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ASME Conference Presenter Attendance Policy and Archival Proceedings

2017;():V009T00A001. doi:10.1115/GT2017-NS9.
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This online compilation of papers from the ASME Turbo Expo 2017: Turbomachinery Technical Conference and Exposition (GT2017) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Oil and Gas Applications

2017;():V009T27A001. doi:10.1115/GT2017-63005.

The paper discusses the interaction between a centrifugal compressor and the process, and as a result, the control requirements for centrifugal compressor packages. The focus is on variable speed, upstream and midstream applications. The impact of the interaction between system characteristics and compressor characteristics, both under steady state and transient conditions is explained. Also considered are concepts to optimize and control the units. Special attention is given to the issue of surge avoidance.

Additionally, the impact of the process and how the process dynamics interact with the compressor is analyzed, categorized, and explained.

Commentary by Dr. Valentin Fuster
2017;():V009T27A002. doi:10.1115/GT2017-63025.

Gas turbine performance has been analyzed for a fleet of GE LM2500 engines at two Statoil offshore fields in the North Sea. Both generator drive engines and compressor driver engines have been analyzed, covering both the LM2500 base and plus configurations, as well as the SAC and DLE combustor configurations. Several of the compressor drive engines are running at peak load (T5.4 control), and the production rate is thus limited to the available power from these engines. The majority of the engines discussed run continuously without redundancy, implying that gas turbine uptime is critical for the field’s production and economy.

Previous studies and operational experience have emphasized that the two key factors to minimize compressor fouling are the optimum designs of the inlet air filtration system and the water wash system. An optimized inlet air filtration system, in combination with daily online water wash (at high water-to-air ratio), are the key factors to achieve successful operation at longer intervals between offline washes and higher average engine performance. Operational experience has documented that the main gas turbine recoverable deterioration is linked to the compressor section.

The main performance parameter when monitoring compressor fouling is the gas turbine compressor efficiency. Previous studies have indicated that inlet depression (air mass flow at compressor inlet) is a better parameter when monitoring compressor fouling, whereas instrumentation for inlet depression is very seldom implemented on offshore gas turbine applications. The main challenge when analyzing compressor efficiency (uncorrected) is the large variation in efficiency during the periods between offline washes, mainly due to operation at various engine loads and ambient conditions.

Understanding the gas turbine performance deterioration is of vital importance. Trending of the deviation from the engine baseline facilitates load-independent monitoring of the gas turbine’s condition. Instrument resolution and repeatability are key factors for attaining reliable results in the performance analysis. A correction methodology for compressor efficiency has been developed, which improves the long term trend data 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, as well as gas turbine performance and response.

Commentary by Dr. Valentin Fuster
2017;():V009T27A003. doi:10.1115/GT2017-63061.

Centrifugal compressor impellers and shafts are subject to severe fluctuating axial and radial forces when operating in surge. These forces can cause severe damage to the close clearance components of a centrifugal compressor such as the thrust and radial bearings, inter-stage and dry gas seals, and balance piston. Being able to accurately quantify the cyclic surge forces on the close clearance components of the compressor allows the user to determine whether an accidental surge event, or emergency shutdown (ESD) transient, has caused damage requiring inspection, repair, or part replacement. For the test, a 700 Hp (∼520 kW) industrial air centrifugal compressor was operated in surge at speeds ranging from 7,000 to 13,000 rpm and pressure ratios from 1.2 to 1.8. The axial surge forces were directly measured using axial load cells on the thrust bearings. Suction and discharge pressures, proximity probe axial shaft position, flows, and temperatures were also measured. Time domain and frequency plots of axial vibration and dynamic pulsations showed the impact of the operating conditions on surge force amplitudes and frequencies. A surge severity coefficient was also derived as a simple screening tool to evaluate the magnitude of potential damage to a compressor during surge.

Commentary by Dr. Valentin Fuster
2017;():V009T27A004. doi:10.1115/GT2017-63070.

The force acting on centrifugal compressors is an important parameter to be considered throughout the operating life of these turbomachines. When the compressor is operating in surge conditions these forces can become highly dangerous for the mechanical and aerodynamic structures. This instability is usually avoided in industrial applications but the anti-surge system may not react in time when emergency shutdowns or power failures take place. During these rapid transients, surge can develop, generating unsteady forces which can harm the close clearance components of the compressor. Therefore, the capability to predict the characteristics and the dynamics of these surge forces would allow the estimation of the off-design fatigue cycles produced on these components by surge. Currently, no validated method exists to predict the frequency and amplitude of the surge forces and determine the potential damage of these components.

In this paper a lumped parameter model, developed by using the bond graph approach to predict the dynamic surge fluid-dynamic oscillations, is presented. The model requires the geometry and the steady-state performance maps of the compressor as inputs, together with the piping system configuration characteristics.

The simulator, is provided with a supplementary tool to estimate the axial force frequency and amplitude, taking into consideration all the contributions to the axial fluid-dynamic thrust, the stiffness-damping of the thrust bearing and the mass of the rotor.

The model was tuned and validated using the test case axial force data from the Southwest Research Institute facility. The model has shown good agreement with the experimental results which implies that it can offer significant information about the severity of a surge event and the quantification of the machine performance losses together with possible damage to the close clearance components. This study is a first important step that can lead to schedule optimization for maintenance and repair activities.

Topics: Compressors , Surges
Commentary by Dr. Valentin Fuster
2017;():V009T27A005. doi:10.1115/GT2017-63106.

ASME PTC-10 (reaffirmed 2009) serves as an internationally recognized standard factory acceptance and field performance testing for centrifugal compressors. It provides a test procedure to determine the thermodynamic performance of centrifugal compressors for gases conforming to ideal gas laws and for real gases. ASME PTC-10 defines ideal gases as those, which fall within the limits of table 3.3. The ratio of heat capacities is one of the parameters used to determine the limits of departure from ideal gas in table 3.3. However, ASME PTC-10 does not clearly define whether to use the ideal gas or a real gas method to calculate the ratio of heat capacities.

The relationship Ĉp – Ĉv = R, is valid for ideal gases, but not real gases. The validity of Ĉp – Ĉv = R is examined across a typical range of pressures and temperatures and a composition applicable to the natural gas industry.

Isentropic processes of ideal gases are accurately described with a simple relationship with the ratio of heat capacities. However, for real gases, that relationship is not valid and a more complex relationship is required for similar accuracy.

Thermodynamic relationships used in calculating isentropic and polytropic exponents are summarized. Limitations for real and ideal gas calculation methods are described. The deviations of real gas isentropic and polytropic volume and temperature exponents from ideal gas calculation methods are presented.

Commentary by Dr. Valentin Fuster
2017;():V009T27A006. doi:10.1115/GT2017-63212.

As cost of damages to the compression systems in oil and gas industry can lead to significant capital cost loss and plant downtime, these valuable assets must be carefully protected to achieve a high level of production and operational reliability. In recent years, several research activities have been conducted to develop knowledge in analysis, design and optimization of compressor anti surge control system. Since, the anti-surge control testing on a full scale compressor are limited for possible consequences of failure and also the experimental facility can be expensive to set up control strategies and logics, design process often involves analyses using compression system dynamic simulation. Such Simulator enables the designer to test the new control logic and see the results before implementing it on governor system. This would increase the reliability and prevents undesirable costs resulting from practical trial and error process. Taking into account its own requirements and market demand, a high fidelity compression system dynamic simulation environment has developed by MAPNA Turbine (TUGA) to verify the anti-surge control system design and test the control logic across the all operating range of the compressor performance. Typical control scenarios that have to be considered are process control, starting and stopping, and emergency shutdowns. Having such simulator is also deemed to be essential to serve other applications during all stages of system life cycle, including but not limited to the educational tool for operators training, Site Acceptance Test (SAT) and Factory Acceptance Test (FAT) and compression plant design optimization.

This research focuses on developing and validating a physics-based, modular, non-linear and one-dimensional dynamic model of a compression system: centrifugal compressor and its surrounding process equipment like scrubber, cooler, a recycle line with a control valve and check valve. The mathematical approach of the model is based on laws of conservation and the included ordinary differential equations (ODEs) which describe the system dynamics, is solved by using advanced computational method in an in-house FORTRAN code. Compressor characteristics maps generated from company compressor test bench are used to determine compressor pressure ratio and efficiency. All equipment and inlet/outlet accessories as well as test instructions follow the requirements of PTC10. The simulation within a wide range of operating conditions allows a parametric study to be performed and the optimal values of the control parameters to be selected. In order to check the validity of the model, the simulation results are then compared with experimental data taken on the company industrial compressor test facility and also with operational field measurement.

Commentary by Dr. Valentin Fuster
2017;():V009T27A007. doi:10.1115/GT2017-63332.

This paper documents the set-up and validation of nonlinear autoregressive exogenous (NARX) models of a heavy-duty single-shaft gas turbine. The considered gas turbine is a General Electric PG 9351FA located in Italy.

The data used for model training are time series data sets of several different maneuvers taken experimentally during the start-up procedure and refer to cold, warm and hot start-up. The trained NARX models are used to predict other experimental data sets and comparisons are made among the outputs of the models and the corresponding measured data.

Therefore, this paper addresses the challenge of setting up robust and reliable NARX models, by means of a sound selection of training data sets and a sensitivity analysis on the number of neurons. Moreover, a new performance function for the training process is defined to weigh more the most rapid transients. The final aim of this paper is the set-up of a powerful, easy-to-build and very accurate simulation tool which can be used for both control logic tuning and gas turbine diagnostics, characterized by good generalization capability.

Topics: Gas turbines
Commentary by Dr. Valentin Fuster
2017;():V009T27A008. doi:10.1115/GT2017-63385.

In rotating equipment, thrust bearings aid to balance axial loads and control shaft position. In turbomachinery, axial loads depend on shaft speed and pressure rise/drop on the impellers. This paper details a water lubricated test rig for measurement of the performance of hydrostatic thrust bearings (HTBs). The rig contains two water lubricated HTBs (105 mm outer diameter), one is the test bearing and the other a slave bearing. Both bearings face the outer side of thrust collars of a rotor. The paper shows measurements of HTB axial clearance, flow rate, and recess pressure for operation with increasing static load (max. 1.4 bar) and supply pressure (max. 4.14 bar) at a rotor speed of 3 krpm (12 m/s OD speed). Severe angular misalignment, static and dynamic, of the bearing surface against its collar persisted and affected all measurements. The HTB axial clearance increases as the supply pressure increases and decreases quickly as the applied load increases. The reduction in clearance increases the flow resistance across the film lands thus reducing the through flow rate with an increase in recess pressure. In addition, an estimated bearing axial stiffness increases as the operating clearance decreases and as the supply pressure increases. Predictions from a bulk flow model qualitatively agree with the measurements. Alas they are not accurate enough. The differences likely stem from the inordinate tilts (static and dynamic) as well as the flow condition. The test HTB operates in a flow regime that spans from laminar to incipient turbulent. Quantification of misalignment at all operating conditions is presently a routine practice during operation of the test rig.

Commentary by Dr. Valentin Fuster
2017;():V009T27A009. doi:10.1115/GT2017-63409.

Statistical parametric methodologies are widely employed in the analysis of time series of gas turbine sensor readings. These methodologies identify outliers as a consequence of excessive deviation from a statistically-based model, derived from available observations. Among parametric techniques, the k-σ methodology demonstrates its effectiveness in the analysis of stationary time series. Furthermore, the simplicity and the clarity of this approach justify its direct application to industry. On the other hand, the k-σ methodology usually proves to be unable to adapt to dynamic time series, since it identifies observations in a transient as outliers.

As this limitation is caused by the nature of the methodology itself, two improved approaches are considered in this paper in addition to the standard k-σ methodology. The two proposed methodologies maintain the same rejection rule of the standard k-σ methodology, but differ in the portions of the time series from which statistical parameters (mean and standard deviation) are inferred. The first approach performs statistical inference by considering all observations prior to the current one, which are assumed reliable, plus a forward window containing a specified number of future observations. The second approach proposed in this paper is based on a moving window scheme.

Simulated data are used to tune the parameters of the proposed improved methodologies and to prove their effectiveness in adapting to dynamic time series. The moving window approach is found to be the best on simulated data in terms of True Positive Rate (TPR), False Negative Rate (FNR) and False Positive Rate (FPR). Therefore, the performance of the moving window approach is further assessed towards both different simulated scenarios and field data taken on a gas turbine.

Commentary by Dr. Valentin Fuster
2017;():V009T27A010. doi:10.1115/GT2017-63410.

The reliability of gas turbine health state monitoring and forecasting depends on the quality of sensor measurements directly taken from the unit. Outlier detection techniques have acquired a major importance, as they are capable of removing anomalous measurements and improve data quality. To this purpose, statistical parametric methodologies are widely employed thanks to the limited knowledge of the specific unit required to perform the analysis. The backward and forward moving window (BFMW) k-σ methodology proved its effectiveness in a previous study performed by the authors, to also manage dynamic time series, i.e. during a transient. However, the estimators used by the k-σ methodology are usually characterized by low statistical robustness and resistance.

This paper aims at evaluating the benefits of implementing robust statistical estimators for the BFMW framework. Three different approaches are considered in this paper. The first methodology, k-MAD, replaces mean and standard deviation of the k-σ methodology with median and mean absolute deviation (MAD), respectively. The second methodology, σ-MAD, is a novel hybrid scheme combining the k-σ and the k-MAD methodologies for the backward and the forward windows, respectively. Finally, the bi-weight methodology implements bi-weight mean and bi-weight standard deviation as location and dispersion estimators.

First, the parameters of these methodologies are tuned and the respective performance is compared by means of simulated data. Different scenarios are considered to evaluate statistical efficiency, robustness and resistance. Subsequently, the performance of these methodologies is further investigated by injecting outliers in field data sets taken on selected Siemens gas turbines.

Results prove that all the investigated methodologies are suitable for outlier identification. Advantages and drawbacks of each methodology allow the identification of different scenarios in which their application can be most effective.

Commentary by Dr. Valentin Fuster
2017;():V009T27A011. doi:10.1115/GT2017-63411.

Anomaly detection in sensor time series is a crucial aspect for raw data cleaning in gas turbine industry. In addition to efficiency, a successful methodology for industrial applications should be also characterized by ease of implementation and operation.

To this purpose, a comprehensive and straightforward approach for Detection, Classification and Integrated Diagnostics of Gas Turbine Sensors (named DCIDS) is proposed in this paper. The tool consists of two main algorithms, i.e. the Anomaly Detection Algorithm (ADA) and the Anomaly Classification Algorithm (ACA). The ADA identifies anomalies according to three different levels of filtering based on gross physics threshold application, inter-sensor statistical analysis (sensor voting) and single-sensor statistical analysis. Anomalies in the time series are identified by the ADA, together with their characteristics, which are analyzed by the ACA to perform their classification. Fault classes discriminate among anomalies according to their time correlation, magnitude and number of sensors in which an anomaly is contemporarily identified. Results of anomaly identification and classification can subsequently be used for sensor diagnostic purposes.

The performance of the tool is assessed in this paper by analyzing two temperature time series with redundant sensors taken on a Siemens gas turbine in operation. The results show that the DICDS is able to identify and classify different types of anomalies. In particular, in the first dataset, two severely incoherent sensors are identified and their anomalies are correctly classified. In the second dataset, the DCIDS tool proves to be capable of identifying and classifying clustered spikes of different magnitudes.

Topics: Sensors , Gas turbines
Commentary by Dr. Valentin Fuster
2017;():V009T27A012. doi:10.1115/GT2017-63456.

Turbodrill is a type of hydraulic axial turbomachine that rotates a bit by the action of the drilling fluid on turbine blades, which converts the hydraulic power provided by the high pressure from drilling fluid into mechanical power through turbine stages. The evaluation of hydraulic turbine performance characteristics are important to define feasible rotational speed and mass flow to attend the bit torque requirements during drilling through the post-salt and salt layers. As a result, optimum operational parameters are proposed for gaining the required rotational speed and torque for post-salt environments. The turbine motor presented in this study was established by design methods based on classical aeronautical turbomachinery blade profile to supply 30k Newton-meters (Nm) of torque requested by a polycrystalline diamond compact (PDC) bit to power the complex heterogeneous layer of rock. The performance evaluation of this innovative hydraulic turbine with 200 stages was carried out using computational fluid dynamics (CFD). The simulation considers two different drilling fluid types, sea water and brine. Besides, different flow rates were considered to investigate how velocity vectors, pressure profile, output power and other performance parameters are affected. Due the large amount of data, the first and second stages of the turbine have been used to predict the performance characteristics. This assumption gives interesting results and avoids too heavy computational costs. A commercial CFD solver (ANSYS CFX 15.0®) was used to calculate the governing equations based on Reynolds-Averaged Navier-Stokes (RANS equations) with the addition of turbulence model. The two-equation Shear-Stress Transport (SST) turbulence model was used to account the effects of flow eddy viscosity.

Commentary by Dr. Valentin Fuster
2017;():V009T27A013. doi:10.1115/GT2017-63563.

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 decrease in compressor performance.

In literature there are some studies related to the fouling phenomena in transonic compressors, but in industrial applications (heavy-duty compressors, pump stations, etc.) the subsonic compressors are widespread. It is highly important for the manufacturer to gather information about the fouling phenomenon related to this type of compressor.

This paper presents three-dimensional numerical simulations of the micro-particle ingestion (0.15 μm – 1.50 μm) in a multistage (i.e. eight stage) subsonic axial compressor, 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.

The adopted computational strategy allows the evaluation of particle deposition in a multistage axial compressor thanks to the use of a mixing plane approach to model the rotor/stator interaction. The compressor numerical model and the discrete phase model are set up and validated against the experimental and numerical data available in literature.

The number of particles and sizes are specified in order to perform a quantitative analysis of the particle impacts on the blade surface. The blade zones affected by particle impacts and the kinematic characteristics (velocity and angle) of the impact of micrometric and sub-micrometric particles with the blade surface are shown. Both blade zones affected 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 the fouling phenomenon.

The results show that micro-particles tend to follow the flow by impacting on the compressor blades at full span. The suction side of the blade is only affected by the impacts of the smallest particles. Particular fluid dynamic phenomena, such as corner separations and clearance vortices, strongly influence the impact location of the particles.

The impact and deposition trends decrease according to the stages. The front stages appear more affected by particle impact and deposition than the rear ones.

Commentary by Dr. Valentin Fuster
2017;():V009T27A014. doi:10.1115/GT2017-64062.

This paper presents the design of a turbine system that powers an alternator in a Positive Mud Pulse Telemetry (P-MPT) system, a Measurement While Drilling tool (MWD) used in the petroleum industry. With rig rates exceeding $1 million per day and wells that are drilled at depths of over 30,000 ft (9144 m), operators need to have an MWD tool that can provide continuous power while drilling without interruption. With our design of the turbine component for the P-MPT system, it allows the signals of directional drilling information to be continuously transmitted while concurrently drilling.

This paper presents the turbine system for a P-MPT system with the testing and analysis for operational conditions. The turbine system consists of a stationary vein positioned 0.25 inches (0.635 centimeters) apart from a rotating vein that is connected to the shaft-alternator system. The turbine system was manufactured with 3D printing. An experimental wind tunnel was built to simulate a downhole drilling environment by applying density scaling techniques to model flow in drilling fluid. The testing results of the turbine system are presented and discussed, including differential pressure, no-load rotation speed (RPM), stall torque, and power. CFD analyses were performed. The wind tunnel experimental data were validated by the CFD analyses. The results show that the turbine system design is functional for the P-MPT system.

Topics: Design , Turbines , Telemetry
Commentary by Dr. Valentin Fuster
2017;():V009T27A015. doi:10.1115/GT2017-64182.

This paper addresses the commissioning of an industrial 9-axes oil-free integrated motor-compressor that is installed on an off-shore oil rig. The magnetic bearing system and advanced software tools are shown to support the machine mechanical assembly, testing, and long term operation protection. The magnetic bearings are used as measurement devices to reduce commissioning time and, because of the high hard- and software integration level of machine and measurement, to improved commissioning quality and reliable long term operation. Measurement data from the commissioning of the machine are presented.

Commentary by Dr. Valentin Fuster
2017;():V009T27A016. doi:10.1115/GT2017-64245.

Gas compressor stations represent a huge potential for exhaust heat recovery. Typical installations consist of open cycle configurations with multiple gas turbine units, usually operated under part-load conditions during the year with limited conversion efficiency. At least, one of the installed unit serves as back-up to ensure the necessary reserve power and the safe operation of the station. Organic Rankine Cycle (ORC) has been proven as an economical and environmentally friendly solution to recover waste heat from gas turbines, improving the overall energy system performance and reducing the CO2 emissions.

In this context, taking as reference typical gas compressor stations located in North America, the paper investigates the potential benefit of ORC application, as bottomer section of gas turbines, in natural gas compression facilities. Thus, ORC converts gas turbines wasted heat into useful additional power that can be used inside the compression facility reducing the amount of consumed natural gas and, consequently, the environmental emissions, or directed to the grid, thus furthermore earning economic benefits.

Different case studies are examined with reference to two typical compressor station size ranges: a “small-medium” and a “medium-high” size range. Two different gas turbine models are considered according to most common manufacturers. Typical gas compressor stations and integrated cycle configurations are identified. Based on Turboden experience in development and production of ORCs, specific design options and constraints, layout arrangements and operating parameters are examined and compared in this study, such as the use of an intermediate heat transfer fluid, the type of organic fluid, the influence of superheating degree and condensation temperature values.

Emphasis is given on thermodynamic performance of the integrated system by evaluating thermal energy and mechanical power recovery. Several key performance indexes are defined such as, the ORC power and efficiency, the specific power recovery per unit of compression power, the integrated system net overall power output and efficiency, the ORC expander and heat exchangers size parameters, the carbon emission savings, etc. The performed comparison of various configurations shows that: (i) the energy recovery with ORC can be remarkable, adding up to more than 35% of additional shaft power to the compression station in the best configuration; (ii) the ORC condensation temperature value has a significant impact on the ORC bottomer cycle and on the integrated system performance; (iii) in case of Cyclopentane, keeping the same ORC cycle operating parameters, the max specific power recovery is achieved in the direct configuration case, (iv) the bottomer cycle size can be reduced with the use of a refrigerant fluid (R1233zd(E)), compared to hydrocarbon fluids; (v) the max environmental benefit can be up to 120 kg CO2/h saved per MW of installed compression power.

Commentary by Dr. Valentin Fuster
2017;():V009T27A017. doi:10.1115/GT2017-64327.

Air/oil separator plays an important role in the aero-engine lubricating oil system, and connects oil cavity and atmospheric environment, where it is used to separate oil from oil-gas two-phase flow and reduce the consumption of lubricating oil. Oil-gas separation efficiency and flow resistance are two key performance parameters of the separator. This paper focuses on how to improve the separation efficiency of one certain air/oil separator, which has many venting holes, under the condition of keeping the stability of flow resistance. Based on the mathematical model, a large number of numerical calculations were carried out using the ANSYS-Fluent. The characteristics of oil-gas separation efficiency, flow resistance were achieved firstly, although the venting holes number changes from 4 to 15, but the holes total flow area is constant, under the same operate condition. In addition, the separation efficiency for single diameter oil droplet was also calculated when the venting holes number changes. The results show that increase the venting holes can effectively reduce the minimum oil droplets diameter which could be separated, improve the separation efficiency and maintain steady flow resistance at the same time. The result of this study may provide an idea or method for the optimization and improvement of Air/oil separator with similar structure.

Commentary by Dr. Valentin Fuster
2017;():V009T27A018. doi:10.1115/GT2017-64374.

Wet gas compression of gas/condensate/water provides a business opportunity for oil and gas producers. There are several opportunities of particular note: 1) As well tail-end production commences, the installation of sub-sea compressors will provide enhanced oil recovery and, if the subsea compressor is capable of handling liquids, the subsea process complexity can be dramatically reduced, thus decreasing capital investments and possibly operational costs. 2) Topside and Onshore projects can also be dramatically simplified. This is the case for both new installations and modification projects for which wet gas compression is a suitable solution.

However, there are several challenges that need to be addressed before wet gas compression, by means of centrifugal compressors, can be considered as a robust commercial solution for future projects. This relates to the robustness of the mechanical design, effects on electrical systems, and issues related to performance. This paper will focus on challenges related to performance prediction and testing.

For conventional dry gas compressor design, performance prediction is usually undertaken by the compressor manufacturer, utilising in-house know-how in impeller design and selection. This specialised knowledge is potentially unsuitable for predicting wet gas performance in the design phase; hence, a wet gas compressor design may not meet design requirements specified by the customer.

It is typical that agreements on performance testing of centrifugal compressors state that these are to be conducted according to an international standard such as ASME PTC10 or ISO 5389. These standards require that the compressed gas is dry.

However, for wet gas compressors, no such internationally established standards exist for performance evaluation. Several of the requirements stipulated in the standards are challenging to apply to wet conditions and they do not ensure similar conditions. Such parameters including the maximum permissible deviation in the specific volume ratio, Mach number and Reynolds number. It is clear that the path towards a standard for wet gas performance testing will require a substantial amount of effort in order to establish new requirements related to wet gas similarity. Based on wet gas compressor test experience, challenges and requirements related to low pressure inert fluid, compared with full pressure actual fluid tests, are analysed and discussed.

Topics: Compression
Commentary by Dr. Valentin Fuster
2017;():V009T27A019. doi:10.1115/GT2017-64425.

Since the beginning of the 1950s, manufacturers and operators have struggled to understand, reduce and eliminate compressor fouling and its effects on gas turbine operation. Several devices (inertial separators, barriers, filters, etc.) and strategies (on-line and off-line washing, manual cleaning, etc.) have been adopted in order to limit and/or eliminate the foulants which stick to the compressor blade and vane surfaces.

The state of the power plant design and installation and environmental conditions determine the rate of fouling and, in turn, gas turbine performance losses. The types of contaminant (organic or inorganic), their concentration and their ability to stick are variable depending on the weather conditions. Desert, tropical, rural, and off-shore conditions are characterized by different foulants with different characteristics which determine compressor fouling.

In this paper, an analysis of the influence of third substances at the particle/surface interface is presented. The analysis is carried out on two different compressor rotors, transonic and subsonic. Firstly, a sensitivity analysis is proposed related to the particle diameter and foulant mixture in order to highlight the influence of air humidity due to environmental conditions or the pressure drop after the filtration stages. The effects of a water electrolytic solution (generated by the presence of inorganic matter) and a water surfactant solution (used in the case of washing) are also considered. In this case, the properties of the mixture substance (solid particles bound by a liquid film) are considered. Secondly, using previous numerical analyses (particle-laden flow with a Eulerian-Lagrangian approach) as a starting point, the variation in particle sticking ability is evaluated against the presence of third substances (water solutions and oily substances) and the particle kinematic characteristics using a sticking model based on an energy balance equation.

The results show the influence of the third substance on particle sticking capability using a susceptibility-to-fouling criterion. Particularly in the presence of humid conditions, sticking capability increases with respect to dry conditions, even though the major effects are due to the mixture viscosity and not only to the presence of liquid water. The sticking capability of the mixture varies according to particle diameter as a function of the particle normal velocity. The results are presented in order to easily quantify the effects of the presence of a third substance at the particle/surface interface according to the type of liquid phase involved in the sticking process.

Commentary by Dr. Valentin Fuster
2017;():V009T27A020. doi:10.1115/GT2017-64541.

Wet Gas Compression (WGC) continues to be an important topic as oil and gas production is driven further out into the ocean and moves critical equipment to the ocean floor. In the last year, significant milestones have been reached for WGC by the installation of the first wet gas compressor off the coast of Norway. Even with this achievement, there is a lack of understanding of the physics behind WGC and there are deficiencies in the ability to predict the compressor performance. Understanding the two phase flow structure inside the compressor is important for validating WGC simulations and being able to predict compressor performance. This paper reviews the results from a test program focused on characterizing the flow inside the compressor by using flow visualization. An open impeller centrifugal compressor was outfitted with windows to view the flow inside the compressor at the inlet, inside the impeller and in the diffuser section. Testing was conducted with an ambient suction pressure at various compressor speeds, flow rates, and liquid volume fractions. Images and videos were captured at the different conditions in order to observe the two phase flow structure. The general patterns and trends that characterize wet gas flow are discussed in this paper.

Commentary by Dr. Valentin Fuster
2017;():V009T27A021. doi:10.1115/GT2017-64689.

Hydraulic fracturing treatments are used to produce oil and gas reserves that would otherwise not be accessible using traditional production techniques. Fracturing treatments require a significant amount of water, which has an associated environmental impact. In recent work funded by the Department of Energy (DOE), an alternative fracturing process has been investigated that uses natural gas as the primary fracturing fluid. In the investigated method, a high-pressure foam of natural gas and water is used for fracturing, a method than could reduce water usage by as much as 80% (by volume). A significant portion of the work focused on identifying and optimizing a mobile processing facility that can be used to pressurize natural gas sourced from adjacent wells or nearby gas processing plants.

This paper discusses some of the evaluated processes capable of producing a high-pressure (10,000 psia) flow of natural gas from a low-pressure source (500 psia). The processes include five refrigeration cycles producing liquefied natural gas as well as a cycle that directly compresses the gas. The identified processes are compared based on their specific energy as calculated from a thermodynamic analysis. Additionally, the processes are compared based on the estimated equipment footprint and the process safety. Details of the thermodynamic analyses used to compare the cycles are provided. This paper also discusses the current state of the art of foam fracturing methods and reviews the advantages of these techniques.

Commentary by Dr. Valentin Fuster
2017;():V009T27A022. doi:10.1115/GT2017-64698.

Controlling risks associated with fires and explosions from leaks of flammable fluids at oil and gas facilities is paramount to ensuring safe operations. The gas turbine is a significant potential source of ignition; however, the residual risk is still not adequately understood.

A model has been successfully developed and implemented in the commercial Computational Fluid Dynamics (CFD) code ANSYS CFX. This model is based on a combination of standard models, User Defined Functions (UDFs) and the CFX Expression Language (CEL). Prediction of ignition is based on a set of criteria to be fulfilled while complex kinetics is handled computationally easy by means of a reaction progress variable.

The simulation results show a good agreement with the trends experimentally observed in other studies. It is found that the hot surface ignition temperature (HSIT) increases with increase in velocity and turbulence but decreases with increase in initial mixture temperature and pressure.

The model shows a great potential in reliable prediction of the risk of hot surface ignition within gas turbines in the oil and gas industry. In the future, a dedicated experimental study will be performed not only to improve the understanding of the risk of hot surface ignition but also to collect experimental data under well-defined conditions to further validate or refine the model.

Commentary by Dr. Valentin Fuster
2017;():V009T27A023. doi:10.1115/GT2017-64783.

Adopting the innovative technology found in a compressor able to compress a mixture of natural gas and condensate has great potential for meeting future challenges in subsea oil and gas production. Benefits include reduced size, complexity and cost, enhanced well output, longer producing life and increased profits, which in turn offer opportunities for exploiting smaller oil and gas discoveries or extending the commercial life of existing fields.

Introducing liquid into a centrifugal compressor creates several thermodynamic and fluid-mechanical challenges. The paper reviews some of the drive mechanisms involved in wet gas compression and views them in the context of the test results presented. An inlet guide vane (IGV) assembly has been installed in a test facility for wet gas compressors and the effect of wet gas on IGV performance documented. The impact of changes in IGV performance on impeller and diffuser has also been documented. The results have been discussed and correction methods compared.

Commentary by Dr. Valentin Fuster
2017;():V009T27A024. doi:10.1115/GT2017-64785.

Improving offshore gas production requires the process compressor to be moved closer to the wellhead. This will yield such benefits as enhanced well output, longer well life and the possibility of exploiting smaller fields. However, the harsh environment, remote location and variable two-phase characteristics of an untreated gas stream pose increased challenges for operational performance and robustness. Several methods are available to ensure that a process compressor maintains constant outlet pressure regardless of inlet stream properties and flow.

Two pressure-ratio control methods — variable inlet guide vanes (IGV) and variable speed — have been investigated. Their effect on diffuser stability has been tested and analysed in dry and wet conditions. Increased diffuser stability in wet conditions with IGV has been discussed and results are presented.

Commentary by Dr. Valentin Fuster
2017;():V009T27A025. doi:10.1115/GT2017-64894.

Nowadays, the operative range limit of compressors is still a key aspect of the research into turbomachinery. In particular, the study of the mass flow rate lower limit represents a significant factor in order to predict and avoid the inception of critical working conditions and instabilities such as stall and surge. The importance of predicting and preventing these dangerous phenomena is vital since they lead to a loss of performance and severe damage to the compression system and the compressor components. The identification of the typical precursors of these two types of compressor unstable behaviors can imply many advantages, in both stationary and aeronautic applications, such as i) avoiding the loss of production (in industry) and efficiency of systems and ii) reducing the cost of maintenance and repairing. Many approaches can be adopted to achieve this target, but one of the most fascinating is the vibro-acoustic analysis of the compressor response during operation. At the Engineering Department of the University of Ferrara, a test bench, dedicated to the study of the performance of an aeronautic turboshaft engine multistage compressor, has been equipped with a high frequency data acquisition system. A set of triaxle accelerometers and microphones, suitable for capturing broad-band vibration and acoustic phenomena, were installed in strategic positions along the compressor and the test rig. Tests were carried out at different rotational speeds, and with two different piping system layouts, by varying the discharge volume and the position of the electric control valve. Moreover, two different methodologies were adopted to lead the compressor towards instability. This experimental campaign allowed the inception of compressor stall and surge phenomena and the acquisition of a great amount of vibro-acoustic data which were firstly processed through an innovative data analysis technique, and then correlated to the thermodynamic data recorded. Subsequently, the precursor signals of stall and surge were detected and identified demonstrating the reliability of the methodology used for the study of compressor instability. The results of this paper can provide a significant contribution to the knowledge of the inception mechanisms of these instabilities. In particular, the experimental data can offer a valid support to the improvement of surge and stall avoidance (or control) techniques since it presents an alternative way of analyzing and detecting unstable compressor behavior characteristics by means of non-intrusive measurements.

Commentary by Dr. Valentin Fuster
2017;():V009T27A026. doi:10.1115/GT2017-64906.

RT61 is a three-stage industrial power turbine which couples with industrial RB211-24GT gas generator. This power turbine was designed and developed by the turbomachinery group in Mount Vernon, Ohio. It was designed based on the 3-D vortex theory during the early 2000 for increased power output and efficiency. It was also designed for low weight with modular construction for ease in maintainability. The industrial RB211-GT61 product serves both oil & gas and power generation market.

Recent drop in crude oil prices has posed significant challenges to the oil & gas customers. To increase the profitability, the entire oil & gas industry (upstream, midstream, and downstream) is looking for opportunities to decrease the operating cost. This served as the main motivation for the life extension of the RT61 power turbine. In order to reduce customer life cycle cost, Siemens Energy, Inc., has extended the life of its most efficient power turbine from 50,000 hours to 100,000 hours.

This paper discusses the efforts taken in extending the meantime between overhaul lives of various RT61 industrial power turbine components from 50,000 hours to 100,000 hours. Measures taken to increase the reliability and minimize the product life cycle cost are presented. New coatings were incorporated for the stage 1 vane and blade for oxidation protection. The thermal characteristics of the power turbine were validated using a comprehensive thermocouple survey of the casings.

Commentary by Dr. Valentin Fuster
2017;():V009T27A027. doi:10.1115/GT2017-65094.

The continuous demand for oil and gas forces the petroleum industry to develop new and cost-efficient technologies in order to increase recovery and exploit existing fields. Subsea wet gas compression of the unprocessed well stream is a powerful tool in increasing production capacity and utilizing remote regions.

Wet gas compressors are particularly useful for handling gas-dominated multiphase flows. Although a limited amount of research has been done on the field, previous studies have revealed that the liquid phase has considerable impact on the compressor performance, from both a fluid mechanical and a thermodynamic perspective.

Being able to ensure stable and predictable compressor behavior in subsea installations is challenging. As the reservoir pressure drops during production, the compressor enters a region where it is far more susceptible to inlet slugging. Inlet slugging may lead to internal compressor damage, including damage to seals, bearings and compressor blades. It needs to be stressed that a slug in this context does not entail 100% liquid holdup upfront of the compressor, but a substantial increase of liquid content in a gas dominated multiphase flow.

An experimental investigation has been carried out on an advanced wet gas test rig, consisting of a shrouded centrifugal impeller, a vaneless diffuser and a circular volute. The paper explores the possibilities of creating realistic inlet slugging scenarios and documents its impact on the compressor system. Slugging was introduced either by collection of liquid in a negatively sloped inlet pipe (i.e. terrain slug simulation) or by sudden opening of an inlet liquid valve upstream of a symmetrical injection manifold, so to create a more “homogenous” slug. The compressor ability to handle such slugs is analyzed. Furthermore, the drive capability to respond to sudden changes in torque requirement is documented. The experimental results form a basis for future dynamic simulation.

Commentary by Dr. Valentin Fuster
2017;():V009T27A028. doi:10.1115/GT2017-65235.

This paper compares various methods employed to determine centrifugal compressor performance. Results show that classic Schultz style end point analyses prescribed by ASME PTC 10 [1], [2] can be augmented by numerical stepwise techniques that provide consistently accurate results without limiting assumptions. A small stage method very closely approaching the classical definition of polytropic analysis is implemented in which the accuracy of the results only depends upon the quality of the applied fluid equation of state. It is no longer necessary to distinguish between ideal and real gas methods since real gas methods are easy to implement using today’s computational tools.

Topics: Compressors
Commentary by Dr. Valentin Fuster

Supercritical CO2 Power Cycles

2017;():V009T38A001. doi:10.1115/GT2017-63056.

The Naval Nuclear Laboratory has been operating the Integrated System Test (IST) with the objective of demonstrating the ability to operate and control a supercritical carbon dioxide (sCO2) Brayton power cycle over a wide range of conditions. The IST is a two shaft recuperated closed sCO2 Brayton cycle with a variable speed turbine-driven compressor and a constant speed turbine-driven generator designed to output 100 kWe. This paper presents a thermal-hydraulic lead control strategy for operation of the cycle over a range of operating conditions along with predicted and actual IST system response to power level changes using this control strategy.

Commentary by Dr. Valentin Fuster
2017;():V009T38A002. doi:10.1115/GT2017-63058.

Supercritical carbon dioxide (sCO2) power cycle designs are typically highly recuperated, transferring heat from the high temperature turbine exhaust stream to the compressor discharge stream thereby increasing overall cycle efficiency. Compact heat exchangers are preferred for this application due to their high surface area-to-volume ratio enabling much smaller heat exchangers as compared to conventional designs. However, compact heat exchangers have a higher metal density than conventional heat exchangers which could result in thermal lag during rapid temperature transients.

The Naval Nuclear Laboratory has been operating the Integrated System Test (IST) with the objective of demonstrating the ability to operate and control an sCO2 Brayton power cycle over a wide range of conditions. Rapid turbomachinery startups and power transients result in thermal transients on the recuperator. This paper presents thermal transients observed in the IST recuperator during loop startup and power transients and illustrates the time to achieve thermal equilibrium following the transients.

Commentary by Dr. Valentin Fuster
2017;():V009T38A003. doi:10.1115/GT2017-63090.

A multi-stage centrifugal compressor model is presented with emphasis on analyzing use of an exit flow coefficient vs. an inlet flow coefficient performance parameter to predict off-design conditions in the critical region of a supercritical carbon dioxide (CO2) power cycle. A description of the performance parameters is given along with their implementation in a design model (number of stages, basic sizing, etc.) and a dynamic model (for use in transient studies). A design case is shown for two compressors, a bypass compressor and a main compressor, as defined in a process simulation of a 10 megawatt (MW) supercritical CO2 recompression Brayton cycle. Simulation results are presented for a simple open cycle and closed cycle process with changes to the inlet temperature of the main compressor which operates near the CO2 critical point. Results showed some difference in results using the exit vs. inlet flow coefficient correction, however, it was not significant for the range of conditions examined. This paper also serves as a reference for future works, including a full process simulation of the 10 MW recompression Brayton cycle.

Commentary by Dr. Valentin Fuster
2017;():V009T38A004. doi:10.1115/GT2017-63148.

Recently there has been increased interest in the use of carbon dioxide (CO2) in closed loop power cycles. As these power cycles capitalize on the non-ideal gas behavior of CO2, their analysis both at the system level and at the detailed component level requires an advanced equation of state. Commonly used analytical equations of state as BWRS (BenedictWebbRubin equation of State) or Peng-Robinson are known to have high errors near the critical point and are thus unsuitable for the analysis of cycles or components where the flow conditions approach the critical point. An accurate equation of state is required at all phases of the development process from high level cycle calculations to the detailed component CFD. The NIST RefProp software package provides accurate CO2 fluid properties across the thermodynamic space but suffers from high computational over-head.

This study is presented in two parts. Part I (this part) of this paper describes an approach to creating a tabular representation of the equation of state that is applicable to any fluid. This approach is applied to generating an accurate, fast and robust tabular representation of the RefProp CO2 properties and an error analysis is performed to meet the accuracy requirements. The paper also discusses two approaches used to define speed of sound in the two-phase region and their sensitivity analysis on the 3D compressor flow. Part II of the paper details the numerical simulations of a supercritical CO2 centrifugal compressor using the tabular approach.

This paper shows that table resolution can be tailored to match the accuracy requirements while minimizing the time used to evaluate the tabulated thermo-physical functions. Error analysis are shown to demonstrate the level of accuracy possible with this approach.

Commentary by Dr. Valentin Fuster
2017;():V009T38A005. doi:10.1115/GT2017-63149.

This paper is presented in two parts. Part I (Tabular fluid properties for real gas analysis) describes an approach to creating a tabular representation of the equation of state that is applicable to any fluid. This approach is applied to generating an accurate and robust tabular representation of the RefProp CO2 properties. Part II (this paper) presents numerical simulations of a low flow coefficient supercritical CO2 centrifugal compressor developed for a closed loop power cycle. The real gas tables presented in part I are used in these simulations. Three operating conditions are simulated near the CO2 critical point: normal day (85 bar, 35C), hot day (105 bar, 50 C) and cold day (70 bar, 20C) conditions. The compressor is a single stage overhung design with shrouded impeller, 155 mm impeller tip diameter and a vaneless diffuser. An axial variable inlet guide vane (IGV) is used to control the incoming swirl into the impeller. An in-house three-dimensional computational fluid dynamics (CFD) solver named TACOMA is used with real gas tables for the steady flow simulations. The equilibrium thermodynamic modeling is used in this study. The real gas effects are important in the desired impeller operating range. It is observed that both the operating range (minimum and maximum volumetric flow rate) and the pressure ratio across the impeller are dependent on the inlet conditions. The compressor has nearly 25% higher operating range on a hot day as compared to the normal day conditions. A condensation region is observed near the impeller leading edge which grows as the compressor operating point moves towards choke. The impeller chokes near the mid-chord due to lower speed of sound in the liquid-vapor region resulting in a sharp drop near the choke side of the speedline. This behavior is explained by analyzing the 3D flow field within the impeller and thermodynamic quantities along the streamline. The 3D flow analysis for the flow near the critical point provides useful insight for the designers to modify the current compressor design for higher efficiency.

Commentary by Dr. Valentin Fuster
2017;():V009T38A006. doi:10.1115/GT2017-63187.

The capability to utilize dry air cooling by which heat is directly rejected to the air atmosphere heat sink is one of the benefits of the supercritical carbon dioxide (sCO2) energy conversion cycle. For the selection and analysis of the heat exchanger options for dry air cooling applications for the sCO2 cycle, two leading forced air flow design approaches have been identified and analyzed for this application; an air cooler consisting of modular finned tube air coolers; and an air cooler consisting of modular compact diffusion-bonded heat exchangers. The commercially available modular finned tube air cooler is found to be more cost effective and is selected as the reference for dry air cooling.

Commentary by Dr. Valentin Fuster
2017;():V009T38A007. doi:10.1115/GT2017-63279.

A transient model of a commercially-available 7.3MWe sCO2 power cycle was developed using an implicit 1-D Navier Stokes solver. The power cycle underwent extensive factory testing, which validated the component and system performance, and the design and performance of the control system. The model structure simulates the as-tested configuration of the power cycle, with major components consisting of water-cooled heat rejection heat exchanger, turbine-driven compressor, recuperator, primary heat exchanger, power turbine, gearbox and generator. Subsystem models are developed to validate individual component (compressor, drive turbine, power turbine, heat exchangers etc.) model performance against design data. The component models are then assembled into the full system model. For the system transient simulation, the input parameters (or boundary conditions) are taken from the test data. More than eight hours of test data is used in the present transient simulation. The simulation results, including thermodynamic state points, component performance and system performance, are compared against the corresponding test data.

Commentary by Dr. Valentin Fuster
2017;():V009T38A008. doi:10.1115/GT2017-63311.

The direct-fired supercritical CO2 (sCO2) cycle is currently considered as a zero-emission power generation concept. It is of interest to know how to optimize various components of this cycle using computational tools, however, a comprehensive effort on this area is currently lacking. In this work, the behavior of thermal properties of sCO2 combustion at various reaction stages has been investigated by coupling real gas CHEMKIN (CHEMKIN-RG) with an in-house Premixed Conditional Moment Closure (PCMC) code and the high pressure Aramco-2.0 kinetic mechanism. Also, the necessary fundamental information for sCO2 combustion modelling is reviewed.

The Soave-Redlich-Kwong equation of state (SRK EOS) is identified as the most accurate EOS to predict the thermal states at all turbulence levels. Also, an empirical model for the compression factor Z is proposed for sCO2 combustors, which is a function of mixture inlet conditions and the reaction progress variable. This empirical model is validated between the operating conditions 250–300 bar, inlet temperatures of 800–1200 K and within the current designed inlet mole fractions and the accuracy is estimated to be less than 0.5% different from the exact relation. For sCO2 operating conditions the compression factor Z always decreases as the reaction progresses and this leads to the static pressure loss between inlet and exit of the sCO2 combustor.

Further, a review of high pressure viscosity and thermal conductivity models of mixtures and pure-components are presented from the literature and suggestions are made for their adoptability in sCO2 combustor simulations. The thermal properties such as specific heats, speed of sound, pressure exponent and isothermal compressibility are accurately quantified.

Commentary by Dr. Valentin Fuster
2017;():V009T38A009. doi:10.1115/GT2017-63322.

For the concentrating solar power (CSP) applications, the supercritical carbon dioxide (s-CO2) power cycle is beneficial in many aspects, including higher cycle efficiencies, reduced component sizing, and potential for the dry cooling option, in comparison to the conventional steam Rankine cycle. Increasing number of investigations and research projects are involved in improving this technology to realize the s-CO2 cycle as a candidate to replace the conventional power conversion systems.

In this conceptual study, an isothermal compressor, a turbomachine which undergoes the compression process at constant temperature to minimize compression work, is applied to the s-CO2 power cycle layout. To investigate the cycle performance changes of adopting the novel technology, a framework for defining the efficiency of the isothermal compressor is revised and suggested. This study demonstrates how the compression work for the isothermal compressor is reduced compared to that of the conventional compressor under varying compressor inlet conditions.

Furthermore, the recompression Brayton cycle layout using s-CO2 as a working fluid is evaluated for the CSP applications. Results show that for compressor inlet temperatures (CIT) near the critical point, the simple recuperated Brayton cycle with an isothermal compressor performs better than the given reference recompression cycle by 6–10% points in terms of cycle thermal efficiency. For higher CIT values, the recompression cycle using an isothermal compressor can perform above 50% in thermal efficiency. Adopting an isothermal compressor in the s-CO2 layout, however, can imply larger heat exchange area for the compressor which requires further detailed design for realization in the future.

Commentary by Dr. Valentin Fuster
2017;():V009T38A010. doi:10.1115/GT2017-63549.

As a part of Sodium-cooled Fast Reactor (SFR) development, the supercritical CO2 (S-CO2) Brayton cycle is considered as an alternative power conversion system to eliminate sodium-water reaction (SWR) from the current conventional steam Rankine cycle is utilized with SFR. The leakage flow of S-CO2 from turbo-machinery via seal becomes one of important issues since not only it influences the cycle efficiency due to parasitic loss but also it is important for evaluating the system safety under various operating conditions. Thus, a transient simulation for estimating the critical flow in a turbo-machinery seal is essential to predict the leakage flow rate and calculate the required total mass of working fluid in a S-CO2 power system.

This paper briefly reviews the advantages of supercritical CO2 Brayton cycle relative to a typical steam Rankine cycle used in the Sodium-cooled Fast Reactor. In addition, this paper describes the test data using a CO2 critical flow experimental facility with three orifice configurations to model the flow resistance of a rotating shaft labyrinth seal. This data is used for validation of an existing transient analytical tool developed for transient hydraulic system analysis. Contained within this code is the analysis approach of Henry/Fauske from 1971 for two phase critical flow of one-component mixtures. This paper presents prediction of transient pressure, temperature, and flow profiles for critical and sub-critical flows of CO2 relative to blowdown test data to show that reasonable results are obtained. Similar analyses relative to test results of three orifice configurations are conducted and it shows that multiple orifices increase the time to equalize pressure in the blowdown system and therefore equates to higher flow resistance and lower leakage.

Commentary by Dr. Valentin Fuster
2017;():V009T38A011. doi:10.1115/GT2017-63570.

Rankine and Brayton cycles are common energy conversion cycles and constitute the basis of a significant proportion of global electricity production. Even a seemingly marginal improvement in the efficiency of these cycles can considerably decrease the annual use of primary energy sources and bring a significant gain in power plant output. Recently, supercritical Brayton cycles using CO2 as the working fluid have attracted much attention, chiefly due to their high efficiency. As with conventional cycles, improving the compressor performance in supercritical cycles is major route to increasing the efficiency of the whole process. This paper numerically investigates the flow field and performance of a supercritical CO2 centrifugal compressor. A thermodynamic look-up table is coupled with the flow solver and the look-up table is systematically refined to take into account the large variation of thermodynamic properties in the vicinity of the critical point. Effects of different boundary and operating conditions are also discussed. It is shown that the compressor performance is highly sensitive to the look-up table resolution as well as the operating and boundary conditions near the critical point. Additionally, a method to overcome the difficulties of simulation close to the critical point is explained.

Commentary by Dr. Valentin Fuster
2017;():V009T38A012. doi:10.1115/GT2017-63639.

Supercritical carbon dioxide (sCO2) Brayton cycles hold great promise as they can achieve high efficiencies — in excess of 50% — even at relatively moderate temperatures of 700–800 K. However, this high performance is contingent upon high-effectiveness recuperating and heat rejection heat exchangers within the cycle. A great deal of work has gone into development of these heat exchangers as they must operate not only at elevated temperatures and very high pressures (20–30 MPa), but they must also be compact, low-cost, and long-life components in order to fully leverage the benefits of the sCO2 power cycle.

This paper discusses the mechanical design and qualification for a novel plate-fin compact heat exchanger designed for sCO2 cycle recuperators and waste heat rejection heat exchangers, as well as direct sCO2 solar absorber applications. The architecture may furthermore be extended to other very high pressure heat exchanger applications such as pipeline natural gas and transcritical cooling cycles. The basic heat exchanger construction is described, with attention given to those details which have a direct impact on the durability of the unit. Modeling and analysis of various mechanical failure modes — including burst strength, creep, and fatigue — are discussed. The design and construction of test sections, test rigs, and testing procedures are described, along with the test results that demonstrate that the tested design has an operating life well in excess of the 100,000 cycles/90,000 hour targets. Finally, the application of these findings to a set of design tools for future units is demonstrated.

Commentary by Dr. Valentin Fuster
2017;():V009T38A013. doi:10.1115/GT2017-63696.

For the application of waste heat recovery (WHR), supercritical CO2 (S-CO2) Brayton power cycles offer significant suitable advantages such as compactness, low capital cost and applicable to a broad range of heat source temperatures. The current study is focused on thermodynamic modelling and optimization of Recuperated (RC) and Recuperated Recompression (RRC) S-CO2 Brayton cycles for exhaust heat recovery from a next generation heavy duty simple cycle gas turbine using a genetic algorithm. The Genetic Algorithm (GA) is mainly based on bio-inspired operators such as crossover, mutation and selection. This non-gradient based algorithm yields a simultaneous optimization of key S-CO2 Brayton cycle decision variables such as turbine inlet temperature, pinch point temperature difference, compressor pressure ratio. It also outputs optimized mass flow rate of CO2 for the fixed mass flow rate and temperature of the exhaust gas. The main goal of the optimization is to maximize power out of the exhaust stream which makes it single objective optimization. The optimization is based on thermodynamic analysis with suitable practical assumptions which can be varied according to the need of user. Further the optimal cycle design points are presented for both RC and RRC configurations and comparison of net power output is established for waste heat recovery.

Commentary by Dr. Valentin Fuster
2017;():V009T38A014. doi:10.1115/GT2017-63707.

This paper presents the selection of a system configuration and off-design control method for an integrally geared compander based on cycle modeling and optimization. The goal of the cycle modeling was to determine a cycle configuration that would reach an efficiency of 50% at design conditions and to optimize off-design control to maintain high efficiencies. The compander is being developed for use in a concentrated solar power supercritical carbon dioxide power plant with expected turbine inlet temperatures of 705°C and utilizing dry cooling leading to compressor inlet temperature varying between 35°C and 55°C. The compander is unique as it consists of eight turbomachinery stages on four pinions all being driven by a single bull gear. The separate stages offer the opportunity to consider a variety of flow splits and cycle configurations including intercooling and multiple stages of reheat.

Cycle modeling was conducted in two stages: on-design and off-design. On-design modeling was simulated with all components operating at their design point. This was used to compare the performance of different cycle configurations and design temperatures. Off-design modeling was then performed to investigate the temperature dependence of the cycle efficiency and power output and to develop a control strategy. Strategies considered and discussed include: turbine bypass, compressor recycle, inlet guide vanes, and inventory control. To determine the best operating conditions for each configuration and control strategy, a genetic algorithm was implemented to optimize the cycle performance across the range of operating temperatures being considered. The final selection of cycle configuration, design temperature and control strategy is also presented.

Commentary by Dr. Valentin Fuster
2017;():V009T38A015. doi:10.1115/GT2017-64042.

Commercially available and cost effective finned tube air coolers are an enabling technology that makes practical dry air cooling for the supercritical carbon dioxide (sCO2) Brayton cycle by which heat is directly rejected from CO2 to the air atmospheric heat sink. With dry air cooling, sCO2 Brayton cycle conditions need to be re-optimized to increase the main compressor inlet temperature and pressure (e.g., 35 °C and 8.2 MPa) relative to water cooling to limit the air cooler size to a practical value, and to increase the compressor outlet pressure (e.g., 25 MPa) to maintain a high efficiency. With reoptimization, the plant efficiency for the AFR-100 Sodium-Cooled Fast Reactor Nuclear Power Plant (NPP) is similar to that with once-through water cooling, while the NPP capital cost per unit output electrical power ($/kWe) is roughly estimated to be only 2 % greater. For the AFR-100 application, no unique benefit is identified for the sCO2 Brayton cycle relative to the superheated steam cycle with respect to the capability to use dry air cooling.

Commentary by Dr. Valentin Fuster
2017;():V009T38A016. doi:10.1115/GT2017-64044.

With the increasing interest in solar and geothermal power plants as well as waste heat recovery systems from many technologies, the whole world is more focused on gas power cycles. Especially, the supercritical carbon dioxide (S-CO2) cycles are very interesting for these applications. This is due to many advantages of the S-CO2 cycles over the other cycles such as a steam-water cycle or helium cycle. On the other hand, S-CO2 cycles have also disadvantages. One of the disadvantages is presence of impurities in the cycles. The big question is the effect of these impurities in the CO2, which can occur as impurities or can be suitably added to the pure CO2. From the previous research, it is obvious that binary mixtures affect the cycle as they influence cycle component design and thus the overall efficiency of the power cycle. The biggest effect of mixtures is on the heat exchangers and compressor, which operate close to the critical point. The positive effect of the binary mixtures is observed in the recuperative heat exchanger. On the other hand, negative effects occurs in the cooler. Therefore, the Czech Technical University in Prague (CTU) conducted research on supercritical carbon dioxide cycles, which is focused on the effect of the gaseous admixtures in S-CO2 on the different cycle components. The main goal of this paper is to describe the effect of gaseous admixtures on the efficiency of the cycles and their effect on each component. The first part of the study is focused on the calculation of the basic cycles for binary mixtures and description of the effect on the compressor and the cycle efficiency. The second part of the study is focused on the calculation of the basic cycles for multicomponent mixtures. In this part, the effect of the mixtures for different compositions and amounts of the individual mixture components will be presented. The calculations are performed for pure CO2 and then for selected multicomponent mixtures. A basic multicomponent mixture includes mixtures from technology of carbon capture and storage. Other multicomponent mixtures are combinations of previously investigated gaseous admixtures such as He, CO, O2, N2, H2, CH4 and H2S. The last part of the study is focused on the optimization of individual basic cycles for different amount of admixtures in CO2. The result of this study defines the optimum composition of multicomponent mixtures and describes their effect on the cycle efficiency for the particular utilization of S-CO2 cycle.

Commentary by Dr. Valentin Fuster
2017;():V009T38A017. doi:10.1115/GT2017-64261.

Direct supercritical CO2 (sCO2) power cycles have received considerable attention in recent years as an efficient and potentially cost-effective method of capturing CO2 from fossil-fueled power plants. These cycles combust natural gas or syngas with oxygen in a high pressure (200–300 bar), heavily-diluted sCO2 environment, such that the fluid entering the turbine is 90–95% CO2, with the balance composed primarily of H2O, CO, O2, N2 and Ar. After recuperation of the turbine exhaust thermal energy, water is condensed from the cycle, and the remainder is recompressed for either return to the combustor or for enhanced oil recovery (EOR) or storage. The compression power requirements vary significantly, depending on the proximity of the operating conditions to the CO2 critical point (31 °C, 73.7 bar), as well as to the level of working fluid dilution by minor components. As this has a large impact on cycle and plant thermal efficiency, it is crucial to correctly capture the appropriate thermo-physical properties of these sCO2 mixtures when carrying out performance simulations of direct sCO2 power plants. These properties are also important to determining how water is removed from the cycle, and for accurate modeling of the heat exchange within the recuperator.

This paper presents a quantitative evaluation of ten different property methods that can be used for modeling direct sCO2 cycles in Aspen Plus®. REFPROP is used as the de facto standard for analyzing indirect sCO2 systems, where the closed nature of the cycle leads to a high purity CO2 working fluid. The addition of impurities due to the open nature of the direct-sCO2 cycle, however, introduces uncertainty to the REFPROP predictions. There is a limited set of mixtures available for which REFPROP can be reliably used and there are a number of species present in a coal-fired direct-fired sCO2 cycle that REFPROP cannot accommodate. Even with a relatively simplified system in which the trace components are eliminated, simulations made using REFPROP require computation times that often preclude its use in parametric studies of these cycles. Consequently, a series of comparative analyses were performed to identify the best physical property method for use in Aspen Plus® for direct-fired sCO2 cycles. These property methods are assessed against several mixture property measurements, and offer a relative comparison to the accuracy obtained with REFPROP. This study also underscores the necessity of accurate property modeling, where cycle performance predictions are shown to vary significantly with the selection of the physical property method.

Commentary by Dr. Valentin Fuster
2017;():V009T38A018. doi:10.1115/GT2017-64287.

KIER (Korea Institute of Energy Research) has developed three supercritical carbon dioxide power cycle test loops since 2013. After developing a 10 kWe-class simple un-recuperated Brayton cycle, a second sub-kWe small-scale experimental test loop was manufactured to investigate the characteristics of the supercritical carbon dioxide power cycle, for which a high speed radial type turbo-generator was also designed and manufactured. Using only one channel of the nozzle, the partial admission method was adopted to reduce the rotational speed of the rotor so that commercial oil-lubricated bearings can be used. This was the world’s first approach to the supercritical carbon dioxide turbo-generator. After several tests, operation of the turbine for power production of up to 670 W was successful. Finally, an 80 kWe-class dual Brayton cycle test loop was designed. Before completion of the full test loop, a 60 kWe axial type turbo-generator was first manufactured and our previous 10 kWe-class test loop was upgraded to drive this turbo-generator. Due to leakage flow through the mechanical seal, a make-up loop was also developed. After assembling all test loops, a cold-run test and a preliminary operation test were conducted. In this paper, the power generating operation results of the sub-kWe-class test loop and the construction of the tens of kWe-class test loop which drives an axial type turbo-generator are described.

Commentary by Dr. Valentin Fuster
2017;():V009T38A019. doi:10.1115/GT2017-64349.

Power generation cycle — typically Brayton cycle — to use CO2 at supercritical state as working fluid have been researched many years because this cycle increase thermal efficiency of cycle and decrease turbomachinery size. But small turbomachinery make it difficult to develop proto type Supercritical Carbon dioxide (S-CO2) cycle equipment of lab scale size. KIER (Korea Institute of Energy Research) have been researched S-CO2 cycle since 2013. This paper is about 60kWe scale and sub-kWe class turbo generator development for applying to this S-CO2 cycle at the lab scale. A design concept of this turbo-generator is to use commercially available components so as to reduce development time and increase reliability. Major problem of SCO2 turbine is small volume flow rate and huge axial force. High density S-CO2 was referred as advantage of S-CO2 cycle because it make small turbomachinery possible. But this advantage was not valid in lab-scale cycles under 100kW because small amount volume flow rate means high rotating speed and too small diameter of turbine to manufacture it. Also, high inlet and outlet pressure make huge axial force. To solve these problem, KIER have attempt various turbines. In this paper, these attempts and results are presented and discussed.

Commentary by Dr. Valentin Fuster
2017;():V009T38A020. doi:10.1115/GT2017-64418.

Since the renewed interest in supercritical carbon dioxide cycles, a large number of cycle layouts have been proposed in literature. These analyses, which are essentially theoretical, consider different operating conditions and modelling assumptions and thus the results are not comparable.

There are also works that aim to provide a fair comparison between different cycles in order to assess which one is most efficient. These analyses are very interesting but, usually, combine thermodynamic and technical restrictions thus making it difficult to draw solid and general conclusions with regards to which the cycle of choice in the future should be .

With this background, the present work provides a systematic thermodynamic analysis of twelve supercritical carbon dioxide cycles under similar working conditions, with and without technical restriction in terms of pressure and/or temperature. This yields very interesting conclusions regarding which the most interesting cycles are amongst those proposed in literature. Also, useful recommendations are extracted from the parametric analysis with respect to the directions that must be followed when searching for more efficient cycles.

The analysis is based on efficiency and specific work diagrams with respect to pressure ratio and turbine inlet temperature in order to enhance their applicability to plant designs driven by fuel economy and/or footprint.

Commentary by Dr. Valentin Fuster
2017;():V009T38A021. doi:10.1115/GT2017-64560.

Printed circuit heat exchangers (PCHEs) have an important role in supercritical CO2 (sCO2) Brayton cycles because of their small footprint and the high level of recuperation required for this power cycle. Compact heat exchangers like PCHEs are a rapidly evolving technology, with many companies developing various designs. One technical unknown that is common to all compact heat exchangers is the flow distribution inside the headers that affects channel flow uniformity. For compact heat exchangers, the core frontal area is often large compared with the inlet pipe area, increasing the possibility of flow maldistribution. With the large area difference, there is potential for higher flow near the center and lower flow around the edges of the core. Flow maldistribution increases pressure drop and decreases effectiveness. In some header geometries, flow separation inside the header adds to the pressure drop without increasing heat transfer.

This is the first known experiment to test for flow maldistribution by direct velocity measurements in the headers. A PCHE visualization prototype was constructed out of transparent acrylic for optical flow measurements with Particle Image Velocimetry (PIV). The channels were machined out of sheets to form many semi-circular cross sections typical of chemically-etched plates used in PCHE fabrication. These plates were stacked and bolted together to resemble the core geometry. Two header geometries were tested, round and square, both with a normally-oriented jet.

PIV allows for velocities to be measured in an entire plane instantly without disturbing the flow. Small particles of approximately 10 micrometers in diameter were added to unheated water. The particles were illuminated by two laser flashes that were carefully timed, and two images were acquired with a specialized digital camera. The movement of particle groups was detected by a cross-correlation algorithm with a result of about 50k velocity measurements in a plane. The velocity distribution inside the header volume was mapped using this method over many planes by traversing the PCHE relative to the optical equipment. The level of flow maldistribution was measured by the spatially-changing velocity coming out of the channels. This effect was quantified by the coefficient of variation proposed by Baek et al. The relative levels of flow maldistribution in the different header geometries in this study were assessed. With highly-resolved velocity measurements, improvements to header geometry to reduce flow maldistribution can be developed.

Commentary by Dr. Valentin Fuster
2017;():V009T38A022. doi:10.1115/GT2017-64625.

Supercritical CO2 power systems offer significant power density advantages along with high efficiencies, compared to traditional Rankine or Brayton cycles. Of the several viable configurations, the recompression cycle has higher efficiency compared to the simple recuperated cycle for source temperatures above 500°C. It also provides a good trade-off between efficiency and plant complexity. This paper explores the dependence of critical operational parameters on source and sink-temperature, which is then used as a means to generate guidelines for developing recompression sCO2 power plants. The maximum source temperature in the analysis is restricted to 565°C to take advantage of the existing materials and technologies associated with industrial steam turbines. However, the methodology described herein is applicable for any other source temperature range.

An important source of thermal efficiency degradation in power plants is attributable to heat exchangers. Analysis presented in this work directly relates the optimum operational parameters of the recompression cycle to the operation of the low temperature recuperator. Thermodynamic analysis confirms that a recompression fraction of 0.25 and pressure ratio of 2.5 is as an optimum design point for the recompression cycle. The penalty in efficiency and power while operating the plant in off-design conditions for a fixed recompression fraction and pressure ratio is highlighted.

Topics: Optimization , Cycles
Commentary by Dr. Valentin Fuster
2017;():V009T38A023. doi:10.1115/GT2017-64631.

It is found that the ideal gas assumption is not proper for the design of turbomachinery blades using supercritical CO2 (S-CO2) as working fluid especially near the critical point. Therefore, the inverse design method which has been successfully applied to the ideal gas is extended to applications for the real gas by using a real gas property lookup table. A fast interpolation lookup approach is implemented which can be applied both in superheated and two-phase regimes. This method is applied to the design of a centrifugal compressor blade and a radial-inflow turbine blade for a S-CO2 recompression Brayton cycle. The stage aerodynamic performance (volute included) of the compressor and turbine is validated numerically by using the commercial CFD code ANSYS CFX R162. The structural integrity of the designs is also confirmed by using ANSYS Workbench Mechanical R162.

Commentary by Dr. Valentin Fuster
2017;():V009T38A024. doi:10.1115/GT2017-64641.

On a ten-year timescale, Carbon Capture and Storage could significantly reduce carbon dioxide (CO2) emissions. One of the major limitations of this technology is the energy penalty for the compression of CO2 to supercritical conditions, which can require up to 15% of the plant’s gross power output. To reduce the power requirements supercritical carbon dioxide compressors must operate at reduced temperatures and near saturation where phase change effects are important. Non-equilibrium condensation can occur in the high-speed flow at the leading edge of the compressor, causing performance and stability issues. The characterization of the fluid at these conditions is vital to enable advanced compressor designs at enhanced efficiency levels but the analysis is challenging due to the lack of data on the metastable fluid properties.

In this paper we assess the metastable behavior and nucleation characteristics of high-pressure subcooled carbon dioxide during the expansion in a Laval nozzle. The assessment is conducted with numerical calculations, supported and corroborated by experimental measurements. The Wilson line is determined via optical measurements in the range of 41 and 82 bar and near the critical point. The state of the metastable fluid is fully characterized through pressure and density measurements, with the latter obtained in a first of its kind laser interferometry set up. In a systematic analysis the inlet conditions of the nozzle are moved close to the critical point to allow for large gradients in fluid properties and reduced margin to condensation. The results of calculations using a direct extrapolation of the Span and Wagner equation of state model are compared with the experimental measurements. The analysis suggests that the direct extrapolation using the Span and Wagner model yields results within 2% of the experimental data, with improved accuracy at conditions away from the critical point. The results are applied in a pre-production supercritical carbon dioxide compressor and are used to define inlet conditions at reduced temperature but free of condensation. Full-scale compressor experiments demonstrate that the new inlet conditions can reduce the shaft power input by 16%.

Commentary by Dr. Valentin Fuster
2017;():V009T38A025. doi:10.1115/GT2017-64908.

This paper presents a method to evaluate the off-design performance of a heat exchanger without specifying detailed heat exchanger geometry. Presently, off-design heat exchanger performance evaluation is often done by assuming one of the terms in a lumped volume approach is constant (such as UA, temperature difference, ε etc.) or by producing a draft heat exchanger geometry to evaluate the local heat transfer coefficients in off-design operation.

As opposed to these approaches, the method presented in this paper manages to accurately predict off-design heat exchanger performance with very limited information. The method relies on a single parameter beyond the design operating conditions, namely the conductance ratio which is the product of heat transfer coefficient and area on both sides of the heat exchanger. The method is particularly powerful as it allows for the exploration of different off-design scenarios for a given on-design operating point.

The paper presents a theoretical introduction of the method along with a validation using data provided by BMPC and Alfa Laval for different types of heat exchangers and working fluids, including supercritical CO2. The method is then used to model the off-design performance of a simple recuperated sCO2 cycle, showing its ability to capture the off-design performance of a heat exchanger without specifying its detailed geometry and the impact of conductance ratio on off-design cycle performance.

Commentary by Dr. Valentin Fuster
2017;():V009T38A026. doi:10.1115/GT2017-64933.

Increasing the efficiency of coal-fired power plants is vital to reducing electricity costs and emissions. Power cycles employing supercritical carbon dioxide (sCO2) as the working fluid have the potential to increase power cycle efficiency by 3–5% points over state-of-the-art oxy-combustion steam-Rankine cycles operating under comparable conditions. To date, the majority of studies have focused on the integration and optimization of sCO2 power cycles in waste heat, solar, or nuclear applications.

The goal of this study is to demonstrate the potential of sCO2 power cycles, and quantify the power cycle efficiency gains that can be achieved versus the state-of-the-art steam-Rankine cycles employed in oxy-fired coal power plants. Turbine inlet conditions were varied among the sCO2 test cases and compared with existing Department of Energy (DOE)/National Energy Technology6 Laboratory (NETL) steam base cases. Two separate sCO2 test cases were considered and the associated flow sheets developed. The turbine inlet conditions for this study were chosen to match conditions in a coal-fired ultra-supercritical steam plant (Tinlet = 593°C, Pinlet = 24.1 MPa) and an advanced ultra-supercritical steam plant (Tinlet = 730°C, Pinlet = 27.6 MPa). A plant size of 550 MWe, was selected to match available information on existing DOE/NETL bases cases.

The effects of cycle architecture, combustion-air preheater temperature, and cooling source type were considered subject to comparable heat source and reference conditions taken from the steam Rankine reference cases. Combinations and variants of sCO2 power cycles — including cascade and recompression and variants with multiple reheat and compression steps — were considered with varying heat-rejection subsystems — air-cooled, direct cooling tower, and indirect-loop cooling tower. Where appropriate, combustion air preheater inlet temperature was also varied.

Through use of a multivariate nonlinear optimization design process that considers both performance and economic impacts, curves of minimum cost versus efficiency were generated for each sCO2 test case and combination of architecture and operational choices. These curves indicate both peak theoretical efficiency and suggest practical limits based on incremental cost versus performance. For a given test case, results for individual architectural and operational options give insight to cost and performance improvements from step-changes in system complexity and design, allowing down selection of candidate architectures. Optimized designs for each test case were then selected based on practical efficiency limits within the remaining candidate architectures and compared to the relevant baseline steam plant. sCO2 cycle flowsheets are presented for each optimized design.

Commentary by Dr. Valentin Fuster
2017;():V009T38A027. doi:10.1115/GT2017-64952.

Direct fired oxy-fuel combustion as a heat source for supercritical carbon dioxide (sCO2) power cycles is a promising method for providing the needed thermal energy input. The method of combustion has the potential to provide efficient power generation with integrated carbon capture at up to 99% of generated CO2. One of the highest efficiency power cycles being considered for sCO2 cycles in the recompression cycle. In the recompression sCO2 power cycle, the amount of energy recovered from the recuperation is roughly five times the energy added via the combustor. Because of this high degree of recuperation in sCO2 power cycles, the inlet temperature of the combustor is much higher than a more traditional combustor design. This elevated combustor temperature leads to some unique design challenges and approaches which are quite different from a traditional combustion system. A combustor designed for these conditions has never been built, and thus the design of the combustor discussed in this paper started from a clean slate.

This necessitates a large degree of fundamental research which might not be necessary for a more well understood traditional combustor design process. Building on previous thermodynamic and chemical kinetics studies, a reduced order reaction kinetics model was used with ANSYS CFX software to explore various auto-ignition type combustor geometries. A discussion of some geometries and the modelling techniques used is presented. Various injector configurations were examined and metrics were used to compare the various configurations. By utilizing the CFD flow field results, a preliminary design for a 1MW-class oxy-fuel combustor was developed.

In the past, little experimental research has been conducted on combustion within carbon dioxide at pressures above 200 bar. In order to confirm the validity of the auto-ignition style combustor a small bench top test rig was constructed to test the oxy-fuel combustion at the full pressure and temperature. This system was designed to validate some of the fundamental properties of the combustion. This includes a gas sampling system and a measurement of auto-ignition delay. Preliminary, data from a bench top scale, sCO2 oxy-fuel combustor experiment will be presented.

The results from this work will enable future development of sCO2 power cycles which enable 99% carbon capture, while maintaining overall cycle efficiency which is competitive with natural gas combined cycle power plants.

Commentary by Dr. Valentin Fuster
2017;():V009T38A028. doi:10.1115/GT2017-64958.

In order to maintain viability as a future power-generating technology, concentrating solar power (CSP) must reduce its levelized cost of electricity (LCOE). The cost of CSP is assessed with the System Advisor Model (SAM) from the National Renewable Energy Laboratory (NREL). The performance of an integrally geared compressor-expander recuperated recompression cycle with supercritical carbon dioxide (sCO2) as the working fluid is modeled. A comparison of the cycle model to the integrated SAM cycle performance is made. The cycle model incorporates innovative cycle control methods to improve the range of efficiency, including inventory control. The SAM model is modified to accommodate the predicted cycle performance. The ultimate goal of minimizing the LCOE is targeted through multiple approaches, including the cost of the power block, the impact of system scale, the sizing of the thermal system relative to the power block system, the operating approach for changes in ambient temperature and availability of sunlight. Through reduced power block cost and a detailed cycle model, the LCOE is modeled to be 5.98 ȼ/kWh, achieving targeted techno-economic performance. The LCOE of the CSP system is compared to the cost of hybrid solar and fossil-fired systems. An analysis is made on the efficacy of a fossil backup system with CSP and how that relates to potential future costs of carbon dioxide emissions.

Commentary by Dr. Valentin Fuster
2017;():V009T38A029. doi:10.1115/GT2017-65066.

A lifetime model is being developed for supercritical CO2 (sCO2) compatibility for the 30 year duty life for concentrated solar power (CSP) applications at >700°C to achieve higher efficiencies than other power cycles. Alloys 740H, 282, 625 and Fe-base alloy 25 are being evaluated in 500-h cycles at 1 bar and 300 bar, and 10-h cycles in 1 bar industrial grade CO2 at 700°–800°C. For comparison, companion experiments are being conducted in 1 bar air or O2. Using mass change, all of the alloys showed low mass gains with parabolic rate constants below the performance metric after 1000 h. However, alloy 25 showed a higher rate at 700°C in 300 bar sCO2 and did not follow an Arrhenius relationship. After 1500 h in 1 bar CO2, a much faster rate was observed for alloy 25 due to the formation of Fe2O3, but a similar increase was not observed in 300 bar CO2. Oxide thickness measurements have been completed after 1000 h in each condition. Only minor differences were noted between the 1 and 300 bar exposures. Up to 4,000 h exposures in 10-h cycles has not resulted in evidence of scale spallation but very small mass losses for alloy 625 were consistently observed. As longer exposures times are completed, quantification of the reaction products as a function time will be used to better model the degradation rate and additional characterization techniques will be included to further develop the model.

Commentary by Dr. Valentin Fuster
2017;():V009T38A030. doi:10.1115/GT2017-65169.

CSP plants using supercritical CO2 (sCO2) power cycle can potentially achieve high thermal conversion efficiency at low capital cost due to compact turbomachinery and other components. An sCO2 expander and improved heat exchanger is expected to provide a major stepping stone for achieving CSP power at $0.06/kW-hr LCOE, energy conversion efficiency > 50%, and total power block cost < $1,200/kW installed. However the life limiting mechanisms of these turbomachines in high pressure, high temperature sCO2 environment are not well understood. To understand the effect of high pressures, high temperatures and sCO2 chemical kinetics on crack initiation, crack propagation and low cycle fatigue (LCF) life of these turbomachines, a novel experimental setup is developed. Advanced microstructure and spectroscopic analyses are conducted that shed light on some key differences between various Ni base alloys in terms of oxidation morphology, chemical species diffusion and trapping, the formation of protective corrosion resistant layers and changes in surface properties. An experimental technique for low cycle fatigue experiments in high pressure, high temperature supercritical CO2 environment is developed. The test setup allows for pressurized LCF testing of alloys being considered for MW scale sCO2 turbine development. Results show that the LCF life remains the same (within the scatter band) irrespective of the location of crack initiation site whether at the OD (non shot-peened bars in air and sCO2), or at the ID (shot peened bars). Total fatigue life, for all conditions, lie within the normal variation in LCF results (± 2X life variation). No significant LCF life debit is observed in IN718 by sCO2 at 550 °C, 0.7% max strain, 20 cpm. Similar conclusion is reached during 0.6% max strain tests. The effect of sCO2 is found not to be significantly more damaging than air at these strain levels. However, the results can be different for lower % max strains due to longer exposure times involved, resulting from larger number of cycles to failure. Similarly at higher temperatures and/or longer hold-times, sCO2 environment may be more aggressive, resulting in lower total fatigue life.

Commentary by Dr. Valentin Fuster
2017;():V009T38A031. doi:10.1115/GT2017-65172.

Supercritical carbon dioxide (sCO2) power cycles require high compressor efficiency at both the design-point and over a wide operating range. Increasing the compressor efficiency and range helps maximize the power output of the cycle and allows operation over a broader range of transient and part-load operating conditions. For sCO2 cycles operating with compressor inlets near the critical point, large variations in fluid properties are possible with small changes in temperature or pressure. This leads to particular challenges for air-cooled cycles where compressor inlet temperature and associated fluid density are subject to daily and seasonal variations as well as transient events.

Design and off-design operating requirements for a wide-range compressor impeller are presented where the impeller is implemented on an integrally-geared compressor-expander (IGC) concept for a high temperature sCO2 recompression cycle. In order to satisfy the range and efficiency requirements of the cycle, a novel compressor stage design incorporating a semi-open impeller concept with a passive recirculating casing treatment is presented that mitigates inducer stall and extends the low flow operating range. The stage design also incorporates splitter blades and a vaneless diffuser to maximize efficiency and operating range. These advanced impeller design features are enabled through the use of direct metal laser sintering (DMLS) manufacturing. The resulting design increases the range from 45% to 73% relative to a conventional closed impeller design while maintaining high design point efficiency.

Commentary by Dr. Valentin Fuster
2017;():V009T38A032. doi:10.1115/GT2017-65217.

The Allam Cycle is a high performance oxy-fuel, supercritical CO2 power cycle that offers significant benefits over traditional fossil and hydrocarbon fuel-based power generation systems. A major benefit arises in the elimination of costly pre-combustion acid gas removal (AGR) for sulfur-(SOx) and nitrogen-based (NOx) impurities by utilizing a novel downstream cleanup process that utilizes NOx first as a gas phase catalyst to effect SOx oxidation, followed by NOx removal. The basic reactions required for this process, which have been well-demonstrated in several facilities for the cleanup of exhaust gasses, ultimately convert SOx and NOx species to sulfuric, nitric and nitrous acids for removal from the supercritical CO2 stream. The process results in simplified and significantly lower cost removal of these species and utilizes conditions inherent to the Allam Cycle that are ideally suited to facilitate this process.

8 Rivers Capital and the Energy & Environmental Research Center (EERC), supported by the state of North Dakota, the US Department of Energy (DOE) and an Industrial consortium from the State of North Dakota, are currently working together to test and optimize this novel impurity removal process for pressurized, semi-closed supercritical CO2 cycles, such as the Allam Cycle. Both reaction kinetic modeling and on-site testing have been completed. Initial results show that both SOx and NOx can be substantially removed from CO2-rich exhaust gas containing excess oxygen under 20 bar operating pressure utilizing a simple packed spray column. Sensitivity of the removal rate to the concentration of oxygen and NOx was investigated. Follow-on work will focus on system optimization to improve removal efficiency and removal control, to minimize metallurgy and corrosion risks from handling concentrated acids, and to reduce overall CAPX/OPEX of the system.

Commentary by Dr. Valentin Fuster

Wind Energy

2017;():V009T49A001. doi:10.1115/GT2017-63129.

Damp air with high humidity combined with foggy, rainy weather as well as icing in winter weather often found to cause turbine performance degradation, and it is more concerned with off-shore wind farm development. To address and understand the high humidity effects on wind turbine performance, our study has been conducted with spread sheet analysis on damp air properties investigation for air density and viscosity, then CFD modeling study using Fluent were carried out on Airfoil and Blade aerodynamic performance effects investigation due to water vapor partial pressure of mixing flow and water condensation around leading edge and trailing edge of airfoil. It is found that the high humidity effects with water vapor mixing flow and water condensation thin film around airfoil may have insignificant effect directly on airfoil/blade performance; however the indirect effects such as blade contamination and icing due to the water condensation may have significant effects on turbine performance degradation. Also found the foggy weather with micro water droplet (including rainy weather) may cause higher drag that lead to turbine performance degradation. It is found that at high temperature, the high humidity effect on air density cannot be ignored for annual energy production calculation. As qualitative validation, the CFD result was compared and correlated with field observation in foggy day of cold weather. The blade contamination and icing phenomenon need to be further investigated in the future study, and blade surface properties such as high surface energy coating effects on water condensation and icing will be investigated for anti-icing coating agent development.

Commentary by Dr. Valentin Fuster
2017;():V009T49A002. doi:10.1115/GT2017-63161.

The current development of wind turbines is moving toward larger and more flexible units, which can make them prone to fatigue damage induced by aeroelastic vibrations. The estimation of the total life of the composite components in a wind turbine requires the knowledge of both low and high cycle fatigue (LCF and HCF) data. The first aim of this study is to produce a validated numerical model, which can be used for aeroelastic analysis of wind turbines and is capable of estimating the LCF and HCF loads on the blade. The second aim of this work is to use the validated numerical model to assess the effects of extreme environmental conditions (such as high wind speeds) and rotor over-speed on low and high cycle fatigue.

Numerical modelling of this project is carried out using the Computational Fluid Dynamics (CFD) & aeroelasticity code AU3D, which is written at Imperial College and developed over many years with the support from Rolls-Royce. This code has been validated extensively for unsteady aerodynamic and aeroelastic analysis of high-speed flows in gas turbines, yet, has not been used for low-speed flows around wind turbine blades. Therefore, in the first place the capability of this code for predicting steady and unsteady flows over wind turbines is studied. The test case used for this purpose is the Phase VI wind turbine from the National Renewable Energy Laboratory (NREL), which has extensive steady, unsteady and mechanical measured data.

From the aerodynamic viewpoint of this study, AU3D results correlated well with the measured data for both steady and unsteady flow variables, which indicated that the code is capable of calculating the correct flow at low speeds for wind turbines.

The aeroelastic results showed that increase in crosswind and shaft speed would result in an increase of unsteady loading on the blade which could decrease the lifespan of a wind turbine due to HCF. Shaft overspeed leads to significant increase in steady loading which affects the LCF behaviour. Moreover, the introduction of crosswind could result in significant dynamic vibration due to forced response at resonance.

Topics: Wind turbines
Commentary by Dr. Valentin Fuster
2017;():V009T49A003. doi:10.1115/GT2017-63362.

Three versions of a generic horizontal axis wind turbine nacelle model were tested in a wind tunnel. Force and wake velocity measurements were performed at a Reynolds number of 4.2e5 based on the complete length of the nacelle and the spinner. The effect of main body corner shape was examined as well as that of the spinner. It was found that rounded corners can lead to significant reduction in the aerodynamic forces experienced by the nacelle. At the same time, however, smooth curve surfaces cause instabilities in the separation location, which lead to sudden drops in the lift force. In terms of aeroelastic behavior this is an undesirable feature that leads to negative aerodynamic damping and structures prone to galloping.

Commentary by Dr. Valentin Fuster
2017;():V009T49A004. doi:10.1115/GT2017-63557.

Many energy supply systems around the world are currently undergoing a phase of transition that is mainly characterized by a continuing increase in installed renewable power generation capacities. Aiming at a better integration of these additional capacities, operators of wind farms in Germany are obliged to bindingly forecast their power production. In order to maintain the continuous stability of the electricity grid, deviations from these forecasts have to be compensated by the grid operator, who charges the wind farm operators accordingly. An alternative way to compensate for forecast errors is the utilization of flexible and dispatchable energy conversion and storage units by the wind farm operator.

Heat-driven combined heat and power (CHP) units with heat storage systems offer the potential of limited short-term load adjustments to compensate forecast errors while simultaneously fulfilling their main task of providing heat.

The main objectives of the present study are the evaluation of the main technical parameters and the economic viability of the described application.

The study utilizes a theoretical scenario incorporating a gas turbine as a CHP unit providing heat for an industrial process, a heat storage and an associated wind farm. A generic wind farm power generation forecast error model is developed, providing realistic forecast errors for the study. Detailed models of all system components are developed and integrated into a common simulation environment, allowing for simulations of the overall system operation with varying heat storage capacities. The simulation results show that the combination of a heat-driven CHP gas turbine and a heat storage system makes a significant contribution to the compensation of the wind farm power generation forecast errors. Distinct correlations between the heat storage capacity and the remaining forecast errors are identified. The net balance energy costs resulting from the remaining demand for balance energy after the partial forecast error compensation are investigated as the main parameter for the economic viability. No distinctive correlation between the heat storage capacity and the net balance energy costs can be identified. This is the result of the stochastic character of the net balance energy price.

Commentary by Dr. Valentin Fuster
2017;():V009T49A005. doi:10.1115/GT2017-63573.

Large and flexible wind turbine blades may be susceptible to severe blade deformations coupled with dynamic stall. To advance prediction capability for this problem a general deforming mesh computational fluid dynamics (CFD) method has been developed for calculating flows with moving or deforming boundaries using an elastic spring analogy. The method has been evaluated against experimental data for flow around a pitching NACA0012 airfoil in the deep dynamic stall regime where flow is highly separated, and compared with other authors’ CFD simulations for pitching airfoil. The effects of varying the reduced frequency are also investigated. During the upstroke the present results are in generally good agreement with experiment and other CFD studies. During the downstroke some differences with experiment and other CFD models are apparent. This may be due to the sensitivity of the separated flow to modelling assumptions and experimental conditions. Overall, the degree of agreement between CFD and experiment is considered encouraging.

Commentary by Dr. Valentin Fuster
2017;():V009T49A006. doi:10.1115/GT2017-63643.

Wind turbine industry has a special need for accurate post stall airfoil data. While literature often covers incidence ranges [−10deg,+25deg] smaller machines experience a range of up to 90 deg for horizontal axis and up to 360 deg for vertical axis wind turbines (VAWTs). The post stall data of airfoils is crucial to improve the prediction of the start-up behavior as well as the performance at low tip speed ratios. The present paper analyzes and discusses the performance of the symmetrical NACA 0021 airfoil at three Reynolds numbers (Re = 100k, 140k, and 180k) through 180 deg incidence. The typical problem of blockage within a wind tunnel was avoided using an open test section. The experiments were conducted in terms of surface pressure distribution over the airfoil for a tripped and a baseline configuration. The pressure was used to gain lift, pressure drag, moment data. Further investigations with positive and negative pitching revealed a second hysteresis loop in the deep post stall region resulting in a difference of 0.2 in moment coefficient and 0.5 in lift.

Commentary by Dr. Valentin Fuster
2017;():V009T49A007. doi:10.1115/GT2017-63653.

During the commissioning and stand-still cycles of wind turbines, the rotor is often stopped or even locked leaving the rotor blades at a standstill. When the blades are at a stand still, angles of attack on the blades can be very high and it is therefore possible that they experience vortex induced vibrations. This experiment and analysis helps to explain the different regimes of flow at very high angles of attack, particularly on moderately twisted and tapered blades. A single blade was tested at two different flow velocities at a range of angles of attack with flow tuft visualisation and hotwire measurements of the wake. Hotwire wake measurements were able to show the gradual inception and ending of certain flow regimes. The power spectral densities of these measurements were normalized in terms of Strouhal number based on the projected chord to show that certain wake features have a relatively constant Strouhal number. The shedding frequency appears then to be relatively independent of chord taper and twist. Vortex generators were tested but were found to have little influence in this case. Gurney flaps were found to modify the wake geometry, stall onset angles and in some cases the shedding frequency.

Commentary by Dr. Valentin Fuster
2017;():V009T49A008. doi:10.1115/GT2017-63691.

In the present work, a narrow span-wide rectangular channel (referred to as a slot) is introduced and drilled near the leading edge of a finite-span cambered airfoil to study its impact on the overall aerodynamic performance. These slots are proposed to have two legs, where the first leg starts from the vicinity of the leading edge, and the second leg exits from the pressure-side of the airfoil. NACA 4412 is used as the baseline airfoil profile, and the influence of several geometrical parameters of the slots at different angles of attack (AoAs) ranging from 0 to 16 degrees are investigated on the lift and drag coefficients. The influence of slot’s length, width, and exit angles are studied, and it is demonstrated that longer and narrower slots that exit more aligned with the pressure-side streams are generally more suitable, and can result in better performance over the entire range of AoA. For the best case considered, a lift coefficient improvement as large as 15% is observed, while the drag penalty is insignificant.

Furthermore, the inlet angle and the vertical position of slots are independently varied within reasonable ranges to characterize the slots further. Computational fluid dynamics (CFD) is used for modeling and analysis. Simulations are performed at the chord-based Reynolds number of 1.6E6. Results are validated against published data and the results from a set of wind-tunnel experiment.

Commentary by Dr. Valentin Fuster
2017;():V009T49A009. doi:10.1115/GT2017-64004.

The evolution of the wake of a wind turbine contributes significantly to its operation and performance, as well as to those of machines installed in the vicinity. The inherent unsteady and three-dimensional aerodynamics of Vertical Axis Wind Turbines (VAWT) have hitherto limited the research on wake evolution. In this paper the wakes of both a troposkien and a H-type VAWT rotor are investigated by comparing experiments and calculations. Experiments were carried out in the large-scale wind tunnel of the Politecnico di Milano, where unsteady velocity measurements in the wake were performed by means of hot wire anemometry. The geometry of the rotors was reconstructed in the open-source wind-turbine software QBlade, developed at the TU Berlin. The aerodynamic model makes use of a lifting line free-vortex wake (LLFVW) formulation, including an adapted Beddoes-Leishman unsteady aerodynamic model; airfoil polars are introduced to assign sectional lift and drag coefficients. A wake sensitivity analysis was carried out to maximize the reliability of wake predictions. The calculations are shown to reproduce several wake features observed in the experiments, including blade-tip vortex, dominant and submissive vortical structures, and periodic unsteadiness caused by sectional dynamic stall. The experimental assessment of the simulations illustrates that the LLFVW model is capable of predicting the unsteady wake development with very limited computational cost, thus making the model ideal for the design and optimization of VAWTs.

Commentary by Dr. Valentin Fuster
2017;():V009T49A010. doi:10.1115/GT2017-64105.

Wake shielding in wind farms caused by the interaction of upstream energy-depleted wakes and down-stream turbines substantially reduces individual turbine efficiency and overall wind farm performance. A method is studied to alleviate this problem using shaft tilting to steer wakes upward and reduce the interaction with downstream turbines. Simulations have been conducted to verify this method and to assess its effectiveness. These simulations employ a specially developed hybrid free wake method that combines a Constant Circulation Contour Model, suitable for downwind far-wake evolution, with a Vortex Lattice Method, leading to accurate blade air-loads calculation, including unsteady effects, stall, and reduced complexity. The interaction of two inline tipped axis turbines has been analyzed to assess the advantages and challenges of wake steering in a system of turbines. Beyond the traditional HAWT, two unconventional turbine configurations have been studied with the intent to further increase wake ascent.

Topics: Simulation , Wakes , Turbines
Commentary by Dr. Valentin Fuster
2017;():V009T49A011. doi:10.1115/GT2017-64125.

In this paper, a slotted tip structure is experimentally analyzed. A wind turbine with three blades, of which the radius is 301.74mm, is investigated by the PIV method. Each wind turbine blade is formed with a slots system comprising four internal tube members embedded in the blade. The inlets of the internal tube member are located at the leading edge of the blade and form an inlet array. The outlets are located at the blade tip face and form an outlet array. The near wake flow field of the wind turbine with slotted tip and without slotted tip are both measured. Velocity field of near wake region and clear images of the tip vortex are captured under different wake ages.

The experimental results show that the radius of the tip vortex core is enlarged by the slotted tip at any wake age compared with that of original wind turbine. Moreover, the diffusion process of the tip vortex is accelerated by the slotted tip which lead to the disappearance of the tip vortex occurs at smaller wake age. The strength of the tip vortex is also reduced indicating that the flow field in the near wake of wind turbine is improved. The experimental data are further analyzed with the vortex core model to reveal the flow mechanism of this kind of flow control method. The turbulence coefficient of the vortex core model for wind turbine is obtained from the experimental data of the wind turbine with and without slotted tip. It shows that the slotted tip increases the turbulence strength in the tip vortex core by importing airflow into the tip vortex core during its initial generation stage, which leads to the reduction of the tip vortex strength.

Therefore, it is promising that the slotted tip can be used to weaken the vorticity and accelerate the diffusion of the tip vortex which would improve the problem caused by the tip vortex.

Commentary by Dr. Valentin Fuster
2017;():V009T49A012. doi:10.1115/GT2017-64137.

The Savonius rotor appears to be particularly promising for the small-scale applications because of its design simplicity, good starting ability, and insensitivity to wind directions. There has been a growing interest in recent times to harness wind energy in an efficient manner by developing newer blade profiles of Savonius rotor. The overlap ratio (OR), one of the important geometric parameters, plays a crucial role in the turbine performance. In a recent study, an elliptical blade profile with a sectional cut angle (θ) of 47.5° has demonstrated its superior performance when set at an OR = 0.20. However, this value of OR is ideal for a semicircular profile, and therefore, requires further investigation to arrive at the optimum overlap ratio for the elliptical profile. In view of this, the present study attempts to make a systemic numerical study to arrive at the optimum OR of the elliptical profile having sectional cut angle, θ = 47.5°. The 2D unsteady simulation is carried out around the elliptical profile considering various overlap ratios in the range of 0.0 to 0.30. The continuity, unsteady Reynolds Averaged Navier-Stokes (URANS) equations and two equation eddy viscosity SST (Shear Stress transport) k-ω model are solved by using the commercial finite volume method (FVM) based solver ANSYS Fluent. The torque and power coefficients are calculated as a function of tip speed ratio (TSR) and at rotating conditions. The total pressure, velocity magnitude and turbulence intensity contours are obtained and analyzed to arrive at the intended objective. The numerical simulation demonstrates an improved performance of the elliptical profile at an OR = 0.15.

Commentary by Dr. Valentin Fuster
2017;():V009T49A013. doi:10.1115/GT2017-64364.

This paper describes a method of creating inhomogeneous inflow conditions to a horizontal axis wind turbine installed in the settling chamber of the large wind tunnel of the TU Berlin. Thereby, a steady gust (i.e. spatial velocity gradient, constant over time) is created which covers half of the swept area of the turbine. For purposes of analysis, a hotwire traversing system was used and measurements were correlated to on-blade angle of attack, velocity and blade root bending moment measurements. Moreover, the paper presents wake measurements at one downstream plane behind the turbine.

Commentary by Dr. Valentin Fuster
2017;():V009T49A014. doi:10.1115/GT2017-64385.

Wind energy has had a major impact on the generation of renewable energy. While most research and development focuses on large, utility-scale wind turbines, a new application is in the field of small wind turbines in the urban environment. A major design challenge for these urban wind turbines is the noise generated during operation. This study examines the power production and the noise generated by two small-scale wind turbines tested in a small wind tunnel. Both rotors were designed using the Blade-Element Momentum Theory and either the NREL S823 or the Eppler 216 airfoils. Point noise measurements were taken using a 1/4” microphone at three locations downstream of the turbine: 16% of the diameter (two chord lengths), 50% of the diameter, and 75% of the diameter. At each horizontal location downstream of the turbine, a vertical traverse was performed to analyze the sound pressure level from the tip of the turbine blades down to the hub. The rotor designed with the Eppler 216 airfoil showed a 9% increase in power production and decrease of up to 7 dB(A).

Commentary by Dr. Valentin Fuster
2017;():V009T49A015. doi:10.1115/GT2017-64475.

This paper presents a strategy to model the aerodynamic Gurney flap effect on two-dimensional airfoils and subsequently on the rotor blade performance of horizontal axis wind turbines.

The first part consists of the parametric investigation of 26 airfoil polar data-sets, derived from different, but comparable, wind tunnel experiments. They are evaluated and processed in terms of the lift and drag increase caused by Gurney flaps in comparison to each Baseline configuration. Consequently, a model is developed, transforming Baseline- into Gurney flap polar data for varying flap-heights. The results of the emerging Gurney Flap Polar Calculator are validated against the experimental lift and drag curves.

In the second part, the blade design of the NREL 5 MW Reference Turbine is modified by implementing polar data-sets of varying Gurney flap-heights, which are imported into the rotor simulation software QBlade. Thereupon, blade optimization strategies are examined regarding the two main Gurney flap applications on rotor blades: the retrofit and the design solution. The optimized retrofit solution on existing blades indicates power performance improvements, albeit at the expense of increasing structural loads. The optimized design solution on to-be-constructed blades, on the other hand, suggests chord-length reductions, while keeping the performance characteristics on a similar or even enhanced level.

It is concluded that aerodynamic improvements are achieved by relatively small Gurney flap-heights, which are applied at specific blade positions.

Topics: Blades , Wind turbines
Commentary by Dr. Valentin Fuster
2017;():V009T49A016. doi:10.1115/GT2017-64701.

Due to the rapid progress in high-performance computing and the availability of increasingly large computational resources, Navier-Stokes computational fluid dynamics (CFD) now offers a cost-effective, versatile and accurate means to improve the understanding of the unsteady aerodynamics of Darrieus wind turbines and deliver more efficient designs.

In particular, the possibility of determining a fully resolved flow field past the blades by means of CFD offers the opportunity to both further understand the physics underlying the turbine fluid dynamics and to use this knowledge to validate lower-order models, which can have a wider diffusion in the wind energy sector, particularly for industrial use, in the light of their lower computational burden.

In this context, highly spatially and temporally refined time-dependent three-dimensional Navier-Stokes simulations were carried out using more than 16,000 processor cores per simulation on an IBM BG/Q cluster in order to investigate thoroughly the three-dimensional unsteady aerodynamics of a single blade in Darrieus-like motion. Particular attention was payed to tip losses, dynamic stall, and blade/wake interaction. CFD results are compared with those obtained with an open-source code based on the Lifting Line Free Vortex Wake Model (LLFVW). At present, this approach is the most refined method among the “lower-fidelity” models and, as the wake is explicitly resolved in contrast to BEM-based methods, LLFVW analyses provide three-dimensional flow solutions. Extended comparisons between the two approaches are presented and a critical analysis is carried out to identify the benefits and drawbacks of the two approaches.

Commentary by Dr. Valentin Fuster
2017;():V009T49A017. doi:10.1115/GT2017-64723.

The analysis of wind turbine wakes’ structure and interaction with other machines installed in the same array or park has become a key element in the current wind energy research due to the notable impact that wakes can have on the actual energy production of the turbines themselves.

The present frontier of the research in this field is leading to the massive use of large-eddy simulations to completely solve the flow field surrounding the rotors. By doing so, however, enormous calculation resources are needed, which are often not available in an industrial context and also generally not compatible with extended optimization analyses (e.g. for a park layout definition). In this latter case, several cases need to be solved in a reasonable amount of time and therefore more computationally efficient methods are still needed.

Within this context, the present study reports a comparative analysis between three different techniques to analyse the wind turbine wakes’ structure and their mutual interaction. In particular, a state-of-the-art 3D RANS calculation of the famous NREL Phase VI rotor was used as a benchmark for comparison with two other methods. The first one is based on the Virtual Blade Model (VBM) of the commercial solver ANSYS® FLUENT®, in which a 3D RANS calculation of the flow field is carried out for the outer domain, while the effect of the rotating blades on the fluid is simulated through a body force, which acts inside a disk of fluid with an area equal to the swept area of the turbine. The value of the body force is time-averaged over a cycle from the forces calculated by a simplified Blade Element Method. In the present study, a stall delay model was also implemented within the VBM module. The second one is instead based on the even more simple approach with an Actuator Disk Model (ADM), in which the turbine presence is actually modelled as a sink of momentum in the main flow. Cross-comparisons between the techniques are shown, both in terms of single wake description and of wake-turbine interaction, leading to the conclusion that the VBM model may represent a valuable and computationally affordable tool in many wind energy applications.

Topics: Wakes , Wind turbines
Commentary by Dr. Valentin Fuster
2017;():V009T49A018. doi:10.1115/GT2017-64733.

Darrieus Vertical Axis Wind Turbines (VAWTs) have been recently identified as the most promising solution for new types of applications, such as small-scale installations in complex terrains or offshore large floating platforms. To improve their efficiencies further and make them competitive with those of conventional horizontal axis wind turbines, a more in depth understanding of the physical phenomena that govern the aerodynamics past a rotating Darrieus turbine is needed. Within this context, Computational Fluid Dynamics (CFD) can play a fundamental role, since it represents the only model able to provide a detailed and comprehensive representation of the flow. Due to the complexity of similar simulations, however, the possibility of having reliable and detailed experimental data to be used as validation test cases is pivotal to tune the numerical tools.

In this study, a two-dimensional U-RANS computational model was applied to analyze the wake characteristics on the mid plane of a small-size H-shaped Darrieus VAWT. The turbine was tested in a large-scale, open-jet wind tunnel, including both performance and wake measurements. Thanks to the availability of such a unique set of experimental data, systematic comparisons between simulations and experiments were carried out analyzing the structure of the wake, and correlating the main macro-structures of the flow to the local aerodynamic features of the airfoils in cycloidal motion. In general, good agreement on the turbine performance estimation was constantly appreciated.

Commentary by Dr. Valentin Fuster
2017;():V009T49A019. doi:10.1115/GT2017-65145.

Nowadays, the growth in the diameter of the rotors and the power capacity of the machinery require the application of constant monitoring to predict failures and reduce maintenance activities. These activities prevent emergency stops and failures that require the replacement of components; however, these operations involve a high cost.

The system presented in this paper integrates a fast data processing method that can analyze monitoring signals in real-time. The overall system includes a set of sensors for the measurement of acceleration, angular velocity, temperature, voltage, electrical current, and oil quality. The system presented is able to produce a frequency spectrum with a length of 8192 points. The FFT module was implemented in a FPGA (Field Programmable Gate Array) and it generates in real time, the frequency spectrum. This system adjusts the dominant frequencies to the wind turbine velocity.

Topics: Failure
Commentary by Dr. Valentin Fuster
2017;():V009T49A020. doi:10.1115/GT2017-65153.

The direct proportionality of streamline curvature to the pressure gradient normal to it causes the dependence of surface pressure loading on geometry curvature. This allows for the use of geometry curvature as a direct and aerodynamically meaningful interface to modify and improve performance of wind turbine sections. A novel blade parameterization technique driven by specification of meanline second derivative and a thickness distribution is presented. This technique is implemented as T-Blade3 which is an already existing in-house open-executable. The second derivative which is indicative of curvature, is used, enabling exploration of a large design space with minimal number of parameters due to the use of B-spline control points, capable of producing smooth curves with only a few points. New thickness and curvature control capabilities have been added to TBlade3 for isolated and wind turbine airfoils. The parameterization ensures curvature and slope of curvature continuity on the airfoil surface which are critical to smooth surface pressure distribution. Consequently, losses due to unintentional pressure spikes are minimized and likelihood of separation reduced. As a demonstration of the parameterization capability, Multi-Objective optimization is carried out to maximize wind turbine efficiency. This is achieved through an optimization tool-chain that minimizes a weighted sum of the drag-to-lift ratios over a range of angles of attack and sectional Reynolds numbers using a Genetic Algorithm. This allows for radial Reynolds number variation and ensures efficiency of wind turbine blade with twist incorporated. The tool-chain uses XFOIL to evaluate drag polars. This is implemented in MATLAB and Python in serial and in parallel with the US Department of Energy optimization system, DAKOTA. The Python and DAKOTA versions of the code are fully open-source. The NREL S809 horizontal axis wind turbine laminar-flow airfoil which is 21% thick has been used as a benchmark for comparison. Hence, the optimization is carried out with the same thickness-to-chord ratio. Drag coefficient improvement ranging from 17% to 55% for Cl between 0.3 and 1 was achieved.

Commentary by Dr. Valentin Fuster

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