiency is then affected for the high blade to jet speed ratio conditions, which is very important at low engine loads during urban driving conditions. Since turbocharger performance is very sensitive for the overall engine efficiency, a very accurate measurement of the characteristic maps is desired to evaluate the thermodynamic behavior of the turbocharger and to ensure best possible matching. This paper presents a methodology to extend the turbine performance at low expansion ratio and to characterize the adiabatic efficiency in a wide operating range. This enables measuring turbines on a hot-gas test bench at very high blade to jet speed ratio and very low turbine flow to develop, improve, and validate reliable turbocharger models that can be used for full engine simulations. The industrial applicability has been proven from very low turbine power up to negative turbine power output simply based on using inlet guide vanes (IGV) upstream of the compressor. By generating a swirl in the compressor wheel rotating direction and pressurizing the inlet air, the compressor can be run as a turbine. Thus, the compressor provides power to the shaft and the turbine can be driven with very low flow power. The test campaign has been realized under quasi-adiabatic conditions to limit the heat transfer. While measuring at three different oil temperatures, the impact of remaining internal heat transfer has been taken into account. A turbocharger heat transfer model has also been used to correct residual heat flows from the obtained data set for all oil temperatures.

Commentary by Dr. Valentin Fuster
2017;():V008T26A005. doi:10.1115/GT2017-63368.

State of the art car engines are fed by compressed air, coming from a turbocharger compressor, to increase the power to weight ratio and to allow downsizing the combustion engine. The used compressor is driven by a radial turbine taking advantage of the hot and pressurized exhaust gases of the engine. Thus, the turbine acts under highly unsteady conditions, working at very different turbine map regions. In urban driving the turbine faces even higher changes due to frequent acceleration and deceleration so that extremely low mass flow can occur. However, the flow behavior in turbocharger turbines at these extreme off-design conditions is rather unknown. So the development of physically-based models for extrapolating the usually narrow experimental turbine maps and advanced measurements to increase the range of turbine maps has been in the focus of many researchers. To provide valuable information about those flow characteristics, this paper supplies a detailed analysis at low mass flow in a radial turbocharger turbine. The turbine has been experimentally characterized under steady flow from normal operating working conditions up to extreme off-design points, where the turbine could even work with negative efficiency. Since heat transfer significantly affects the turbine efficiency calculation when turbine power is low, the experiments have been executed under quasi-adiabatic conditions and residual heat fluxes have further been corrected. This paper takes advantage of these data to validate adiabatic CFD simulations in a wide operating range, from optimum efficiency point up to negative turbine power. Stationary and transient three-dimensional CFD simulations of the turbocharger turbine have been performed. The numerical campaign covers a wide range of operating conditions, providing different flow patterns. The obtained results show that the secondary flow field changes appreciably with mass flow rate. At low mass flows, a further backflow region develops over the entire circumference close to the hub, significantly constricting the effective turbine area and provoking mass flow instability. The highlighted flow phenomena will allow to improve state of the art extrapolation models and might help designers to understand turbine flow operating under extreme off-design conditions.

Commentary by Dr. Valentin Fuster
2017;():V008T26A006. doi:10.1115/GT2017-63370.

Turbochargers are a key technology for reducing the fuel consumption and CO2 emissions of heavy-duty internal combustion engines by enabling greater power density, which is essential for engine downsizing and downspeeding. This in turn raises turbine expansion ratio levels and drives the shift to air systems with multiple stages, which also implies the need for interconnecting ducting, all of which is subject to tight packaging constraints.

This paper considers the challenges in the aerodynamic optimization of the exhaust side of a two-stage air system for a Caterpillar 4.4-litre heavy-duty diesel engine, focusing on the high pressure turbine wheel and interstage duct. Using the current production designs as a baseline, a genetic algorithm-based aerodynamic optimization process was carried out separately for the wheel and duct components in order to minimize the computational effort required to evaluate seven key operating points.

While efficiency was a clear choice for the cost function for turbine wheel optimization, the most appropriate objective for interstage duct optimization was less certain, and so this paper also explores the resulting effect of optimizing the duct design for different objectives. Results of the optimization generated differing turbine wheel and interstage duct designs depending on the corresponding operating point, thus it was important to check the performance of these components at every other operating point, in order to determine the most appropriate designs to carry forward. Once the best compromise high pressure turbine wheel and interstage duct designs were chosen, prototypes of both were manufactured and then tested together against the baseline designs to validate the CFD predictions. The best performing high pressure turbine design, wheel A, was predicted to show an efficiency improvement of 2.15 percentage points, for on-design operation. Meanwhile, the optimized interstage duct contributed a 0.2 and 0.5 percentage-point efficiency increase for the high and low pressure turbines, respectively.

Commentary by Dr. Valentin Fuster
2017;():V008T26A007. doi:10.1115/GT2017-63462.

When it comes to evaluating the thermodynamic and fluid dynamic behavior of a turbocharger (TC) and ensuring the best possible matching to the internal combustion engine, a precise measurement of the characteristic maps is required. Measured on a hot-gas test bench under steady-state conditions, the maps of the compressor and turbine give detailed information about the turbocharger performance and provide boundary conditions for simulating the engine processes of a turbocharged engine.

At the moment there is no comprehensive standard on how to measure a TC on a test bench. All existing guidelines such as SAE J1826 & J922 and ASME PTC 10 contain more general information with recommendations, but turbocharger mapping on a hot-gas test bench is highly complex and has a large dependence on heat transfer. The characteristics are not determined directly, but have to be computed using the measured quantities and the thermodynamic properties of the working fluid. For that reason, all influences that may occur during the measurement have to be taken into account for the comparability of different turbochargers or the repeatability for the same turbocharger specimen.

As a consequence, this paper deals with the methodology of thermodynamic computation for a mono-scroll and a twin-scroll turbocharger in general and the impact of different approaches for computing the thermodynamic properties of the individual species on compressor and turbine efficiencies in particular.

For computing the characteristics of a mono-scroll turbocharger, a methodology is used that matches good practice and state-of-the-art technology. For computing the characteristics of a twin-scroll turbocharger, a new methodology is developed to provide a practical approach for the evaluation of pressure ratios and efficiencies of a twin-entry turbine. The use of the closed-loop unit for the extension of the turbine is included.

Computing the thermodynamic state changes, power, and efficiencies of the compressor and turbine requires knowledge of the physicochemical properties of the involved fluids. These fluids are humid air (compressor) and exhaust gas (turbine) and depend on the gas composition, pressure, and especially temperature with respect to the relative humidity.

Turbocharger efficiencies are very sensitive to deviations in the thermodynamic properties of the individual species. Different forms of polynomials with different levels of complexity and precision are available in literature. In this paper, the most commonly used polynomials for computing the thermodynamic properties such as caloric perfect gas, humid air, VDI 4670, NASA9, CHEMKIN, NIST, and SAE J1826 will be used for the characteristic maps of a twin-scroll turbocharger. The results will be discussed in terms of the impact on turbocharger efficiencies and the relevance to achieving a desired quality level for turbocharger mapping.

Commentary by Dr. Valentin Fuster
2017;():V008T26A008. doi:10.1115/GT2017-63526.

Turbomachines are commonly designed for a high mass flow rate. However, because of new cycle concepts, turbomachines are also required to compress or expand at small mass flow rates. One example is the supercritical carbon dioxide Brayton cycle. The mass flow rate can be in the range of one kg/s at an almost high fluid density at the inlet to the compressor. This results in a small through flow area. In this paper, a turbomachine concept is presented that integrates the turbomachine parts into an electrical machine. Specifically, the turbomachine is located in the gap between the rotor and the stator of the electrical machine. In that way, a very compact design can be achieved. This paper aims to explain the basic concept. An aerodynamic design study is performed that demonstrates the important parameters for machine performance. Additionally, the design of the electrical machine is discussed based on a realistic application. Finally, conclusions for further development are drawn.

Commentary by Dr. Valentin Fuster
2017;():V008T26A009. doi:10.1115/GT2017-63658.

The bearing system of turbochargers used in trucks needs to be optimized in order to reduce the frictional losses. This helps in transmitting the exhaust energy more efficiently to the compressor wheel to increase boost pressure. Understanding the thrust loading on the axial bearing helps in optimal design of the bearing and the associated lubrication system. With the advent of twin scroll turbochargers, it is necessary to understand the thrust load behaviour at different operating conditions. This paper pioneers in studying the unsteady axial loads measured on a twin scroll turbocharger mounted on a 6 cylinder, 13 litre diesel engine used in the truck industry along with the corresponding analytical predictions for varied engine speeds and loading conditions. Transient thrust forces were measured using a weakened bearing in the experimental approach along with transient pressure measurments on the turbocharger. The axial bearing weakening required a design trade-off between flexibility and rigidity of the bearing. The results from the experimental and analytical methods provide better understanding of the characteristics of transient thrust forces that act on a turbocharger mounted on an engine of a heavy duty truck along with its design implications. The maximum normalized axial load measured and predicted were −90 N and −100 N, respectively.

Commentary by Dr. Valentin Fuster
2017;():V008T26A010. doi:10.1115/GT2017-63661.

New concepts for power generation are discussed as a response to CO2 emissions from the combustion of fossil fuels. These concepts include low-carbon fuels as well as new fuel supplies will be used, with (biogenic) low-caloric gases such as syngas with an amount of hydrogen, with a share of 50% and even higher. However, hydrogen mixtures have a higher reactivity than natural gas (NG) mixtures, burned mostly in today’s gas turbine combustors.

The authors discuss in this paper the potential of a micro gas turbine (MGT) combustor when operated under unconventional conditions, both in terms of variation in the fuel supplied and concerning the part-load or off-design operation.

In particular, the authors’ methodology relies on an advanced CFD approach that makes use of extended kinetic mechanisms coupled with the turbulent interaction of the reacting species. A preliminary set-up of the combustion model is based on data provided by experimental tests of the micro-turbine.

In the paper, several computational examples are discussed, namely:

- The comparison of combustion stability and efficiency and pollutant production with several fuels.

- The analysis of the combustor response with reduced load.

- The use of the pilot and main injectors for supplying different fuels.

Commentary by Dr. Valentin Fuster
2017;():V008T26A011. doi:10.1115/GT2017-63801.

In this work, the performances of a 100 kW Micro Gas Turbine (MGT) fed by Natural Gas (NG) and three different biomass-derived Synthesis Gases (SGs) have been assessed using a MATLAB® simulation algorithm. The set of equations in the algorithm includes the thermodynamic transformations of the working fluid in each component, the performance maps that describe the turbomachines’ isentropic efficiencies and pressure ratios as a function of the reduced mass flow rate and the reduced rotational speed, the performance and the pressure losses in each component, as well as the consumption of the other auxiliary devices.

The electric power output, achieved using SGs, turns out to be lower or higher with respect to the one produced with the NG, depending on the fuel Lower Heating Value (LHV) but also largely on the variation of the working fluid composition.

In this work, the effect of the steam injection on the MGT performance characteristics has been also investigated. Steam injection allows to obtain higher power and efficiencies using both NG and SGs at the rated rotational speed, mainly thanks to the increase of the turbine enthalpy drop and the reduction of the compressor consumption. Attention must be paid to the risk of the compressor stall, especially when using SGs, as the mass flow rate processed by the compressor decreases significantly.

Moreover, another advantage of adopting the steam injection technique lies in the increased flexibility of the system: according to the users’ needs, the discharged heat can be exploited to generate steam, thus to enhance the electric performances, or to supply thermal power.

Commentary by Dr. Valentin Fuster
2017;():V008T26A012. doi:10.1115/GT2017-63887.

In order to meet the requirements of automobile engines and marine-use diesel engines, turbochargers must be developed with high boost pressure and appreciably high levels of efficiency. The high pressure rise typically achieved in transonic compressors lead to a stage characterized by high inlet relative Mach numbers. Losses generated in transonic compressors are to a large extent due to the formation of shockwaves at the inducer with interactions between the shock, tip leakage vortex and boundary layer. Significant efficiency reduction occurs at the tip region of the impeller due to the complex interaction of the tip clearance flow and shocks, resulting in significant overall performance degradation.

A study has been conducted on the unsteady motion of shockwaves in a transonic centrifugal compressor with vaned diffuser using time-resolved three-dimensional Reynolds average Navier-Stokes simulation. Focus is placed on the impact of the shock motion and post shock unsteadiness on stage performance and impeller-diffuser interaction. The key findings were that the interaction of the shockwave with the tip leakage flow and the boundary layer were the most influential in loss generation with a consequence of increased aerodynamic loss. For the unsteady blade row interaction, the influence of upstream flow unsteadiness on diffuser vanes had significant effect on the flow incidence angle. Periodic jet and wake structure from the impeller and the progressive pressure waves which interacts with the vanes at the interface strongly determines the intensity and position of the vane shock. This has implications on performance in terms of stall inception and static pressure rise across the diffuser.

Commentary by Dr. Valentin Fuster
2017;():V008T26A013. doi:10.1115/GT2017-63923.

Regulated two-stage (RTS) turbocharging system is an effective way to enhance power density and reduce pollutant of internal combustion engine for increasingly stringent demands of fuel consumption and emission regulation. Due to achieving high boost pressure with great system efficiency and controllable characteristic in wide working range, the RTS turbocharging system improves output power at low speed condition and reduces pumping loss at high speed condition. Composing of two turbochargers and control valves, the RTS turbocharging system is matched with engine at a design point and regulated by adjusting control valves to meet the engine requirement of intake pressure and flow at other working conditions. Calibration of the control valves under all operating conditions by plentiful experiments is significant for turbocharging system, particularly that matched with diesel engine for vehicle. Moreover, when an automobile run on the plateau, the intake air flow will decrease and combustion in cylinder will deteriorate obviously. Compared with other turbocharging system, two-stage turbocharging system is more suitable to the offset power loss of engine. Regulating boost system under different operation conditions draws more attention to engine performance recovery so that the workload of calibration raises rapidly in consideration of altitude factor. Though much work has been done in calibration at various altitudes, there are few, if any, discussion on open-closed boundary of control valves to simplify the calibration process. In this paper, it aims to present a regulation boundary model of control valve at different altitudes to guide the calibration and a series of experiments for RTS system can be saved. Firstly, a thermodynamics analysis of the RTS turbocharging system is conducted and typical regulation methods are compared in terms of the adjustment capacity and efficiency characteristics of turbocharging system, which indicates that high-stage turbine bypass is the optimum regulation method. Then, a regulation boundary model for the RTS turbocharging system at different altitudes is deduced, according to the relation of equivalent turbine area and engine operating condition. The regulation boundaries of different altitudes are obtained by iterative computation of the model, and the whole working mode of the RTS system is divided into a fully closed area and a regulated area. Experiments are carried out to verify the regulation boundary model at sea level condition. Brake torque, efficiency of the RTS system and temperature before high-stage turbine are primary parameters for verification in this article. The maximum error shows up with a value of 3.65% brake torque at 2200rpm. While a one-dimensional simulation model is built up to validate the regulation boundary model of the plateau. All the errors are smaller than 3% at various altitudes. It results that model is accurate enough to predict the regulation boundary of the RTS system. By the calculation of regulation boundary model, the brake torque at regulation boundary will decrease if the engine speeds up. It also manifests that fully closed area will argument if the automobile climbs up to high operating altitude, especially under high speed condition.

Commentary by Dr. Valentin Fuster
2017;():V008T26A014. doi:10.1115/GT2017-64007.

The use of ceramics in gas turbines potentially allows for very high cycle efficiency and power density, by increasing operating temperatures. This is especially relevant for sub-megawatt gas turbines, where the integration of complex blade cooling greatly affects machine capital cost. However, ceramics are brittle and prone to fragile, catastrophic failure, making their current use limited to static and low-stress parts. Using the inside-out ceramic turbine (ICT) configuration solves this issue by converting the centrifugal blade loading to compressive stress, by using an external high-strength carbon-polymer composite rim. This paper presents a superalloy cooling system designed to protect the composite rim and allow it to withstand operating temperatures up to 1600 K. The cooling system was designed using one-dimensional (1D) models, developed to predict flow conditions as well as the temperatures of its critical components. These models were subsequently supported with computational fluid dynamics and used to conduct a power scalability study on a single stage ICT. Results suggest that the ICT configuration should achieve a turbine inlet temperature (TIT) of 1600 K with a composite rim cooling-to-main mass flow rate ratio under 5.2% for power levels above 350 kW. A proof of concept was performed by experimental validation of a small-scale 15 kW prototype, using a commercially available bismaleimide-carbon (BMI-carbon) composite rim and Inconel® 718 nickel-based alloy. The combination of numerical and experimental results show that the ICT can operate at a TIT of 1100 K without damage to the composite rim.

Commentary by Dr. Valentin Fuster
2017;():V008T26A015. doi:10.1115/GT2017-64178.

This paper presents the development approach, design and evaluation of three turbocharger compressors with variable geometry for heavy duty engines. The main goal is the improvement of fuel economy without sacrifices regarding any other performance criteria. In a first step, a vaned diffuser parameter study shows that efficiency improvements in the relevant operating areas are possible at the cost of reduced map width. Concluding from the results three variable geometries with varying complexity based on vaned diffusers are designed. Results from the hot gas test stand and engine test rig show that all systems are capable of increasing compressor efficiency and thus improving fuel economy in the main driving range of heavy duty engines. The most significant differences can be seen regarding the engine brake performance. Only one system meets all engine demands while improving fuel economy.

Commentary by Dr. Valentin Fuster
2017;():V008T26A016. doi:10.1115/GT2017-64190.

In times of stringent emission standards for automotive and truck applications, exhaust gas recirculation (EGR) is used in IC engines to reduce NOx emissions by recirculating a portion of an engine’s exhaust gas. The amount of exhaust gas determined for EGR is withdrawn from the exhaust gas route and routed back into the combustion chamber. The recirculated exhaust gas acts as an inert gas and, when mixed with the pre-combustion mixture, helps to decrease the combustion temperature and thus NOx emissions.

Designed for a diesel engine within a truck application, the turbine in this particular research project is fed by two cylinder groups, however, only the exhaust gas of one group is recirculated. The reduced mass flow in the small turbine scroll (EGR-scroll) through EGR withdrawal, along with the increased pressure required for EGR transport, leads to a massive reduction in the mass flow parameter of the EGR-scroll.

The common turbocharger design process has been based on steady admission rather than unsteady admission given through the pulsating nature of multi-cylinder admission. This has lead to diverging results of turbochargers performing well on steady hot gas test rigs compared to performing badly in the final tests on the engine itself.

In this paper however, unequal admission resulting from pulsating admission is taken into account. Based on unsteady admission, a methodology is proposed for steady computations with unequal admissions, and a thorough 3D CFD loss analysis is to be presented to understand the turbine behaviour, reveal the regions for improvements, and provide a framework for further development.

Topics: Turbochargers
Commentary by Dr. Valentin Fuster
2017;():V008T26A017. doi:10.1115/GT2017-64218.

Standard test rigs for basic research on turbochargers usually do not provide the capability of periodically changing, instantaneous process values, which are characteristic for the real application of these turbines. The challenge of testing the performance potential of turbocharger turbines under pulsating inflow conditions is mainly originated by the complex compatibility of two main issues that need to be implemented at a test facility: Firstly, a special device is required that reproducibly provides real engine-like exhaust gas pulsations with some variability representing different engine operating conditions. Secondly, appropriate real time measurement techniques for all significant transient values are required to measure both, instantaneous turbine inflow conditions and turbine power output. This paper presents a new developed test rig that enables a preferably high overlap between the above mentioned supply of approximately real engine exhaust gas conditions and the fundamental and scientifically based attempt of unsteady gas flow examinations.

Commentary by Dr. Valentin Fuster
2017;():V008T26A018. doi:10.1115/GT2017-64250.

A demonstration test with the aim to show the potential of ammonia-fired power plant is planned using a micro gas turbine. 50kW class turbine system firing kerosene is selected as a base model. Over 40kW of power generation was achieved by firing ammonia gas only. Over 40kW of power generation was also achieved by firing mixture of ammonia and methane. However ammonia gas supply increases NOx in the exhaust gas dramatically. NOx concentration in the exhaust gas of gas turbine reached at over 600ppm. In the case of the gas turbine operation firing kerosene-ammonia with 31kW of power generation at 75,000rpm of rotating speed, the LHV (Lower Heating Value) ratio of ammonia to the total supplied fuel was changed from 0% to 100% in detail. NO emission increases rapidly to around 400ppm with ammonia at 7% of LHV ratio of ammonia. Then NO emission increases gradually to 600ppm with ammonia at 27% of LHV ratio of ammonia. NO emission has the peak around 60% of LHV ratio of ammonia. NO emission decreases below 500ppm at 100% of LHV ratio of ammonia. The gas turbine operation firing methane-ammonia with 31kW of power generation at 75,000rpm of rotating speed was also tried. NO emission increases rapidly to around 470ppm with ammonia at 7% of LHV ratio of ammonia. Then NO emission increases gradually to 600ppm with ammonia around 30% of LHV ratio of ammonia. NO emission has the peak at 65% of LHV ratio of ammonia. NO emission decreases below 500ppm at 100% of LHV ratio of ammonia. Since the ammonia flame in the prototype combustor seems to be inhomogeneous, ammonia combustion in the prototype combustor may have high NOx region and low NOx region. Therefore there is a possibility of low-NOx combustion.

Flame observation was planned to know combustion state for improvement toward the low NOx combustor. Flame observation from the combustor exit was available by extending the combustor exit with the adaptor of the bent coaxial tubes and the quartz window. Swirling flames of ammonia, methane and methane-ammonia were observed near the center axis of the combustor. Flame observation at 39.1kW of power generation was succeeded. In the case of the flame observation, fuel consumption increased due to increase of the heat loss from the combustor. The emissions of NO and NH3 clearly depend on the combustion inlet temperature at 75,000rpm of rotating speed. The emissions of NO and NH3 in the case of the flame observation setting corresponds to the emission in the case of the normal setting at the condition that the power output is 11.2kW lower.

Commentary by Dr. Valentin Fuster
2017;():V008T26A019. doi:10.1115/GT2017-64283.

The measured performance maps of turbochargers which are commonly used for the matching process with a combustion engine are influenced by heat transfer and friction phenomena. Internal heat transfer from the hot turbine side to the colder compressor side leads to an apparently lower compressor efficiency at low to mid speeds and is not comparable to the compressor efficiency measured under adiabatic conditions.

The product of the isentropic turbine efficiency and the mechanical efficiency is typically applied to characterize the turbine efficiency and results from the power balance of the turbocharger. This so-called ‘thermo-mechanical’ turbine efficiency is strongly correlated with the compressor efficiency obtained from measured data.

Based on a previously developed one-dimensional heat transfer model, non-dimensional analysis was carried out and a generally valid heat transfer model for the compressor side of different turbochargers was developed.

From measurements and ramp-up simulations of turbocharger friction power, an analytical friction power model was developed to correct the thermo-mechanical turbine efficiency from friction impact.

The developed heat transfer and friction model demonstrates the capability to properly predict the adiabatic (aerodynamic) compressor and turbine performance from measurement data obtained at a steady-flow hot gas test bench.

Commentary by Dr. Valentin Fuster
2017;():V008T26A020. doi:10.1115/GT2017-64329.

Throughout the world there is pressure to increase distributed energy generation. Driving factors include for example political and environmental concerns in developed countries and reliability in places where centralized grid does not either exist or is too unreliable. The energy generation based on renewable fuels such as biogas is also usually decentralized. To answer this demand, the number of small-scale gas turbine combined heat and power (CHP) installations have increased. Due to its nature, the required power output of distributed generation is highly variable. The power output of decentralized power plant needs to follow the local consumption power need and thus it needs to be efficiently controlled. Therefore, the requirement for variable output necessitates that small-scale gas turbines are often run at part-loads.

Previously, most of the installed small-scale gas turbines have been single-spool units with either fixed or variable speed shafts. Control schemes and part-load performance are somewhat different for the two setups. Recently, a two-spool gas turbine where the spools can be controlled independently has been proposed as a feasible alternative. The possibility to produce the desired power output with two spools, both having their own generator, which can be controlled independently of each other, offers significantly more possibilities for the control. Therefore, it might also offer better part-load performance.

In this paper, the control schemes of three different small-scale gas turbines are compared. Especially, the part-load electrical efficiency is studied. The studied gas turbines are: a single-spool fixed speed, a single-spool variable speed driven, and a two-spool variable speed driven gas turbine. The part-load performance of different machines is studied and then compared against each other. Furthermore, some estimations are given on how the part-load performance of each machine fares against certain load profiles.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2017;():V008T26A021. doi:10.1115/GT2017-64359.

Automotive turbochargers play an important role in improving fuel economy, reducing emissions and improving drivability. Key to the improvement of the turbocharger performance is compressor efficiency. Compressors used in turbochargers are typically operated in a wide range of speed and flow. This wide operating range is a challenge to the design and improving the performance is often a fine balance between required efficiencies towards the surge, choke regions apart from having a comfortable speed margin for high altitude operations. In this study an existing compressor that best matched a 180hp commercial diesel engine application is chosen and its performance is further improved towards the lower flow region. Improvement is carried out through a set of designed experiments using a combination of Preliminary Design (PD) and Computational Fluid Dynamics (CFD) tools. Mechanical integrity of the wheel is ensured using Finite Element Analysis. A prototype is made out of the improved design and tested in an in-house gas stand. Predicted efficiency improvements are reflected in gas stand tests. Efficiency improvements in the lower flow range are observed over 7% while there is an acceptable drop (3.7%) near the peak power side. The improved compressor also shows higher part load efficiencies.

Commentary by Dr. Valentin Fuster
2017;():V008T26A022. doi:10.1115/GT2017-64361.

Decentralized electricity and heat production is a rising trend in small-scale industry. There is a tendency towards more distributed power generation. The decentralized power generation is also pushed forward by the policymakers.

Reciprocating engines and gas turbines have an essential role in the global decentralized energy markets and improvements in their electrical efficiency have a substantial impact from the environmental and economic viewpoints. This paper introduces an intercooled and recuperated three stage, three-shaft gas turbine concept in 850 kW electric output range. The gas turbine is optimized for a realistic combination of the turbomachinery efficiencies, the turbine inlet temperature, the compressor specific speeds, the recuperation rate and the pressure ratio. The new gas turbine design is a natural development of the earlier two-spool gas turbine construction and it competes with the efficiencies achieved both with similar size reciprocating engines and large industrial gas turbines used in heat and power generation all over the world and manufactured in large production series. This paper presents a small-scale gas turbine process, which has a simulated electrical efficiency of 48% as well as thermal efficiency of 51% and can compete with reciprocating engines in terms of electrical efficiency at nominal and partial load conditions.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2017;():V008T26A023. doi:10.1115/GT2017-64490.

Decentralized heat and power (CHP) production constitutes a promising solution to reduce the primary energy consumption and greenhouse gas emissions. Here, micro gas turbine (MGT) based CHP systems are particularly suitable due to their low pollutant emissions without exhaust gas treatment.

Typically, the electrical power demand for single houses ranges from 1 to several kWel. However, downsizing turbocharger components of a conventional MGT CHP system can reduce electrical efficiencies since losses like seal and tip leakages, generally do not scale proportionally with size. By introducing an inverted Brayton Cycle (IBC) based MGT this potential can be exploited. The IBC keeps the volumetric flows constant while mass flow and thermodynamic work are scaled by the ratio of pressure level. Since the performance of turbocharger components is mainly driven by the volumetric flow they should be applicable for both cycles. Hence, smaller power outputs can be achieved.

The overall aim of this work, is the development of a recuperated inverted MGT CHP unit for a single family house with 1 kWel. This paper presents an experimental study of the applicability and feasibility of a conventional MGT operated in IBC mode. The demonstrator was based on a single shaft, single stage conventional MGT. Reliable start up and stable operation within the entire operating range from 180 000 rpm to 240 000 rpm are demonstrated. The turbine outlet pressure varied between 0,5 bar (part load) and 0,3 bar absolute (full load). All relevant parameters such as pressure losses and efficiencies of the main components are investigated.

Moreover, the power output and the mechanical and thermal losses were analyzed in detail. Although the results indicated that the mechanical and heat losses have a high influence on the performance and economic efficiency of the system, the prototype shows great potential for further development.

Commentary by Dr. Valentin Fuster
2017;():V008T26A024. doi:10.1115/GT2017-64562.

The present work summarizes the design process of a new continuous closed-loop hot transonic linear cascade. The facility features fully modular design which is intended to serve as a test bench for axial micro-turbomachinery components in independently varying Mach and Reynolds numbers ranges of 0 – 1.3 and 2·104 – 6·105 respectively. Moreover, for preserving heat transfer characteristics of the hot gas section, the gas to solid temperature ratio (up to 2) is retained. This operational environment has not been sufficiently addressed in prior art, although it is critical for the future development of ultra-efficient high power or thrust devices.

In order to alleviate the dimension specific challenges associated with micro-turbomachinery, the facility is designed in a highly versatile manner, and can easily accommodate different geometric configurations (pitch, ±20° stagger angle, ±20° incidence angle), absent of any alterations to the test section. Owing to the quick swap design, the vane geometry can be easily replaced without manufacturing or re-assembly of other components. Flow periodicity is achieved by the inlet boundary layer suction and independently adjustable tailboard mechanisms. Enabling test-aided design capability for micro gas turbine manufacturers, aero-thermal performance of various advanced geometries can be assessed in engine relevant environments.

Commentary by Dr. Valentin Fuster
2017;():V008T26A025. doi:10.1115/GT2017-64628.

This work presents experimental and numerical investigations into the vibrations of turbocharger rotors on full-floating ring bearings with a circumferential oil-groove. The pressure distribution in the fluid-film bearings is calculated through the Reynolds equation using a highly efficient global Galerkin approach with suitable trial and test functions. The numerical efficiency of the method is markedly increased as the resultant linear system is solved symbolically, establishing a semi-analytical solution. The temperature in the oil-film may increase due to the mechanical power dissipation, affecting the pressure distribution and the load capacity of the bearing. Therefore, a reduced thermal energy model is implemented together with the Reynolds equation to account for the variable oil-viscosity and for the thermal expansion of the surrounding solids. The thermal energy balance equations are implemented in a transient form, i.e. including the time dependent temperature term. The corresponding system of nonlinear differential equations is efficiently solved, leading to a further significant reduction in simulation times. The hydrodynamic bearing model including the thermal effects is finally coupled with the equations of motion of a turbocharger rotor and numerical run-up simulations are compared with experimental results. The comparisons show that the numerical model captures adequately the dynamics of the system, giving precise information about the frequencies and the amplitudes of the synchronous and the self-excited subsynchronous rotor vibrations.

Commentary by Dr. Valentin Fuster
2017;():V008T26A026. doi:10.1115/GT2017-64695.

Approximately 30% of the energy from an internal combustion engine is rejected as heat in the exhaust gases. An inverted Brayton cycle (IBC) is one potential means of recovering some of this energy, in order to improve the overall system efficiency. When a fuel is burnt, water and CO2 are produced and expelled as part of the exhaust gases. In an IBC, in order to reduce compression work, the exhaust gases are cooled before compression up to ambient pressure. If coolant with a low enough temperature is available, it is possible to condense some of the water out of the exhaust gases, further reducing compressor work.

In this study the condensation of exhaust gas water is studied. The results show that the IBC can produce an improvement of approximately 5% in BSFC at the baseline conditions chosen and for a compressor inlet temperature of 310 K. The main factors that influence the power output are heat exchanger pressure drop, turbine expansion ratio, coolant temperature and turbine inlet temperature. A lower coolant temperature significantly increases power output, particularly when condensation occurs. Larger turbine expansion ratios produce more power and slightly lower the temperature at which condensation onset occurs. The system is very sensitive to heat exchanger pressure drop, as larger pressure drops increase the compressor pressure ratio whilst leaving the turbine expansion ratio unchanged. Higher turbine inlet pressures can also increase net power, but the higher exhaust backpressures may increase engine pumping losses.

Finally, for conditions when condensation is possible, the water content of the exhaust gas has a significant influence on power output. The hydrogen to carbon ratio of the fuel has the most potential to vary the water content and hence the power generated by the system. If there is no condensation, water content has a small impact on performance. The effect on power in the condensing region is predominantly due to reduced mass flow in the compressor.

Commentary by Dr. Valentin Fuster
2017;():V008T26A027. doi:10.1115/GT2017-64732.

Engine downsizing is a modern solution for the reduction of CO2 emissions from internal combustion engines. This technology has been gaining increasing attention from industry. In order to enable a downsized engine to operate properly at low speed conditions, it is essential to have a compressor stage with very good surge margin. The ported shroud, also known as the casing treatment, is a conventional way used in turbochargers to widen the working range. However, the ported shroud works effectively only at pressure ratios higher than 3:1. At lower pressure ratio its advantages for surge margin enhancements are very limited. The variable inlet guide vanes are also a solution to this problem. By adjusting the setting angles of VIGVs, it is possible to shift the compressor map towards the smaller flow rates. However, this would also undermine the stage efficiency, require extra space for installing the IGVs, and add costs. The best solution is therefore to improve the design of impeller blade itself to attain high aerodynamic performances and wide operating ranges. This paper reports a recent study of using inverse design method for the redesign of a centrifugal compressor stage used in an electric supercharger, including the impeller blade and volute. The main requirements were to substantially increase the stable operating range of the compressor in order to meet the demands of the downsized engine. The 3D inverse design method was used to optimize the impeller geometry and achieve higher efficiency and stable operating range. The predicted performance map shows great advantages when compared with the existing design. To validate the CFD results, this new compressor stage has also been prototyped and tested. It will be shown that the CFD predictions have very good agreement with experiments and the redesigned compressor stage has improved the pressure ratio, aerodynamic efficiency, choke and surge margins considerably.

Commentary by Dr. Valentin Fuster
2017;():V008T26A028. doi:10.1115/GT2017-64743.

As governments around the world ramp up their efforts to reduce CO2 emissions, downsizing internal combustion engines has become a dominant trend in the automotive industry. Air charging systems are being utilised to increase power density and therefore lower emissions by downsizing internal combustion engines. Turbocharging represents the majority of these air charging systems, which are commonly adopted for commercial and passenger vehicles. The process of matching turbomachinery to an engine during early-stage development is important to achieving maximum engine performance in terms of power output and the reduction of emissions.

Despite on-engine conditions providing highly unsteady gas flows, current turbocharger development commonly uses performance maps that are produced from steady state measurements. There are other significant sources of error to be found in early stage turbocharger performance prediction, such as the omission of heat transfer effects, and the use of data extrapolation methods to cover the entire operating range of a device from limited data sets. Realistic engine conditions provide a complex heat transfer scenario, which is dependent upon load history and the component layout of the engine bay. Heat transfer effects are particularly prevalent at low engine loads, whilst pulsating effects are significant at both high and low engine speeds (and therefore exhaust pulse frequency). Compressor maps are often provided by manufacturers with a level of heat transfer corresponding to a gas stand test, not realistic engine conditions. This causes a mismatch when using the aforementioned maps in commercial engine codes. This reduces the quality of overall engine performance predictions, since as the temperature of the exhaust gas on the turbine side rises, the performance prediction increasingly deviates from the usual adiabatic assumption used in simulations.

In the present work, a one-dimensional unsteady flow model has been developed to predict the performance of a vaneless turbine under pulsating inlet conditions, with scope to account for heat transfer effects. Flow within the volute is considered to be one-dimensional and unsteady, with mass addition and withdrawal used to simulate the gas flow between the volute and rotor. Rotor passages are also treated as one-dimensional and unsteady, with the equations being solved by the method of characteristics. This model is able to simulate the circumferential feeding of the rotor from the casing, unlike many previous zero and one-dimensional models. Building upon previous work, the basis of this code has been constructed in C++ with future integration with other modern gas dynamics codes in mind. By providing the appropriate instantaneous operating conditions at specified time intervals, a code such as this could theoretically negate the need for maps produced by steady-state data.

Commentary by Dr. Valentin Fuster
2017;():V008T26A029. doi:10.1115/GT2017-64825.

Despite engine turbocharging being a widespread technology, there are still drawbacks present in current turbocharging systems stemming from the apparent mismatch between the periodic operation of a piston engine operating in conjunction with an essentially steady-state, rotordynamic machine (turbocharger). The primary issue remains the provision of adequate transient response thereby suppressing the issue of turbocharger lag (turbo-lag) or the poor initial response of the turbocharger to driver-commanded, engine operating point changes due to its inertia. Another problem is engine-turbocharger matching and operation under pulsating conditions in the exhaust manifold and generally unsteady engine operating conditions. The exhaust flows of internal combustion engines are characterized by pulsating flows at constant engine speeds (local pulsating effect) as well as “global” unsteadiness during engine transient events. Because of the volute volume and the length of the flow path, this unsteadiness generates a phase shift between mass flow, temperature and pressure at rotor inlet, and a stronger circumferential variation of the rotor inlet condition than in steady flow conditions. The shift and the variation increase the losses in the turbine, resulting in lower turbine efficiency.

The current paper develops original concept work carried out at Brunel University London to develop an innovative fluid-dynamic design for an axial turbine for turbocharger application. An axial flow turbine coupled with a specially-designed, outflow volute, arranged in a non-classical way, are the target of this work. CFD analysis and 1D simulation of an engine coupled with the innovative turbine have been performed to highlight the design potential.

Commentary by Dr. Valentin Fuster
2017;():V008T26A030. doi:10.1115/GT2017-64839.

Engine oil lubricated (semi) floating ring bearing (S)FRB systems in passenger vehicle turbochargers (TC) operate at temperatures well above ambient and must withstand large temperature gradients that can lead to severe thermo-mechanical induced stresses. Physical modeling of the thermal energy flow paths and an effective thermal management strategy are paramount to determine safe operating conditions ensuring the TC component mechanical integrity and the robustness of its bearing system. On occasion, the selection of one particular bearing parameter to improve a certain performance characteristic could be detrimental to other performance characteristics of a TC system. The paper details a thermohydrodynamic model to predict the hydrodynamic pressure and temperature fields and the distribution of thermal energy flows in the bearing system. The impact of the lubricant supply conditions (pressure and temperature), bearing film clearances, oil supply grooves on the ring ID surface are quantified. Lubricating a (S)FRB with either a low oil temperature or a high supply pressure increases (shear induced) heat flow. A lube high supply pressure or a large clearance allow for more flow through the inner film working towards drawing more heat flow from the hot journal, yet raises the shear drag power as the oil viscosity remains high. Nonetheless, the peak temperature of the inner film is not influenced much by the changes on the way the oil is supplied into the film as the thermal energy displaced from the hot shaft into the film is overwhelming. Adding axial grooves on the inner side of the (S)FRB improves its dynamic stability, albeit increasing the drawn oil flow as well as the drag power and heat flow from the shaft. The predictive model allows to identify a compromise between different parameters of groove designs thus enabling a bearing system with a low power consumption.

Commentary by Dr. Valentin Fuster
2017;():V008T26A031. doi:10.1115/GT2017-64927.

Engine downsizing and down-speeding are essential in order to meet future US fuel economy mandates. Turbocharging is one technology to meet these goals. Fuel economy improvements must, however, be achieved without sacrificing performance. One significant factor impacting drivability on turbocharged engines is typically referred to as, “Turbo-Lag”. Since Turbo-lag directly impacts the driver’s torque demands, it is usually perceptible as an undesired slow transient boost response or as a sluggish torque response. High throughput turbochargers are especially susceptible to this dynamic and are often equipped with variable geometry turbines (VGT) to mitigate some of this effect. Assisted boosting techniques that add power directly to the TC shaft from a power source that is independent of the engine have been shown to significantly reduce turbo-lag. Single unit assisted turbochargers are either electrically assisted or hydraulically assisted. In this study a regenerative hydraulically assisted turbocharger (RHAT) system is evaluated. A custom designed RHAT system is coupled to a light duty diesel engine and is analyzed via vehicle and engine simulations for performance and energy requirements over standard test cycles. Supplier provided performance maps for the hydraulic turbine, hydraulic turbo pump were used. A production controller was coupled with the engine model and upgraded to control the engagement and disengagement of RHAT, with energy management strategies. Results show some interesting dynamics and shed light on system capabilities especially with regard to the energy balance between the assist and regenerative functions. Design considerations based on open loop simulations are used for sizing the high pressure accumulator. Simulation results show that the proposed RHAT turbocharger system can significantly improve engine transient response. Vehicle level simulations that include the driveline were also conducted and showed potential for up to 4% fuel economy improvement over the FTP 75 drive cycle. This study also identified some technical challenges related to optimal design and operation of the RHAT. Opportunities for additional fuel economy improvements are also discussed.

Topics: Turbochargers
Commentary by Dr. Valentin Fuster
2017;():V008T26A032. doi:10.1115/GT2017-64960.

To secure the highly challenging 2°C climate change limit, the automotive sector is expected to further improve the efficiency of the internal combustion engines. Over the past decade, internal combustion engine downsizing through turbocharging has become one of the major solutions that the industry has offered to fulfil its carbon commitment. Although the various new turbocharging technologies has changed the sluggish image of conventional turbocharged engines, the turbocharger system is far from perfect. From the perspective of engine energy flow, the copious amount of waste energy is habitually harvested by the turbine with low efficiency, subsequently the turbine power transmitted to the compressor is used solely to charge the engine. When this power for charging is excessive for the set engine operating condition, it either is consumed by throttling or is directly discharged through the wastegate, both as a pure enthalpy loss. To more efficiently harness the waste energy without deteriorating other engine performance parameters, a full electric turbocharging technology is proposed by Aeristech Ltd. The system is composed of an electric turbo generator and an electric compressor connected only through electrical system. Without the constraint of a mechanical turboshaft, the compressor and the turbine can be operated at different speeds. The electrically driven compressor can be free floating when boost is not required and the motor can provide the boost promptly only when higher load is requested. Meanwhile, the electric turbine can be controlled by the generator to operate at any set speed, allowing maximum efficiency for energy harvesting. This paper presents a simulation study of the capability of the decoupled eTurbocharging system to charge a highly boosted 2 litre gasoline engine. The single stage eTurbocharger configuration and the eTurbocharger plus a mechanical turbocharger configuration were evaluated and compared. The simulation results have revealed that the two stage eTurbocharging system has the potential to reduce CO2 emission in the proximity of 1 percent in different drive cycles compared to conventional wastegate turbocharger and the benefit would be much higher for future real world driving cycle. The single stage configuration was shown to be impractical in that the power level of the turbine generator will not only limit the engine power output, but also have negative impact on the system design, cooling and cost implied. Meanwhile, the two stage configuration where the eCompressor acts as a supplementary boost device at low end and transient device came out as a better solution with overall advantage in manageable power level, system efficiency, transient response and implied cost.

Commentary by Dr. Valentin Fuster

Steam Turbines

2017;():V008T29A001. doi:10.1115/GT2017-63059.

Ultra-supercritical (USC) coal-fired steam turbines, as the representative of fuel-efficient power generation technology, occupies a large portion of power generation capacity in China. During the design process of USC steam turbines, high temperature creep deformation is a focusing topic, usually predicted by finite element method (FEM). In-service test data of a specific USC power plant during nine-year operation are used to validate the predicted results based on FEM. Toward this end, the components in HP turbine of a specific USC steam turbine (600 °C/26.2MPa) is chosen for the FEM modeling. The measurement of the creep behavior inducing plastic deformation of the components was carried out during the major overhaul of the power plant. Comparison of the results between the simulation and the measurement disclosed an accepted agreement, and the accumulated test data and the experience of the simulation method would be of great help for the further design of the components and the operation of the power plant.

Commentary by Dr. Valentin Fuster
2017;():V008T29A002. doi:10.1115/GT2017-63130.

The aim of this paper is to present advances in the blading design for large steam turbines — ‘Controlled Flow’ technology. The purpose of the design is to improve the turbine efficiency in a cost neutral manner, adding value for the customer. Controlled Flow is a 3D design philosophy which passes more flow through the efficient middle sections of the blade, and less flow through the comparatively inefficient regions near the endwalls. It has been used for the Impulse Technology Blading ITB guide blades. The current improvement builds on the previous successful Controlled Flow design but incorporates the following new features:

- Ultra High Lift for the mean section (at significantly reduced axial width)

- Ultra High Back Surface Deflection for the mean section

- forward leading edge sweep.

The new guide delivers the same radial distribution of absolute fluid exit angle to the runner as the previous design.

Confirmatory model turbine tests demonstrated that the new guide delivered a stage efficiency improvement of 0.35%, above an already very high datum level.

The endwall sections of the guide are kept the same which maintains the mechanical strength of the diaphragm (same stress and deflection). Therefore, the new design can easily ‘slot-in’ and replace the previous design.

The following will be described in detail:

- History of the designs/background and design philosophy

- Flow physics

- Stage optimization and performance prediction

- Probabilistic analysis and robustness of the design

- Confirmatory model turbine testing and validation (comparison with design predictions).

Commentary by Dr. Valentin Fuster
2017;():V008T29A003. doi:10.1115/GT2017-63224.

In 2013, with the operation of 1000MW steam turbine unit in a power plant, the exhaust steam temperature of intermediate pressure (IP) steam turbine exceeded the normal value. After overhaul of the IP steam turbine, a long crack was found through the drain hole of the IP inlet diffuser. The reheat steam leaked out from inside the diffuser to the exhaust cavity. It caused the exhaust temperature to exceed the design value, and reduced the efficiency of the units. Also, if the high temperature steam, which is close to 600°C, came into contact with the outer casing, a significant reduction in life would be obtained. In order to find the root cause of the crack, many efforts have been done within each department of the shanghai turbine plant (STP). Such as material detection, design, manufacture, assembling, etc.

In this paper, a three-dimensional finite element diffuser model was built and the stress state during startup and operation was obtain. It was found that the drain hole at the inside and the groove at the outside of the diffuser are dangerous regions because of the stress concentration. That’s the primary reason for causing the diffuser crack. Then the fracture mechanics software ZenCrack was used to analyze the diffuser crack route. This crack route consistently matched the actual diffuser crack.

Commentary by Dr. Valentin Fuster
2017;():V008T29A004. doi:10.1115/GT2017-63269.

The performance of the axial-radial diffuser downstream of the last low-pressure steam turbine stages and the losses occurring subsequently within the exhaust hood directly influences the overall efficiency of a steam power plant. It is estimated that an improvement of the pressure recovery in the diffuser and exhaust hood by 10% translates into 1% of last stage efficiency [11].

While the design of axial-radial diffusers has been the object of quite many studies, the flow phenomena occurring within the exhaust hood have not received much attention in recent years. However, major losses occur due to dissipation within vortices and inability of the hood to properly diffuse the flow. Flow turning from radial to downward flow towards the condenser, especially at the upper part of the hood is essentially the main cause for this.

This paper presents a detailed analysis of the losses within the exhaust hood flow for two operating conditions based on numerical results. In order to identify the underlying mechanisms and the locations where dissipation mainly occurs, an approach was followed, whereby the diffuser inflow is divided into different sectors and pressure recovery, dissipation and finally residual kinetic energy of the flow originating from these sectors is calculated at different locations within the hood.

Based on this method, the flow from the topmost sectors at the diffuser inlet is found to cause the highest dissipation for both investigated cases. Upon hitting the exhaust hood walls, the flow on the upper part of the diffuser is deflected, forming complex vortices which are stretching into the condenser and interacting with flow originating from other sectors, thereby causing further swirling and generating additional losses.

The detailed study of the flow behavior in the exhaust hood and the associated dissipation presents an opportunity for future investigations of efficient geometrical features to be introduced within the hood to improve the flow and hence the overall pressure recovery coefficient.

Commentary by Dr. Valentin Fuster
2017;():V008T29A005. doi:10.1115/GT2017-63280.

The influence of a cylindrical strut shortly downstream of the bladerow on the vibration behavior of the last stage rotor blades of a single stage LP model steam turbine was investigated in the present study. Steam turbine retrofits often result in an increase of turbine size, aiming for more power and higher efficiency. As the existing LP steam turbine exhaust hoods are generally not modified, the last stage rotor blades frequently move closer to installations within the exhaust hood. To capture the influence of such an installation on the flow field characteristics, extensive flow field measurements using pneumatic probes were conducted at the turbine outlet plane. In addition, time-resolved pressure measurements along the casing contour of the diffuser and on the surface of the cylinder were made, aiming for the identification of pressure fluctuations induced by the flow around the installation. Blade vibration behavior was measured at three different operating conditions by means of a tip timing system.

Despite the considerable changes in the flow field and its frequency content, no significant impact on blade vibration amplitudes were observed for the investigated case and considered operating conditions. Nevertheless, time-resolved pressure measurements suggest that notable pressure oscillations induced by the vortex shedding can reach the upstream bladerow.

Commentary by Dr. Valentin Fuster
2017;():V008T29A006. doi:10.1115/GT2017-63325.

The 1500-r/min 1905mm (75inch) ultra-long last three stage blades for half-speed large-scale nuclear steam turbines of 3rd generation nuclear power plants have been developed with the application of new design features and Computer-Aided-Engineering (CAE) technologies.

The last stage rotating blade was designed with an integral shroud, snubber and fir-tree root. During operation, the adjacent blades are continuously coupled by the centrifugal force. It is designed that the adjacent shrouds and snubbers of each blade can provide additional structural damping to minimize the dynamic stress of the blade. In order to meet the blade development requirements, the quasi-3D aerodynamic method was used to obtain the preliminary flow path design for the last three stages in LP (Low-pressure) casing and the airfoil of last stage rotating blade was optimized as well to minimize its centrifugal stress. The latest CAE technologies and approaches of Computational Fluid Dynamics (CFD), Finite Element Analysis (FEA) and Fatigue Lifetime Analysis (FLA) were applied to analyze and optimize the aerodynamic performance and reliability behavior of the blade structure.

The blade was well tuned to avoid any possible excitation and resonant vibration. The blades and test rotor have been manufactured and the rotating vibration test with the vibration monitoring had been carried out in the verification tests.

Commentary by Dr. Valentin Fuster
2017;():V008T29A007. doi:10.1115/GT2017-63401.

The determination of the aerodynamic damping is a major task in predicting flutter stability and therefore safety margins for turbine operation. Throughout the current work the energy method is employed to predict the aerodynamic damping for a last stage rotor blade numerically. The focus is put on the prediction of the aerodynamic damping with different traveling wave mode representations and on the influence of the blade fixation at the root. The Fourier transformation-method, the influence-coefficients-method and a direct traveling wave mode calculation are employed.

The investigated rotor geometry was taken from the open literature, a root was designed and an iterative process was installed to determine the cold blade geometry. It became apparent, that the influence-coefficients-method is capable of predicting the overall stability curve computationally efficient, whereas the Fourier-transformation-method showed advantages in the identification of the least stable point for a finer mesh. Nevertheless, all methods predicted a potential flutter risk for the current operating point. The influence of the additional blade root with a completely fixed support on the aerodynamic damping is minor.

Commentary by Dr. Valentin Fuster
2017;():V008T29A008. doi:10.1115/GT2017-63404.

An evaluation method for CFD simulations is presented, which allows an in-depth analysis of different loss mechanisms applying the approach of entropy creation proposed by Denton. The entropy creation within each single mesh element is determined based on the entropy flux through the cell faces and therefore the locations, where losses occur, can be identified clearly. By using unique features of the different loss mechanisms present in low pressure steam turbines, the losses are categorized into boundary layer, wake mixing and shock losses as well as thermodynamic wetness losses.

The suitability of the evaluation method is demonstrated by means of steady state CFD simulations of the flow through a generic last stage of a low pressure steam turbine. The simulations have been performed on streamtubes extracted from three-dimensional simulations representing the flow at 10 % span. The impact of non-equilibrium steam effects on the overall loss composition of the stator passage is investigated by comparing the results to an equilibrium steam simulation. It is shown, that the boundary layer losses for the investigated case are of similar magnitude, but the shock and wake losses exhibit significant differences.

Commentary by Dr. Valentin Fuster
2017;():V008T29A009. doi:10.1115/GT2017-63405.

Today’s power market asks for highly efficient turbines which can operate at a maximum flexibility, achieving a high lifetime and all of this on competitive product investments. In line with the demands for reduction of CO2, the machine efficiencies are continuously increased. To further increase efficiencies, deeper insight into the single components is required to better understand the individual contributions to the overall performance. This work focuses on the measurement of leakages through High Pressure Steam Turbine gland sealing. Gland sealing technologies are frequently in focus of research and development activities. Various concepts were and are still developed and proven to work in test rigs. Typically such test rigs form an idealized environment with ideal manufacturing and low tolerances compared to real plant components. For this work real gland sealing components from a power plant were taken, a test rig developed around them and the different leakage path massflow rates determined. The advantage of using real plant components is i.e. in the real life relevance of the gained results by considering real design manufacturing tolerances, surface qualities etc.. By testing numerous gland segments from manufacturing and installing them in different orders, even statistical relevant information may be gained with respect to the influence of manufacturing on the seal tightness. This work presents the development of a test rig around a real plant gland seal and the tested leakages through the various paths of the tested component. The here gained information enables further development and optimization of advanced sealing technologies.

Commentary by Dr. Valentin Fuster
2017;():V008T29A010. doi:10.1115/GT2017-63466.

This paper presents the findings of the original nozzle of the fourth stage in a 7 stage LP steam turbine where an obvious geometric feature is the extremely thick leading edge (LE). A cascade test was carried out to investigate the mechanism of loss reduction. A detailed comparison study was carried out using a conventional thinner leading edge design and the original thicker LE profile. The studies reveal that the overall loss in the original design is significantly lower than the counterpart of the thinner LE option together with a much wider range of incidence for which the vane is of low loss. This design philosophy is then successfully cloned to the first stage and third stage nozzles in a seven stage LP steam turbine. The analysis indicates that the obvious advantages of the new designs over the conventional thinner option are on not only the reduction of the profile loss, but the reduction of the blade count which has a significant implication on manufacturing cost. The numerical studies reveal that the idea behind this thick LE design philosophy is to minimise the profile loss without incurring a significant penalty on diffusion loss or at the worst separation. A detail investigation on the stage 2 nozzle indicates that this concept only works for a reasonably high aspect ratio blading where the secondary loss is limited.

Commentary by Dr. Valentin Fuster
2017;():V008T29A011. doi:10.1115/GT2017-63502.

High performance of the last stage long blade plays an important role on the aerodynamic performance of low pressure cylinder for steam turbines. Aerodynamic optimization design of the last stage long blade for the maximization total-total isentropic efficiency with constraints of mass flow rate and leaving velocity using self-adaptive differential evolution algorithm is presented in this work. The aerodynamic performance of last stage is evaluated using three-dimensional Reynolds-Averaged Navier-Stokes (RANS) computations. Six two-dimensional airfoils along the span and three controlling points for the radial foil of blade using B-Spline functions are used to parameterize the three-dimensional profiles of the stator and rotor blade of the last stage, respectively. Self-adaptive differential evolution algorithms is developed to optimize the maximization total-total isentropic efficiency of last stage. The results show that the total-total isentropic efficiency of the optimized last stage is higher 1.68% than that of the referenced design. Furthermore, the aerodynamic performance of the five stages low pressure cylinder with three extractions coupled with the optimized last stage and referenced design is analyzed and compared. The detailed flow field and aerodynamic parameters of the optimized last stage are also illustrated.

Commentary by Dr. Valentin Fuster
2017;():V008T29A012. doi:10.1115/GT2017-63547.

In order to achieve the highest power plant efficiency, original equipment manufacturers (OEMs) continuously increase turbine working parameters (steam temperatures and pressures), improve components design and modify start-up cycles to reduce time while providing more frequent start-up events. All these actions result in much higher levels of thermo-stresses, a lifetime consumption of primary components and an increased demand for accurate thermo-structural and LCF simulations.

In this study, some aspects of methodological improvement are analyzed and proposed in the frame of an integrated approach for steam turbine components thermo-structural analysis, reliability and lifetime prediction. The full scope of the engineering tasks includes aero/thermodynamic flow path and secondary flows analysis to determine thermal boundary conditions, detailed thermal/structural 2D and 3D FE models preparation, components thermal and stress-strain simulation, rotor-casing differential expansion and clearances analysis, and finally, turbine unit lifetime estimation. Special attention is paid to some of the key factors influencing the accuracy of thermal stresses prediction, specifically, the effect of ‘steam condensation’ on thermal BC, the level of detailing for thermal zones definition, thermal contacts and mesh quality in mechanical models. These aspects have been studied and validated against test data, obtained via a 30 MW steam turbine for combined cycle application based on actual start-up data measured from the power plant. The casing temperatures and rotor-stator differential expansion, measured during the commissioning phase of the turbine, were used for methodology validation. Finally, the evaluation of the steam turbine HPIP rotor lifetime by means of a low cycle fatigue approach is performed.

Commentary by Dr. Valentin Fuster
2017;():V008T29A013. doi:10.1115/GT2017-63550.

During start-up and coast-down of the turbine train large last stage moving blades may show elevated amplitude levels. In cases where the excitation mechanism results from a rotor imbalance, it is well known that the occurring amplitude level can be influenced by the turbine set-up as well as the mode of operation during start-up and coast-down. For example, by increasing speed gradients the exposure time in which the blade row is operated at or close to speed-synchronous resonance is reduced and hence the excitation is minimized.

To evaluate the blade response during transient operation an analytical model is developed which allows for an accurate description of the exciting inertia forces resulting from rotor imbalance. In the present paper, the effect of a small-scale variation of the tuned blades’ resonance frequency and intentional mistuning of the blade row, (Siewert & Stüer, 2014), on the obtainable resonance amplitudes and the potentially accumulated fatigue is assessed. The results show a significant increase in robustness against transient excitation by application of a dedicated tuning, allowing for enhanced flexible operation of large last stage moving blades.

Commentary by Dr. Valentin Fuster
2017;():V008T29A014. doi:10.1115/GT2017-63555.

Nowadays, steam turbines in conventional power plants deal with an increasing number of startups due to the high share of fluctuating power input of renewable generation. Thus, the development of new methods for flexibility improvements, such as reduction of the start-up time and its costs, have become more and more important. At the same time, fast start-up and flexible steam turbine operation increase the lifetime consumption and reduce the inspection intervals. One possible option to prevent these negative impacts of a flexible operation is to keep the steam turbine warm during a shut down and a startup. In order to do so, General Electric has developed a concept for warm-keeping respectively pre-warming of a high-pressure (HP) / intermediate-pressure (IP) steam turbine with hot air: After a certain cool-down phase, air is passed through the turbine while the turbine is rotated by the turning engine. The flow and the rotational direction can be inverted to optimize the warming operation. In order to fulfill the requirements of high flexibility in combination with reduced costs and thermal stresses during the start-up, a detailed investigation of the dominant heat transfer effects and the corresponding flow structure is necessary: Complex numerical approaches, such as Conjugate Heat Transfer (CHT), can provide this corresponding information and help to understand the physical impact of the flow phenomena.

The aim of the present work is thus to understand the predominant heat transport phenomena in warm-keeping operation and to gain detailed heat transfer coefficients within the flow channel for blade, vane and shrouds. A multitude of steady-state simulations were performed to investigate the different warm-keeping operation points. Data from literature was recomputed in good agreement to qualitatively validate the numerical model in windage operation. Furthermore, the steady-state simulations were compared with transient Computational Fluid Dynamics (CFD) simulations to verify that the flow in warming operation can be simulated with a steady-state case. The transient calculations confirm the steady-state results. A variation of the mass flow rate and the rotational speed was conducted to calculate a characteristic map of heat transfer coefficients. The Conjugate Heat Transfer simulations provide an insight into the flow structure and offer a comparison with the flow phenomena in conventional operation. In addition, the impact of the flow phenomena on the local heat transfer was investigated.

Commentary by Dr. Valentin Fuster
2017;():V008T29A015. doi:10.1115/GT2017-63576.

The last-stage blade (LSB) rows and exhaust hood in low-pressure (LP) steam turbine sections are key elements of the entire LP turbine part. The cold end section affects significantly the whole LP turbine efficiency and overall turbine performance due to huge steam expansion. This expansion is strongly coupled with the diffuser and exhaust hood, which transforms kinetic energy at the stage exit into potential energy. Mentioned mechanism leads to expansion line prolongation between the stage inlet and diffuser outlet and higher turbine power output.

An experimental investigation of the flow field in the exhaust hood is very economically and procedurally expensive and not commonly feasible. Nowadays, capable numerical simulations can provide relatively fast and accurate results on any studied model. On the other hand, the flow behavior inside the LSB and the exhaust hood is very complex and it is still challenging to investigate the whole system using CFD codes.

The purpose of this paper is to validate complex three-dimensional CFD methodology of the flow field in the operating 1 090 MW steam turbine exhaust hood with radial diffuser and condenser neck. The exceptional contribution of this paper is the fact that unique data obtained by measurement on operating Nuclear Power Plant (NPP) steam turbine are available. The comparison is focused mainly on the pressure, velocity and steam wetness distribution along the LSB height at the stage exit/diffuser inlet. Wall static pressures and the pressure recovery coefficient of the exhaust hood were also determined and compared with experimental data. The complete CFD study helps to understand the flow behavior inside the whole exhaust throat and locate critical parts that negatively affect aerodynamic design.

Commentary by Dr. Valentin Fuster
2017;():V008T29A016. doi:10.1115/GT2017-63592.

Flexible operations of steam turbines with faster startups and shutdowns are required to accommodate emerging renewable power generations, needing more advanced prediction tools for transient thermal design and analysis. A major challenge is the time scale disparity. For a natural cooling, the physical process is typically in hours or tens of hours, but on the other hand, the time step sizes typically usable tend to be very small (in seconds or sub-seconds) due to the numerical stability requirement for natural convection as often observed. A general issue to be addressed is what time step sizes can be and should be used in terms of stability as well as accuracy.

In the present work, the impact of the temporal gradient in unsteady flow and its modelling is examined in relation to numerical stability and modelling accuracy for natural convectio n. A source term based dual timing for mulation is adopted and implemented in a commercial code, which is shown to be numerically stable for very large time steps for natural convection analysis. Furthermore, a loosely coupled partitioned procedure is developed to combine this enhanced flow solver together with a solid conduction solver for solving transient conjugate heat transfer problems for natural convection. This allows very large computational time steps to be used without any stability issues, and thus enables to assess the impact of using different time step sizes entirely in terms of the temporal accuracy requirement. Computational case studies demonstrate that the present method is more stable at a markedly shortened computational time than the baseline solver. The method is also shown to be more accurate than the commonly adopted quasi-steady methods when unsteady effects are non-negligible.

Commentary by Dr. Valentin Fuster
2017;():V008T29A017. doi:10.1115/GT2017-63608.

Many power plant components and joint connections are subjected to complex thermo-mechanical loading paths under severe temperature environments over a long period. An important part in the lifetime assessment is the reliable prediction of stress relaxation using improved creep modeling to avoid possible integrity or functionality issues and malfunction in such components.

The aim of this work is to analyze the proposed constitutive model for advanced high chromium steels with the goal of predicting stress relaxation over the long term. The evolution equations of the constitutive model for inelastic material behavior are introduced to account for hardening and softening phenomena. The material properties were identified for 9–12%CrMoV steels in the creep range.

The model is applied to the stress relaxation analysis of power plant components. The results for long-term assessment, which are encouragingly close to reality, will be presented and discussed. An outlook on further developments of the model and assessment procedure is also provided.

Commentary by Dr. Valentin Fuster
2017;():V008T29A018. doi:10.1115/GT2017-63630.

Last stage blade rows of modern low pressure steam turbines are subjected to high static and dynamic loads. The static loads are primarily caused by the centrifugal forces due to the steam turbine’s rotational speed. Dynamic loads can be caused by instationary steam forces, for example. A primary goal in the design of modern and robust blade rows is to prevent High Cycle Fatigue caused by dynamic loads due to synchronous or non-synchronous excitation mechanisms. Therefore, it is important for the mechanical design process to predict the blade row’s vibration response. The vibration response level of a blade row can be limited by means of a damping element coupling concept. Damping elements are loosely assembled into pockets attached to the airfoils. The improvement in the blade row’s structural integrity is the key aspect in the use of a damping element blade coupling concept. In this paper, the vibrational behavior of a last stage blade row with damping elements is analyzed numerically. The calculation results are compared to results obtained from spin pit measurements for this last stage blade row coupled by damping elements.

Commentary by Dr. Valentin Fuster
2017;():V008T29A019. doi:10.1115/GT2017-63665.

Stress corrosion cracking in steam turbines had been an old problem though some modern steam turbines have almost eliminated this problem by several methods. The methods include design modification to reduce the stress levels below the threshold stress level for stress corrosion cracking, inducing compressive stress by different means and using pure steam [1, 2]. Some of the older steam turbine discs are prone to stress corrosion cracking. Two cases where such machines experienced stress corrosion cracking in their discs are discussed here.

The row 6 disc of an integral steam turbine rotor developed cracks in the root sections. Some of the cracks were mechanically opened for the evaluation. Evaluation of the fracture surfaces with a scanning electron microscope showed evidence of intergranular mode of cracking. Optical microscopy of a cracked root confirmed intergranular mode of cracking. In addition, it showed branching of cracks. Based on these findings, it was concluded that stress corrosion cracking was the reason for the cracks. In addition, finite element analysis was used to calculate the stress distribution in the blade root of the disc. The location of the maximum equivalent stress coincided perfectly with that of the actual crack location in the disc root section. Unfortunately, redesign of the root geometry to minimize the local stress concentration is very difficult due to the size limitation of the blade roots. Small amount of chlorine was identified on the fracture surface and the chlorine could have come from the steam used. The customer was advised to analyze their steam quality and to improve the quality of the steam if needed. The cracked portion was removed from the disc and weld-build up to machine new root sections with the same type of roots.

Root section of the row 6 disc of another steam turbine developed failure. This disc had radial entry type blades. Portion of the disc root and some blades were liberated from the disc due to the cracking. The fracture surface had heavy oxide layer on it. Evaluation of the fracture surface with a scanning electron microscope revealed intergranular mode of failure. Energy dispersive spectroscopy analysis of the fracture surface found oxides on the fracture surface. Optical microscopy showed secondary cracking and branched cracking. All these evidences confirmed that the failure occurred due to stress corrosion cracking. In addition, it was suspected that forging was not heat treated properly due to measured lower toughness and different microstructure. The lower toughness was believed to be a result of improper heat treatment rather than that of embrittlement. Methods to mitigate the risk of stress corrosion cracking were proposed.

Commentary by Dr. Valentin Fuster
2017;():V008T29A020. doi:10.1115/GT2017-63667.

This paper investigates the validity of the current industrial procedure of measuring optimized blade profiles in a wind tunnel under air condition although they are applied in a steam turbine. Therefore, it is important to analyze the possibility of using air-measured profile data for optimizing steam turbine blades. To this end, experimental data is collected using the cylindrical datum blade of a steam turbine in a three-stage high pressure steam turbine and in an annular air cascade wind tunnel. Three-dimensional CFD simulations are separately performed for both setups and show a good agreement with the experimental data. The numerical simulations can therefore be assumed to represent the real flow conditions.

Firstly, for analyzing aerodynamic transferability, two optimized profiles are measured in the annular air cascade wind tunnel at Reynolds number of 6 × 105. These profile sections are designed for high and intermediate pressure applications by employing an optimizer. The optimization is performed with the focus on reducing the profile loss for steam conditions. The experimental data verifies that the losses of the optimized profiles are reduced significantly compared to the datum blade profile measured in the same air rig. Secondly, the air-measured optimized blade profiles are used to design a 3D-optimized blade. In a numerical investigation, this optimized blade is analyzed in the steam turbine by applying steam conditions. The outlet Reynolds number of the 2nd stage is 8 × 105. This configuration is compared with the numerical results of the datum blade profile simulations.

The relative isentropic total-to-total efficiency is increased by 0.6% due to the use of the optimized rotor blades. The benefit persists also for a maximum outlet Reynolds number of 9 × 106.

Commentary by Dr. Valentin Fuster
2017;():V008T29A021. doi:10.1115/GT2017-63902.

Taking a CPR 1000 nuclear steam turbine HP casing as an example, this paper carried out the numerical study to investigate on the deformation of the casing during shutdown.

Based on ProE software, a 3D model for the HP casing was established at first. Secondly, the heat transfer coefficient of every surface of HP casing of the shutdown was calculated by using convection heat transfer coefficient formula in detail, ignoring radiative heat transfer inside the casing. Then, the finite element method was used to calculate and analyze the shutdown of HP casing’s temperature field. In the calculation process, the third boundary condition was used to apply steam temperature and heat transfer coefficient to each area of the HP casing. Finally, the calculation results of the temperature field were used as the input conditions to calculate the stress and strain of the casing, and the possible reasons for the HP casing’s deformation were obtained.

Commentary by Dr. Valentin Fuster
2017;():V008T29A022. doi:10.1115/GT2017-63946.

The solid particle erosion (SPE) of flow passage is a universal problem in modern high-parameter steam turbines. With the continuous improvement of the working parameters of the steam turbine, the problem of SPE is becoming more serious. This problem is caused by the ferric oxide exfoliations carried by steam from the inner wall of the boiler tube into the steam turbine flow passage, causing the stator blades, the rotor blades, and the shroud to be eroded under impingement and scuffing failure. The SPE cannot only destroy the blade profile, increase the roughness of the blade surface, and affect the aerodynamic performance of the blade, but it can also shorten the maintenance cycle, prolong the maintenance downtime, and even increase the cost for steam turbine maintenance thereby reducing the unit efficiency and safety.

In order to simulate SPE in the governing stage of a high-parameter steam turbine, this study adopts the Lagrange method and the Finnie erosion model. The motion characteristics of five different kinds of solid particle, including the solid particle trajectory, are thoroughly analyzed. The regulation of the erosion distribution in the radial and axial directions to the stator and rotor blades is studied to present the mechanism of SPE.

Simulated results show that before their collision with the blades, the particles of the small diameters flow with the main stream, and their trajectories are close to the steam streamlines. By contrast, the particles of the large diameters are hardly influenced by the external factors, and their trajectories are close to the straight line. The SPE distribution of the stator and rotor blades varies with the particle diameter. The eroded area in the stator blade is mainly located at the leading edge and the pressure surface, particularly the mid-rear part of the pressure surface, whereas no eroded area can be observed in the suction surface. The small particles greatly affect the erosion distribution of the stator blade. The eroded area in the rotor blade is primarily at the mid-rear part of the pressure surface and the suction surface, which is close to the leading edge. The eroded area takes on a typical slop shape, and the erosion position has an obvious upward trend.

The proposed research reveals both the motion characteristics of the solid particles and the distribution regulation of the SPE in the steam turbine flow passage. The analysis results provide references for the governing stage of a high-parameter steam turbine to prevent SPE.

Commentary by Dr. Valentin Fuster
2017;():V008T29A023. doi:10.1115/GT2017-63964.

Kinetic energy recovery is a key objective for low pressure exhaust hood design and optimization. Numerical simulation of the exhaust hood helps the engineers to explore and confirm the causes of the loss in the hood. Many studies have suggested that it is necessary for the simulation to include the last stage blade to get a realistic assessment. For the sole exhaust hood study, the inlet boundary condition is hard to set precisely like the downstream flow of the last stage blade. And the studies have also shown that the performances generated from the simulations may vary evidently between the sole exhaust hood and exhaust hood with last stage blade. It is obvious that the blade influences the exhaust hood, but the exact effect factors of the blade and the way they work are not thoroughly discussed. This paper has conducted many numerical tests to audit the influence of the common effect factors of the last stage blade. The internal flow field of the exhaust hood was numerically investigated using three-dimensional Reynolds-Averaged Navier-Stokes (RANS) solutions based on the ANSYS-CFX. In the first part of the paper, the tests are conducted by changing each effect factor of the inlet boundary condition for sole exhaust hood studies. These factors include the mass flow flux, the angle of the exit flow of the last stage, both the circumferential and the radial ones, and the speed and position of the jet-flow downstream of the seal over the shroud of the bucket. The tests show that each factor has its own distinctive style and extent for influence. Some of them may maximize the performance at some certain point, and some may deteriorate the performance rapidly beyond a threshold. And some factors may change the performance insignificantly within a wide range. However, these influences are not good enough to be consistent with the difference between the sole exhaust hood and the hood with blade simulations. In the second part of this paper, the focus locates on the direction of the jet-flow of the bucket seal. The tests prove that this direction is the prominent factor to influence the exhaust hood performance. Some extra tests for the seal have also been conducted to analyze this factor. The static pressure recovery for the simulation with labyrinth seal is about only half of the sole exhaust hood simulation. The discussion of these tests show that the seal jet is the main cause for this performance dive, and explain how the seal jet direction changes the flow field of the exhaust hood. It also suggests that the procedure to optimize the seal design is not mature yet, for some nature of the jet-flow remains unclear. It may need more detailed study in the future.

Commentary by Dr. Valentin Fuster
2017;():V008T29A024. doi:10.1115/GT2017-64021.

Blade failure caused by flutter is a major problem in the last stage of modern steam turbines. It is because rotor at this stage always has a large scale in spanwise, which provides low structural frequency as well as supersonic tip speeds. Since most of the unsteady aerodynamic work is done in the tip region, transonic tip-leakage flow that influences the tip region flow could have a remarkable effect on the aerodynamic stability of rotor blades. However, few research had been done on the tip-leakage flow influence on flutter characteristic based on full-scale steam turbine numerical models. In this paper, an open 3D steam turbine stage model designed by Durham University was applied, which was widely analyzed and representative for the last stage of modern industrial steam turbines. The average Mach number at the rotor outlet is 1.1. URANS simulation carried by both numerical software CFX and LUFT code is applied, and the two solvers show an agreement on steady and unsteady results. The numerical results indicate that the influence of tip leakage flow on blade stability is based on two types of flow mechanisms. Both mechanisms act on the suction side of near tip region. The first type of mechanism is produced by the reduction of passage shock near the leading edge, and the other type of mechanism at the rear of blade is caused by the interaction between tip leakage vortex and trailing edge shock of the neighbor blade. In conclusion, tip leakage flow has a significant influence on steam turbine flutter boundary prediction and requires further analysis in the future.

Commentary by Dr. Valentin Fuster
2017;():V008T29A025. doi:10.1115/GT2017-64047.

All turbine blades have mistuned structures caused by manufacturing variations within the manufacturing tolerance, such as the geometrical deviations and variance of material properties. The mistuning effect has a known tendency to increase the dynamic stress, but it is also known to be difficult to predict the maximum vibration response before the operation. This paper studies the blade vibration of grouped blades in a low-pressure steam turbine. The study objectives are to characterize the vibration behavior of the grouped blade structure and to evaluate the maximum response of all blades in a stage by experiments.

An experimental investigation is carried out in a vacuum chamber, and blades are excited by an air jet during start-up and shut-down. The circumferential blade amplitude distribution is measured by non-contact sensors and strain gauges. The circumferential blade amplitude distribution is found to differ depending on vibration modes and nodal diameters, but the relative tendency is almost the same for all types of operation at each mode and all nodal diameters.

Therefore, the median of all experimental results obtained with the non-contact sensors are used in a comparison with calculation results and two theoretical curves obtained using equations from the literature. In comparing the measurement results and the calculation results, the circumferential blade amplitude distribution is not the same with all modes and nodal diameters. However, the maximum amplitude magnification is about 1.5–1.8, and all measurement results are lower than the results for the two theoretical equations. This means the maximum response compared to the tuned blade can be evaluated on the safe side by the two theoretical equations.

Commentary by Dr. Valentin Fuster
2017;():V008T29A026. doi:10.1115/GT2017-64133.

As the landscape of energy production in Europe and other developed countries undergoes a rapid shift towards renewable energies, such as offshore wind, the use of HVDC (High-Voltage Direct Current) technology is increasing. HVDC converter stations have been reported to potentially cause sub-synchronous torsional interaction (SSTI) with turbo-generators in the grid. This phenomenon implies a negative electrical damping at the turbo-generator which may result in non-attenuating torsional vibrations. This is especially an issue in power grids where the close proximity of the power stations to the HVDC converter stations cannot be avoided. Therefore, in order to ensure safe and stable operation, monitoring of torsional vibrations is required.

In this article, a proof-of-concept study of touchless torque sensing at a nuclear power train in Gösgen, Switzerland, is presented. All three sub-synchronous natural frequencies were detected by the torque sensor during a load rejection test and show very good agreement with theoretical predictions based on finite element calculations. Possibilities to devise monitoring and eventually protection systems for torsional vibrations based on this technology are also discussed.

Commentary by Dr. Valentin Fuster
2017;():V008T29A027. doi:10.1115/GT2017-64141.

Nowadays, the development of turbines tends to enlarge the capacity and increase the corresponding parameters. Turbine inlet valve is an important part of turbine governing system. Consequently, the high pressure turbine requires good performance of the inlet valve. In this paper, the aerodynamic performance of a real ultra-supercritical power unit turbine inlet combined valve is analyzed in detail via numerical method. At the same time, the shape design of valve plug is improved by means of specific effective methods, including geometrical analysis and quick selection of the angles. A porous medium model is adopted to deal with the strainer structure and it has a good effect on the numerical simulation. The SST turbulence model is finally selected for calculation to obtain reliable results. The results show that the flow in the combined valve presents obvious flow separation nearby the valve plug and downstream of the throat. According to the analysis of pressure and static entropy, it can be concluded that the main pressure loss is concentrated in the strainer and control valve chamber, and an obvious vortex appears in control valve chamber with energy dissipation. Suitable optimization theory plays an effective role in the research. In this process, much attention has been paid to the decrease of pressure loss. Parabola and quick opening shapes are adopted to improve the shape of valve plug and a series of shapes with different angles are tested. The two best optimization models are selected and their results are analyzed. The results show good performance and the pressure loss coefficient reduces from 2.0% to 1.7%.

Commentary by Dr. Valentin Fuster
2017;():V008T29A028. doi:10.1115/GT2017-64278.

The expansion of steam flow and the condensation phenomena in an LP turbine depend on both the flow passage shape and the operating conditions. This paper presents the quantification of the influence of local geometrical details of the steam turbine blade including blade surface tapering, dimple inclusion and trailing edge shapes on flow expansion and condensation phenomena. For this purpose, the wet-steam model of ANSYS FLUENT, based on the Eulerian-Eulerian approach, was used. The mixture of vapor and liquid phases was solved by compressible Reynolds-averaged Navier-Stokes equations. The low inlet superheat case of White et al. [1] which is conducted with planar stator cascade was used as reference for this study. Various modifications including blade trailing edge shapes, blade shape modification via blade pressure and suction surfaces’ tapering, and addition of dimple feature to the blade pressure surface were applied to the blade profile. The presented results revealed that the applied blade shape modifications affected nucleation and droplet growth processes, shock wave structures and entropy generation rates. The influence of blade shape on loss generation was presented by calculating the Markov energy loss coefficients. The presented analysis exhibits that the blade shape alteration influences the overall loss generation that occur due to the irreversible heat and mass transfer during the condensation process.

Commentary by Dr. Valentin Fuster
2017;():V008T29A029. doi:10.1115/GT2017-64281.

Liberalized electricity market conditions and concentrating solar power technologies call for increased power plant operational flexibility. Concerning the steam turbine component, one key aspect of its flexibility is the capability for fast starts. In current practice, turbine start-up limitations are set by consideration of thermal stress and low cycle fatigue. However, the pursuit of faster starts raises the question whether other thermal phenomena can become a limiting factor to the start-up process. Differential expansion is one of such thermal properties, especially since the design of axial clearances is not included as part of start-up schedule design and because its measurement during operation is often limited or not a possibility at all.

The aim of this work is to understand differential expansion behavior with respect to transient operation and to quantify the effect that such operation would have in the design and operation of axial clearances. This was accomplished through the use of a validated thermo-mechanical model that was used to compare differential expansion behavior for different operating conditions of the machine. These comparisons showed that faster starts do not necessarily imply that wider axial clearances are needed, which means that the thermal flexibility of the studied turbine is not limited by differential expansion. However, for particular locations it was also obtained that axial rubbing can indeed become a limiting factor in direct relation to start-up operation. The resulting approach presented in this work serves to avoid over-conservative limitations in both design and operation concerning axial clearances.

Topics: Steam turbines
Commentary by Dr. Valentin Fuster
2017;():V008T29A030. doi:10.1115/GT2017-64285.

The successful demonstration of the “Aerostatic Seal” in a half scale rotating facility is described in this paper. The Aerostatic seal is a novel dynamic clearance seal specifically designed for steam turbine secondary gas path applications. The seal responds to radial rotor excursions, so a reduced clearance can be maintained compared to conventional labyrinth seal without damage to the seal. This enables increased turbine performance through reduced leakage and increased tolerance of turbine transient events typically found during start up. The seal is an extension of the existing retractable seal design already deployed in commercial steam turbines.

The seal was tested in the Durham Rotating Seals Rig, which was developed specifically to test this device. The rig featured a rotor designed to run with large eccentricities to model high speed radial rotor excursions, and the seal was instrumented to measure the real time seal response to the rotor.

The experimental campaign has conclusively demonstrated the ability of the seal to dynamically respond to the rotor position. The key result is that the seal is able to track the rotor position at high speed, and hence maintain a mean seal clearance that is lower than the rotor eccentricity. Overall this work marks a key milestone in the development of the Aerostatic Seal, and leads the way to testing in a steam environment and application in steam turbine plant.

Commentary by Dr. Valentin Fuster
2017;():V008T29A031. doi:10.1115/GT2017-64561.

The aim of this work is to provide an insight into the performance reduction of a 1.5 axial steam turbine stage working under extreme incidence conditions at the inlet. In particular, the main object of the study is the propagation of the loss cores across the blade rows, so as to assess how such operating conditions affect the full machine. Experimental data have been used to validate an unsteady three-dimensional numerical simulation, which provided the tools to investigate the flowfield in detail. To do so, the 1.5 turbine stage installed in the Low Speed Test Rig at Politecnico di Milano has been tested with design and off-design inlet conditions by modifying the IGV orientation. The inter-stage flowfield was investigated by traversing pressure probes in three different axial planes, downstream of each blade row. The numerical simulation has been carried out at University of Florence. The experimental data from probes traversing was used as boundary conditions so as to match as closely as possible the actual operative parameters of the stage. Data from flange-to-flange measurements on the test rig were also used to compare the stage efficiency. After the successful validation of the numerical results, the loss cores propagation study itself was carried out. Using CFD results, the unsteady nature of the separation occurring on the first stator in off-design condition is investigated. Subsequently, a detailed analysis of the propagation of the loss cores is presented, including loss coefficients calculation and entropy trends along the machines axial coordinate. The main outcome is that at the machine exit the loss structures appear to be mainly mixed out and, therefore, subsequent stages would operate under conditions not far from the nominal ones.

Commentary by Dr. Valentin Fuster

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