ASME Conference Presenter Attendance Policy and Archival Proceedings

2016;():V009T00A001. doi:10.1115/GT2016-NS9.

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presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.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

Oil and Gas Applications

2016;():V009T24A001. doi:10.1115/GT2016-56025.

Strong pressure pulsations into the suction or discharge of a centrifugal compressor can move its operating point into operational instability regions such as surge, rotating stall, or choke. This is of special operational and safety concern in mixed pipeline compressor stations where many centrifugal compressors operate in series or parallel with reciprocating compressors. Over the last 30 years, several authors have discussed the impact of piping flow pulsations on centrifugal compressor stability and specifically, on the impact on surge margin and performance. For example, Sparks (1983), Kurz et al., (2006), and Brun et al. (2014) provided analysis and numerical predictions on the impact of discrete and periodic pressure pulsation on the behavior of a centrifugal compressor. This interaction came to be known as the “Compressor Dynamic Response (CDR) theory.” CDR theory explains how pulsations are amplified or attenuated by a compression system’s acoustic response characteristic superimposed on the compressor head-flow map. Although the CDR Theory describes the impact of the nearby piping system on the compressor surge and pulsation amplification, it provides only limited usefulness as a quantitative analysis tool, primarily due to the lack of numerical prediction tools and test data for comparison. Recently, Brun et al. (2014) utilized an efficient 1-D transient Navier-Stokes flow solver to predict CDR in real life compression systems. Numerical results showed that acoustic resonances in the piping system can have a profound impact on a centrifugal compressor’s surge margin. However, although interesting, the fundamental problem with both Spark’s and Brun’s approach was that no experimental data was available to validate the analytical and numerical predictions.

In 2014, laboratory testing of reciprocating and centrifugal compressor mixed operation was performed in an air loop at Southwest Research Institute’s (SwRI®) compressor laboratory. The specific goal was to quantify the impact of periodic pressure and flow pulsation originating from a reciprocating compressor on the surge margin and performance of a centrifugal compressor in a series arrangement. This data was to be utilized to validate predictions from Sparks’ CDR theory and Brun’s numerical approach. For this testing, a 50 hp single-stage, double-acting reciprocating compressor provided inlet pulsations into a two-stage 700 hp centrifugal compressor operating inside a semi-open recycle loop which uses near atmospheric air as the process gas. Tests were performed over a range of pulsation excitation amplitudes, frequencies, and pipe geometry variations to determine the impact of piping impedance and resonance response. Detailed transient velocity and pressure measurements were taken by a hot wire anemometer and dynamic pressure transducers installed near the compressor’s suction and discharge flanges. Steady-state flow, pressure, and temperature data were also recorded with ASME PTC-10 compliant instrumentation. This paper describes the test facility and procedure, reports the reduced test results, and discusses comparisons to predictions. Results provided clear evidence that suction pulsations can significantly reduce the surge margin of a centrifugal compressors and that the geometry of the piping system immediately upstream and downstream of a centrifugal compressor will have an impact on the surge margin reduction. In severe cases, surge margin reductions of over 30% were observed for high centrifugal compressor inlet suction pulsation. Pulsation impact results are presented as both flow versus surge margin and operating map ellipses. Some basic design rules were developed from the test results to relate predicted flow pulsation amplitudes to corresponding reductions in surge margin.

Topics: Compressors , Surges
Commentary by Dr. Valentin Fuster
2016;():V009T24A002. doi:10.1115/GT2016-56027.

Fouling of compressor blades is an important mechanism leading to performance deterioration in gas turbines over time. Experimental and simulation data are available for the impact of specified amounts of fouling on performance, as well as the amount of foulants entering the engine for defined air filtration systems and ambient conditions.

This study provides experimental data on the amount of foulants in the air that actually stick to a blade surface for different conditions of the blade surface. Quantitative results both indicate the amount of dust as well as the distribution of dust on the airfoil, for a dry airfoil, as well as airfoils that were wet from ingested water, as well as different types of oil. The retention patterns are correlated with the boundary layer shear stress. The tests show the higher dust retention from wet surfaces compared to dry surfaces. They also provide information about the behavior of the particles after they impact on the blade surface, showing that for a certain amount of wet film thickness, the shear forces actually wash the dust downstream, and off the airfoil. Further, the effect of particle agglomeration of particles to form larger clusters was observed, which would explain the disproportional impact of very small particles on boundary layer losses.

Topics: Compressors , Blades
Commentary by Dr. Valentin Fuster
2016;():V009T24A003. doi:10.1115/GT2016-56066.

Optimized operation of gas turbines is discussed for six LM2500PE engines at a Statoil North Sea offshore field. Three engines are generator drivers whilst three engines are compressor drivers. Two of the compressor drive engines are running at peak load (T5.4 control), hence the production rate is limited by the available power from these engines. All of the six engines discussed run continuously without redundancy, gas turbine uptime is therefore critical for the field’s production and economy. The performance and operational experience with upgraded inlet air filter systems and online water wash at high water-to-air ratio, as well as successful operation at longer intervals and higher average engine performance are described.

For North Sea operation, a key property of the filter system is the ability to handle high humidity and high salt-content through the harsh environment in these waters. The upgraded filter systems analyzed in this paper is a 2-stage system (vane separator stage upstream of the high-efficiency-filter stage), which is a simplified design versus the old traditional 3-stage systems (louvre upstream and vane separator downstream of the filter stage). These 2-stage systems rely on an efficient upstream vane separator to remove the vast majority of water from the airflow before it reaches the high-efficiency filters. The high-efficiency filters are especially designed to withstand moisture. Deposit analysis from the downstream side of the filters has been performed. Extensive testing of both new and used filter elements, of different filter grade and operated at different intervals, has been performed on a filter test rig facility onshore.

All six engines have historically been operated with 4-month intervals between maintenance stops. Online wash is performed daily between the maintenance stops at full load (i.e. normal operating load for the subject engine). As a result of successful development and pilot testing of new filters and optimized filter change intervals, as well as successful online water wash, the engine operating intervals are now extended to 6 months with very low deterioration rate.

Understanding the gas turbine performance deterioration is of vital importance. Trending of its deviation from the engine baseline facilitates load-independent monitoring of the gas turbine’s condition. Instrument resolution and repeatability are key factors in order to get reasonable results from the performance analysis. Improvement of the package instrumentation has been implemented on three of the analyzed engines, for better performance monitoring. As a result of these analyses, a set of monitoring parameters is suggested for effective diagnostics of compressor degradation. Avenues for further research and development are proposed in order to further increase the understanding of the deterioration mechanisms and the gas turbine performance and response.

Commentary by Dr. Valentin Fuster
2016;():V009T24A004. doi:10.1115/GT2016-56112.

The natural gas midstream gathering and pipeline landscape has become much more dynamic in recent years. Some of the attributes contributing to these continuous changes in operations are due to increased supply from shale gas explorations in North America and increasing natural gas demand in Asia. These changes require new pipelines and compressor stations to be built or existing pipelines and compressor stations to be modified to match new required operating conditions.

Economic factors such as initial capital investment and life cycle costs are very important considerations in the decision process to evaluate the benefits of building new stations or modifying existing stations. This paper presents a discussion of some of the more fundamental factors to be considered in evaluating the economics of station optimization projects, and also introduces a variety of options to manage the lifecycle of the centrifugal gas compressors units and stations.

Commentary by Dr. Valentin Fuster
2016;():V009T24A005. doi:10.1115/GT2016-56138.

Due to environmental regulations, Nitrogen oxides (NOx), Carbon Monoxide (CO) and Sulfur Dioxide (SO2) emissions are key issues for gas turbine plants. Regulators are becoming more and more involved and they often require complete and real-time emission information.

The measurements can be done with gas analyzers, this technology is called CEMS: Continuous Emissions Monitoring System. An alternative method [1][2] is to use a calculation based on the turbine instrumentation. This technology is called PEMS: Predictive Emissions Monitoring System. But these technologies do not provide all the information required by the regulator.

GRTgaz, the main gas transmission company in France managing 44 turbines spread over 27 stations across France, has decided to monitor its emissions by PEMS for many years. Two years ago, GRTgaz developed successfully its own PEMS equations, organized answers to regulators around this technology and decided to spread the technology across its gas turbine fleet. The complete intellectual path followed is described in the paper GT2014-25242. This 2016 3-part paper describes the PEMS project steps forward.

In the first part of the paper, a review is done of the PEMS equations used at GRTgaz for NOx and CO concentrations. The various lean premixed combustion turbines differ in terms of combustion design, control and instrumentation. These differences are analyzed considering their influence on combustion and their impact on the PEMS results accuracy.

In order to comply with regulators requirements a calibration of the PEMS results is done every quarter. The results of the first 2 stations equipped with PEMS are described in this first part.

The second part of the paper introduces the smoke developed and the neutral air flow to complete the real time calculation required by the regulators: SO2 concentration and the mass flowrates for NOx, CO and SO2. The final calculation integrates the mass flowrate in order to elaborate the total mass emitted into the atmosphere over different time periods.

The last part deals with developing personnel involvement, managing the data and compiling the results given to regulators. These aspects were more difficult to implement than expected. The importance of these aspects should not be underestimated because the scientific credibility of PEMS cannot be confirmed without them.

Commentary by Dr. Valentin Fuster
2016;():V009T24A006. doi:10.1115/GT2016-56159.

In oil and gas applications, gas-liquid mixtures of a process fluid are commonplace and the phases of the mixtures are separated upstream of pump or compressor machinery. Considering compressors, the separation of phases is important because the liquid causes the compressor to operate significantly different than with dry to affect the range, performance, and durability of the machine. Even with separation equipment, liquid can be ingested in a compressor by liquid carryover from the separator or condensation of the process gas. Additionally, there is no single definition of what is considered a wet gas.

In this paper, the definition of wet gas from multiple applications is reviewed and a general definition for wet gas is formulated. The effects of wet gas on reciprocating, screw-type, and centrifugal compressors are reviewed to provide insight into how their operation is affected. The limited information for screw compressors is supplemented with multiphase effects in screw pumps.

Commentary by Dr. Valentin Fuster
2016;():V009T24A007. doi:10.1115/GT2016-56463.

In the last years, the increased demand of the energy market has led to the increasing penetration of renewable energies in order to achieve the primary energy supply. However, simultaneously natural gas still plays a key role in the energy market, mainly as gaseous fuel for stationary energy generation, but also as liquefied fuel, as an alternative to the diesel fuel, in vehicular applications. Liquefied Natural Gas (LNG) is currently produced in large plants directly located at the extraction sites.

In this study, the idea of realizing plug & play solutions to produce LNG directly at vehicle’s filling stations has been investigated. A novel process of LNG production for filling stations has been analyzed, consisting in a single stage Joule-Thompson isenthalpic expansion process, with intercooled compression. Furthermore, the presented layout has been developed with the purpose of optimizing the energy consumption of the plant, obtaining moderately pressurized LNG.

With the aim of investigating the feasibility of this novel LNG generation process, a thermodynamic analysis has been carried out and presented in this study. Moreover, the minimization of energy consumption has been investigated with a parametric analysis, in order to optimize the LNG production and to maximize the efficiency of the process. Furthermore, novel performance indicators have been defined, in order to account the efficiency of the LNG production process. Results of the optimization analysis show that, with the proposed layout, an energy consumption equal to about 1.9 MJ/kg of produced LNG can be achieved.

Commentary by Dr. Valentin Fuster
2016;():V009T24A008. doi:10.1115/GT2016-56576.

Multiphase flow introduces many challenges in turbomachines analysis and operation, as each of the phases responds differently to the forces in the hydraulic channel, and the mechanical and thermal interaction among the phases has to be taken into account too. Especially when very high gaseous fractions need to be covered, due to the concurrent physics involved and transient phenomena, a proper machine characterization cannot be limited to an overall description of the performance, but it needs to rely on advanced analysis tools which can reveal the local phenomena responsible for performance degradation and instabilities.

The flow regimes vary from a homogeneous distribution of fine bubbles, evenly dispersed and carried away by the main flow, to more complex flow patterns, especially if bubbles coalesce and adhere to a wide portion of the channel wall.

Tests are performed on a multiphase pump facility recently designed by the authors, which allows a complete optical access to the pump channels and fine adjustments in the inlet configuration and the tip clearance gap.

The investigation focuses on the effect of the mixture inlet pressure and machine rotational speed on performance and stability. Increasing values of the inlet pressure reduce the density ratio between the phases, thus making them become more coupled; this results in a milder performance degradation and a wider stable operating range.

Accurate flow visualization through the pump transparent casing complements the study, allowing an immediate and detailed local phenomena description.

The results are presented under performance and surging curves, showing the gas-handling capability dependence on the actual conditions.

These analyses fully characterize the machine behavior, define the operating zones which should be avoided, and will serve later to implement a control strategy to keep the machine in a safe regime.

Commentary by Dr. Valentin Fuster
2016;():V009T24A009. doi:10.1115/GT2016-56599.

A progressive cavity pump (PCP) is a positive displacement pump and has been used as an artificial lift method in the oil and gas industry for pumping fluid with solid content and high viscosity. In a PCP, a single-lobe rotor rotates inside a double-lobe stator. Articles on computational works for flows through a PCP are limited because of transient behavior of flow, complex geometry and moving boundaries. In this paper, a 3D CFD model has been developed to predict the flow variables at different operating conditions. The flow is considered as incompressible, single phase, transient, and turbulent. The dynamic mesh model in Ansys-Fluent for the rotor mesh movement is used, and a user defined function (UDF) written in C language defines the rotor’s hypocycloid path. The mesh deformation is done with spring based smoothing and local remeshing technique. The computational results are compared with the experiment results available in the literature. Thepump gives maximum flowrate at zero differential pressure.

Commentary by Dr. Valentin Fuster
2016;():V009T24A010. doi:10.1115/GT2016-56646.

In the design and testing of gas compressors, the correct determination of the thermodynamic properties of the gas, such as enthalpy, entropy, and specific volume from pressure, temperature, and composition, plays an important role. Due to the wide range of conditions encountered, pressure, specific volume and temperature (p-v-T) equations of state (EOS) are used to determine the isentropic or polytropic efficiency, the work input, and capacity of a compressor configuration. However, accurate equations of state may be lacking for some more complicated gas mixtures. Experimentally determined thermodynamic state information is more accurate and is needed to validate, correct, or supplant existing equations of state.

The methodology for calculating enthalpy and entropy from experimental data is presented including the full step-by-step derivation from first principles. The thermodynamic relations or calculus properties used in each step of the derivation are clearly identified to provide traceability and encourage verification. Results are presented in three forms to match all possible tabulation formats of temperature, pressure, and volume data. Calculation methods for other useful but unmeasured thermodynamic properties such as the specific heat at constant pressure and the ratio of specific heats are also given. The methodology is demonstrated in two examples. The first is a verification case where REFPROP, an equation of state software, is used to generate input data, and the enthalpy and entropy values calculated from the input data are shown to match those given directly by REFPROP. The second is a practical demonstration where the methodology is used with actual experimental data.

Topics: Entropy , Enthalpy
Commentary by Dr. Valentin Fuster
2016;():V009T24A011. doi:10.1115/GT2016-56778.

A single stage cryogenic liquid turbine expander is developed as a replacement for traditional Joule-Thomson valves used in the large-scale internal compression air-separation unit for the purpose of energy saving. Similar to the conventional hydraulic turbine, detrimental swirling and cavitation flow is also encountered at turbine expander impeller exit and its successive diffuser tube, but due to significant thermodynamic effect of cryogenic fluid flow, it is much more complicated than the conventional hydraulic turbine.

In the present study, cavitating flow mechanism of the turbine expander is investigated first with a combination of the homogenous multiphase mixture model and the Rayleigh-Plesset model, where the former treats liquid and gas as a continuum mixture and the latter depicts the bubble dynamics. Then sensitivity study is conducted for the impeller fairing cone geometry on suppression of cavitating flow. The following are demonstrated: with a use of the fairing cone, flow behavior near and downstream the impeller exit is significantly improved, where the low static pressure region is reduced and the local temperature rise decreases, subsequently the cavitating flow is effectively suppressed. The cavitating flow is sensitive to a tuning of the fairing cone geometry, and an optimal design of the cone geometry is essential.

Commentary by Dr. Valentin Fuster
2016;():V009T24A012. doi:10.1115/GT2016-56780.

LNG expander is developed and used as a replacement of a J-T valve in liquefaction process of natural gas to reduce significantly the energy consumption in the LNG plant. Similar to conventional hydraulic turbines, the unexpected cavitation also occurs in the LNG expander. In the present study, cavitating flow in two-phase LNG expander is investigated. With the justified Rayleigh-Plesset cavitation model, cavitating flow characteristics is investigated for the LNG expander in the entire stage environment including an annular bend, nozzle ring, and radial inflow impeller. On the basis of cavitating flow analysis, a coaxial rotating exducer is developed and fitted downstream to the impeller, so as to reduce the cavitation in impeller and subsequently prevent impeller damage.

The following are demonstrated: (1) without exducer, significant cavitating flow is encountered at the impeller trailing edge and also in half streamline-wise region, and they are resulted from the viscous dissipation and flow separation; (3) with exducer, the impeller cavitation has diminished entirely but it has occurred in the successive exducer; (3) a use of exducer enhances the energy conversion capability of the rotors, but reduces the overall temperature drop and efficiency of the expander; (4) the design optimization of exducer is required to suppress the exducer cavitation, which also needs to be incorporated with the impeller design to achieve a better match between rotor/stator, so as to maximize the design optimization benefits.

Commentary by Dr. Valentin Fuster
2016;():V009T24A013. doi:10.1115/GT2016-56950.

Knowledge of compressor system behaviour during a trip is essential to obtain reliable operation. Experience at a gas treatment plant has shown that the export compressors (43 MW) may enter the unstable area of the performance characteristics and suffer from surge and rotating stall in situations with voltage drops in the electricity grid.

A detailed dynamic simulation model has been established to analyse the impact of different mechanical, process and control tuning aspects. The electric motor driven compressor, the protection valves and piping have been modelled and dynamic analyses performed to reveal the transient response.

The main challenges are related to the compressor system/process design and to the compressor and variable speed drive (VSD) response during power dip or compressor trip. Based on experiences detailed sensitivity analyses have been performed, covering the impact of: compressor discharge volumes, performance characteristics — head rise to surge, dimensions of the recycle piping as well as the sizing, characteristics and trip signal delay of the recycle protection valve(s).

This paper reports experience from analyses of the export compressor and driver behaviour during the first 3 seconds of an electric power/voltage dip. It is partly based on earlier tests and dynamic simulations performed for the facility, and partly on new technology challenges and opportunities. The main objective has been to study the compressor system sensitivity related to variation in available driver power during power dip and its impact on the transient rundown trajectory and recovery phase.

Commentary by Dr. Valentin Fuster
2016;():V009T24A014. doi:10.1115/GT2016-57020.

Turboexpanders provide the most efficient solution when it is required to reduce the pressure of a fluid stream. By expanding high pressure fluid, energy in the high pressure fluid entering the turboexpander can be efficiently used for either driving a booster compressor or for electrical power generation. While the plants are designed to operate without the need for the power produced by turboexpander, the work recovered from the expansion is a bonus, which increases the plant thermal efficiency.

This paper is intended to explain the benefits of utilizing a turboexpander in LNG liquefaction applications. Also, in absence of a published API standard for a turboexpander-generator package, this paper provides recommendations on factory acceptance tests.

Commentary by Dr. Valentin Fuster
2016;():V009T24A015. doi:10.1115/GT2016-57076.

While proper design and maintenance of dry gas seals is a well-understood topic, dry gas seal failures are still relatively common. These failures can result in frequent repairs and costly downtime. Although several case studies of individual failures are available, relatively few large-scale dry gas seal failure studies exist. Based on a review of existing literature, very little has been published on failure statistics aimed at improving seal reliability. As a part of an industry-funded dry gas seal reliability project, a failure database has been populated with information provided by both end users and original equipment manufacturers. The database includes details on dry gas seal and separation seal configuration, seal gas supply, operating history, conditions at time of failure, and failure symptoms, including any results from failure analyses performed by the survey respondent. In total, eight companies contributed 194 failures. Of these, 144 cases had root causes provided.

From this database of failures, statistical analysis is used to determine common reasons behind dry gas seal failures in gas compression service. Failure trends are identified based on data collected, and corresponding recommendations are provided for improving dry gas seal reliability.

Commentary by Dr. Valentin Fuster
2016;():V009T24A016. doi:10.1115/GT2016-57117.

Performance curves in 2nd quadrant are important to size protection equipment of both compressor and surrounding system. With reverse flow also the level and frequency of pressure fluctuations in different operating points is important to estimate blade loading and possible presence of excitation frequencies. The capability of performance predictive tools (either CFD or correlations based methods) as also mechanical design criteria are generally poor in 2nd quadrant and suffer for the scarcity and inadequacy of validation data.

The second quadrant branch for a centrifugal compressor has been experimentally tested after the standard characterization in direct flow. A test arrangement has been designed, with a booster compressor connected in parallel with the tested stage, forcing the flow to be stable in reverse flow. The compressor characteristics have been measured with static and dynamic instrumentation. Present experience showed that when machine is operated in the stable reverse flow condition, pressure fluctuations and vibration are higher with respect to the values measured in nominal direct flow operating conditions. The increase is in the order of 10–20% of the corresponding value in direct flow. The same can be stated also for axial thrust and secondary flows that increase when the gas flows in reverse direction but the increase is in the order of 10–15%. Thus in 2nd quadrant, compressor equipment (in particular impeller blades) and all other system devices experience unusual loading levels but the additional loads are not big enough to cause relevant damaging if sustained for limited time periods. This result may allow simplifying the design of system layout: in particular, if during ESD no surging cycle is expected but only a reverse flow sustained steadily by the system (which is actually the most typical experience), the additional ASV’s, such as hot or cold gas by-pass valves, may be reduced in size or eventually removed optimizing the plant BOP.

Topics: Compressors
Commentary by Dr. Valentin Fuster
2016;():V009T24A017. doi:10.1115/GT2016-57124.

The sizing of surge protection devices for both compressor and surrounding system may require the knowledge of performance curves in 2nd quadrant with a certain level of accuracy. In particular two performance curves are usually important: the pressure ratio trend versus flow rate inside the compressor and the work coefficient or power absorption law. The first curve allows estimating mass flow in the compressor given a certain pressure level imposed by system, while the second is important to estimate the time required to system blow down during ESD (emergency shutdown). Experimental data are routinely not available in the early phase of anti-surge protection devices and prediction methods are needed to provide performance curves in 2nd quadrant starting from the geometry of both compressor and system.

In this paper two different approaches are presented to estimate performance curves in 2nd quadrant: the first is a simple 1D approach based on velocity triangle and the second is a full unsteady CFD computation. The two different approaches are applied to the experimental data more deeply investigated in part I by Belardini E.[3]. The measurement of compressor behavior in 2nd quadrant was possible thanks to a dedicated test arrangement in which a booster compressor is used forcing stable reverse flow.

The 1D method showed good agreement with experiments at design speed. In off-design condition a correlation for deviation angle was tuned on experimental data to maintain an acceptable level of accuracy. With very low reverse flow rates some discrepancies are still present but this region plays a secondary role during the dynamic simulations of ESD or surge events.

The unsteady CFD computation allowed a deeper insight into the fluid structures, especially close to very low flow rates when the deviation of the 1D method and the experimental data is higher. An important power absorption mechanism was identified in the pre-rotation effect of impeller as also the higher impact of secondary flows.

These two methods are complementary in terms of level of complexity and accuracy and to a certain extent both necessary. 1D methods are fast to be executed and more easily calibrated to match the available experiments, but they have limited capability to help understanding the underlying physics. CFD is a more powerful tool for understanding fluid structures and energy transfer mechanisms but requires computational times not always suitable for a production environment. 1D method can be used for standard compressor and applications for which consolidated experience have been already gathered while CFD is more suitable during the development of new products or up to front projects in general.

Topics: Compressors
Commentary by Dr. Valentin Fuster
2016;():V009T24A018. doi:10.1115/GT2016-57168.

Flow instability conditions, in particular during surge and stall phenomena, have always influenced the operational reliability of turbo-compressors and have attracted significant interest resulting in extensive literature. Nowadays, this subject is still one of the most investigated because of its high relevance on centrifugal and axial compressor operating flow range, performance and efficiency. Many researchers approach this important issue by developing numerical models, whereas others approach it through experimental studies specifically carried out in order to better comprehend this phenomenon. The aim of this paper is to experimentally analyze the stable and unstable operating conditions of an aeronautic turbo-shaft gas turbine axial-centrifugal compressor installed on a brand new test-rig properly designed for this purpose.

The test facility is set up in order to obtain i) the compressor performance maps at rotational speeds up to 25,000 rpm and ii) the compressor transient behavior during surge. By using two different test rig layouts, instabilities occurring in the compressor, beyond the peak of the characteristic curve, are identified and investigated.

These two types of analysis are carried out thanks to pressure, temperature and mass flow sensors located in strategic positions along the circuit. These measurement sensors are part of a proper control and acquisition system, characterized by an adjustable sampling frequency. Thus, the desired operating conditions of the compressor, in terms of mass flow and rotational speed and transient of these two parameters are regulated by this dedicated control system.

Topics: Compressors , Surges
Commentary by Dr. Valentin Fuster
2016;():V009T24A019. doi:10.1115/GT2016-57340.

The quality and purity of the air entering a gas turbine is a significant factor influencing its performance and life. Foulants in the ppm range which are not captured by the air filtration system usually cause deposits on blading, which results in a severe drop in the performance of the compressor.

Through the interdisciplinary approach proposed in this paper, it is possible to determine the evolution of the fouling phenomenon through the integration of studies in different research fields: (i) numerical simulation, (ii) power plant characteristics and (iii) particle-adhesion characteristics. In fact, the size of the particles, their concentrations and adhesion ability, and filtration efficiency represent the major contributors to performing a realistic quantitative analysis of fouling phenomena in an axial compressor. The aim of this work is the estimation of the actual deposits on the blade surface in terms of location and quantity. This study combines the impact/adhesion characteristic of the particles obtained through a CFD and the real size distribution of the contaminants in the air swallowed by the compressor.

The blade zones affected by deposits are clearly reported by using easy-to-use contaminant maps realized on the blade surface, in terms of contaminant mass.

The analysis has shown that particular fluid-dynamic phenomena and airfoil shape influence the pattern deposition. The use of a filtration system decreases the contamination of the blade and the charge level of the electrostatic seems to be less important than the air contaminant concentration. From these analyses, some guidelines for proper installation and management of the power plant (in terms of filtration systems and washing strategies) can be drawn up. Characterization of the air contaminants in the power plant location represents the most important step in improving the management of the gas turbine power plant.

Commentary by Dr. Valentin Fuster
2016;():V009T24A020. doi:10.1115/GT2016-57376.

The introduction of variable inlet guide vanes (VIGVs) upfront of a compressor stage affects performance and permits tuning for off-design conditions. This is of great interest for emerging technology related to subsea compression. Unprocessed gas from the wellhead will contain liquid condensate, which affects the operational condition of the compressor. To investigate the effect of guide vanes on volume flow and pressure ratio in a wet gas compressor, VIGVs are implemented upfront of a centrifugal compressor stage to control the inlet flow direction. The guide vane geometry and test rig setup have previous been presented. This paper documents how changing the VIGV setting affects compressor performance under dry and wet operating conditions. The reduced performance effect and operating range at increased liquid content are of specific interest. Also documented is the change in the VIGV effect relative to the setting angle.

Commentary by Dr. Valentin Fuster
2016;():V009T24A021. doi:10.1115/GT2016-57697.

Due to the huge amount of power connected to centrifugal compressors’ applications, even small rangeability increases of the stages would provide significant energy and money savings. In particular, industrial manufacturers pay lot of interest in better understanding the instabilities that in many cases define the minimum flow limit of their stages, but they are often hampered in the research by the short time-to-market. On the other hand, academia has historically found difficulties in approaching the problem due to the lack of dedicated experimental facilities.

In this study, the concept design of a new research test rig is presented. The rig will be able to test impellers in field-like conditions (original mass flow and peripheral Mach numbers up to 0.7), operating in open-loop configuration with ambient inlet conditions. In view of systematic test campaigns, a modular design will allow to easily replace any component of the asset and even to modify the flowpath after the impeller, so that the influence of each component can be estimated. As a research academic facility, the rig is characterized by some new design solutions, oriented to minimize the mechanical complexity, the energy consumption, the overall dimensions, and, finally, the cost. Moreover, it will be equipped with advanced experimental measurement instrumentation, e.g. a PIV system or fast response aerodynamic pressure probes.

The paper illustrates the conceptual design of the rig, including the selection of the best architecture and layout, the drivetrain assessment and the rotordynamic verification. Computational fluid-dynamic analyses are also presented, aimed at verifying the flow uniformity in the discharge sections and the thermal stability of the system during the tests.

Topics: Compressors , Design
Commentary by Dr. Valentin Fuster
2016;():V009T24A022. doi:10.1115/GT2016-57771.

Compression trains for oil and gas applications must meet, now more than ever, the requirement of versatility. Production rates and compression demands of extraction fields significantly change during their operational life; this has pushed customers to ask for equipments designed to efficiently operate all over their lifespan in order to comply with energy saving and pollution reduction needs. For this reason modular simulation codes turn out to be the best choice compared with dedicated tools for specific compression plant configuration, since they provide flexibility without losing accuracy.

This paper presents the implementation, within a previously developed modular tool, of a design and off-design procedure for compression plant simulation. This tool is based on a wide library of elementary components analytically defined through equations that model their physical behaviour. For impellers, descriptive equations represent an in-house database of real stages characteristic curves, for all the other elementary components the equations arise from fundamental mechanical and thermodynamic laws. Physical properties of real gases are assessed by the use of suitable thermodynamic libraries. An implemented trust-region Gauss-Newton method, called TRESNEI, has been adopted to solve the mathematical model.

Numerical calculations, performed on two real compression train arrangements, have been devoted to validate the code over design and off-design simulation mode. Results have been compared with those obtained with a pre-existing in-house tool and with experimental data. Comparisons show both a satisfactory agreement between numerical and experimental data and a perfect matching between the simulation codes.

Topics: Design , Compression , Trains
Commentary by Dr. Valentin Fuster
2016;():V009T24A023. doi:10.1115/GT2016-57800.

Acoustically induced vibration (AIV) is a high-frequency vibration phenomenon that can occur downstream of pressure-reducing devices such as control valves, restriction orifices, and pressure relief or safety valves in compressor piping systems. These vibrations can lead to high cycle fatigue failures of downstream piping at side branches or welded supports. Existing methods for screening and analyzing acoustically induced vibration are not well-grounded in the underlying physics and thus do not provide a methodology for evaluating a variety of mitigation strategies. Modeling of acoustically induced vibration is computationally challenging, as it requires the interaction between tens or hundreds of higher-order acoustic modes with a similar number of piping shell modes.

In order to obtain better insight into the underlying physics of AIV and to characterize the effectiveness of several mitigation methods, full-scale blow-down testing was performed at Southwest Research Institute. Tests were performed using 20 MPa nitrogen gas vented at 28 kg/s through a 3×4” pressure safety valve and multiple header pipe sizes ranging from 12” to 36”. Test configurations included baseline piping geometry at each size and several AIV mitigations including stiffening rings, viscous damping wrap, and internal acoustic mode disruptors. Test results from strain gauges, accelerometers, and dynamic pressure transducers show a broadband multimodal response with dynamic stresses up to 3 kHz near the safety valve tailpipe connection to the test header, and various mitigations reduced dynamic stresses by 8–52% depending on the piping and type of mitigation.

Acoustic and structural finite element models were analyzed in order to determine the coincident modes that match in both axial/circumferential shape and natural frequency and compare coincident frequencies with measure stresses. The results show that observed peak stress frequencies do not generally correlate well with predicted coincident modes, and that flow-induced turbulence excites frequencies below piping shell modes that can also result in significant stresses that combine with AIV.

Commentary by Dr. Valentin Fuster
2016;():V009T24A024. doi:10.1115/GT2016-57803.

Solid particle ingestion is one of the principal degradation mechanisms in the compressor and turbine sections of gas turbines. In particular, in industrial applications, the micro-particles not captured by the air filtration system can cause deposits on blading and, consequently, result in a performance drop of the compressor.

This paper presents three-dimensional numerical simulations of the micro-particle ingestion (0.15 μm – 1.50 μm) in a transonic axial compressor stage, carried out by means of a commercial computational fluid dynamic code. Particles of this size can follow the main air flow with relatively little slip, while being impacted by flow turbulence. It is of great interest to the industry to determine which zones of the compressor blades are impacted by these small particles. Particle trajectory simulations use a stochastic Lagrangian tracking method that solves the equations of motion separately from the continuous phase.

A particular computational strategy is adopted in order to take into account the presence of two subsequent annular cascades (rotor and stator) in the case of particle ingestion. The proposed strategy allows the evaluation of particle deposition in an axial compressor stage thanks to its capability of accounting for the rotor/stator interaction. NASA Stage 37 is considered as a case study for the numerical investigation. The compressor stage numerical model and the discrete phase model are set up and validated against the experimental and numerical data available in literature.

The blade zones affected by particle impact and the kinematic characteristics of the impact of micrometric and sub-micrometric particles with the blade surface are shown. Both blade zones interested by particle impact and deposition are analyzed.

The particle deposition is established by using the quantity called sticking probability, adopted from literature. The sticking probability links the kinematic characteristics of particle impact on the blade with fouling phenomenon.

The results show that micro-particles tend to follow the flow by impacting at full span with a higher impact concentration on the pressure side of rotor blade and stator vane. Both rotor blade and stator vane suction side are affected only by the impact of smaller particles (up to 1 μm). Particular fluid dynamic phenomena, such as separation, shock waves and tip leakage vortex, strongly influence the impact location of the particles. The kinematic analysis shows a high tendency of particle adhesion on the suction side of the rotor blade, especially for particles with a diameter equal to 0.15 μm.

Commentary by Dr. Valentin Fuster
2016;():V009T24A025. doi:10.1115/GT2016-57961.

This paper investigates the control challenges in the operation of centrifugal compressors in series configuration, which arise from the interactions of the different control loops for process and anti-surge control. The investigation is carried out with simulations, where two compressors in series are considered to represent the simplest possible case for systems analysis. The simulations are based on a well-known nonlinear compressor model to represent the plant behavior as close to reality as possible. The paper starts with the derivation of linear transfer functions from step response analysis, which are used to develop both advanced decentralized control solutions using loop decoupling schemes as well as a model predictive controller (MPC). The various alternative control strategies are then investigated by introducing a typical downstream disturbance case that pushes the compressors towards surge conditions while creating interactions among all control loops. The simulation results are analyzed to determine the advantages and the shortcomings of the different control strategies. The MPC based approach stands out as a high performance alternative to traditional control schemes relying on simple feedback and feedforward elements.

Commentary by Dr. Valentin Fuster
2016;():V009T24A026. doi:10.1115/GT2016-57976.

In the last few years wet compression has received special attention from the oil and gas industry. Here, the development and implementation of new subsea solutions are important focus areas to increase production and recovery from existing fields. This new technology will contribute to exploitation of small and remote fields and access in very deep water. In this regard liquid tolerance represents a viable option to reduce the cost of a subsea compression station bringing considerable simplification to the subsea process itself. However, the industry may experience some drawbacks: the various levels of liquid presence may create operational risk for traditional compressors; the liquid may cause mechanical damage because of erosion and corrosion of the internal units and the compressor performance might be affected too. The experimental investigation conducted in the study considers dry and wet conditions in a laboratory setup to understand how the presence of liquid influences the stage performance. The test campaign has been carried out at the Norwegian University of Science and Technology, NTNU, in Trondheim, to assess the performance and operating range of a tridimensional impeller when processing a mixture of gas and liquid phases. Experimental results allowed validating the OEM internal prediction code for compressors’ performance in wet conditions. Finally, the effect of liquid on machine operability has been assessed through a left-limit investigation by means of dynamic pressure probes readings in order to evaluate the stall/surge behaviour for different values of liquid mass fraction.

Commentary by Dr. Valentin Fuster
2016;():V009T24A027. doi:10.1115/GT2016-57984.

Flow evolving in turbomachinery applications is turbulent and laden with particles, such as dust, ash, dirt, etc. This would affect the behaviour of the turbine components given that solid particles can impact and then bounce off, deposit or erode their surfaces. Erosion and deposition phenomena may seriously affect the components performance, because they alter the blade profile and hence the flow field. It is thus clear that the prediction of these phenomena would be of great help form both design optimization and maintenance of turbomachinery. Besides experiments, in the last decade CFD became one of the main tool for studying flow evolution within turbine components, phenomena that involve them, and prediction of problems. In particle-laden flows, CFD is used to simulate the flow field, but also solid particle transport and dispersion, impact mechanics, adhesion or rebound, and erosion. Several approaches can be used depending on the kind of application studied and information expected. Particle transport can be simulated adopting a single or cluster of particle tracking approach (Crowe et al, 2006). Since to have a statistically independent results a large number of simulated particles is needed, the for-mer approach can be used when the domain size is not too large; moreover the instantaneous flow field is needed, otherwise turbulent dispersion of particles has to be accounted for. The cluster of particles approach (i.e., Particle Cloud Tracking model, Baxter 1989) overcomes some of these problems, since it usually uses a model for particle dispersion, computing very few trajectories to simulate a large number of particles. Particle impact/rebound and deposit/erosion are modelled using one of the available choice. For instance, impact mechanics can be modelled according to the Johnson-Kendall-Roberts theory (1971) if the particle temperature is not large enough to modify the physical properties of the particles, or the Thornton and Ning variation (1997). When the effect of temperature becomes relevant, a temperature based sticking model is used, such as that of Walsh et al. (1990). Erosion can be studied according to the model of Tabackof (1979). Aim of this study is showing how CFD can be used to simulate particle deposition/erosion in all the components of a turbine (i.e. fan, turbine, compressor), and predict the most critical regions of a given component. This will be done introducing the numerical models used for some applications, describing reference test cases, and showing/discussing results.

Commentary by Dr. Valentin Fuster

Supercritical CO2 Power Cycles

2016;():V009T36A001. doi:10.1115/GT2016-56038.

The Bechtel Marine Propulsion Corporation (BMPC) Integrated System Test (IST) is a two shaft recuperated closed Brayton cycle using supercritical carbon dioxide (sCO2) as the working fluid. The IST is a simple recuperated Brayton cycle with a variable speed turbine driven compressor and a constant speed turbine driven generator designed to output 100 kWe. The main focus of the IST is to demonstrate operational, control, and performance characteristics of an sCO2 Brayton power cycle over a wide range of conditions.

IST operation has reached the point where the system can be run with the turbine-compressor thermal-hydraulically balanced so that the net power of the cycle is equal to the turbine-generator output. In this operating mode, power level is changed by using the compressor recirculation valve to adjust the fraction of compressor flow that goes to the turbines as well as the compressor pressure ratio. Steady-state operational data and trends are presented at various system power levels from near zero net cycle power to maximum operating power using a simplified thermal-hydraulic based control method. Confirmation of stable steady-state operation of the system with automatic thermal-hydraulic control is also discussed.

Commentary by Dr. Valentin Fuster
2016;():V009T36A002. doi:10.1115/GT2016-56426.

Nuclear Power Institute of China (NPIC) has started the investigation on supercritical carbon dioxide (S-CO2) power cycles since 2011. The aim of this project is to understand the feasibility and economics of coupling S-CO2 power cycle with Gen IV nuclear reactors, industrial waste heat, solar power, and so on. Up to now, the pre-concept design, economics analyses, and feasibility evaluation has been accomplished. The focus in the next step is the technology demonstrations by constructing and testing an integral test loop of S-CO2 Brayton cycle. With subcontractors and collaborators from China and UK, NPIC starts the design of a MWe scale Integral Test Loop in mid-2015. This paper presents some preliminary design results on this Integral Test Loop, including cycle layouts, design heat balance, and components considerations. Future planning on SCO2 power cycle research and development in NPIC is discussed.

Commentary by Dr. Valentin Fuster
2016;():V009T36A003. doi:10.1115/GT2016-56431.

We recently proposed a numerical method for simulating flows of supercritical CO2 based on a preconditioning method and the thermophysical models programed in a program package for thermophysical properties of fluids (PROPATH). In this study, this method is applied to the investigation of cascade channel. Numerical results obtained by assuming supercritical pressure conditions indicate that the normal shock generated in the cascade channel deeply depends on the pressure condition. In particular, the speed of sound varying with the pressure variation at the supercritical state is a key thermophysical property which changes the flow field in the cascade channel. In addition, we also simulate those flows with nonequilibrium condensation in which the inlet pressure and temperature approaching to those of the critical point are specified. Then a nonequilibrium condensation model developed by our group is further applied to the numerical method. CO2 condensation observed in a case indicates that condensation occurs at a local region near the leading edge due to the flow expansion; the droplets soon grow at the local region and streams downward with keeping almost the same mass fraction.

Commentary by Dr. Valentin Fuster
2016;():V009T36A004. doi:10.1115/GT2016-56459.

In this work, the National Energy Technology Laboratory (NETL) in collaboration with the Thermochemical Power Group (TPG) of the University of Genoa have developed a dynamic model of a 10 MW closed-loop supercritical CO2 (sCO2) recompression Brayton cycle plant in the MATLAB-Simulink environment. The sCO2 cycle modeled here is a closed cycle with an external thermal source used to heat the sCO2 working fluid before it is expanded in a turbine. The turbine exhaust heat is recuperated using high- and low-temperature recuperators, with mixing of two compressor outlets between the recuperators (on the cold-side). About two thirds of the low-pressure sCO2 is compressed by a main compressor, after passing through a cooler, while the remaining working fluid flows directly through a bypass compressor. The reference fluid properties (REFPROP) method by the National Institute of Standards and Technology is used to provide the thermodynamic and transport properties for sCO2 over the cycle temperature and pressure range because the sCO2 behavior is highly non-ideal, especially at the inlet of the two compressors. Dynamic simulations have been carried out to assess the behavior of the plant during a typical process disturbance.

Commentary by Dr. Valentin Fuster
2016;():V009T36A005. doi:10.1115/GT2016-56513.

Cycle efficiency is one of the critical parameters linked to the success of implementing a Supercritical Carbon Dioxide (sCO2) power cycle in a Concentrating Solar Power (CSP) plant application. Ambient conditions often change rapidly during operation, making it imperative that the efficiency of the plant cycle be optimized to obtain the maximum power production when sunlight is available. Past analyses have shown that operating the cycle at the critical point provides the optimum efficiency for dry operation. However, operation at this point is challenging due to the dramatic changes in thermophysical properties of CO2 near the critical point and the risk of the fluid having a two-phase, gas-liquid state. As a result, there is a high likelihood that liquid can form upstream of the primary compressor in the sCO2 power cycle. This paper explores the potential for liquid formation when operating near the critical point and looks at the influence of liquid on the compressor performance. The performance impact is based on industry experience with wet gas compression in power generation and oil and gas applications. Options for mitigating liquid effects are also investigated, such as upstream heating, separation, or compressor internal controls (blade surface gas ejection). The conclusions of the paper focus on the risk, estimated impact on performance, and summary of mitigation techniques for liquid CO2 entering a sCO2 compressor.

Commentary by Dr. Valentin Fuster
2016;():V009T36A006. doi:10.1115/GT2016-56532.

A Sodium-cooled Fast Reactor (SFR) has receiving attention as one of the promising next generation nuclear reactors because it can recycle the spent nuclear fuel produced from the current commercial nuclear reactors and accomplish higher thermal efficiency than the current commercial nuclear reactors. However, after shutdown of the nuclear reactor core, the accumulated fission products of the SFR also decay and release heat via radiation within the reactor. To remove this residual heat, a decay heat removal system (DHRS) with supercritical CO2 (S-CO2) as the working fluid is suggested with a turbocharger system which achieves passive operational capability. However, for designing this system an improved S-CO2 turbine design methodology should be suggested because the existing methodology for designing the S-CO2 Brayton cycle has focused only on the compressor design near the critical point.

To develop a S-CO2 turbine design methodology, the non-dimensional number based design and the 1D mean line design method were modified and suggested. The design methodology was implemented into the developed code and the code results were compared with existing turbine experimental data. The data were collected under air and S-CO2 environment. The developed code in this research showed a reasonable agreement with the experimental data. Finally using the design code, the turbocharger design for the suggested DHRS and prediction of the off design performance were carried out. As further works, more effort will be put it to expand the S-CO2 turbine test data for validating the design code and methodology.

Commentary by Dr. Valentin Fuster
2016;():V009T36A007. doi:10.1115/GT2016-56537.

The Queensland Geothermal Energy Centre of Excellence is investigating the use of supercritical CO2 closed loop Brayton cycles in the Concentrated Solar Thermal power cycle plant. One of the important components in the turbomachinery within the plant are seals. As the cycle is closed loop and operating at high pressures, dry gas seals have been recommended for future use in these systems. One of the main challenges of using supercritical CO2 dry gas seals is that operating conditions are near the critical point. In the supercritical region in the vicinity of the critical point (304 K, 7.4 MPa), CO2 behaves as a real-gas, exhibiting large and abrupt non-linear changes in fluid and transport properties and high densities. To correctly predict the seal operation and performance, the interaction between this real gas and the seal rotor (primary ring) and the seal stator (mating ring) need to analysed and investigated in detail, as they can lead to significant changes in flow and seal performance. Results from this paper show that increased centrifugal effects caused by higher gas densities can reduce the pressure in the sealing dam region. This adversely affects the loading capacity of the dry gas seal. However, it also benefits seal performances by reducing the leakage rate. The current work presents an investigation of the supercritical CO2 dry gas seals operating close to the critical point with an inlet pressure and temperature of 8.5Mpa and 370K respectively and a speed of 30000 RPM. Results highlighting the effects of the groove length or dam to groove ratio on the performance of the dry gas seal are presented. The seal is simulated using Computational Fluid Dynamics to study the flow behaviour of the supercitical CO2 in the dry gas seal. Supercritical CO2 fluid properties are based on the fluid database REFPROP. The numerical model was validated with previous work and good agreement was demonstrated.

Commentary by Dr. Valentin Fuster
2016;():V009T36A008. doi:10.1115/GT2016-56820.

The Supercritical Carbon Dioxide (SCO2) Brayton cycle has been getting more and more attentions all over the world in recent years for its high cycle efficiency and compact components. The compressor is one of the most important components in the cycle. Different from traditional working fluid, SCO2 has a risk of condensation at the impeller inlet because of the particular properties near the critical point. In order to determine the possibility of the condensation, a concept called “Condensation Margin (CM)” suited for SCO2 is introduced. It is associated with the total and saturated thermodynamic conditions. A design parameter called velocity ratio at the impeller inlet (IVR) is defined to control the state of working fluid at impeller inlet based on CM. In terms of different constraints and design requirements, such as impeller efficiency, operating range and processing technic, especially in small size cases, the design parameters at the impeller outlet are explored by establishing a function of outlet width, the number of blades, rotating speed, outlet tangential velocity coefficient and outlet meridional velocity coefficient. A preliminary design result of a low-flow-coefficient SCO2 centrifugal compressor is presented as an example of the application of the design parameters exploration results; then CFD simulation is performed, and consistent results are obtained compared with exploration results.

Commentary by Dr. Valentin Fuster
2016;():V009T36A009. doi:10.1115/GT2016-56847.

This paper explores the feasibility of a direct coupled turbo-compressor power block for a simple recuperated S-CO2 Brayton cycle. The turbine inlet temperature is fixed at 600°C and the maximum working pressure is restricted to 300 bar due to material constraints to enable use of conventional steel alloys. Analysis is performed for a single stage radial flow turbine and a centrifugal compressor configuration. Mean-line flow is individually analyzed for the turbine and compressor to generate contour maps of optimum operating speeds for a range of power levels at various isentropic efficiencies. While performing the mean-line analysis real gas properties and friction coefficients of S-CO2 have been considered. The mean-line flow code is coupled with thermodynamic model of the simple recuperated S-CO2 Brayton cycle for generating a range of optimum operating conditions where direct coupled power blocks can be used.

Commentary by Dr. Valentin Fuster
2016;():V009T36A010. doi:10.1115/GT2016-57100.

With the efforts of many researchers and engineers on the Supercritical CO2 (S-CO2) Brayton cycle technology development, the S-CO2 Brayton cycle is now considered as one of the key power technologies for the future. Since S-CO2 Brayton cycle has advantages in economics due to high efficiency and compactness of system, various industries have been trying to develop technologies on the design and analysis of S-CO2 Brayton cycle components. Among various technical issues on the S-CO2 Brayton cycle technology development, treatment of thermodynamic property near the critical point of S-CO2 is very important since the property shows non-linear variation which causes large error on design and analysis results for ideal gas based methodologies.

Due to the special behavior of thermodynamic property of CO2 near the critical point, KAIST research team has been trying to develop a S-CO2 compressor design and analysis tool to reflect real gas effect accurately for better design and performance prediction results. The main motivation for developing an in-house code is to establish turbomachinery design methodology based on general equations to improve accuracy of design and analysis results for various working fluids including S-CO2. One of the key improvements of KAIST_TMD which is an in-house tool for S-CO2 turbomachinery design and analysis is the conversion process between stagnation condition and static condition. Since fluid is moving with high flow velocity in a compressor, the conversion process between stagnation and static condition is important and it can have an impact on the design and analysis results significantly. A common process for the conversion is based on the specific heat ratio which is typically a constant from ideal gas assumption. However, specific heat ratio cannot be assumed as a constant for the case of S-CO2 compressor design and analysis because it varies dramatically near the critical point.

Thus, in this paper, sensitivity study results on the state condition conversion between stagnation and static conditions with different approaches will be presented and further analysis on impact of the selected approaches on the final impeller design results will be discussed.

Commentary by Dr. Valentin Fuster
2016;():V009T36A011. doi:10.1115/GT2016-57132.

CO2 in a transcritical CO2 cycle can not easily be condensed due to its low critical temperature (304.15K). In order to increase the critical temperature of working fluid, an effective method is to blend CO2 with other refrigerants to achieve a higher critical temperature. In this study, a transcritical power cycle using CO2-based mixtures which blend CO2 with other refrigerants as working fluids is investigated under heat source. Mathematical models are established to simulate the transcritical power cycle using different CO2-based mixtures under MATLAB® software environment. A parametric analysis is conducted under steady-state conditions for different CO2-based mixtures. In addition, a parametric optimization is carried out to obtain the optimal design parameters, and the comparisons of the transcritical power cycle using different CO2-based mixtures and pure CO2 are conducted. The results show that a raise in critical temperature can be achieved by using CO2-based mixtures, and CO2-based mixtures with R32 and R22 can also obtain better thermodynamic performance than pure CO2 in transcritical power cycle. What’s more, the condenser area needed by CO2-based mixture is smaller than pure CO2.

Commentary by Dr. Valentin Fuster
2016;():V009T36A012. doi:10.1115/GT2016-57151.

As the demand to develop more efficient energy systems increases, ways to generate power from waste heat are under investigation. The supercritical carbon dioxide recovery cycle (S-CO2 cycle) has been considered a viable candidate as a bottoming cycle for “waste heat to power” (WHP) applications, such as the utilization of gas turbine outlet heat.

One major limitation to the system is that the S-CO2 cycle operates at a low expansion ratio, which leads to a higher turbine outlet temperature. This waste heat should be recuperated in order for the overall cycle efficiency to increase. Such limitation leads to a larger recuperator, higher volume flow rate, lower temperature gradient at the heater, and more complex cycle layouts for WHP applications. These constraints ultimately lead to the increase of hardware costs, which can degrade economics of the system.

To solve the existing problems regarding the use of S-CO2 cycle for WHP applications, the possibility of using an isothermal compressor in place of a conventional compressor in a simple Brayton cycle is investigated. This solution, named the iso-Brayton cycle, though the compressor technology is still under development, seems promising because it does not require an additional heat exchanger as one of the cycle components. Furthermore, the compressing work is minimized during an isothermal compression process.

To analyze the cycle performance of the iso-Brayton cycle, it is compared with a reference cycle, the simple recuperated Brayton cycle. The parameters of cycle net efficiency and cycle net work (or net usable work) are calculated using the KAIST-CCD in-house code.

Commentary by Dr. Valentin Fuster
2016;():V009T36A013. doi:10.1115/GT2016-57460.

Three supercritical carbon dioxide (CO2) power cycle experimental loops have been developed in Korea Institute of Energy Research (KIER) from 2013. As the first step, a 10 kWe-class simple un-recuperated Brayton power cycle experimental loop was designed and manufactured to test its feasibility. A 12.6 kWe hermetic turbine-alternator-compressor (TAC) unit which is composed of a centrifugal compressor, a radial turbine and the gas foil bearings was manufactured. The turbine inlet design temperature and pressure were 180 °C and 130 bar, respectively. Preliminary operation was successful at 30,000 RPM which all states of the cycle existed in the supercritical region. Second, a multi-purpose 1 kW-class test loop which operates as a transcritical cycle at a temperature of 200 °C was developed to concentrate on the characteristics of the cycle, control and stability issues of the cycle. A high-speed turbo-generator was developed which is composed of a radial turbine with a partial admission nozzle and the commercial oil-lubricated angular contact ball bearings. Finally, a 60 kWe-class Brayton cycle is being developed which is composed of two turbines and one compressor to utilize flue-gas waste heat. As the first phase of development, a turbo-generator which is composed of an axial turbine, a mechanical seal and the oil-lubricated tilting-pad bearings was designed and manufactured.

Commentary by Dr. Valentin Fuster
2016;():V009T36A014. doi:10.1115/GT2016-57481.

Centrifugal compressors are one of the best choices among compressors in supercritical Brayton cycles. A supercritical CO2 centrifugal compressor increases the pressure of the fluid which state is initially very close to the critical point. When the supercritical fluid is compressed near the critical point, wide variations of fluid properties occur. The density of carbon dioxide at its critical point is close to the liquid density which leads to reduction in compressor work.

The investigated compressor is a centrifugal compressor tested in the Sandia supercritical CO2 compression loop. In order to get results with the high accuracy and take into account the non-linear variation of the properties near the critical point, the CFD flow solver is coupled with a lookup table of properties of fluid. Behavior of real gas close to its critical point and the effect of the accuracy of the real gas model on the compressor performance are studied in this paper and the results are compared with the experimental data from the Sandia compression facility.

Commentary by Dr. Valentin Fuster
2016;():V009T36A015. doi:10.1115/GT2016-57620.

At Sandia National Laboratories (SNL), The Nuclear Energy Systems Laboratory / Brayton Lab has been established to research and develop subsystems and demonstrate the viability of the closed Brayton cycles (CBC), and in particular, the recompression CBC. The ultimate objective of this program is to have a commercial-ready system available for small modular reactors. For this objective, R&D efforts must demonstrate that, among other things, component and the system behavior is understood and control is manageable, and system performance is predictable. Research activities that address these needs include investigating system responses to various anticipated perturbations, and demonstrating that component and system performance is understood. To these ends, this paper presents system response to a perturbation, and turbomachinery performance results during steady state operation. A long duration test, with an extensive period at steady state, was completed in the simple CBC configuration. During this period, a cooling perturbation was initiated. Data from this test are presented and evaluated to explain the sequence of events following the perturbation. It was found that a cascading series of events ensued, starting with the fluid condensing effect of the cooling perturbation. The explanation of events emphasizes the highly interactive and nonlinear nature of CBC’s. The comparisons of measured and predicted turbomachinery performance yielded excellent results and give confidence that the predictive methods originally envisioned for this system work well.

Commentary by Dr. Valentin Fuster
2016;():V009T36A016. doi:10.1115/GT2016-57644.

Supercritical carbon dioxide cycles are recently very perspective and they are researched all around the world. CO2 is an interesting medium for applications in many technologies, from nuclear energy through geothermal, solar and waste heat recovery systems. However, S-CO2 cycles have several issues which have to be researched, one of them being the presence of the so called pinch point in the heat exchangers design. Therefore, the Czech Technical University (CTU) conducts research on supercritical carbon dioxide cycles, which are focused on the effect of the gaseous admixtures in S-CO2 on different cycle components. The research is primarily focused on the pinch point shift within heat exchangers caused by gaseous admixtures. Previous work has shown that the pinch point can be removed with the addition of small amounts of another gases. However, it is also important to describe the effect on the performance of the cycles. This is the main topic of this paper. One of the reasons for this research is the positive effects on components are possible. The first part of the study is focused on the development of computational code for calculation of the basic S-CO2 cycles with mixtures. The second part of the study is focused on the calculation of basic cycles for binary mixtures. The calculation will be performed for pure CO2 and some binary mixture. He, CO, O2, N2, Ar will be used for the calculation as the most common admixtures, furthermore H2, CH4 and H2S will be used as well. The last part of the study will be focused on the optimization of individual cycles for different amount of admixtures in CO2. The result of this study will define the optimum ratio of admixtures and description of their effect on cycle efficiency.

Commentary by Dr. Valentin Fuster
2016;():V009T36A017. doi:10.1115/GT2016-57670.

U.S. Department of Energy (DOE) has recently sponsored research programs to develop megawatt scale supercritical CO2 (sCO2) turbine for use in concentrated solar power (CSP) and fossil based applications. To achieve the CSP goal of power at $0.06/kW-hr LCOE and energy conversion efficiency > 50%, the sCO2 turbine relies critically on extremely low leakage film riding seals like dry gas seal (DGS). Although DGS technology has been used in other applications before. making it successful for stringent conditions of an sCO2 turbo-expander is challenging. This paper presents results from a multi-scale coupled physics model that predicts the performance of DGS under a typical sCO2 turbine mission cycle and addresses some of the risks specific to operation in sCO2. Real gas equations of state are incorporated in the models to capture large discontinuities in fluid properties close to the critical point. A novel experimental setup is developed to observe and characterize transition of CO2 through liquid-vapor and supercritical phases. Coupled fluid-structure-thermal interaction model investigates the effect of aerodynamic and thermal perturbations on the structural and rotordynamic instabilities. Dynamic instabilities arising from sonic transition in thin sCO2 film of DGS pose additional challenges while the large surface roughness changes due to sCO2 corrosion warrant further design considerations. Effectiveness of features like spiral grooves in converting fluid momentum into pressure rise in the thin film and also in achieving local flow reversals is investigated. Effect of various design features on the optimal performance is quantified and insights for a successful DGS operation in a sCO2 turbomachine are provided.

Commentary by Dr. Valentin Fuster
2016;():V009T36A018. doi:10.1115/GT2016-57695.

U.S. Department of Energy (DOE) has recently sponsored research programs to develop Megawatt scale supercritical CO2 (sCO2) turbine for use in concentrated solar power (CSP) and fossil based applications. From rotordynamics perspective, the extremely high power densities associated with this sCO2 turbine (similar to those of rocket turbo pump) and the long shaft of the turbine require mid-span support from bearings to achieve the high rotational speeds needed to achieve desired turbine efficiency targets. The mid-span bearings, if oil lubricated, need two sets of seals to isolate the working fluid from lubricating oil and represent a combined parasitic load of as much as 5% of the power plant output. These parasitic loads can be avoided by the use of hybrid gas bearing (HGB) which uses turbine working fluid itself as the lubricant for generating hydrostatic + hydrodynamic pressure and hence can operate in a high temperature, high pressure sCO2 environment. A successful and reliable implementation of HGB is expected to provide significant efficiency improvement in the sCO2 turbine for modular plants in 10MW scale. However, the behavior of hybrid gas bearing with supercritical CO2 as the working fluid has never been studied before. Hence a careful investigation of its hydrodynamic and rotordynamic characteristics with perturbations associated with sCO2 fluid and sCO2 turbine is warranted. In this paper the use of HGB to bring the critical speed ratio of highly power dense 10 MW sCO2 turbine into a manageable regime is investigated. Further, the performance characteristics of HGB in high pressure, high temperature sCO2 are investigated with a coupled fluid-structure interaction model. Differences between load capacity and stiffness characteristics for traditional operation in air versus operation in highly dense and viscous sCO2 are highlighted. Using Bayesian probabilistic models, insights into key design parameters needed to optimize HGB performance are obtained. A 3D fracture mechanics and crack propagation model is also developed to predict life of HGB under typical sCO2 turbine mission cycle for CSP application.

Commentary by Dr. Valentin Fuster
2016;():V009T36A019. doi:10.1115/GT2016-58066.

The Allam Cycle is a semi-closed, recuperated, oxy-fuel, supercritical carbon dioxide (sCO2) Brayton power cycle, offering advantages over simple cycle and combined cycle arrangements. The Allam Cycle uniquely combines oxy-combustion with a substantially elevated operating pressure, high sCO2 recirculation flow, high gross turbine efficiency, and inventive low- and high-grade heat recuperation. As a result, the core Allam Cycle meets or exceeds the achievable net efficiencies of existing high efficiency combined cycle plants not equipped for carbon capture, while capturing substantially all CO2 emissions at purities and pressures necessary for downstream CO2 reuse and/or sequestration. Additionally, with minor alterations, the core cycle can operate with a variety of organic fuels. A 50MWt natural gas-fired demonstration of the core cycle is currently under development by 8 Rivers, NET Power, CB&I, Exelon, and Toshiba.

This paper addresses the coal syngas-fired variant of the Allam Cycle system, extending beyond high-level feasibility analyses conducted in previous studies. The paper explores in detail the unique considerations, possible hurdles, and advantages of integrating a commercially-available coal gasifier with the Allam Cycle. In particular, the paper analyzes five (5) primary technical optimizations that drive the Allam Cycle’s advantages in efficiency and cost over conventional baselines. These include: (1) a simpler overall process, requiring fewer critical integration points while still providing for efficient high- and low-grade heat recuperation; (2) high efficiencies regardless of coal rank and type used — further, the efficiency drop when using low-rank coal in an Allam Cycle arrangement is smaller than IGCC arrangements; (3) high efficiencies regardless of syngas composition (such as H2:CO ratio), particularly when compared to gasification in the chemical industry and IGCC with carbon capture and sequestration; (4) the ability to utilize a singular, cost-effective post-combustion SOX/NOX removal mechanism; and (5) considerable water savings versus IGCC and SCPC baselines, with the ability to run substantially water free with only minor impacts to overall efficiency.

Commentary by Dr. Valentin Fuster
2016;():V009T36A020. doi:10.1115/GT2016-58137.

Supercritical CO2 (sCO2) radial inflow turbine are an enabling technology for small scale concentrated solar thermal power. They are a research direction of the Australian Solar Thermal Research Initiative (ASTRI). This study uses the 1D meanline design code TOPGEN, to explore the radial turbine design space under consideration of sCO2 real gas properties. TOPGEN maps a parametric design space defined by flow and head coefficient.

The preliminary design code is used explore the feasibility, geometry and performance of sCO2 turbines in the 100kW to 200kW range in order to assess feasible design spaces and to investigate turbine scaling. Turbines are scaled with respect to power, while maintaining specific speed constant and with respect to speed. This analysis shows that both scaling approaches change the feasible design space and that both geometric constraints such as blade height or operational constraints such as blade natural frequency can significantly limit the design space.

Detailed analysis of four shortlisted designs shows that turbine efficiencies close to 85% can be attained for 100kW and 200kW output powers, even when operating at reduced rotor speeds. This work provides new insight towards the design of small scale radial turbines for operation with sCO2 and highlights scaling issues that may arise when testing sub-scale turbine prototypes.

Commentary by Dr. Valentin Fuster
2016;():V009T36A021. doi:10.1115/GT2016-58144.

The forced response of high-pressure sCO2 radial-inflow turbine blisk is studied with regards to internal mistuning and inherent characteristics of supercritical Brayton cycle. A novel preliminary meanline analysis led to the generation of turbine designs for the sCO2 Brayton cycle with respect to concentrating solar power (CSP) applications. Details of mentioned study are published in a separate paper. The sCO2 turbine with a pressure ratio of 2.2 and the mild inlet temperature of 560 C is studied for rotational speed ranging between 75000 and 125000 RPM.

Aiming to achieve an enhanced understanding of the fluid-structure-interaction in sCO2 radial-inflow turbine, a numerical method capable of predicting the forced responses of tuned and intentionally mistuned blisks due to aerodynamic excitation is presented. The numerical work involves the simulation of the transient flow field, and then the unsteady aerodynamic excitation forces on the blades are determined by modelling various resonance condition, including the influence of the operating condition and stator number. Performing the forced response of the structure, the transient and spatially resolved pressure distribution is used as a boundary condition in an FE model. As a result, the response amplifications of sCO2 turbines are eventually compared.

The similar geometrical turbine was designed and manufactured to be operated in subcritical state for the sake of validation. The results of the subcritical turbine are derived by means of experimental and numerical analyses. To update the effect of mistuning in the FE model, blade by blade measurements using the example of a subcritical turbine blisk is performed and results of well correlated FRFs are used as inputs to adjust the blade individual Young’s modulus. The tendency of results is approved by previous works done in subcritical state. The structural damping information to be considered in the update process is taken from results of an experimental modal analysis and the aerodynamic damping induced by blade vibration is computed using an energy balance technique.

It has been found that increase of the maximum forced response beyond that of the sCO2 turbine with higher rotational speed is not significant due to the existence of high pressure-density sCO2. This implies an occurrence of high aerodynamic damping which would cause a low vibrational amplitude in case of a mistuned blisk. Considering aeroelastic coupling, in supercritical turbine with small mistuning, no change of maximum response magnitudes is achieved for the fundamental bending mode; however, with large mistuning pattern, aerodynamic damping can cause significantly better response level. This result indicates considerable contrast with responses obtained from subcritical model which would be expressed by either characteristic or state of working fluid.

Commentary by Dr. Valentin Fuster

Wind Energy

2016;():V009T46A001. doi:10.1115/GT2016-56190.

In urban areas, it is preferable to use small wind turbines which may be integrated to a building in order to supply the local grid with green energy. The main drawback of using wind turbines in urban areas is that the air flow is affected by the existence of nearby buildings, which in conjunction with the variation of wind speed, wind direction and turbulence may adversely affect wind energy extraction. Moreover, the efficiency of a wind turbine is limited by the Betz limit. One of the methods developed to increase the efficiency of small wind turbines and to overcome the Betz limit is the introduction of a converging – diverging shroud around the turbine. Several researchers have studied the effect of shrouds on Horizontal Axis Wind Turbines, but relatively little research has been carried out on shroud augmented Vertical Axis Wind Turbines.

This paper presents the numerical study of a shrouded Vertical Axis Wind Turbine. A wide range of test cases, were examined in order to predict the flow characteristics around the rotor, through the shroud and through the rotor – shroud arrangement using 3D Computational Fluid Dynamics simulations. The power output of the shrouded rotor has been improved by a factor greater than 2.0. The detailed flow analysis results showed that there is a significant improvement in the performance of the wind turbine.

Commentary by Dr. Valentin Fuster
2016;():V009T46A002. doi:10.1115/GT2016-56290.

A coupling of the Lifting Line Free Vortex Wake (LLFVW) model of the open source wind turbine software QBlade and the wind turbine structural analysis tool FAST has been achieved. FAST has been modified and compiled as a dynamic library, taking rotor blade loading from the LLFVW model as input. Most current wind turbine aeroelastic simulations make use of the Blade Element Momentum (BEM) model, based upon a number of simplifying assumptions which are often violated in unsteady situations. The purpose of the implemented model is to improve accuracy under unsteady conditions. The coupling has been thoroughly validated against the NREL 5MW reference turbine. The turbine is compared under both steady conditions and three unsteady operating conditions to the BEM code AeroDyn. The turbine has been simulated operating at a constant RPM and with a variable-speed, variable blade-pitch-to-feather controller. Under steady conditions the agreement between the LLFVW and AeroDyn is demonstrated to be very good. The LLFVW produces different predictions for rotor power, blade deflection and blade loading during transient conditions. A number of important observations have been made which illustrate the necessity of a higher fidelity aerodynamic model. The validation and results are considered as a step towards the implementation of an open-source, high fidelity aeroelastic tool for wind turbines.

Commentary by Dr. Valentin Fuster
2016;():V009T46A003. doi:10.1115/GT2016-56338.

Nowadays, ensuring access to energy is one of the serious challenges the world confronts. For those who live in poverty, a shortage of access to energy services desperately influences and undermines health, affects education and development. The problem of energy access for the poor countries has become even more intense because of the impacts of climate change, the global financial crisis and volatile energy prices. Then a use of another sources of energy such as wind power could be a good alternative in these countries.

This paper represents a design of a vertical axis wind turbine that will produce an output power of 883 W for 9 m/s wind speed from a synchronous generator. The project involves the design and the sizing of all the components of this wind turbine. Oil barrels will be used as blades, Filippini was one of the first ones who developed a wind turbine architecture with half cylinders. Later, some modifications were introduced which leads to Thiès rotor and C-rotor.

A simple design was established to facilitate its implementation and reduce its cost. This wind turbine was designed to be used in developing countries. These facts had led to think about an uncomplicated conception and use accessible and cheap equipment which could be available all over the world.

Afterwards, a scaled prototype was realized to make some tests in order to examine its efficiency, some modifications were done to observe the feedback. The procedure of the design of the wind turbine was accomplished from the beginning to the end, no step has been skipped. Later, an estimation of cost was completed. The initial cost remains lower than the cost of a wind turbine in the market. Finally, this wind turbine could be constructed easily with accessible materials. An implementation of this machine in developing countries could help people in their lives. An economical model applied to an African country shows that using one turbine could save about 1400 € per year.

Commentary by Dr. Valentin Fuster
2016;():V009T46A004. doi:10.1115/GT2016-56359.

Wind Turbine Array Boundary Layer (WTABL) is a relatively simple, yet useful theoretical conceptualization to study very large wind farms in atmospheric boundary layer (ABL). In the current paper, we perform a high-fidelity LES investigation of a 3 × 3 wind turbine array in a WTABL framework, with a main focus on extending the work beyond the simple analytical model and providing a rigorous fundamental understanding of the dynamic behaviour of length scales, their scaling laws and the anisotropic structure of the energy containing eddies responsible for power generation from the wind turbines. This is accomplished by studying the components of energy and shear-stress spectra in the flow. This knowledge can potentially provide an efficient way to control the wind farm power output as well as serve as a stepping stone to design efficient low order numerical models for predicting farm power and dynamics at reduced computational expense.

Commentary by Dr. Valentin Fuster
2016;():V009T46A005. doi:10.1115/GT2016-56377.

Small horizontal axis wind turbines (sHAWTs) are mostly designed by smaller companies with no or just small possibilities of aerodynamic testing and hence, airfoil selection is often based on published performance data and minimal or no experimental testing from the blade designer’s side. This paper focuses on the aerodynamic consequences resulting from an unqualified airfoil selection and accumulating surface soiling. The high performance low Reynolds profile FX 63-137 is compared to an Eppler-338 wing section as well as to a high performance utility scale wind turbine airfoil, AH 93-W-174 -1ex. We extensively investigated these three different airfoils within the low Reynolds regime between 50,000 and 200,000. This regime is especially important for the starting behavior of a wind turbine, i.e. a quick speed up, and is crucial for small wind turbines because they have more frequent start/stop events. A Reynolds number of 200 k is additionally the operational regime of some sHAWT under the 5–10 kW level.

The present study discusses not only the low Reynolds performance of the smooth profiles but investigates the influence of surface soiling. This ranges from 2D disturbances, such as a 0.2mm thin tripwire or several zigzag tapes, up to the simulation of massive sand build up by covering the entire leading edge region with a 40 grit sand paper. The experiments reveal that even small surface soiling has an impact and massive roughness leads in some cases to the loss of 50% in lift coefficient. The experimental data is used to simulate a sHAWT in different stages of debris. While the peak power was reduced by two thirds compared to the clean configuration the annual energy production has halved under certain conditions.

Commentary by Dr. Valentin Fuster
2016;():V009T46A006. doi:10.1115/GT2016-56679.

The present paper analyzes the effect of passive flow control (PFC) with respect to the retrofitting on small horizontal axis wind turbines (sHAWT). We conducted extensive wind tunnel studies on an high performance low Reynolds airfoil using different PFC elements, i.e. vortex generators (VGs) and Gurney flaps. QBlade, an open source Blade Element Momentum (BEM) code, is used to study the retrofitting potential of a simulated small wind turbine. The turbine design is presented and discussed. The simulations include the data and polars gained from the experiments and give further insight into the effects of PFC on sHAWT. Therefore several different blades were simulated using several variations of VG positions. This paper discusses their influence on the turbine performance. The authors focus especially on the start-up performance as well as achieving increased power output at lower wind speeds. The vortex generators reduce the risk of laminar separation and enhance the lift in some configurations by more than 40% at low Reynolds numbers.

Commentary by Dr. Valentin Fuster
2016;():V009T46A007. doi:10.1115/GT2016-56685.

Interest in vertical-axis wind turbines (VAWTs) is experiencing a renaissance after most major research projects came to a standstill in the mid 90’s, in favour of conventional horizontal-axis turbines (HAWTs). Nowadays, the inherent advantages of the VAWT concept, especially in the Darrieus configuration, may outweigh their disadvantages in specific applications, like the urban context or floating platforms.

To enable these concepts further, efficient, accurate, and robust aerodynamic prediction tools and design guidelines are needed for VAWTs, for which low-order simulation methods have not reached yet a maturity comparable to that of the Blade Element Momentum Theory for HAWTs’ applications. The two computationally efficient methods that are presently capable of capturing the unsteady aerodynamics of Darrieus turbines are the Double Multiple Streamtubes (DMS) Theory, based on momentum balances, and the Lifting Line Theory (LLT) coupled to a free vortex wake model. Both methods make use of tabulated lift and drag coefficients to compute the blade forces.

Since the incidence angles range experienced by a VAWT blade is much wider than that of a HAWT blade, the accuracy of polars in describing the stall region and the transition towards the “thin plate like” behaviour has a large effect on simulation results. This paper will demonstrate the importance of stall and post-stall data handling in the performance estimation of Darrieus VAWTs. Using validated CFD simulations as a baseline, comparisons are provided for a blade in VAWT-like motion based on a DMS and a LLT code employing three sets of post-stall data obtained from a wind tunnel campaign, XFoil predictions extrapolated with the Viterna-Corrigan model and a combination of them. The polar extrapolation influence on quasi-steady operating conditions is shown and azimuthal variations of thrust and torque are compared for exemplary tip-speed ratios (TSRs). In addition, the major relevance of a proper dynamic stall model into both simulation methods is highlighted and discussed.

Commentary by Dr. Valentin Fuster
2016;():V009T46A008. doi:10.1115/GT2016-56728.

Computational Fluid Dynamics (CFD) simulations are becoming increasingly important to enhancing the understanding of rotor aerodynamics and improving blade design for wind turbines. The present study addresses the effect of turbulence treatment on the CFD-based performance assessment of wind turbines by successively increasing the modeling depth. A process for 2D and 3D CFD simulations is described, which is based on the geometry of the NREL 5MW reference wind turbine. It is shown that the main differences between fully turbulent computations and transition model simulations with and without additional curvature correction model occur in the inner blade region, and increase in 3D flow regimes. Literature and the findings of the present study lead to the conclusion that simulations with the transition model in conjunction with the curvature correction model should be preferred. The resulting power output of this setup is also in good agreement with the Blade Element Momentum (BEM) calculation.

Commentary by Dr. Valentin Fuster
2016;():V009T46A009. doi:10.1115/GT2016-56836.

This study introduces strategic methods for improving the aerodynamic performance of wind turbines. It was completed by combining different optimization methods for each part of the wind turbine rotor. The chord length and pitch angle are optimized by a torque-matched method (TMASO), whereas the airfoil shape is optimized by the genetic algorithm (GA). The TMASO is implemented to produce an improved design of a reference turbine (NREL UAE Phase V). The GA is operated to generate a novel airfoil design that is evaluated by automatic interfacing for the highest gliding ratio (GR). The adopted method produces an optimized wind turbine with an 11% increase of power coefficient (Cp) with 30% less of the corresponding tip speed ratio (TSR). Furthermore, the optimized wind turbine shows reduced tip loss effect.

Commentary by Dr. Valentin Fuster
2016;():V009T46A010. doi:10.1115/GT2016-57010.

The process of surface erosion due to particle collision has been the focus of a number of investigations with regards to gas turbine engines, aircraft, reentry missiles, pipelines carrying coal slurry, etc. Recently, increased interest in wind energy by countries in the Saharan regions of the Middle East and North Africa (MENA) brings about some concern about leading edge erosion of wind turbines operating under such dusty conditions. Leading edge erosion can have a detrimental impact on the extracted energy as it changes the blade surface roughness causing premature/unpredictable separation. Though erosion may not be easily avoided; it may be mitigated via using airfoil families characterized by low roughness sensitivity. In this paper, a model of an airfoil erosion subjected to sand blasting is developed using the discrete phase modeling capability in ANSYS-FLUENT along with the DNV erosion model. The effect of various flow parameters, such as angle of attack, and particle size, on the extent of erosion is investigated for a number of airfoil designs. The developed model is used as a predictive tool to assess the power deterioration of eroded wind blades.

Commentary by Dr. Valentin Fuster
2016;():V009T46A011. doi:10.1115/GT2016-57184.

This paper describes the introduction of an unsteady aerodynamics model applicable for horizontal and vertical axis wind turbines (HAWT/VAWT) into the advanced blade design and simulation code QBlade, developed at the HFI of the TU Berlin. The software contains a module based on lifting line theory including a free vortex wake algorithm (LLFVW) which has recently been coupled to the structural solver of FAST to allow for time-resolved aeroelastic simulations of large, flexible wind turbine blades. The aerodynamic model yields an accuracy improvement with respect to Blade Element Momentum (BEM) theory and a more practical approach compared to higher fidelity methods such as Computational Fluid Dynamics (CFD) which are too computationally demanding for load case calculations. To capture the dynamics of flow separation, a semi-empirical method based on the Beddoes-Leishman model now extends the simple table lookups of static polar data by predicting the unsteady lift and drag coefficients from steady data and the current state of motion. The model modifications for wind turbines and the coupling to QBlade’s vortex method are described. A 2D validation of the implementation is presented in this paper to demonstrate the capability and reliability of the resulting simulation scheme. The applicability of the model is shown for exemplary HAWT and VAWT test cases. The modelling of the dynamic stall vortex, the empiric model constants as well as the influence of the dynamic coefficients on performance predictions are investigated.

Commentary by Dr. Valentin Fuster
2016;():V009T46A012. doi:10.1115/GT2016-57667.

The assessment of robust CFD techniques is casting new light on the aerodynamics of airfoils rotating around an axis orthogonal to flow direction, with particular reference to flow curvature effects and stall mechanisms. In particular, Darrieus wind turbines’ designers are taking profit from these new discovers to improve the aerodynamic design of the rotors, in view of an increase of the overall efficiency and a reduction of the structural stresses on the blades.

A controversial design parameter for Darrieus turbines, especially in case of small-size rotors, is represented by the location of the blade-spoke connection along the chord.

The most common solution is indeed to place the connection at approximately airfoil’s quarter chord, i.e. where the pressure center is commonly located for low incidence angles. In some cases, however, the blade is connected at middle chord due to symmetry or aesthetic reasons. In some small turbines, innovative designs have even disregarded this parameter. Even if one can argue that the blade connection point is about to have some aerodynamic effects on the turbine’s performance, the real impact of this important design parameter is often not fully understood.

The present study makes use of extensive CFD simulations on a literature case study, using a NACA 0021 airfoil, to assess the influence of the blade-spoke connection point. In particular, the differences in terms of power coefficient curve of the turbine, optimal tip-speed ratio, torque profiles and stresses on the connection are analyzed and discussed. Detailed flow analyses are also shown for azimuthal positions of particular interest. Results on the selected case study showed that the middle-chord blade-spoke connection point seems to guarantee a higher performance of the rotor, even if additional solicitation is applied to the connection itself. It is further shown that the same performance can indeed be obtained with the airfoil attached at quarter chord and properly pitched. By doing so, the stresses are contained and the performance is maximized.

Topics: Blades , Wind turbines
Commentary by Dr. Valentin Fuster
2016;():V009T46A013. doi:10.1115/GT2016-57679.

Recent studies have demonstrated that, when rotating around an axis orthogonal to the flow direction, airfoils are virtually transformed into equivalent airfoils with a camber line defined by their arc of rotation. In these conditions, the symmetric airfoils commonly used for Darrieus blades actually behaves like virtually cambered ones or, equivalently, rotors have to be manufactured with counter-cambered blades in order to have the performance of a symmetric airfoil.

To complete these analyses, the present study focuses the attention on the airfoils’ aerodynamics during the start-up of the rotors. This phase of turbines’ functioning is indeed of particular interest since it actually defines the cut-in speed of the rotors and then notably impacts on the annual energy production, especially in case of small-size machines.

In the work, unsteady CFD simulations have been carried out in start-up like conditions on three airfoils, i.e. a NACA 0018 and two modified profiles based on the same airfoil. The modified profiles have been conformally transformed to fit their camber lines to the arc of a circle, such that the ratio of the airfoil chord to the circle’s radius is 0.114 or 0.25.

The study demonstrates that all the conventional theories based on one-dimensional aerodynamic coefficients (e.g. blade element momentum models) are affected by an intrinsic error in evaluating the starting torque profiles. Symmetric airfoils in fact exhibit a counter-intuitive non symmetric starting torque over the revolution. Conversely, airfoils compensated for the virtual camber effect show a substantially different starting torque profile, with a more symmetric distribution between the upwind and the downwind halves. This behavior is due to the effect of the pitching moment, which is usually neglected in lumped parameters models. At very low revolution speeds, its contribution becomes significant due to the very high angles of attack experienced by the blade. In particular, the pitching moment is non symmetric between the upwind and the downwind halves of the revolution. For upwind azimuthal positions the pitching moment reduces the overall torque output, while it changes sign in the downwind section, increasing the torque.

The importance of accounting for the pitching moment contribution in low-order models (e.g. a blade element momentum model) is finally discussed by comparing the predicted torque profiles with those obtained by CFD.

Commentary by Dr. Valentin Fuster
2016;():V009T46A014. doi:10.1115/GT2016-57701.

The category of small wind turbines is a rapidly growing market. The U. S., Europe (UK), and China are of particular interest and seeing the most growth. This paper examines the category of small wind starting with the variety of definitions found in the literature. Growth world-wide, with an emphasis on these major markets, is analyzed for trends and predicted development. The focus is on fixed pitch, small horizontal axis wind turbines, with a direct drive DC generator in the 1–10 kW class. To understand small wind turbines it is necessary to discuss design tools available for design. Included in this design discussion is the necessity for computational fluid dynamic models as well as experimentally testing both open rotors and wind tunnel models. In order for small wind turbines to continue to improve, better technologies are necessary. For design, wind turbines must be optimized for peak performance to include startup/cut-in speeds and other modifications. These wind turbines will rely on new and purposely designed airfoils; however, for low Reynolds number conditions actual airfoil data are needed as many of the computational tools do not accurately predict separation. Increasingly, noise is an issue, especially if these wind turbines will be sited in populated urban areas. An analysis of noise generation as well as design techniques for reducing noise is necessary for future designs. Important discussions on the technologies particular to small wind turbines should include the topics of aerodynamics and structures/materials. Future applications of small wind turbines seem bright. Small wind turbines are contributing to the concept of distributed generation and helping to reduce the carbon footprint. Urban environments are becoming more accepted for small wind turbines which lead to studies of flow fields in and around buildings. Of particular note are hybrid systems which combine wind with other energy generation systems such as solar, internal combustion engines, and diesel engines to name a few. These systems are advantageous for the homeowner, small business, cell phone towers, remote locations, and backup emergency power systems (to include lighting). Lastly, the concept of energy storage must be addressed in the context of small wind turbines, especially those turbines used in an isolated application. Permitting and government incentives are critical to the future success of these wind turbines.

Commentary by Dr. Valentin Fuster
2016;():V009T46A015. doi:10.1115/GT2016-57762.

In this paper the aerodynamics and performance of two Vertical Axis Wind Turbines are discussed, on the basis of a wide set of experiments performed at Politecnico di Milano (Italy). A H-shaped and a Troposkien Darrieus turbine for micro-generation, characterized by the same swept area and blade section, are tested in real-scale. Performance measurements show that the Troposkien rotor outperforms the H-shaped turbine, mostly related to the larger midspan section of the Troposkien rotor (resulting by the constraint of constant swept area) and to the non-aerodynamic struts of the H-shaped rotor. These features are consistent with the character of the wakes shed by the turbines, measured by means of hot wire anemometry on several surfaces downstream of the models. The morphology of H-shape and Troposkien rotor wakes exhibit relevant differences, especially in the three-dimensional character and time-periodic evolution in the blade tip region. In particular, large-scale vortices dominate the tip region of the wake shed by the H-shape turbine; these vortices pulsate significantly during the period, due to the periodic fluctuation of the blade aerodynamic loading. Conversely, the highly tapered shape of the Troposkien rotor prevents the onset of tip vortices, but also induces a dramatic spanwise reduction of tip speed ratio, promoting the onset of local dynamic stall marked by high periodic and turbulent unsteadiness in the tip region of the wake. The way in which these mechanisms affect the wake evolution and mixing process for the two classes of turbines is investigated for different tip speed ratios, highlighting some relevant implications in the framework of wind energy exploitation.

Commentary by Dr. Valentin Fuster
2016;():V009T46A016. doi:10.1115/GT2016-57867.

Driven by the increasing fossil fuel prices, global warming and climatic change, the world is currently witnessing an increasing development in renewable energy technologies, particularly those of wind energy. As such, engineers around the world are trying to optimize the design of wind turbines to maximize the captured energy while simultaneously minimizing the cost. This work aims to develop a mathematical tool to be used to compare different wind turbine designs and hence to reach the ultimate goal of an optimized wind turbine rotor designed specifically to operate in the Saharan regions of North Africa and the Middle East. As a case study, the main aerodynamic and structural parameters of the NREL 5MW virtual rotor have been optimized for the wind conditions prevalent at the Zaafarana site in Egypt. Specifically, the airfoil chord lengths and twist angles — smoothed using Bezier curves — as well as the layup sequence of the spar caps have been considered i n the optimization process which was carried out using a Genetic Algorithm (GA) developed i n MATLAB and coupled with NREL’s FAST Modularization Framework. The results showed that the NREL 5MW wind turbine design optimized for the site specific wind conditions of Zaafarana using airfoil families with low-sensitivity to dust accumulation, achieved a drop of 2.41% of the Levelized Cost of Energy of Energy (LCOE) over that of the baseline design. The developed turbine rotor design is tested for structural integrity commensurate with IEC standards.

Commentary by Dr. Valentin Fuster

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