ASME Conference Presenter Attendance Policy and Archival Proceedings

2016;():V008T00A001. doi:10.1115/GT2016-NS8.

This online compilation of papers from the ASME Turbo Expo 2016: Turbomachinery Technical Conference and Exposition (GT2016) 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presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.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

2016;():V008T23A001. doi:10.1115/GT2016-56167.

A double-sided centrifugal compressor consists of two impellers whose inlets are non-balanced, with one side of the impeller connected to the straight duct, and the other connected to the bending duct. This leads to the differences in the inlet conditions of the double-sided impeller, resulting in the differences in the flow structures of the rear impeller along the circumferential direction. In this work, aiming at analyzing the flow structures of the rear impeller, diffuser and volute internal in three flow rate conditions, the internal flow field of the double-sided centrifugal compressor was calculated in a numerical method. It is found that the inlet bending duct results in significant inlet axial velocity difference of the rear impeller along circumferential direction. The axial velocity differences at high span positions become more obvious with the increase of the flow rate. Moreover, the jet-wake structures among the blade passage outlets are also various. At the high static pressure zones of the volute, corresponding blade passage wake regions increase and their sizes are also influenced by the inlet distortion. The circumferential distributions of the static pressure in the diffuser agree well with that in the volute. In the diffuser, the non-uniform degrees of the static pressure distributions are roughly the same at different radius positions and are weakening with the decrease of the flow rate.

Commentary by Dr. Valentin Fuster
2016;():V008T23A002. doi:10.1115/GT2016-56169.

In turbocharger systems, the inlets of centrifugal compressor often connect with bent ducts, producing a non-uniform flow field distribution at the compressor impeller inlet, which degrades the compressor performance and deteriorate the flow structure of the compressor significantly.

In present work, a group of typical bent ducts is designed by adjusting the torsion angle of the U-shaped duct to investigate the effects of these bent ducts on the performance and flow field of the compressor. The experimental tests of the compressor with various inlet bent duct configurations were carried out to obtain the aerodynamic performance and pressure distributions. The experimental performance curves showed the bent ducts affect the aerodynamic performance and surge margin of the compressor.

To understand these effects fundamentally, the unsteady flow calculations were conducted to capture the detailed flow in the bent ducts and compressor internal. The flow distortion distributions and swirl patterns in various bent ducts were compared by numerical calculation results and analyzed in theory. The results showed the total pressure distorted region at the duct exit expands along the circumferential direction and distortion degree is weakened with the torsion angle of bent duct increases. Moreover, the swirl distortion patterns vary in different inlet ducts. The further analysis showed that an appropriate bent duct configuration is helpful for improving the surge margin of the compressor effectively. By observing the static pressure in the impeller inlet and shroud region, it found that the bent ducts produce non-uniform static pressure at impeller inlet and reduce the static pressure in shroud wall. It also found that the change of the pressure in compressor internal has some relation with the swirl structure of bent duct exit.

Commentary by Dr. Valentin Fuster
2016;():V008T23A003. doi:10.1115/GT2016-56274.

In this study, the viability, performance, and characteristics of a turbojet-to-turbofan conversion through the use of a continuously variable transmission (CVT) are investigated. By an in-house thermodynamic simulation code, the performance of the simple cycle turbojet, a direct shaft joined turbofan, and a CVT coupled turbofan with variable bypass are contrasted. The baseline turbojet and turbofan findings are validated against a commercial software. The comparison indicates high quantitative agreement.

Analyzing the results of the turbofan engine equipped with a variable bypass and CVT, it is observed that both the thrust and the efficiency are increased. The augmented thrust is observed to be an artifact of enhanced component matching and wider operational range introduced by variable bypass capability. On the other hand, the introduction of CVT contributes to the reduction in fuel consumption. Therefore, the current research suggests that adaptation of a micro-turbojet into a variable cycle micro turbofan will greatly improve the performance and efficiency of existing engines, in addition to providing a pathway towards extended use in various applications.

Topics: Turbofans , Turbojets
Commentary by Dr. Valentin Fuster
2016;():V008T23A004. doi:10.1115/GT2016-56328.

In turbomachinery design the accurate prediction of the life cycle is one of the most challenging issues. Traditionally, life cycle calculations for radial turbine wheels of turbochargers focus on mechanical loads such as centrifugal and vibration forces. Due to the increase in exhaust gas temperatures in the last years, thermomechanical fatigue in the turbine wheel came more into focus. In order to account for the thermally induced stresses in the turbine wheel as a part of the standard design process, a fast method is required for predicting metal temperatures.

In order to develop a suitable method, the mechanisms have to be understood that cause the thermal stresses. Thus, in a first step a detailed analysis of the temperature fields is conducted in the present paper. Extensive numerical simulations of a thermal shock process are carried out and validated by experimental data from a test rig. Based on the results the main heat transfer mechanisms are identified, that are crucial for the critical thermal stresses in transient operation. It is shown that these critical stresses mainly depend on local 3D flow structures.

With this understanding, a fast method to calculate the transient temperatures in a radial turbine was developed. It is based on a standard method for transient fluid/solid heat transfer. This standard method was modified in order to achieve a sufficient accuracy in the calculation of the investigated heat transfer processes. The results show a good agreement with experimental data and with the results of the extensive numerical calculations.

Commentary by Dr. Valentin Fuster
2016;():V008T23A005. doi:10.1115/GT2016-56342.

One of critical concerns in a variable geometry turbine (VGT) design program is shock wave generated from nozzle exit at small open conditions with high inlet pressure condition, which may potentially lead to forced response of turbine wheel, even high-cycle fatigue issues and damage of inducer or exducer. Though modern turbine design programs have been well developed, it is difficult to eliminate the shock wave and all the resonant crossings that may occur within the wide operating range of a VGT turbine for automotive applications. This paper presents an option to mitigate intensity of the shock wave induced excitation using grooves on nozzle vane surface before the shock wave. Two kinds of turbines in which nozzle vanes with and without grooves were numerically simulated to obtain a three-dimensional flow field inside the turbine. The predicted performances from steady simulations were compared with test data to validate computational mesh and the unsteady simulation results were analyzed in detail to predict the responses of both shock wave and aerodynamic load acting on turbine blade surface. Compared with the original design, an introduction of grooves on nozzle vane surface mitigates the shock wave while also obviously reduces the amplitudes of alternating aerodynamic load on the turbine blades.

Commentary by Dr. Valentin Fuster
2016;():V008T23A006. doi:10.1115/GT2016-56381.

Vortex generators are known to be effective in augmenting the mixing of fuel jets with air. The configuration investgated in this study is a tubular air passage with fuel injection from one single orifice placed in the side wall. In the range of typical gas turbine combustor inlet temperatures, the performance vortex generator premixers (VGPs) have already been investigated for natural gas as well as for blends of natural gas and hydrogen. However, for highly reactive fuels, the application of VGPs in recuperated gas turbines is particularly challenging because the high combustor inlet temperature leads to potential risk with regard to premature self-ignition and flame flashback. As the current knowledge does not cover the temperature range far above the self-ignition temperature, an experimental investigation of the operational limits of VGPs is currently being conducted at the Thermodynamics Institute of the Technical University of Munich, which is particularly focused on reactive fuels and the thermodynamic conditions present in recuperated gas turbines with pressure ratios of 4–5. For the study presented in the paper, an atmospheric combustion VGP test rig has been designed, which facilitates investigations in a wide range of operating conditions in order to comply with the situation in recuperated micro gas turbines, namely global equivalence ratios between 0.2 and 0.7, air preheating temperatures between 288K and 1100K, and air bulk flow rates between 6–16 g/s. Both the entire mixing zone in the VGP and the primary combustion zone of the test rig are optically accessible. High speed OH* chemiluminescence imaging is used for the detection of the flashback and blow-off limits of the investigated VGPs. Flashback and blow-off limits of hydrogen in a wide temperature range covering the auto-ignition regime are presented, addressing the influences of equivalence ratio, air preheating temperature and momentum ratio between air and hydrogen on the operational limits in terms of bulk flow velocity. It is shown that flashback and blow-off limits are increasingly influenced by auto-ignition in the ultra-high temperature regime.

Commentary by Dr. Valentin Fuster
2016;():V008T23A007. doi:10.1115/GT2016-56444.

Decentralized power and heat generation is a growing trend throughout the world. In smaller applications, electrical power output less than few megawatts, reciprocating engines have dominated the market. In recent years, small sized gas turbines have emerged as challengers for the reciprocating engines. The small gas turbines have a growing share of the decentralized energy market, which itself is rapidly growing. Hence, improvements in small gas turbine efficiency have a significant impact from the economic and environmental perspective.

In this paper, the design of a high efficiency 400 kW gas turbine prototype is described and discussed. The prototype is a two-spool, recuperated and intercooled gas turbine where both spools comprise of a radial compressor and turbine, a permanent magnet electric generator, an axial and two radial active magnetic bearings and two safety bearings.

The prototype design was divided into five categories and each of the categories are discussed. The categories were: the process design, the turbomachinery design, the generator and electrical design, bearing design and rotor dynamic analysis, and mechanical design. The design of recuperator, intercooler, and combustion chamber were outsourced. Hence, they are not discussed in this paper.

The prototype design process showed the readiness of the chosen technological selections, as well it showed that the type of machine under discussion can be designed and manufactured.

Commentary by Dr. Valentin Fuster
2016;():V008T23A008. doi:10.1115/GT2016-56466.

The use of turbochargers on both gasoline and diesel engines is started to become a common strategy to comply with stringent limits on CO2. The main action towards lowering fuel consumption of powertrains is achieved by reduction of engine size and number of cylinders, annexed to the lower friction. However, this is directly linked to the worsening of deliverable output power under the natural aspirated configuration. Therefore, turbocharging is often adopted to overcome this problem where useful energy contained in the exhaust gasses is used to increase the air density at the intake. The increase in power from a natural aspirated configuration is a direct consequence of higher fuel quantity to be injected.

In order to pursue a systematic evaluation of the powertrain system, engine, turbochargers and auxiliary components are included into 1D models. Several conditions can be simulated without the need of an extensive test plan. In 1D software like Ricardo Wave, turbochargers performance are imposed as input. These are previously measured in appropriate turbocharger gas-stand where hot or cold air is blown through the turbine while load on compressor is controlled by adjusting a back pressure valve. Compressor and turbine maps are generated for constant speed lines which are corrected for total temperature. Pressure ratio, mass flow and isentropic efficiency are also monitored as parameters to characterize performance maps of turbomachinery. In gas-stands, steady flow conditions are imposed at compressor and turbine. However, in turbocharged engines, pulsating flows induced by the engine valvetrain disturb continuously turbocharger conditions during the engine cycle. In fact, the effects that the conditions of the engine air-path could have on the turbocharger operations are excluded from the system modelling.

In this study, an appropriate engine gas-stand has been developed in order to improve the accuracy on estimating the turbine extraction power in 1D powertrain simulations. In addition, future analyses on turbocharger transient operations could be investigated. The compressor outlet has been disconnected from the 2.2L Diesel engine intake so that the load on turbocharger and engine can be independently controlled. In order to extend the engine capability in delivering mass flow and pressure at the turbine inlet, an external boost rig has been installed with the capability to control pressure, mass flow and temperature at the engine intake. In a first instance, a 1D model of the system including turbomachinery, Diesel engine and boost rig has been developed using the commercial platform Ricardo Wave. In this way, a preliminary DoE study of the entire system has been performed in order to evaluate the effects of parameters and actuators on the turbocharger operations. Additionally, the control of the rig has been tested by confirming the previous DoE study. Approaches to create turbochargers maps are shown. Last section of the paper focuses on turbine pulsations and the interpretation of efficiency calculated in experiments and simulations.

Commentary by Dr. Valentin Fuster
2016;():V008T23A009. doi:10.1115/GT2016-56479.

Rotor-dynamics of Micro Gas Turbines (MGTs) under 30 kW have been a critical issue for the successful development of reliable engines during the last decades. Especially, no consensus has been reached on a reliable MGT arrangement under 10 kW with rotational speeds above 100,000 rpm, making the understanding of the rotor-dynamics of these high speed systems an important research area. This paper presents a linear rotor-dynamic analysis and comparison of three mechanical arrangements of a 6 kW MGT intended for utilising Concentrated Solar Power (CSP) using a parabolic dish concentrator. This application differs from the usual fuel burning MGT in that it is required to operate at a wider operating speed range. The objective is to find an arrangement that allows reliable mechanical operation through better understanding of the rotor dynamics for a number of alternative shaft-bearings arrangements. Finite Element Analysis (FEA) was used to produce Campbell diagrams and to determine the critical speeds and mode shapes. Experimental hammer tests using a new approach based on optical sensing technology were used to validate the rotor-dynamic models. The FEA simulation results for the natural frequencies of a shaft arrangement were within 5% of the measurements, while the deviation for the shaft-bearings arrangement increased up to 16%.

Commentary by Dr. Valentin Fuster
2016;():V008T23A010. doi:10.1115/GT2016-56508.

This work presents the theoretical and experimental rotordynamic evaluations of a rotor-air foil bearing system supporting a large overhung mass for high speed application. The proposed system highlights the compact design of a single shaft rotor configuration with turbomachine components arranged on one side of the bearing span. In this work, low speed tests up to 45krpm are performed to measure the lift off speed and to check the bearing manufacturing quality. Rotordynamic performance at high speeds is evaluated both analytically and experimentally. In the analytical approach, simulated imbalance responses are studied using both rigid and flexible shaft models with bearing forces calculated from transient Reynolds equation along with rotor motion. The simulation predicts that the system experiences small synchronous rigid mode vibration at 20krpm and bending mode at 200krpm. A high speed test rig is designed to experimentally evaluate the rotor-air foil bearings system. The high speed tests are operated up to 160krpm. The vibration spectrum indicates that the rotor-air foil bearing system operates under stable conditions. The experimental waterfall plots also show very small sub-synchronous vibrations with frequency locked to the system natural frequency. Overall, this work demonstrates the potential capability of air foil bearings in supporting a shaft with a large overhung mass at high speed.

Topics: Foil bearings
Commentary by Dr. Valentin Fuster
2016;():V008T23A011. doi:10.1115/GT2016-56574.

Turbocompounding is a promising waste heat recovery technology to improve fuel economy of internal combustion engines and comply with increasingly stringent emission regulations. The performance of a turbocompound engine is significantly influenced by the matching of the turbocharger turbine and the power turbine. Conventionally, the matching of the turbocharger turbine and power turbine is carried out at a single operating point. Single-point matched turbocompound systems tends to have poor performance at off-design operating conditions, which restrained the fuel-saving potential of turbocompound engines under driving cycle conditions.

In the present study, a multipoint matching method for turbocompound systems was developed, which was essentially an optimization process of the swallowing capacities of the turbocharger turbine and the power turbine to achieve best performance at multiple matching points. In order to improve the performance of turbocompound engines under driving cycle conditions, common operating points of a driving cycle were used as matching points in the matching process. Common operating points under a driving cycle were determined by clustering the dynamic profiles of the driving cycle.

A simulation study was carried out to examine the effectiveness of the multipoint matching method. The performance of the multipoint matched turbocompound system was compared against two single-point matched turbocompound systems under stationary operating points and driving cycle conditions respectively. According to the simulation results, the multipoint matched turbocompound system could attain satisfactory BSFC benefit under a wider operating range when compared with the two single-point matched turbocompound systems. The multipoint matched turbocompound engine showed the largest reduction in fuel consumption under driving cycle conditions.

Commentary by Dr. Valentin Fuster
2016;():V008T23A012. doi:10.1115/GT2016-56705.

An effective measure to improve the surge margin of a centrifugal compressor, without sacrificing efficiency, is to implement a recirculating casing treatment inside the compressor cover. However, introduction of an additional sound propagation path directly over the rotating impeller blades exposes the inherently unsteady internal flow-field as an added potential noise source, which is of concern for automotive applications. The present study conducts performance and acoustic measurements of a new compressor which was designed to achieve high isentropic efficiency over a wide flow range, featuring an impeller with splitter blades and a vaneless diffuser. A dual-port active casing treatment (ACT) was also incorporated into the compressor cover to independently extend both the low and high flow rate operating regions of the compressor. The slot of the first (surge) port is positioned between the main and splitter blades of the impeller, similar to passive casing treatments that are already widely adopted. This port extends the low-flow boundary of the compressor operating range by reducing flow separation on the suction surface of the main blades near the shroud. The slot of the second (choke) port is located just behind the splitter blades, and it is studied in both the open and closed positions. This second port allows for increased air flow near choke, due to the slot position just downstream of the aerodynamic throat of the compressor. The current ACT design leaves the surge port open at all times, while the choke port is only opened when the compressor operates near choke conditions. In addition to comparing experimental results from this new compressor in both configurations (choke port open and closed), measurements from a comparable (baseline) compressor without splitter blades and a single-port shroud are utilized to assess the acoustics of the new design. Acoustic measurements were completed over the low to mid-speed operating range, which is a region heavily weighted in customer drive cycles for light and medium duty vehicles. The conscientious design of the impeller and surge slot of the new compressor to minimize flow separation on the suction surface of the inducer blades is shown to not only improve efficiency and extend the low-flow operating range, but (with the choke port closed) broadband noise is significantly reduced in the mid to high flow rate operating region. At low flow rates, the new compressor (with the choke port closed) is slightly louder than the baseline compressor at the inlet duct measurement location, but essentially equal to the baseline compressor at the external microphone location near the compressor inlet duct opening. When the choke port of the new compressor is open, broadband noise increases slightly relative to the closed configuration. More importantly, the peak sound pressure level at (main) blade-pass frequency is reduced by opening the choke port, and the operating region of elevated tonal noise shifts from mid to high flow rates.

Commentary by Dr. Valentin Fuster
2016;():V008T23A013. doi:10.1115/GT2016-56770.

Turbocharger performance maps used for the matching process with a combustion engine are measured on test benches which do not exhibit the same boundary conditions as the engine. However, these maps are used in engine simulations, ignoring that the compressor and turbine aerodynamic performance is rated on the basis of quantities which were measured at positions which do not coincide with the respective system boundaries of the turbomachinery.

In the operating range of low to mid engine speeds, the ratio between the heat flux and the work done by the turbine and the compressor is much greater than at high speeds where heat transfer phenomena on the compressor side can usually be neglected. Heat losses on the turbine side must be taken into account even at higher shaft speeds when dealing with isentropic turbine efficiencies.

Based on an extensive experimental investigation a one-dimensional heat transfer model is developed. The compressor and turbine side are treated individually and divided into sections of inlet, wheel, outlet, diffuser and volute. The model demonstrates the capability to properly account for the impact of heat transfer and thereby improves the predictive accuracy of temperatures relevant for the matching process.

Commentary by Dr. Valentin Fuster
2016;():V008T23A014. doi:10.1115/GT2016-56808.

Small unmanned aircraft are currently limited to flight ceilings below 20,000 ft due to the lack of an appropriate propulsion system. One of the most critical technological hurdles for an increased flight ceiling of small platforms is the impact of reduced Reynolds number conditions at altitude on the performance of small radial turbomachinery. The current article investigates the influence of Reynolds number on the efficiency and pressure ratio of two small centrifugal compressor impellers using a one-dimensional meanline performance analysis code. The results show that the efficiency and pressure ratio of the 60 mm baseline compressor at the design rotational speed drops with 6–9% from sea-level to 70,000 ft. The impact on the smaller 20 mm compressor is slightly more pronounced and amounts to 6–10%. Off-design changes at low rotational speeds are significantly higher and can amount to up to 15%. Whereas existing correlations show a good match for the efficiency drop at the design rotational speed, they fail to predict efficiency changes with rotational speed. A modified version is therefore proposed.

Commentary by Dr. Valentin Fuster
2016;():V008T23A015. doi:10.1115/GT2016-56811.

Small unmanned aircraft are currently limited to flight ceilings below 20,000 ft (6.1 km) due to the lack of an appropriate propulsion system. One of the most critical technological hurdles for an increased flight ceiling of small platforms is the impact of the low Reynolds number conditions at altitude on the performance of small radial turbomachinery. The current article investigates the influence of Reynolds number on the efficiency and pressure ratio of 5 radial turbines using a 1D meanline performance analysis code. The results show that the efficiency at the design rotational speed drops by up to 30% from sea-level to 70,000 feet. However, the bulk of this efficiency loss occurs above approximately 20,000 feet where the flow in most turbines transitions to laminar flow. Existing turbine scaling correlations do not predict this efficiency loss well. However, an adapted method from compressor scaling research is shown to be fairly accurate.

Commentary by Dr. Valentin Fuster
2016;():V008T23A016. doi:10.1115/GT2016-56941.

During the past decade, increasing interest has been shown in micro gas turbines for the high-power and high-energy density. However, due to the small characteristic scale, it is still a key problem to ensure safe and reliable operation of the micro-combustor. A new micro gas turbine combustor with a Γ-shaped porous media dome was investigated in this paper. The volume of the combustor is 2.7 cm3. Dual-zone combustion (combustion zone and dilution zone) was adopted in the combustor. Combustion characteristics of the micro-combustor with different total air mass flow and total equivalence ratios were investigated by experiments at ambient temperature and atmospheric pressure. The results show that the relationship between liner pressure-loss and total air mass flow cannot be fit by a polynomial due to porous media and dilution holes combined influence. The ratio of airflow across porous media dome to total air mass flow increased with increasing total air mass flow. Stable combustion was obtained in this micro combustor as the air mass flow rate was in the range of 0.15∼1.2 g/s. With the increasing total air mass flow, the total equivalence ratios of lean ignition and blow-out limits decreased first, then increased. The exit gas temperature as high as 1460 K and power density 636 MW/m3 were achieved at the total equivalence ratio of 0.5, and total air flow rate of 1.2 g/s, the overall efficiency reached 98.5% in this condition. The results showed that safe and reliable operation can be achieved in this new micro gas turbine combustor with high overall efficiency.

Commentary by Dr. Valentin Fuster
2016;():V008T23A017. doi:10.1115/GT2016-56944.

With the miniaturization of mechanical and electrical systems, the demands for small-scale power resources with high energy density have promoted studies on small-scale combustors as well as methods to achieve stable small-scale combustion. In the present paper, a micro swirl injector for a small-scale combustor was designed to study the shape and stability of swirl flame in a 4 mm diameter quartz tube experimentally. The influences of fuel equivalence ratio, axial average velocity in the tube, and structure of the swirler exit were investigated under atmospheric pressure and ambient temperature. The fuel equivalence ratio was in the range of 0.5 to 3.0 and axial average velocity varied from 0.2 to 6 m/s. Methane was used as fuel. Results show that when the axial average velocity increases to a certain value and is kept constant, the methane flame abruptly changes from a swirl flame outside the tube into a stable spindle flame located at the exit of the swirl injector inside the tube within certain limits of equivalence ratio. The equivalence ratio ranges that the shape transition can happen extends with the increase of axial average velocity to certain limits, which is approximately 0.7–1.1 for methane. While under low axial velocity conditions, the spindle flame cannot be formed by the changing of equivalence ratio under a certain velocity, it can be formed and stabilized in a wide equivalence ratio range by slowly decreasing the axial velocity of a high-velocity spindle flame under a constant equivalence ratio of the premixed mixture. A flare at the exit of the swirl injector has little influence on stability limits while it makes the spindle flame to shift down and anchor nearer to the swirler exit. Flame shapes under the structure are characterized, and a brief explanation is given based on the vortex bursting mechanism and the match between local gas velocity and flame propagation velocity.

Topics: Flames , Swirling flow
Commentary by Dr. Valentin Fuster
2016;():V008T23A018. doi:10.1115/GT2016-56954.

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. A standard combustor is replaced by a prototype combustor which enables a bi fuel supply of kerosene and ammonia gas. Diffusion combustion is employed in the prototype combustor due to its flame stability. Demonstration test firing ammonia gas was achieved using a new facility of large amount of ammonia supply. The gas turbine started firing kerosene and increased its electric power output. After achievement of stable power output, ammonia gas was started to be supplied and its flow rate increased gradually.

41.8kW power output was achieved by firing ammonia gas only. Ammonia gas supply increases NOx in the exhaust gas dramatically. However post-combustion clean-up of the exhaust gas via Selective Catalytic Reduction can reduce NOx successfully.

Commentary by Dr. Valentin Fuster
2016;():V008T23A019. doi:10.1115/GT2016-56982.

The research into natural cooling of a shutdown gas turbine engine is necessary to understand the factors reducing the engine components life by restart. The additional unbalance, bearing overheating and clearance changes are phenomena, which can appear during cooldown and engine restart. In this work, experimental and numerical research for a micro GTE cooldown were conducted.

Numerical conjugate flow & heat-transfer model of the whole engine was created for these purposes. The calculated temperature distribution at an operating point was used as a starting point of the cooldown time. Cooldown calculation included two phases: run-down and natural cooling. The numerical investigation was supported by experimental measurements, which were obtained using a micro gas turbine test rig.

From the results obtained it can be concluded that the allowed time before the restart must generally be based on individual factors such as bearings temperature, the displacement of turbine wheels centers of gravity etc. These factors attain their peak values at different time points after the engine shutdown.

Commentary by Dr. Valentin Fuster
2016;():V008T23A020. doi:10.1115/GT2016-56996.

During the initial layout of a new air-handling system for internal combustion engines, the systems engineers need to make significant assumptions based on experience regarding the compressor performance, operating range, the turbine performance and volute and ducting losses. Typically, the initial sizing occurs before the selection of the turbocharger layout and before actual turbocharger maps are available. On the other hand the system information is key to be able to customize and match the compressor and the turbine performance to meet the specific engine requirements which may be significantly different depending on the engine type application. The current paper proposes a simple method to leverage basic turbomachinery design rules, correlations and databases to provide guidance to the system engineers on the expected performance of the air handling components. Additionally the current paper discusses the trade-offs that could be made on turbocharger performance by the adequate selection of the turbine and compressor couple that addresses the specific engine application needs.

Commentary by Dr. Valentin Fuster
2016;():V008T23A021. doi:10.1115/GT2016-57008.

In this paper the procedure and results of the aeromechanical optimization of a mixed-flow compressor impeller to be used in a 600 N micro gas turbine (MGT) are presented. Today’s unmanned aerial vehicles (UAVs) demand high thrust-to-weight ratios and low engine frontal area. This may be achieved using mixed-flow compressors. The initial impeller design was obtained using a 1-D turbomachinery layout tool. A multi-point optimization of the impeller aerodynamic performance was completed, followed by a mechanical optimization to reduce mechanical stresses in the impeller. A coupled aero-mechanical optimization was concluded with the purpose of increasing the choke limit and reducing stresses while conserving aero-performance. Subsequently, a modal analysis of the rotor was conducted to determine its vibrational characteristics. The optimization process was set up and controlled in an integrated environment that includes a 3-D Navier-Stokes flow solver and a 3-D finite element (FE) structural solver. The optimization process is based on the use of a database, an artificial neural network (ANN), a user-defined objective function and a genetic algorithm (GA). The overall optimization process achieved an increase in pressure ratio (total-to-total) of 30.6% compared to the initial design while the efficiency (isentropic total-to-total) was increased by 5% at design conditions. A decrease in the surge margin was experienced, but the final surge margin was still acceptable (12%). The choke limit was increased meaningfully. This was achieved while also decreasing the peak von Mises stress from far above the material yield strength to 30% below the yield limit.

Commentary by Dr. Valentin Fuster
2016;():V008T23A022. doi:10.1115/GT2016-57041.

Ceramic turbines could substantially increase operating temperatures of turbomachinery without the need of blade cooling, leading to higher conversion efficiency and power density. However, the inherent brittleness and low tensile strength of ceramic materials limits the use of hub-based ceramic turbines. This paper presents a novel inside-out turbine architecture, permitting the use of monolithic ceramic blades. The proposed architecture uses a carbon-polymer composite rim which converts the rotor radial loads to tangential hoop stress. The blades mainly support compressive loads, minimizing tensile stresses within the blade and thus crack propagation. This allows the use of low tensile strength ceramics which cannot be used in standard hub-based turbines. The rotor hub is comprised of two radially flexible C-shaped hubs, which have sufficient compliance to follow radial displacement of the heavily loaded composite rim. The feasibility of the proposed inside-out ceramic turbine is demonstrated by addressing the four key challenges of the architecture using proof-of-concept prototypes, namely: (1) rotor dynamics of the flexible hub; (2) thermal viability of the composite rim (3) local tensile stress in the blades, and (4) thermal shock in the ceramic blades in transient mode. Experimental validation with an alumina blade confirms that this architecture supports the use of low tensile strength, brittle ceramic blades.

Topics: Ceramics , Turbines
Commentary by Dr. Valentin Fuster
2016;():V008T23A023. doi:10.1115/GT2016-57105.

Design and optimization of centrifugal compressors, based on main blades configuration of impeller have been vastly discussed in open literature, but less researches have addressed splitters. In this research, the impeller of a commercial turbocharger compressor is investigated. Here, profiles of main blades are not changed while the effect of changing the configuration of splitters is studied. An optimization study is performed to find the best configuration using genetic algorithm over a complete operating curve of the compressor. CFD codes with experimental support are used to predict the compressor performance. Quantumetric tests beside destructive analysis of two impellers are implemented for material identification and selection which is necessary for manufacturing. After taking into account structural considerations and approving the safety by numerical simulation, the new impeller is manufactured using 5 axis CNC machine. Non destructive tests are performed for identification of any structural defects. The new impeller is then mounted on a turbocharger shaft and tested experimentally in a wide range of operating conditions, which leads to a design having 2.3% improvement in efficiency. This is an important achievement in all applications of centrifugal compressors, especially in turbochargers.

Commentary by Dr. Valentin Fuster
2016;():V008T23A024. doi:10.1115/GT2016-57190.

The downsizing of combustion engines has become the major strategy within the automotive industry to meet the increasing demands in terms of fuel economy and harmful emissions. Furthermore, it is important to fulfil the customers expectations in terms of drivability by increasing the power density and transient performance of the engines. The key technology to reach these ambitious targets is the enhanced utilization of exhaust pulses on turbocharged engines. In four cylinder gasoline engine applications this is mainly realized by the use of double entry turbines or variabilities in the exhaust valve train. During the designing and matching process of double entry turbines it is still a major challenge to predict the turbine power output and accurately model its interaction with the engine. In the past few years, several authors have published measurement and simulation technologies aimed at enhanced modelling quality. Most of these approaches are based on the introduction of different flow conditions which help to describe the thermodynamic performance under various pulsating boundary conditions. Within an average engine cycle, the turbine operates under equal, single and unequal admissions. Furthermore, the evaluation of a turbine interacting with a four cylinder gasoline engine shows that cross flow between both turbine scrolls can occur during the blow-down phase of the cylinders. In this phase, the temperature and pressure upstream of the turbine reach their peak values within the complete engine cycle. Therefore, this phase is most crucial for the conversion of the exhaust energy into mechanical energy, which drives the compressor impeller of the turbocharger.

This work focuses on the results of stationary hot gas measurements and 3D CFD simulations of the cross flow phenomena to gain a deeper understanding of the scroll interaction in double entry turbines and its impact on engine performance. The findings were used to improve the modeling quality of double entry turbines in 1D engine process simulations, especially during the exhaust blow down where cross flow between the dividing wall and the turbine wheel occurs. The new methodology to quantify the amount of cross flow with a hot gas test has shown that the cross flow rate of a twin scroll turbine can reach values as high as 35% of the overall flow rate entering the turbine housing, whereas this value can be significantly reduced by using a segment turbine housing. The new map based turbine model, which enables predictive simulations, covers all engine relevant flow conditions of the turbine including cross flow. This is important because the cross flow has a large impact on the exhaust pulse separation and thus on the residual gas fraction of the cylinders after the gas exchange.

Commentary by Dr. Valentin Fuster
2016;():V008T23A025. doi:10.1115/GT2016-57278.

The present paper presents the multi-disciplinary optimization of a centrifugal compressor for a 100kW micro gas turbine. The high rotational speed fixed by the cycle optimization (75,000 rpm) required a simultaneous analysis of flow aerodynamics and mechanical behavior to account for the high centrifugal stresses the blades are subjected to, while maximizing the aerodynamic performance. A commercial 3D (three dimensional) computational fluid dynamics (CFD) solver adopted for the aerodynamic computations and an open source finite element FEM code for the mechanical integrity calculations have been coupled with metamodels to speed up the optimization process. Home-made scripting modules, which manage multidisciplinary optimization, mesh generation, geometry parameterization and result post-processing have been written and utilized. A sample data-base has been generated on the basis of the parameters selected to describe aerodynamic and mechanical constraints, and an optimization procedure based on a genetic algorithm has been performed. A RANS (Reynold Averaged Navier Stokes) steady approach with a two-equation SST (Shear Stress Transport) model has been adopted for the aerodynamic computations during the optimization procedure. The optimized compressor so achieved showed an important boost in aerodynamic performance, without any penalty in the mechanical behavior, as compared with the preliminary design. The optimized configuration has been tested also by means of URANS (Unsteady Reynolds Averaged Navier Stokes) phase-lag investigations, which confirmed the aerodynamic performance increase predicted by steady RANS calculations.

Commentary by Dr. Valentin Fuster
2016;():V008T23A026. doi:10.1115/GT2016-57523.

The demand for more environmentally friendly and economic power production has led to an increasing interest to utilize alternative fuels. In the past, several investigations focusing on the effect of low-calorific fuels on the combustion process and steady-state performance have been published. However, it is also important to consider the transient behavior of the gas turbine when operating on non-conventional fuels. The alternative fuels contain very often a large amount of dilutants resulting in a low energy density. Therefore a higher fuel flow rate is required, which can impact the dynamic behavior of the gas turbine.

This paper will present an investigation of the transient behavior of the all-radial OP16 gas turbine. The OP16 is an industrial gas turbine rated at 1.9 MW, which has the capability to burn a wide range of fuels including ultra-low-calorific gaseous fuels. The transient behavior is simulated using the commercial software GSP including the recently added thermal network modeling functionality. The steady-state and transient performance model is thoroughly validated using real engine test data.

The developed model is used to simulate and analyze the physical behavior of the gas turbine when performing load sheds. From the simulations it is found that the energy density of the fuel has a noticeable effect of the rotor over-speed and must be considered when designing the fuel control.

Commentary by Dr. Valentin Fuster
2016;():V008T23A027. doi:10.1115/GT2016-57665.

A high speed gas bearing test rig was developed to characterize rotordynamic, thermal, and thrust load performance of gas bearings being developed for an oil-free turboexpander. The radial bearings tested in this paper were tilting pad journal bearings with radial compliance features that allow the bearing bore to increase to accommodate shaft growth, and the thrust bearings were a spiral groove type with axial compliance features. The thrust bearing accounts for over 90% of the combined bearing power consumption, which has a cubic relationship with speed and increases with case pressure. Radial bearing circumferential pad temperature gradients increased approximately with speed to the fourth or fifth power, with slightly higher temperature rise for lower case pressure. Maximum steady state bearing pad temperatures increase with increasing speed for similar cooling mass flow rates; however, only the thrust bearing showed a significant increase in temperature with higher case pressure. The thrust bearings were stable at all speeds, but the load capacity was found to be lower than anticipated, apparently due to pad deformations caused by radial temperature gradients in the stator. More advanced modeling approaches have been proposed to better understand the thrust bearing thermal behavior and to improve the thrust bearing design. Finally, the radial bearings tested were demonstrated to be stable up to the design speed of 130 krpm, which represents the highest surface speed for tilting pad gas bearings tested in the literature.

Commentary by Dr. Valentin Fuster
2016;():V008T23A028. doi:10.1115/GT2016-57675.

For an internal combustion engine, a large quantity of fuel energy (accounting for approximately 30% of the total combustion energy) is expelled through the exhaust without being converted into useful work. Various technologies including turbo-compounding and the pressurized Brayton bottoming cycle have been developed to recover the exhaust heat and thus reduce the fuel consumption and CO2 emission. However, the application of these approaches in small automotive power plants has been relatively less explored because of the inherent difficulties, such as the detrimental backpressure and higher complexity imposed by the additional devices. Therefore, research has been conducted, in which modifications were made to the traditional arrangement aiming to minimize the weaknesses. The turbocharger of the baseline series turbo-compounding was eliminated from the system so that the power turbine became the only heat recovery device on the exhaust side of the engine, and operated at a higher expansion ratio. The compressor was separated from the turbine shaft and mechanically connected to the engine via CVT. According to the results, the backpressure of the novel system is significantly reduced comparing with the series turbo-compounding model. The power output at lower engine speed was also promoted. For the pressurized Brayton bottoming cycle, rather than transferring the thermal energy from the exhaust to the working fluid, the exhaust gas was directly utilized as the working medium and was simply cooled by ambient coolant before the compressor. This arrangement, which is known as the inverted Brayton cycle was simpler to implement. Besides, it allowed the exhaust gasses to be expanded below the ambient pressure. Thereby, the primary cycle was less compromised by the bottoming cycle. The potential of recovering energy from the exhaust was increased as well. This paper analysed and optimized the parameters (including CVT ratio, turbine and compressor speed and the inlet pressure to the bottoming cycle) that are sensitive to the performance of the small vehicle engine equipped with inverted Brayton cycle and novel turbo-compounding system respectively. The performance evaluation was given in terms of brake power output and specific fuel consumption. Two working conditions, full and partial load (10 and 2 bar BMEP) were investigated. Evaluation of the transient performance was also carried out. Simulated results of these two designs were compared with each other as well as the performance from the corresponding baseline models. The system models in this paper were built in GT-Power which is a one dimension (1-D) engine simulation code. All the waste heat recovery systems were combined with a 2.0 litre gasoline engine.

Commentary by Dr. Valentin Fuster
2016;():V008T23A029. doi:10.1115/GT2016-57687.

The accurate prediction of the life cycle is one of the most challenging issues in turbomachinery design. Nowadays life cycle calculations for radial turbines focus on mechanical loads such as centrifugal and vibration forces. Because of enhanced turbine inlet temperatures with inevitably increasing thermal stress, the requirements in the design process of turbine wheels become higher. Therefore, it is desirable to know the temperature profile and thus the thermal stress in the turbine wheel as early as possible in the design process for steady state operating points and transient operation.

This paper reports the development of a fast empirical method to calculate the heat transfer coefficients on the surface of a radial turbine wheel for steady state operating points. In order to do this, steady state Conjugate Heat Transfer (CHT) investigations of a turbocharger turbine wheel for commercial application were performed to model the heat transfer between the fluid and the solid state. These investigations provide a basis for the analysis and characterization of the heat transfer distribution at the turbine wheel and the flow phenomena that cause these. The empirical method for determining heat transfer coefficients of a turbine wheel is developed based on the numerical results. To model the local different heat transfer coefficients the turbine wheel is divided in several surface segments which correspond to the geometry of a radial turbine wheel.

To validate the method the heat transfer coefficients from the empirical model are used as boundary conditions for a subsequent Finite Element Analysis (FEA). The calculated temperatures of the FEA results are compared to those of the CHT simulation and to the experimental data. For operating points with a circumferential velocity of u ≤ 0.75 u0 a good agreement are reached. The deviations increase for higher circumferential velocities. Furthermore the number of surface segments is varied to show the influence of the segmentation level to the temperature profile. It is also possible to reach a good agreement for operating points of u ≥ 0.75 u0 if the blade is segmented over its height. With the presented method a fast prediction of the heat transfer coefficients and the steady state temperature field of the turbine wheel are possible for steady state operating points.

Commentary by Dr. Valentin Fuster
2016;():V008T23A030. doi:10.1115/GT2016-57780.

Ceramic recuperators could enable microturbines to achieve higher fuel efficiency and specific power. Challenges include finding a suitable ceramic fabrication process, minimizing stray heat transfer and gas leakage, mitigating thermal stress, and joining the ceramic parts to neighboring metal components. This paper describes engine and recuperator design concepts intended to address these obstacles. The engine is sized to produce twelve kilowatts of shaft power, and it has a reverse-flow compressor and turbine. Motivations for this layout are to balance axial thrust forces on the rotor assembly; to minimize gas leakage along the rotating shaft; to reduce heat transfer to the compressor diffuser; to enable the use of a simple, single-can combustor; and to provide room for lightweight ceramic insulation surrounding all hot section components. The recuperator is an annular, radial counterflow heat exchanger with the can combustor at the center. It is assembled from segmented wafers made by ceramic injection molding (CIM). These are housed in a pressure vessel to load the walls mainly in compression, and are joined together by flexible adhesives in the cool areas to accommodate thermal expansion. A representative wafer stack was built by laser-cutting, laminating, and sintering tapecast ceramic material. The prototype was tested at temperatures up to 675°C, and the results were used to validate analytical and computational fluid dynamics (CFD) models, which were then used to estimate the effectiveness of the actual design. Turbomachinery efficiencies were also calculated using CFD, and allowances were made for additional losses like bearing friction and gas leakage. Based on these component performance estimates, a cycle model indicates the engine could achieve a net fuel-to-electrical efficiency of 21%, at a core weight including the recuperator of 11 kg, or about 1 kg/kW electric output.

Commentary by Dr. Valentin Fuster
2016;():V008T23A031. doi:10.1115/GT2016-57805.

An entirely new 40-kW-class steam turbine prototype has been constructed and successfully tested for more than 6,500 hours. The two-stage Curtis wheel with a nominal pressure ratio of about 130 has been designed for applications in bottoming water/steam cycles. The axial turbine runs at a rated rotational speed of 36,000 rpm, utilizing magnetic bearings and a permanent magnet synchronous generator which is coupled to the grid by frequency inverters. The integral turbine-generator set has been designed as a hermetically sealed assembly group. The turbine design allows both the turbine’s oil supply system and the bottoming cycle’s feed-water treatment system to be eliminated. The turbine has been designed to allow unmanned operation of the entire cycle with minimum maintenance requirements and reduced costs.

Extensive turbine testing, including rated power, overload and load rejection tests was carried out to verify functionality. Long-term operational capability was also demonstrated, giving particular attention to generator performance. Exemplarily a bottoming cycle, utilizing the turbine, matching the exhaust conditions of internal combustion engines was designed, increasing electrical efficiency from 40.4% to 43.4%, which represents a relative net improvement of 7.5%. In this application a turbine isentropic efficiency of more than 55% is expected, based on the congruence of measurement and calculation.

Commentary by Dr. Valentin Fuster
2016;():V008T23A032. doi:10.1115/GT2016-57828.

The development of a derivative small industrial gas turbine, sometimes termed a “microturbine”, is described. The target markets were the oil and gas upstream and midstream markets, and combined heat and power applications. These applications defined the product objectives. A general description is presented of the baseline recuperated gas turbine from which the new gas turbine is derived. The new gas turbine is a 333kWe recuperated gas turbine with a modernized compressor, variable inlet guide vanes for improved part-power efficiency and emissions, and an improved hot-section. The applicability and advantages of the GT333S features are compared with the market requirements. The development program included compressor rig testing, inlet guide vane development and testing, and engine testing. For the new product, a large number of components remained unchanged from GT250S, including most of the drivetrain. This provided confidence that the reliability of the GT333S, based on the millions of successful operating hours accrued on the GT250S, would translate into good GT333S reliability and availability. Extensive factory testing was performed to demonstrate the robustness of the new engine. The final performance results showed that the gas turbine achieved the program objectives. Some lessons learned from the development program are described.

Commentary by Dr. Valentin Fuster
2016;():V008T23A033. doi:10.1115/GT2016-57843.

Cryogenic Energy Storage (CES) technology which uses liquid air/nitrogen as energy carrier has attracted considerable attention recently due to its high exergy density (762kJ/kg) compared to other energy storage technologies. Liquid air/nitrogen occupies about 1/700 of the volume of its gaseous phase making it easier to store and transport. The stored energy can be recovered through a direct expansion process where the expander design and performance have a major effect on the efficiency of the energy conversion process. In this work the effects of surface roughness, tip clearance and trailing edge thickness on the performance of a small scale (tip diameter 40mm, mass flow rate 0.3 kg/s) axial cryogenic turbine have been investigated using mean line 1D analysis and ANSYS CFX 3D modelling where limited data available in the literature. Results showed that stator surface roughness has the highest impact on the turbine performance, where power output and turbine efficiency were significantly reduced as the roughness increased. For example at 20000RPM (design point) with stator roughness value of 0.5mm the efficiency and power output were 87.2% and 1197.7 W while for the same roughness on rotor blade the efficiency and power output were 89.34% and 1198.59 W. Regarding the effect of tip clearance, the efficiency decreases by 2% as the tip clearance increases from 0.35mm to 1mm.

Commentary by Dr. Valentin Fuster
2016;():V008T23A034. doi:10.1115/GT2016-57855.

Oil-free bearings for automotive turbochargers (TCs) offer unique advantages eliminating oil-related catastrophic TC failures (oil coking, severe bearing wear/seizure, and significant oil leakage, for example) while increasing overall system reliability and reducing maintenance costs. The main objective of the current investigation is to advance the technology of the gas foil bearings (GFBs) for automotive TCs by demonstrating their reliability, durability, and static/dynamic force characteristics desirable in extreme speed and temperature conditions. The paper compares drag friction and on-engine performances of an oil-free TC supported on GFBs against an oil-lubricated commercial production TC with identical compressor and turbine wheels. Extensive coast-down and fast acceleration TC rotor speed tests are conducted in a cold-air driven high-speed test cell. Rotor speed coast-down tests demonstrate that the differences in the identified rotational viscous drag coefficients and drag torques between the oil-free and production TCs are quite similar. In addition, rotor acceleration tests show that the acceleration torque of the oil-free TC rotor, when airborne, is larger than the production TC rotor due to the large mass and moment of inertia of the oil-free TC rotor even though air has lower viscosity than the TC lubricant oil. Separate experiments of the oil-free TC installed on a diesel engine demonstrate the reliable dynamic forced performance and superior rotor dynamic stability of the oil-free TC over the oil-lubricated TC. The post on-engine test inspection of the oil-free TC test hardware reveals no evidence of significant surface wear between the rotor and bearings, as well as no dimensional changes in the rotor outer surfaces and bearing top foil inner surfaces. The present experimental characterization and verified robustness of the oil-free TC system continue to extend the GFB knowledge database.

Commentary by Dr. Valentin Fuster
2016;():V008T23A035. doi:10.1115/GT2016-57971.

This paper aims at proposing a feasible method to determine an appropriate disc spacing distance in the design of Tesla turbines. Therefore, a typical Tesla turbine with seven different disc spacing distances was calculated numerically at different rotational speeds to investigate the influence of disc spacing distance on the aerodynamic performance and flow field of Tesla turbines and further to put forward the method. The results show that the isentropic efficiency of Tesla turbines peaks when the disc spacing distance gets its optimal value, and it decreases quickly as the disc spacing distance decreases from its optimal value. What’s more, the dimensionless parameter Ekman number is applied to determine an appropriate disc spacing distance in the design of Tesla turbines. There’s an optimal value of the Ekman number that Tesla turbines obtain its best performance, and it is influenced by the rotational speed. Meanwhile, the optimal value of the dimensionless rotor inlet tangential velocity difference which decides the rotational speed is also affected by the disc spacing distance. Thus, the determination of the optimal values of the dimensionless rotor inlet tangential velocity difference and the Ekman number is a cyclic iterative process to make them at their optimal values or in their optimal ranges respectively.

Commentary by Dr. Valentin Fuster
2016;():V008T23A037. doi:10.1115/GT2016-58168.

This work investigates the influence of hydrodynamic thrust bearings on the lateral rotor oscillations. Four thrust bearing models are compared in terms of their predictions of the oil-film pressure (Reynolds equation), the oil-film temperature (energy equation) and the load capacity. A detailed thrust bearing model using the generalized Reynolds equation and the 3D energy equation, a model using the standard Reynolds equation with a 2D energy equation, a model where the standard Reynolds equation and the 2D energy equation are decoupled and finally an isothermal thrust bearing model are presented. It is shown that in lower rotational speeds, the four models produce almost the same results. However, as the rotational speed is increased, the necessity for a thermo-hydrodynamic model is demonstrated. Run-up simulations of a turbocharger rotor/bearing system are performed, using an isothermal thrust bearing model with different inlet oil-temperatures. The influence of the oil-temperature of the thrust bearing on the subsynchronous rotor oscillations is investigated. Finally, a thermo-hydrodynamic model is compared with an isothermal in run-up simulations, where the influence of the variable oil-viscosity is discussed.

Commentary by Dr. Valentin Fuster

Steam Turbines

2016;():V008T26A001. doi:10.1115/GT2016-56030.

The design of a low reaction turbine blade profiles was carried out to improve the steam flow efficiency. The blade profiles geometry design for both the stationary and moving blades and reprofiling of them are done using Vista ATBlade, according to the aerodynamic analysis results from the cascade analysis code MISES.

The original stator profile is aft-loaded, and the new one present in this paper is highly-aft-loaded (HAL) to depress the development of secondary flow further, while maintaining even lower profile loss and wider incidence angle tolerance. The newly designed moving blade is more robust compared with the original one, thus it has larger aspect ratio under the same blade section average stress level, and with better incidence tolerant capability as well.

The planar cascade air tests were first carried out to verify the stator profile loss improvement, with a decrease of energy loss coefficient of almost 0.8% obtained under the Reynolds number of about 1e6. Then the annular cascade air tests with fully 360 degrees stator blades installed were conducted to validate the reduction of endwall loss and the profile loss as well, and to measure the mass flow capability (real mass flow/ideal mass flow).

Finally, two three-stage tests for the original blades and the new one were developed to verify the improvement under real multi-stages flow conditions. All the stages for both tests are designed with the hub reaction of about 15%, without interstage swirl, in the design condition. The flow probes at upstream of first stage stator and downstream of last stage moving blade, the hydraulic dynamometer and the flowmeter are used to test the overall efficiency. Three traverse planes are located at the upstream, middle and downstream of the second stage to measure the flow properties using five hole pneumatic probes. The test results showed a increase of overall efficiency of about 1.5%.

The CFD simulations showed very good agreement of mass flow capability with the tests, for both the stator annular and multi-stage tests.

The application of the newly designed blade profiles in SanHe subcritical reheat 300MW steam turbine (16.7MPa/537°C/537°C) retrofit gives the final proof of the efficiency improvement. The measured efficiency showed remarkable performance, with an increase of efficiency of 1.5%–2.2% for both the HP and IP cylinder.

Commentary by Dr. Valentin Fuster
2016;():V008T26A002. doi:10.1115/GT2016-56063.

A well-designed exhaust hood of large steam turbines would recover some kinetic energy from the flow between the last stage blades and condenser, which improves the efficiency of the cylinder. The internal flow field of the exhaust hood was firstly numerical investigated using three-dimensional Reynolds-Averaged Navier-Stokes (RANS) solutions based on the ANSYS-CFX. Then, the effects of the dimensions of the cylinder, bearing cone, diffuser guide, and diffuser ribs on the static pressure recovery performance of the exhaust hood were numerically conducted. The numerical results show that the cylinder length has significantly impact on the static pressure recovery coefficient of the exhaust hood by comparison of the cylinder section area at the fixed bearing cone and diffuser size. The bearing cone and diffuser were optimized to improve the aerodynamic performance of the exhaust hood. The rotationally symmetrical and enlarged diffusers show the different static pressure recovery performance of the exhaust hood. The optimized exhaust hood shows the improved aerodynamic performance by comparison of the initial design. The detailed flow pattern of the initial and optimized exhaust hood is also illustrated and discussed. This paper explicitly shows the interaction, and offers a good strategy for optimization, which has not been thoroughly discussed.

Commentary by Dr. Valentin Fuster
2016;():V008T26A003. doi:10.1115/GT2016-56065.

The transient steam flow inside a curved channel, which is embedded with dual butterfly valves (stop valve & throttle valve), was numerically simulated when the throttle valve was rapidly closed. Two cases with different closing time were particularly compared in terms of the instantaneously varying flow pattern and aerodynamic torque, i.e., 0.5 s and 1.0 s; as the benchmark cases, the flows at different opening angles of the valve disk were also simulated for comparison. To accurately capture the transient flow, a total of 30 meshes corresponding to different opening angles of the throttle valve’s disk were preliminarily built by using ANSYS ICEM CFD 14.0, while mesh deformation and remeshing methods were used as Dynamic Mesh model to get the consecutively updated mesh. The SST model was chosen as turbulence model and ANSYS CFX 14.0 Software Package was used to solve the flow field. The results showed that rapid close of the throttle valve would lead to an overall flow imbalance between the inlet and the outlet, which was deteriorated with the decrease in the closing time.

Commentary by Dr. Valentin Fuster
2016;():V008T26A004. doi:10.1115/GT2016-56088.

In Steam Turbines, under low flow conditions, the flow structure on the long last stage blades is complex. The rotor blades create outward radial flow. Recirculations are setup near the tip in the gap between the fixed and moving blades, and near the hub downstream of the moving blade. The blade carries negative loading and encounters gross flow separations. In this environment, fluctuations in pressure are detected rotating at about half of the rotor speed. Some similarities exist with rotating stall, as found in compressors. In the validation of a new blade design, checks are therefore included to ensure that the rotating excitation does not pass over a natural frequency of the blading. In turn, this can reduce the available design space.

A less restrictive approach is to consider alleviation techniques. A promising candidate is a scheme where steam jets are directed into the flow, onto the LSB, from the outer boundary. Jets have been introduced and tested on a 1/3rd scale multistage steam turbine. The test turbine is both aerodynamically and mechanically representative of a full size machine. The blowing scheme was shown to reduce and then practically eliminate the rotating pressure pattern. 3D CFD computations reveal the major influence of the jets. The solution is elegant because it does not lead to loss of efficiency or design space.

Commentary by Dr. Valentin Fuster
2016;():V008T26A005. doi:10.1115/GT2016-56114.

A method is presented for predicting the energy loss for a 2D turbine cascade blade operating in subsonic regions where the exit Mach number ≤ 0.8. A prediction method based on entropy creation was used to analyze the cascade profile loss mechanism. The basic profile loss model was introduced from the isentropic Mach number distribution along the blade surface and the trailing edge loss model was introduced from available test data, CFD results and available loss models. In addition, the Reynolds number correction curve was applied from previous research. Linear cascade test datasets which represent hub, mid-span and tip sections were used to validate this loss model.

Commentary by Dr. Valentin Fuster
2016;():V008T26A006. doi:10.1115/GT2016-56136.

The turbine blade is one of the most critical components of a steam turbine. The high thermal loads and large centrifugal forces cause extreme stresses on the blade, especially on its root.

This paper focuses on improving the double-T root of a turbine blade of the control stage by decreasing the root’s peak equivalent von-Mises stress. An 18% reduction was achieved in the peak stress by changing the convexity of the contact surface between the root and the groove. The equivalent von-Mises stress was determined in a static structural analysis of a three dimensional finite element model (3D FEM-model) using ANSYS Workbench. This numerical model was developed to include one blade and the associated part of the shaft, whereas the complete circle of blades was considered by applying cyclic symmetry. Furthermore, this paper includes a modal analysis comparing the natural frequencies of the initial FEM-model with the frequencies of the optimized one. The results were established by an investigation of the influence of the FEM-model’s parameters, its material properties, thermal effects, and an additional damping wire in the shroud.

Commentary by Dr. Valentin Fuster
2016;():V008T26A007. doi:10.1115/GT2016-56148.

Liquid films in steam turbines, present in usual operating conditions, play a large but poorly understood part in the wetness-born troubles (power losses and erosion). More knowledge is needed to estimate their impacts and lessen their effects. The aim of this paper is to propose and verify a model to predict these liquid films. This model is based on modified Shallow-Water equations (integral formulation). It takes into account inertia, mass transfer, gravity, gas and wall frictions, pressure, surface tension, droplet impacts, rotational effects and is unsteady. A 2D code has been developed to implement this model. A part of the model has been verified with analytical solutions (Riemann problems and inclined lake at rest), has been confronted with the linear stability of falling liquid film and has been validated with the experiment of Hammitt et al. [1] which involves a sheared film under low-pressure steam turbine conditions.

Commentary by Dr. Valentin Fuster
2016;():V008T26A008. doi:10.1115/GT2016-56179.

Effect of cooling hole structure on the steam cooling performance of the solid components (including the rotor blades, roots and wheel discs) in intermediate pressure turbine stages for an ultra-supercritical steam turbine was numerically investigated using the hybrid method of flow field calculation and conjugated heat transfer. Numerical solutions were carried out for four cooling hole structures, which includes a designed structure and three modified structures. Numerical results presented and compared the steam flow patterns in the disc cavities and temperature distributions in the rotating components for all cooling hole structures. Reducing through flow area of the cooling hole at the bottom of the blade root results in a significant strengthen in the preventing ability to hot main stream ingress of the cooling steam, as well as an increase in the cooling regions of the blade root and wheel disc. Furthermore, the overall solid temperature and temperature gradient can be efficiently reduced by modifying the cooling hole structure. Numerical results also reveal that it is possible to prevent the hot main steam ingress and increase cooling performance by modifying the steam cooling hole structure, which optimizes the flow pattern and flow rate distribution of the cooling steam.

Topics: Pressure , Cooling , Turbines , Steam
Commentary by Dr. Valentin Fuster
2016;():V008T26A009. doi:10.1115/GT2016-56180.

An automated multidisciplinary optimization system for multi-objective design of the long blade turbine stage for steam turbines is developed in this paper. The Self-adaptive Multi-objective Differential Evolution (SMODE) algorithm, cubic non-uniform B-Spline curves based on surface modeling technology for three-dimensional turbine blade parameterization method, aerodynamic and mechanical performance of long blade turbine stage evaluation approach are coupled in the presented multidisciplinary optimization system. The aerodynamic performance of long blade turbine stage design candidates is evaluated using three-dimensional Reynolds-Averaged Navier-Stokes (RANS) solutions. The mechanical performance of the designed long rotor blade is analyzed using Finite Element Analysis (FEA) method based on the software ANSYS. Multi-objective design of long blade turbine stage is conducted using the developed multidisciplinary optimization methodology for the maximization of specific power and minimization of maximum Von Mises Stress with constraints on mass flow rate. The design variables are specified by the stator and rotor blade parameterization method. The Pareto solutions of the multidisciplinary optimization design for the long blade turbine stage are obtained. The aerodynamic and strength performance of obtained Pareto solutions improves obviously by comparison of the referenced design. The dynamic frequency with pre-stress of the referenced and optimized long rotor blade is also calculated in order to avoid resonance. The availability of the presented multidisciplinary optimization system for multi-objective design of long blade turbine stage for steam turbines is also demonstrated.

Commentary by Dr. Valentin Fuster
2016;():V008T26A010. doi:10.1115/GT2016-56314.

The power output of steam turbines is controlled by steam turbine inlet valves. These valves have a large flow capacity and dissipate a huge amount of energy in throttled operation. The dissipation process generates strong pressure fluctuations resulting in high dynamic forces causing valve vibrations. A brief survey of the literature dealing with valve vibrations reveals that vibrational problems and damages mostly occur in throttled operation when high speed jets, shocks, and shear layers are present. As previous investigations reveal that a feedback mechanism between the dynamic flow field and the vibrating valve plug exists, the vibrations are investigated with two-way coupled simulations. The fluid dynamics are solved with a scale-adaptive approach to resolve the pressure fluctuations generated by the turbulent flow. The finite element model solving the structural dynamics considers both frictional effects at the valve packing and contact effects caused by the plug impacting on the valve bushing. As different flow topologies causing diverse dynamic loads exist, the fluid flow and the structural dynamics are simulated at different operating points. The simulations show that differences to the one-way coupled approach exist leading to a change of the vibrational behavior. The physics behind the feedback mechanisms causing this change are analyzed and conclusions regarding the accuracy of the one-way coupled approach are drawn.

Commentary by Dr. Valentin Fuster
2016;():V008T26A011. doi:10.1115/GT2016-56360.

At the 2015 ASME Turbo Expo in Montreal, we presented a paper on unsteady three-dimensional wet-steam flow simulations for the last three stages of a low-pressure real steam turbine. We then focused on the investigation of unsteady wetness in the three-stage blade passages, which was conducted by assuming the same number of blades in the previous study and the real blade number. The obtained results showed that wetness is definitively influenced by the blade number difference between the stator and the rotor. This paper presents a numerical investigation of unsteady pressure forces on the multi-stage blade rows caused by stator-rotor interactions, which include unsteady wakes, vortices, shocks, and wetness. In particular, we investigate the effect of blade number variation on the pressure forces. Our results indicate that unsteady pressure forces are significantly influenced by shocks from the upstream stator trailing edges transferred to the adjacent rotor blade noses. We finally found that the unsteady pressure forces on the rotor blades are strongly influenced by shocks from upstream stator trailing edges near the hub region and the forces result in a time-dependent torque difference between neighboring two rotor blades.

Commentary by Dr. Valentin Fuster
2016;():V008T26A012. doi:10.1115/GT2016-56378.

A validated non-linear uncoupled method for flutter stability analysis was employed to estimate the aerodynamic damping of an HP (High-Pressure) steam turbine blade row.

Usually such blade rows are not affected to flutter instability problems, yet an estimation of the aerodynamic damping can be useful for an accurate aeromechanical characterization of these kind of blade rows. The geometry under investigation is a typical steam turbine blade row at design point. Computational aeroelastic analyses are performed on the more relevant modeshape, sampling the nodal diameters, in order to well describe the typical aeroelastic stability curve. The presence of the tip shroud implies a strong mechanical coupling between adjacent blades resulting in complex modeshapes with high frequency, significantly different from those usually analyzed in the flutter analysis.

The results in term of aerodynamic damping curves are rather different from the usually sinusoidal shape. This is due to the large variation of the frequency over the analyzed nodal diameters, especially at low nodal diameters range. This results are useful to give a better insight in the aeroelastic response of this type of blades.

Commentary by Dr. Valentin Fuster
2016;():V008T26A013. doi:10.1115/GT2016-56518.

The wide use of fir-tree root and groove in turbine structures prompts the expectation to find optimum configurations, which ensure that stresses are in the safe limits to avoid mechanical failure. To perform the optimization, the reasonable characterization of root configuration is required. The existing researches characterized the fir-tree root with straight line, arc or even elliptic fillet, then the parameters of these features were defined as design variables to perform root profile optimization. However, this feature-based optimization technique yields configuration which is only optimal under the feature assumption, the question why choose these feature and whether there is a better feature modeling technique is difficult to answer. In this work, instead of the feature-based method, spline curves technique is involved to characterize the root and groove configuration, and the horizontal coordinates of the control points are selected as design variables, which are modified in the vicinity of their initial values during optimization process. The objective function is to minimize the peak stress in the root and groove regions. With the Multi-island genetic algorithm, the optimal fir-tree root configuration can be obtained with better stress distributions and low stress concentrations. The proposed spline-based optimization approach may shed lights on the conceptual design of blade root and can be easily extended to other industrial equipment design.

Commentary by Dr. Valentin Fuster
2016;():V008T26A014. doi:10.1115/GT2016-56529.

One of the main functions of the steam turbine admission valve is to provide a very fast closing to intercept the supply of mass flow rapidly from the steam entering the turbine and cause destructive overspeed. In order to quickly close the admission valve, at the beginning of the stroke the moving parts of the valve should be accelerated to a high speed; when the stroke ends, a lot of kinetic energy is converted to impact energy. To prevent damage to valve parts, a quick closing buffer system is required to absorb the most of the impact energy. The quick closing buffer system plays an important role in the admission valve as an influencing factor of the dynamic characteristics of the valve. In the past, considering the complex internal structure, the research about quick closing buffer system relied on confirmatory experimental study or analytic method to get a quick closing buffer process. This paper focus on the quick closing process of a high pressure steam admission valve of Shanghai Turbine Plant. The dynamic characteristic of the quick closing buffer system is investigated by means of method of CFD numerical simulation for the first time, in order to find a more convenient and effective way to get the key factors that affect the dynamic characteristics, and the accuracy of the of CFD numerical simulation is verified by test, which are valuable for building an accurate dynamic characteristic analysis model of steam turbine admission valve.

Commentary by Dr. Valentin Fuster
2016;():V008T26A015. doi:10.1115/GT2016-56547.

This work aims at investigating the impact of axial gap variation on aerodynamic performance of a high-pressure steam turbine stage.

Numerical and experimental campaigns were conducted on a 1.5-stage of a reaction steam turbine. This low speed test rig was designed and operated in different operating conditions. Two different configurations were studied, in which blades axial gap was varied in a range from 40% to 95% of the blade axial chord. Numerical analyses were carried out by means of three-dimensional, viscous, unsteady simulations, adopting measured inlet/outlet boundary conditions. Two set of measurements were performed. Steady measurements, from one hand, for global performance estimation of the whole turbine, such as efficiency, mass flow, stage work. Steady and unsteady measurements, on the other hand, were performed downstream of rotor row, in order to characterize the flow structures in this region.

The fidelity of computational setup was proven by comparing numerical results to measurements. Main performance curves and span-wise distributions shown a good agreement in terms of both shape of curves/distributions and absolute values. Moreover, the comparison of two dimensional maps downstream of rotor row shown similar structures of the flow field.

Finally, a comprehensive study of the axial gap effect on stage aerodynamic performance was carried out for four blade spacings (10%, 25%, 40% and 95% of S1 axial chord), and five aspect ratios (1.0, 1.6, 3, 4 and 5). The results pointed out how unsteady interaction between blade rows affects stage operation, in terms of pressure and flow angle distributions, as well as of secondary flows development. The combined effect of these aspects in determining the stage efficiency is investigated and discussed in detail.

Commentary by Dr. Valentin Fuster
2016;():V008T26A016. doi:10.1115/GT2016-56553.

A fast method is presented for evaluating the impact of blade manufacturing variations on the aerodynamic performance of turbomachines. The method consists of two parts. Firstly, an adjoint method is developed to evaluate the aerodynamic sensitivities for multistage turbomachines. This sensitivity information may then be used to perform fast direct Monte Carlo simulations to obtain the statistical distribution of the variations of aerodynamic performances resulting from any given set of manufacturing variations. Secondly, a method is developed to construct reduced-order models for the three-dimensional blades manufacturing variations using the Principal Component Analysis (PCA) method. Monte-Carlo simulations with the adjoint sensitivities can then be applied to the full and individual modes of the blade manufacturing deviations. The proposed method is applied to the last two stages of a low-pressure steam turbine. A total of 29 sets of measured manufacturing deviations of the last-stage rotor blades are used to construct a reduced-order model of the manufacturing variations. The manufacturing variation reduced-order model helps identify origins of the manufacturing deviations connected to the machining processes of the blades. Relations of the statistics of the aerodynamic performance variations such as mean, standard deviation, etc. to the different modes of manufacturing deviations are studied and analyzed.

Commentary by Dr. Valentin Fuster
2016;():V008T26A017. doi:10.1115/GT2016-56640.

The design of an effective diffuser for a given last stage blade of an LP turbine is known to be highly dependent on the size and shape of the exhaust hood in which it is located. For retrofit steam turbines in particular, where a new last stage blade and diffuser are fitted into an existing exhaust hood, the shapes and sizes of the exhaust box have been seen to vary significantly from one contract to the next. An experimental parametric study of diffuser lips and exhaust hood configurations has been run on a model test turbine rig at GE Power to investigate the impact of various geometric parameters on the performance of the diffusers. Improved testing and post-processing methodologies means the diffuser performance has been obtained for a greater number of geometric configurations than was previously typically possible. The results of these experiments are compared with numerical calculations and confirm the accuracy of the standard in-house diffuser design tools. Key geometric parameters are identified from the test data and used to generate improved diffuser design guidelines.

Commentary by Dr. Valentin Fuster
2016;():V008T26A018. doi:10.1115/GT2016-56917.

Effect of leakage flow in honeycomb shroud seals on the aerodynamic performance of steam turbine stages in the high-pressure cylinder was numerically investigated by using the commercial CFD (Computational Fluid Dynamics) software ANSYS CFX11.0. The geometrical parameters of the honeycomb shroud seal, including the sealing clearance, cell depth and cell diameter, were selected as the research objectives to compute the aerodynamic performance of turbine stages at a wide range of dimensions. The numerical results show that the leakage rate in the shroud honeycomb seal is almost linearly increased with increase of sealing clearance. Correspondingly, the total-total isentropic efficiency of turbine stages decreases as well. As the cell depth increases, the total-total isentropic efficiency of the turbine stages is firstly increased and then almost kept constant, and the leakage rate in the honeycomb shroud seal is firstly decreased and then almost kept constant as well. For different honeycomb cell diameters, the leakage rate and stage efficiency are mainly determined by the flow structures in the honeycomb cells and seal outlet region. The present studies also show that, as the cell diameter increases, the total-total isentropic efficiency increases whereas the leakage rate decreases. Among the studied geometrical parameters (i.e. sealing clearance, cell depth and cell diameter), the variation of sealing clearance has a pronounced influence on the mixing loss in the main flow paths, but the variation of cell diameter has less effect on the aerodynamic performance of the turbine stages than that of sealing clearance. If the cell depth is not very small, the variation of cell-depth has a minor effect on the aerodynamic performance in the turbine stages.

Commentary by Dr. Valentin Fuster
2016;():V008T26A019. doi:10.1115/GT2016-56921.

The hole-pattern seal is usually used as a replacement of honeycomb seal due to their similarities in geometry and performance (leakage and rotordynamic). Compared with the honeycomb seal, the hole-pattern seal is easier to be manufactured and installed thus it is welcomed by the manufactures. However, most current literatures about the hole-pattern seal mainly addressed the rotordynamic characteristic of the shaft seal. Almost no research attempts to give an insight into the aerodynamic performance of the hole-pattern seal applied in turbine stages. Therefore, the main objective of the present paper is to investigate how the hole-pattern seal geometries, i.e. sealing clearance, hole-diameter and hole-depth, affect the aerodynamic performance of the steam turbine stages. With the commercial CFD (Computational Fluid Dynamics) software ANSYS CFX11.0, the leakage rates and aerodynamic efficiencies for the two stages with hole-pattern shroud seals were obtained and compared with those configured with honeycomb shroud seals at a range of seal dimensions. The results show that the leakage rate from the hole-pattern shroud seal is a bit higher than that from the honeycomb shroud seal at the same geometrical parameters (i.e. sealing clearance, hole/cell-diameter and hole/cell-depth). However, for these two configurations, the aerodynamic efficiencies are very close at the small sealing clearance cases. Big differences are shown at the large sealing clearance cases due to the difference in hole-area ratio. For the turbine stages with various hole-diameters and hole-depths, the aerodynamic performance of the turbine stages with honeycomb/hole-pattern seals are mainly affected by the flow patterns at the seal outlet if the sealing clearance is fixed. The sealing clearance has little effect on the flow pattern in the cells/holes, but it has a significant effect on the flow fields in the seal outlet chamber, thus affects the secondary flow development in the downstream flow paths.

Commentary by Dr. Valentin Fuster
2016;():V008T26A020. doi:10.1115/GT2016-56939.

The usual ways to measure the aerodynamic forcing function are complex and expensive. The aim of this work is to evaluate the forces acting on the blades using a relatively simpler experimental methodology based on a time-resolved pressure measurement at the rotor discharge. Upstream of the rotor, a steady three holes probe has been used.

The post processing procedures are described in detail, including the application of a phase-locked average and of an extension algorithm with phase-lag. The algorithm for the computation of the force components is presented, along with the underlying assumptions.

In order to interpret the results, a preliminary description of the flow field, both upstream and downstream of the rotor, is provided. This gives an insight of the most relevant features that affect the computation of the forces.

Finally, the analysis of the results is presented. These are first described and then compared with overall section-average results (torque-sensor), and with the results from 3D unsteady simulations (integral of pressure over the blade surface) in order to assess the accuracy of the method. Both the experimental and the numerical results are also compared for two different operating conditions with increasing stage load.

Commentary by Dr. Valentin Fuster
2016;():V008T26A021. doi:10.1115/GT2016-56974.

The performance of the radial diffuser of a low pressure (LP) steam turbine is important to the power output of the turbine. A reliable and robust prediction and optimization tool is desirable in industry for preliminary design and performance evaluation. This is particularly critical during the tendering phase of retrofit projects which typically cover a wide range of OEM and oOEMs designs. This work describes a fast and reliable numerical approach for the simulation of flow in the last stage and radial diffuser coupled with the exhaust hood. The numerical solver is based on a streamline curvature throughflow method and a geometry-modification treatment has been developed for off-design conditions, at which large scale flow separation may occur in the diffuser domain causing convergence difficulty. To take into account the effect of tip leakage jet flow, a boundary layer solver is coupled with the throughflow calculation to predict flow separation on the diffuser lip. The performance of the downstream exhaust hood is modeled by a hood loss model (HLM) that accounts for various loss generations along the flow paths. Furthermore, the solver is implemented in an optimization process. Both the diffuser lip and hub profiles can be quickly optimized, together or separately, to improve the design in the early tender phase. 3D CFD simulations are used to validate the solver and the optimization process. The results show that the current method predicts the diffuser/exhaust hood performance within good agreement with the CFD calculation and the optimized diffuser profile improves the diffuser recovery over the datum design. The tool provides GE the capability to rapidly optimize and customize retrofit diffusers for each customer considering different constraints.

Commentary by Dr. Valentin Fuster
2016;():V008T26A022. doi:10.1115/GT2016-56988.

The design of steam turbine components is driven by high efficiency demands and also requirements for increased operational flexibility due to more renewable energy sources being added to the grid. Therefore, fossil power plants which operate reliably under these conditions must be designed. Robust low pressure (LP) end stage blades are one key factor for modern steam turbine design to meet current and future market requirements.

In operation, LP end stage blades of steam turbines are exposed to complex mechanical load, resulting in stresses mainly due to blade vibration and high centrifugal forces. Design methods accounting for high cycle fatigue (HCF) and low cycle fatigue (LCF) are required for fatigue lifetime calculation. To determine the HCF component strength and to validate the calculation procedure, an HCF component test facility for full-scale LP end stage blades has recently been established at Siemens. Besides the validation of the calculation procedures, the full-scale component tests serve as part of upfront validation to minimize risk for first time implementation of newly developed as well as next generation blades, and to demonstrate operational robustness of the existing fleet.

This paper describes the development and setup of the HCF component test facility for full-scale LP end stage blades at Siemens, the successful execution of HCF component tests with blades of different sizes, surface conditions and materials, and the evaluation of the results. In addition, crack growth and threshold behavior has been investigated in detail. Based on the test results, validation of the corresponding calculation methods has been performed. An outlook on further development of test facilities is provided.

Commentary by Dr. Valentin Fuster
2016;():V008T26A023. doi:10.1115/GT2016-56995.

This paper presents a series of experiments on the Aerostatic Seal, a dynamic clearance seal for steam turbine application first described at the 2015 ASME Turbo Expo (Paper Number GT2015-43471). This dynamic clearance seal moves with rotor excursions and so has the potential to deliver a smaller clearance than traditional seals. The concept is an extension of the retractable seal design which is widely used in existing steam turbines.

The experimental program was carried out in a low cost static test facility using an aerostatic seal design. The seal exhibited a dynamic clearance response and will therefore respond to rotor excursions. 3D CFD was also used to aid the understanding of flow features not captured by the analytical design tool. Adjustments to both the design process and to future seal designs are proposed in the body of the paper.

This paper therefore describes an experimental proof of concept for the aerostatic seal and paves the way for future development in rotating facilities.

Commentary by Dr. Valentin Fuster
2016;():V008T26A024. doi:10.1115/GT2016-57102.

700°C HUSC technology is considered as the next generation of more efficiently coal-fired power generation technology, the heat rate of which can be reduced by more than 8% on the basis of current ultra-supercritical units. That means there is a huge energy saving benefits. With the main steam / reheat steam temperature increasing from 600°C / 620 °C to 700°C/ 720°C, the temperature of extraction steam increases dramatically, especially the first extraction stage after reheater, the temperature of which will increase to 630 ∼ 650 °C. That means a substantial increase in the cost of the initial investment because of the nickel-based material being used in extraction pipe and heaters. With EC system, the extraction steam temperature is reduced sharply because the high temperature extraction steam is moved from the main turbine to a small parallel extraction turbine and the steam source of the small extraction turbine is from the cold reheater. So the highest extraction steam temperature will not exceed 500 °C, and the high temperature risk of heat recovery system will be eliminated completely.

In this paper, exergy theory is introduced to analyze the cycle efficiency of the new thermodynamic system and the conventional one. In order to obtain a better 700 °C high ultra-supercritical thermodynamic system solution, GA method is used to optimize the regenerative system parameters to lower the overall heat consumption. The exergy theory is also used to analyze the reason why optimal solution can bring economic benefits. Finally, the feasibility of the entire system project will be analyzed.

Commentary by Dr. Valentin Fuster
2016;():V008T26A025. doi:10.1115/GT2016-57149.

Since many years the diffuser and exhaust of low pressure (LP) turbines have been in the focus of turbine development and accordingly broadly discussed within the scientific community. The pressure recovery gained within the diffuser significantly contributes to the turbine performance and therefore plenty of care is taken in investigations of the flow as well as optimization within this part of the turbine. However on a plant level the component following the LP turbine is the condenser, which is connected by the condenser neck. Typically the condenser neck is not fully designed to provide additional enthalpy recovery. Due to plant arrangement reasons, often it is full of built-ins like stiffening struts, feed-water heaters, extraction pipes, steam dump devices and others. It is vital to minimize the pressure losses across the condenser neck, in order to keep performance benefit, previously gained within the diffuser. As a general rule, each mbar of total pressure loss in a condenser neck may reduce the gross power output up to 0.1%.

While turbines usually follow a modular approach, the condenser is typically designed plant specific. Therefore, on a plant level it is crucial to identify and evaluate the loss contributors and develop processes and tools which allow an accurate and efficient design process for an optimized condenser neck design. This needs to be performed as a coupled modelling approach, as both, turbine and condenser flow interact with each other. 3-D CFD tools enable a deep insight into the flow field and help to locally optimize the design, as they help to identify local losses and this even for small geometrical design changes. Unfortunately these tools are costly with respect to computational time and resources, if they are used to analyze a full condenser neck with all built-ins. Here 1-D modelling approaches can help to close the gap, as they can provide fast feedback, e.g. in a project tender phase, or can allow to quickly analyze design changes. For this they need a proper calibration and validation. This publication discusses the CFD modelling of a LP steam turbine coupled to a condenser neck and the validity of such calculations against measurement data. In the second publication (Part 2) a simplification of the gained information to a 1-D modelling approach will be discussed.

Commentary by Dr. Valentin Fuster
2016;():V008T26A026. doi:10.1115/GT2016-57161.

Since many years the diffuser and exhaust of low pressure (LP) turbines are in focus of turbine development and accordingly broadly discussed within the scientific community. The pressure recovery gained within the diffuser significantly contributes to the turbine performance and therefore care is taken in flow investigation as well as optimization within this part of the turbine. However, on a plant level the component following the LP turbine is the condenser, which is connected by the condenser neck. Typically the condenser neck is not designed to provide additional enthalpy recovery. For plant design reasons, often numerous built-ins like stiffening struts, extraction pipes, steam dump devices and others are placed into the neck. Here it is vital to keep the pressure losses low, in order not to deteriorate performance, previously gained within the diffuser. Each mbar of total pressure loss in a condenser can reduce the plant power output up to 0.1%.

As discussed in the first publication (Part 1), 3D CFD enables a deep insight into the flow field, which is costly with respect to computational time and resources. But there are phases during project execution, when geometry and/or boundary conditions are not fixed and quick estimation of pressure loss and recovery in the condenser neck is needed for benchmark of designs or design changes (e.g. tender phase). Here 1D modelling approaches can help to close the gap.

Analysis of available 1D correlation of flow around obstacles has shown that these need to be adapted to the flow conditions in a condenser neck of a steam, nuclear or combined-cycle power plant. Therefore, the fluid fields, calculated and discussed in the first publication (Part 1), were analyzed regarding pressure loss created by single obstacles and interaction of built-ins of different size, number and shape. Furthermore, a 1D velocity to be used for 1D calculation was derived from the 3D velocity field. In addition, vacuum-correction-curves were implemented to cover the range of possible operating conditions.

This publication discusses the development of a 1D model for calculation of pressure loss in a condenser neck and the validity of such calculations against measurement and 3D CFD data.

Commentary by Dr. Valentin Fuster
2016;():V008T26A027. doi:10.1115/GT2016-57204.

Today’s challenging requirements on thermal power plant cycle efficiency favor plants with steam temperatures up to 630°C. Cost competition, however, confines the application of particularly cost-intensive materials causing high manufacturing effort such as Ni-based alloys. Alternatives are martensitic steels containing less than 10% Cr. Due to the low Chromium content, these materials are less oxidation-resistant compared to 12% Cr-steels. Increased load-cycling requirements resulting from varying renewable energy production may result in decreased plant efficiency due to increased scaling and spallation of oxide layers during thermal transients. Hence, it is beneficial to protect such components against oxidation using coatings.

Investigations on various coating systems for their potential to serve as oxidation protection are described in this paper. These investigations consisted of a short-term screening program to identify the most promising coatings followed by an extensive test program including long-term steam exposure at high temperatures, thermal cycling, solid particle erosion tests as well as tests under operating conditions on samples and blades.

The test program revealed which coatings appeared to be the most promising solution for power plant applications, showing excellent oxidation protection capability of the base material in steam at high temperatures, structural stability upon thermal cycling and good solid-particle erosion protection. Tests under operational conditions have proven the functionality and stability of the coatings.

Commentary by Dr. Valentin Fuster
2016;():V008T26A028. doi:10.1115/GT2016-57215.

Unsteady flows in steam turbine control valves are becoming more and more a subject of interest for many reasons. In general, steam conditions have increased for both industrial and power plant turbines. Flexible operation conditions of industrial turbines are a field of research ever since, but became more and more interesting for power plants due to the significant rise of renewable energies and their natural inconstant supply.

In this paper the results of experiments in an original sized industrial steam turbine control valve are presented. The experiments with air as fluid focus on the flow in the diffuser and are recorded using PIV techniques. With the results typical flow patterns are derived and allocated to a number of operating points with variation in pressure and opening ratio and therefore also in mass flow.

Commentary by Dr. Valentin Fuster
2016;():V008T26A029. doi:10.1115/GT2016-57247.

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 costs. In order to increase the plant cycle efficiency, during the past years, nominal steam temperatures and pressures have been continuously increased. To fulfill the lifetime requirements and still achieve the product cost requirements, accurate mechanical integrity based assessments on cyclic lifetime became more and more important. For this reason, precise boundary conditions in terms of local temperatures as well as heat transfer coefficients are essential. In order to gain such information and understand the flow physics behind them, more and more complex thermal modelling approaches are necessary, like Computational Fluid Dynamics (CFD) or even Conjugate Heat Transfer (CHT). A proper application of calculation rules and methods is crucial regarding the determination of thermal stresses, thermal expansion, lifetime or creep. The aim is to exploit during turbine developments the limits of the designs with the selected materials. A huge effort especially in validation and understanding of those methodologies was done with detailed numerical investigations associated to extensive measurement studies at onsite turbines in operation. This paper focuses on the validation of numerical models based on CHT calculations against experimental data of a large intermediate pressure steam turbine module regarding the temperature distribution at the inner and outer casing for nominal load as well as transient shut-down.

Commentary by Dr. Valentin Fuster
2016;():V008T26A030. doi:10.1115/GT2016-57258.

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, such as baffles or ribs.

To assess the influence of supporting ribs on the vibration behavior of the last stage rotor blades, tests with two rib configurations were performed in a single stage LP model steam turbine at the Institute of Thermal Turbomachinery and Machinery Laboratory (ITSM) at the University of Stuttgart, Germany.

At design load and overload operating conditions no significant change in blade vibration amplitudes is observed in case of a supporting rib in close vicinity to the rotor blades compared to the reference case without installations. However, at part load operating conditions a remarkable reduction in blade amplitudes is found rather unexpectedly.

The present work shows that changes in the pattern and the frequency content of the flow within the diffuser, caused by the interaction between supporting rib and steam flow is evidently responsible for this.

Commentary by Dr. Valentin Fuster
2016;():V008T26A031. doi:10.1115/GT2016-57321.

Flexibility and availability together with fast startup times become more and more important for steam turbine operation. Exact knowledge about the turbine components stresses and lifetime consumption during transient operation is a prerequisite in order to meet these requirements.

A transient FE model of an intermediate pressure steam turbine rotor was generated, allowing the prediction of temperature and elastic stress field during turbine startup, load changes and shutdown. Operating data of the steam parameters and of a thermocouple inside the wall of the turbine inner casing were used to indirectly validate the thermal FE model in order to reproduce the measured metal temperatures in a proper accuracy.

Subsequently a probabilistic sensitivity study was performed in order to identify the influence of scattering or not well known boundary conditions on the calculated lifetime consumption of the steam turbine rotor during a cold start. This in fact provides information about the accuracy of the prediction. The results of the sensitivity study also help to improve the model accuracy by identifying the boundary conditions with the largest impact on lifetime prediction uncertainty, i.e. the boundary conditions that need further investigation.

Commentary by Dr. Valentin Fuster
2016;():V008T26A032. doi:10.1115/GT2016-57339.

The demand for higher plant cycling operation and reduced life-cycle costs are the main drivers for the design and assessment of turbine components today. Heavy cyclic loading increases the potential of fully utilizing the fatigue capabilities of the material which might lead to crack initiation and subsequent crack propagation.

Fracture mechanics methods and evaluation concepts are widely applied to assess the integrity of components with defects or crack-like findings. The realistic modelling of the failure mechanism plays a key role for the accurate prediction of crack sizes at failure state.

A basic treatment of material toughness typically leads to conservative assessments for components with sufficient ductility. A standard approach to describe material behavior with high ductility is to use the start of stable crack extension as a dimensioning parameter for the analysis. By definition a critical condition for a component is reached when the crack driving force is equal to the characteristic material parameter. On the other hand, advanced analysis methods allow determination of the instability point (ductile tearing analysis).

This paper will discuss two cases for practical analysis from steam turbine design showing clear advantages for service application by using advanced analysis methods.

Commentary by Dr. Valentin Fuster
2016;():V008T26A033. doi:10.1115/GT2016-57374.

Flexible operation at a wide range of operating conditions combined with very high efficiency is a fundamental customer requirement for industrial steam turbines today. The blading of turbines has to be designed with those objectives in mind. Especially for intermediate pressure (IP) blades operating at fixed and variable speed there is high potential for improvement with respect to efficiency. Nonetheless these blades have to satisfy rules of reliability and mechanical integrity, in many cases American Petroleum Institute (API) Standard 612 requirements. In this paper the dynamic behavior of IP blades with an improved and efficient 3D design is investigated numerically and verified by experiments. Shrouded blades are common for IP blade design to ensure a low dynamic stress level. These blades are supposed to form a closed coupling contact between adjacent blades to grant higher stiffness of the blade row and additional contact damping. Natural frequencies and mode shapes mainly remain on stiffness and mass distribution. Contact pressure at assembly state is generated by geometric interferences of the blades. Operational centrifugal forces and untwisting of blades tend to reduce the shroud contact pressure. One focus of this study is the research of different blade shroud contact conditions after the assembly process and the influence on closed shroud conditions during operation. A highly sophisticated numerical 3D model was set up to simulate and predict contact status and dynamic behavior of the blade row. Forced vibrations were imposed on the blade rows in a spin bunker to measure speed dependent frequencies and the effect of reduced shroud forces. Three-dimensional blades as well as cylindrical blades were tested on the same rotor to compare efficiency improved blades with long term industrial proven design. Results of the experiment were in good agreement with results of numerical calculations. A 10 MW steam turbine with one controlled extraction was used for validation. The turbine was operated within and beyond standard operational limits. Amongst others, the predicted contact and dynamic behavior were verified based on centrifugal forces, steam forces and real temperature distributions. Results confirmed that closed shroud contact was maintained at all operating points for properly assembled blade rows.

Topics: Vibration , Blades
Commentary by Dr. Valentin Fuster
2016;():V008T26A034. doi:10.1115/GT2016-57561.

Solid Particle Erosion (SPE) damage can be found on steam turbine stages. These solid particles are caused by the exfoliation of iron oxides formed on the inner surfaces of both boiler tubes and steam pipes which are exposed to elevated temperature. They can damage both fixed and Moving Blades, as well as both outer extension ring and tip seals. Severe SPE damages can be expensive for the utility industry due to reduced efficiency and lost power generation, as well as increased costs of repair or replacement of eroded components. Thus it is very important to understand this phenomenon and propose cost effective solutions to reduce the damage.

This study investigates the effects of SPE damage using particle trajectory calculations. This investigation confirms that “Bounce Back” is the dominant cause for the SPE damage on both Fixed Blade trailing edge and Moving Blade leading edge. The particles do not accelerate at same speed as steam, therefore they travel much slower when they hit the Moving Blade leading edge. Then they are thrown back towards the Fixed Blade to hit the trailing edge. Due to strong centrifugal forces, the particles are also thrown radically outwards and damage both outer extension ring and tip seals.

Based on these enhanced understandings, a practical solution is proposed to reduce SPE damage. It is predicted to have negligible impact on the stage performance. Evidence from the latest inspection demonstrates that this solution is very effective in reducing SPE damage to the Fixed Blade trailing edge.

Commentary by Dr. Valentin Fuster
2016;():V008T26A035. doi:10.1115/GT2016-57575.

A laboratory scale test facility has been developed to investigate deposition in steam turbines under conditions that are representative of those in steam power generation cycles. The facility is an advanced two-reactor vessel test arrangement, which is a more flexible and more accurately controllable refinement to the single reactor vessel test arrangement described previously in ASME Paper No. GT2014-25517 [1]. The commissioning of the new test facility is described in this paper, together with the results from a series of tests over a range of steam conditions, which show the effect of steam conditions (particularly steam pressure) on the amount and type of deposits obtained. Comparisons are made between the test results and feedback/experience of copper fouling in real machines.

Commentary by Dr. Valentin Fuster
2016;():V008T26A036. doi:10.1115/GT2016-57590.

With the tremendous role played by steam turbines in power generation cycle, it is essential to understand the flow field of condensing steam flow in a steam turbine to design an energy efficient turbine because condensation at low pressure (LP) turbine introduces extra losses, and erosion in turbine blades. The turbulence has a leading role in condensing phenomena which involve a rapid change of mass, momentum and heat transfer. The paper presents the influence of turbulence modelling on non-equilibrium condensing steam flows in a LP steam turbine stage adopting CFD code. The simulations were conducted using the Eulerian-Eulerian approach, based on Reynolds-averaged Navier-Stokes equations coupled with a two equation turbulence model, which is included with nucleation and droplet growth model for the liquid phase. The SST k-ω model was modified, and the modifications were implemented in the CFD code. First, the performance of the modified model is validated with nozzles and turbine cascade cases. The effect of turbulence modelling on the wet-steam properties and the loss mechanism for the 3D stator-rotor stage is discussed. The presented results show that an accurate computational prediction of condensing steam flow requires the turbulence to be modelled accurately.

Commentary by Dr. Valentin Fuster
2016;():V008T26A037. doi:10.1115/GT2016-57598.

Condensing nozzle flows have been used extensively to validate wet steam models. Many test cases are available in the literature and in the past a range of numerical studies have dealt with this challenging task. It is usually assumed that the nozzles provide a one- or two-dimensional flow with a fully turbulent boundary layer. The present paper reviews these assumptions and investigates numerically the influence of boundary layers on dry and wet steam nozzle expansions.

For the narrow nozzle of Moses and Stein it is shown that the pressure distribution is significantly affected by the additional blockage due to the side wall boundary layer. Comparison of laminar and turbulent flow predictions for this nozzles suggests that laminar-turbulent transition only occurs after the throat. Other examples are the Binnie nozzle and the Moore nozzles for which it is known that sudden changes in wall curvature produce expansion and compression waves that interact with the boundary layers. The differences between two- and three-dimensional calculations for these cases and the influence of laminar and turbulent boundary layers are discussed.

The present results reveal that boundary layer effects can have a considerable impact on the mean nozzle flow and thus on the validation process of condensation models. In order to verify the accuracy of turbulence modelling a test case that is not widely known internationally is included within the present study. This experimental work is remarkable because it includes boundary layer data as well as the usual pressure measurements along the nozzle centreline. Predicted and measured boundary layer profiles are compared and the effect of different turbulence models is discussed. Most of the numerical results are obtained with the in-house wet steam RANS-solver, Steamblock, but for the purpose of comparison the commercial program ANSYS CFX is also used, providing a wider range of standard RANS-based turbulence models.

Commentary by Dr. Valentin Fuster
2016;():V008T26A038. doi:10.1115/GT2016-57652.

Market requirements for faster and more frequent power unit start-up events result in a much faster deterioration of equipment, and a shorter equipment lifespan. Significant heat exchange occurs between steam and turbine rotors during the start-up process and even more intensive heat exchange takes place during the condensation phase in cold start-up mode, which leads to further thermal stresses and lifetime reduction. Therefore, the accuracy of lifetime prediction is strongly affected and dependent on the accuracy of transient thermal state prediction.

In this study, transient thermal and structural analyses of a 30 MW steam turbine for a combined High and Intermediate pressures (HPIP) rotor during a full cold start cycle is performed and special attention is paid to initial start-up phase with ‘condensation’ thermal BC. All steps for rotor design and the thermal model preparation were done using the AxSTREAM™* software platform. It included the development of a two dimensional model of the rotor, thermal zones and corresponding thermal boundary conditions (heat transfer coefficients and steam temperatures) calculation during turbine start-up and shut down operation. Rotor thermal and structural simulations were done using commercial FE analysis software to evaluate the thermo-stress-strain state of the turbine rotor. Calculation and validation of thermal and structural state of the rotor was done using actual start-up cycle and measured data from a power plant, and it showed good agreement of the calculated and the measured data. Based on the results of thermo-structural analysis, the evaluation of rotor lifetime by means of a low cycle fatigue approach was performed and presented in this paper.

Commentary by Dr. Valentin Fuster
2016;():V008T26A039. doi:10.1115/GT2016-57753.

The largest share of electricity production worldwide belongs to steam turbines. However, the increase of renewable energy production has led steam turbines to operate under part load conditions and increase in size. As a consequence long rotor blades will generate a relative supersonic flow field at the inlet of the last rotor. This paper presents a unique experiment work that focuses at the top 30% of stator exit in the last stage of an LP steam turbine test facility with coarse droplets and high wetness mass fraction under different operating conditions. The measurements were performed with two novel fast response probes. A fast response probe for three dimensional flow field wet steam measurements and an optical backscatter probe for coarse water droplet measurements ranging from 30 up to 110μm in diameter. This study has shown that the attached bow shock at the rotor leading edge is the main source of inter blade row interactions between the stator and rotor of the last stage. In addition, the measurements showed that coarse droplets are present in the entire stator pitch with larger droplets located at the vicinity of the stator’s suction side. Unsteady droplet measurements showed that the coarse water droplets are modulated with the downstream rotor blade-passing period. This set of time-resolved data will be used for in-house CFD code development and validation.

Commentary by Dr. Valentin Fuster
2016;():V008T26A040. doi:10.1115/GT2016-57796.

The quadrature method of moments (QMOM) has recently attracted much attention in representing the size distribution of liquid droplets in wet-steam flows using the n-point Gaussian quadrature. However, solving transport equations of moments using high-order advection schemes is bound to corrupt the moment set, which is then termed a non-realizable moment set. The problem is that the failure and success of the Gaussian quadrature is unconditionally dependent on the realizability of the moment set. First, this article explains the non-realizability problem with the QMOM. Second, it compares two solutions to preserve realizability of the moment sets. The first solution applies a so-called “quasi-high-order” advection scheme specifically proposed for the QMOM to preserve realizability. However, owing to the fact that wet-steam models are usually built on existing numerical solvers, in many cases modifying the available advection schemes is either impossible or not desired. Therefore, the second solution considers correction techniques directly applied to the non-realizable moment sets instead of the advection scheme. These solutions are compared in terms of accuracy in representing the droplet size distribution. It is observed that a quasi-high-order scheme can be reliably applied to guarantee realizability. However, as with all numerical models in an Eulerian reference frame, in general its results are also sensitive to the grid resolution. In contrast, the corrections applied to moments either fail in identifying and correcting the invalid moment sets, or distorts the shape of the droplet size distribution after the correction.

Commentary by Dr. Valentin Fuster
2016;():V008T26A041. doi:10.1115/GT2016-57814.

In this work the thermodynamic analysis of two power cycles operating at ultracritical and supercritical conditions (300 bar, 600°C), and conventional or subcritical (124 bar, 538°C) is made. The supercritical cycle has ten and eight stages of regenerative feed heating. The conventional cycle has seven and six stages of regenerative feed heating. The aim of this analysis is to show the variation of work out, thermal efficiency, the heat rate, specific steam and fuel consumptions and the operating range of the pressure of overheating. For example, supercritical conditions operation of 300 bar and 600°C and a condensation pressure of 0.1107 bar, the maximum pressure reheating is 100 bar, because with a higher pressure, steam quality at the end of the step of expanding the low pressure turbine will be less than 0.88, requiring a second reheating. Additionally, the supercritical Rankine cycles have better thermal efficiency than subcritical cycles, increases in average 6%, and consequently the heat rate and steam and fuel consumption decrease.

Topics: Rankine cycle
Commentary by Dr. Valentin Fuster
2016;():V008T26A042. doi:10.1115/GT2016-57899.

Turbine blade flutter is a concern for the manufacturers of steam turbines. Typically, the length of last stage blades of large steam turbines is over one meter. These long blades are susceptible to flutter because of their low structural frequency and supersonic tip speeds with oblique shocks and their reflections. Although steam condensation has usually occurred by the last stage, ideal gas is mostly assumed when performing flutter analysis for steam turbines.

The results of a flutter analysis of a 2D steam turbine test case which examine the influence of non-equilibrium wet steam are presented. The geometry and flow conditions of the test case are supposed to be similar to the flow near the tip in a steam turbine as this is where most of the unsteady aerodynamic work contributing to flutter is done. The unsteady flow simulations required for the flutter analysis are performed by ANSYS CFX. Three fluid models are examined: ideal gas, equilibrium wet steam (EQS) and non-equilibrium wet steam (NES), of which NES reflects the reality most.

Previous studies have shown that a good agreement between ideal gas and EQS simulations can be achieved if the prescribed ratio of specific heats is equal to the equilibrium polytropic index of the wet steam flow through the turbine.

In this paper the results of a flutter analysis are presented for the 2D test case at flow conditions with wet steam at the inlet. The investigated plunge mode normal to chord is similar to a bending mode around the turbine axis for a freestanding blade in 3D. The influence of the overall wetness fraction and the size of the water droplets at the inlet are examined. The results show an increase of aerodynamic damping for all investigated interblade phase angles with a reduction of droplet size. The influence of the wetness fraction is in comparison of less importance.

Commentary by Dr. Valentin Fuster
2016;():V008T26A043. doi:10.1115/GT2016-57954.

A shutdown operation of a large size steam turbine could possibly cause flashing phenomena of the pooled drain water in low-pressure heaters. The boiled steam is sometimes in the same amount as the main flow in the case where shutdown is executed during low load conditions, and returns to the steam flow path through the extraction lines. A series of experimental works with a subscale model turbine facility has been carried out to investigate the vibration stress behavior, and the steady and unsteady pressures under the flashing back conditions. It was observed that the blades of the two stages before the last stage (L-2) and a stage before the last stage (L-1) endured their peak vibration stresses immediately after the flash-back flow reached the turbine. In the meantime, the vibration stresses of the last stage (L-0) blades were reduced.

In this paper, the behavior of the water droplets and their vaporization in the steam path were mainly investigated. A series of experiment was conducted in which several amounts of controlled sprayed water were continuously supplied into the turbine. The transient steam condition and blade’s vibration stresses were measured at the same time. The results showed the possibility that sprayed water upstream can change the mass flow rate and temperature downstream to avoid the unstable steam flow and overheating of the long blades during low load operation.

Commentary by Dr. Valentin Fuster

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