Technical Papers

Author: Kevin Fairman, Concepts NREC

Abstract

Turbomachinery built for Supercritical Carbon Dioxide (sCO2) service is designed with special consideration for the very high-density characteristics of the working fluid. The fluid density has strong effects upon the dynamic behavior of the machines and makes the machines sensitive to changes in the operating point of the power cycle. The power cycle control must consider the condition of the turbomachinery in order to maintain safe and reliable operation. Concepts NREC is working with the DOE under an SBIR funded program to develop an sCO2 turbomachinery bearing spindle with magnetic bearings. The magnetic bearing has been selected for its diagnostic and measurement feedback capabilities. Magnetic bearings will provide real time information about the condition of the machinery. This type of information can be valuable in component level testing, and for the operational control and monitoring of the machine in service. Condition monitoring of machinery for the sCO2 program can yield great benefits in the development stages, as well as in the future reliability improvement stages of the program. Understanding the machinery response to changes in operations, such as changes to the heat input or fast transients due to upsets, will help the development of process control algorithms for the power cycle. This paper will describe the influences of the high-density fluid upon the machinery configuration and describe the machinery under development for the magnetic bearing spindle for sCO2 turbomachinery.

Authors: Edward Childs, Stephen Kohr, Concepts NREC

Abstract

The design of a turbocharger compressor must meet aerodynamic performance requirements, operate within specified stress and vibration limits, and respond quickly to changes in operating conditions. Design optimization must therefore include static, thermal and modal analysis (including weight and polar moment of inertia calculations) along with aerodynamic analysis (CFD). In some cases, a design optimized for aerodynamic performance only can be optimized separately to meet structural goals, using impeller backface geometry, bore radius and fillet radius inputs, which generally do not impact aerodynamic performance. If, however, impeller geometry inputs such as R1t-R1h, R2, B2 influence both aerodynamic and structural analysis, a coupled optimization is required, and each design must have both CFD and FEA analyses. In this study, a radial compressor with a vaneless diffuser at a single operating point is considered. The aerodynamic parameters for the impeller (BETA1H, BETA1S, BETA2S, main blade count, B2, R2, R1t) and diffuser (Pinch, R3/R2, Rex/R2) comprise in total 10 independent aerodynamic inputs. The aerodynamic objectives are to meet the operating point pressure ratio target and to maximize efficiency. The structural parameters for the backface (shoulder position, shoulder radius, web thickness at outer diameter (OD), OD angle, shoulder angle), bore radius and fillet radius comprise in total 7 independent structural inputs. The main structural objectives are to minimize the polar moment of inertia, and satisfy constraints on allowable maximum stress, deflection and the frequencies of blade vibration (flapping) modes. Successful multi-disciplinary optimization requires both CFD and FEA analysis to complete successfully for each trial design. Initial test runs of the optimization resulted in many geometries for which a valid CFD grid or FEA grid could not be generated. The high percentage of failed runs in the initial DOE impeded the construction of a viable surrogate model. A comprehensive investigation of all failure modes led to prescreening of both CFD and FEA geometry generation, using input constraints. The failure rate was greatly reduced as a result, leading to an improved search. Prior to the geometry screening, the optimizer found a large Pareto frontier between the efficiency and polar moment of inertia objectives. Following the screening, the efficiency and IP objectives became more cooperative. The optimization was carried out using Concepts NREC tools AxCent^® and TurboOPT II™, NUMECA Fine/Turbo, and ESTECO modeFRONTIER^®.

Author: Mark Anderson, Concepts NREC

Abstract

Turbomachinery systems are often subject to variations in ambient conditions and applied loads in operation. Standard maps (perhaps the most common being pressure ratio verses mass flow for compressors) are usually presented in terms of fixed inflow conditions. To account for changes in performance due to varying inlet conditions, compressors maps are often presented in standardized form where the mass flow and rotational speeds are normalized as a function of the reference condition total pressure and temperature. These methods are very widely used, particularly in the turbo charger industry. With these normalized maps, the actual performance of a compressor in a given environment can be deduced simply and easily with very reasonable accuracy in most cases.

The underlaying assumption of this conventional normalization process is that the fluid behaves as a perfect gas. While this is usually sufficient for air compressors, the method is not viable where the fluid properties are not near perfect gas conditions, which is certainly the case for supercritical applications. The highly variable fluid properties near the critical point, and the challenges they present in design, have been well documented in the literature. The two most critical properties to consider in the design process are the density and the speed of sound. The density determines the volumetric flow for a given mass flow and this in turn determines the incidence angle, a primary driver of performance. The speed of sound directly affects the range of the compressor via choking. Choking range can be further complicated by the fact that under certain conditions, the choked state can be reached at Mach numbers less than one. While rare, this situation can occur when the inflow conditions are found close to the liquid side of the saturation dome.

To account for these effects, a new method is proposed to generate normalized maps of performance that can be used to determine actual performance of a wide range of inlet conditions for highly non-linear thermodynamic properties. Although not as simple as the conventional perfect gas method that can be applied in a “back-of-the-envelope” style, the new method can be applied very rapidly using a spreadsheet-based method directly calling high fidelity NIST thermodynamic models. The end result of this tool is that a compressor map that has been painstakingly generated with testing or CFD can be applied to any inlet condition and the range and performance predicted very rapidly with high accuracy.

Authors: Thiago Ebel, Mark Anderson, Parth Pandya, Mat Perchanok, Nick Tiney, Steve P. Gravante

Abstract

When developing a turbocharged internal combustion engine, the choice of turbocharger is usually based on designer experience and existing hardware. However, proper turbocharger design relies on matching the compressor and turbine performance to the engine requirements so that parameters such as boost and back pressure, compressor pressure ratio, and turbine inlet temperatures meet the needs of the engine without exceeding its allowable operating envelope. Therefore, the ultimate measure of a successful turbocharger design is how well it is matched to an engine across various operating conditions. This, in turn, determines whether a new turbocharger is required, or an existing solution can be used.

When existing turbocharger solutions are not viable, the engine designer is at a loss on how to define a new turbocharger that meets the desired performance requirements. A common approach in industry has been to scale the performance of an existing turbocharger (compressor and turbine maps) and take these requirements for Original Equipment Manufacturers to possibly match it with a real machine. However, the assumptions made in a basic scaling process are quite simplistic and generally not satisfactory in this situation. A better approach would be to use a validated meanline model for a compressor and turbine instead, allowing to perform an actual preliminary design of such components. Such approach allows to link the engine performance requirements in a very early stage of te component design project and it guides the designer for the design decisions, such as rotor size, variable geometry nozzles, diameter, or shroud trims and others. Therefore, a feasible solution is more likely with design less iterations.

This paper describes a methodology for an integrated approach to design and analyze a turbocharged internal combustion engine using commercially available state-of-the-art 1D gas dynamics simulation tool linked to two powerful turbomachinery meanline programs. The outputs of this analysis are detailed performance data of the engine and turbocharger at different engine operating conditions.

Two case studies are then presented for a 10-liter diesel truck engine. The first study demonstrates how the programs are used to evaluate an existing engine and reverse engineer an existing turbocharger based only on the available performance maps. Then a second study is done using a similar approach but redesigning a new turbocharger (based on the reverse engineered one) for an increased torque output of the same engine.

Author:  Mark R. Anderson, Concepts NREC

Abstract

It is well known that the fluid properties near the critical point are highly variable and
complicate the design process for compressors operating in this region. This had been well documented in the literature and improved analysis methods using highly accurate thermodynamic models are now routinely included in sCO2 design. Depending on the cycle conditions and heat sources, moderate to strong variation in inflow conditions to the compressor can be expected in many cycles and their effects on compressor performance must be considered. Perhaps less well understood is the potential for even more no n linear behavior when the inflow conditions to the compressor shift toward the liquid side of the saturation curve.
In this work, both 1D and 3D numerical and analytical methods are used to demonstrate highly unexpected behavior of the fluid near the critical point and where the inlet conditions move over to the liquid side of the saturation dome. This paper describes usual fluid dynamic behavior in the saturated liquid region of CO2 which includes choking flows at Mach numbers significantly less than unity as well as shock like behavior at Mach numbers well below the typical sonic point.
These effects are expected to have a strong influence on compressor performance as inlet conditions vary, The applicability of common turbulence and two phase modeling approaches for sCO2 is also
discussed.

Author: Francis A. DiBella, P.E., Concepts NREC

Abstract

Reduction of carbon dioxide (CO2) through sequestration is an energy-intensive process,
requiring as much as 10-15% of a utility’s power generation. Concepts NREC (CN) has developed a hybrid cycle that combines a Supercritical CO2 Brayton cycle (SCO2) with a Pressure Swing Adsorption (PSA) process.
The goal of this hybrid cycle is to significantly reduce the external power required for carbon sequestration, by utilizing the highly efficient SCO2 cycle. The hybrid cycle is particularly attractive to use on a coal-fired power plant in order to reduce the release of CO2 into the environment. As such the proposed cycle may be thought of as a “cross-over” system to enable the continued use of coal in utility power generation systems even as natural gas and/or renewable energy continue to be developed along with the necessary infrastructure of pipelines and large surface area and thus can be phased in to completely replace the coal powered systems. The “Cross-over” concept can be used for countries that continue to plan new coal-fired facilities until natural gas
infrastructure is in place but who are also cognoscente of the need to reduce CO2 emissions.
This hybrid cycle also uses a PSA process to recover CO2 from the exhaust gas stream, and then utilizes the recovered CO2 as the working fluid in an SCO2 cycle. The discharge of the SCO2 cycle is sequestered into an underground geophysical vault, but only after the SCO2 is first passed through a let-down turbine that recovers a significant portion of the energy of compression until the ground vault is pressurized, thus providing additional power generation.
CN estimates that this new hybrid cycle will reduce the required power for CO2 sequestration by at least 30-40%. The power for compressing the sequestered CO2 can be reduced by 80%, if a CO2 pressure let-down turbine is also used. Of course, the cost per kw ($/kw) of such a combined system is also important to the acceptance of the concept. However, the cost must also be balanced against the increasing costs of not reducing the release of CO2 into the atmosphere.
Such cost comparison is left to another study while the goal of this study: the analysis of the thermodynamic viability of the proposed concept is presented and found to be acceptable.
Current independent studies indicate that the new cycle could utilize K-promoted hydrotalcite in a high-pressure, high-temperature PSA system to recover CO2 from utility power plant exhaust gas. The cycle is thought to satisfy the need to reduce the atmospheric release of CO2 from, in particular, coal-fueled power plants, until such time as the coal plants can be replaced by renewable energy sources and/or power plants that use cleaner-burning fuels. To be clear, the thermodynamic analysis offered in this paper is focused on the successful recovery of CO2 from the exhaust gases of a power plant, ostensibly a coal-fired plant where it has the most benefit but also a natural gas fired plant. The recovery of the CO2 by a PSA system has shown some merit, but the use of a PSA system should not be considered to be at the sacrifice of other methods for recovering CO2 from exhaust gases.
It is generally agreed that this reduction is necessary for the sake of future generations; but it is also understood that the deployment of current CO2 reduction techniques is costly. Thus, the technical objective of several research programs sponsored by the DOE has been to reduce the initial cost and power consumption of systems that effectively capture, reduce, or sequester carbon dioxide.
Thus, the proposed hybrid SCO2/PSA cycle provides a technical solution, albeit a futuristic one that requires the further development of the basic SCO2 system as well as the selection of the most viable CO2 recovery methodology. This is seen as at least honoring the spirit, if not the immediate intent of the climate change initiatives agreed upon at the COP21 Conference, as essential for the health of the populations of all nations of the world.

Authors: Francis A. DiBella, Kevin Fairman, Alex Gofer, David Karon, Concepts NREC,, with Zhang Jingxuan, Huang Weiguang, Shanghai Advanced Research Institute (SARI), CAS  Center of Advanced Energy System and Equipment R&D

Abstract

In recent years, supercritical carbon dioxide (sCO2) power cycle has attracted much more attention for its prominent advantages, such as high efficiency, high-power density and wide range of applications. The performance of the sCO2 compressor plays a very important role in the operation of the whole cycle, especially during the part load operation. Because of the non-ideal gas properties of the sCO2 fluid above and near the critical point, the use of the traditional design code for an sCO2 compressor must be verified with the actual testing of the design and possible modification of the design code. However, there are few industrial application
level sCO2 compressor rigs for the experimental research.
In this paper, a 2MW rated sCO2 compressor rig is designed with some test articles considered — test articles include: (1) the compressor impeller, (2) the diffuser, and (3) the volute. This paper will review the final design of the compressor including the AERO and structural design of the sCO2 impeller, as well as a review of the bearings, shaft seal and plans for the instrumentation as required to quantify the performance of the compressor.
SARI (Shanghai Advanced Research Institute) is a multi-disciplinary scientific research institute based in Shanghai, China. SARI is developing a world class sCO2 turbomachinery testing facility. Concepts NREC LLC, known for its turbomachinery software and turbomachinery engineering services, is collaborating with SARI in the development of the compressor rig for SARI’s laboratory testing.

Authors: Meng Soon Chiong, Srithar Rajoo, Richardo F. Martinez-Botas, Torsten Palenschat, Peter Weitzman, Mark Anderson, Thiago Ebel

Abstract

In recent years, supercritical carbon dioxide (sCO2) power cycle has attracted much more attention for its prominent advantages, such as high efficiency, high-power density and wide range of applications. The performance of the sCO2 compressor plays a very important role in the operation of the whole cycle, especially during the part load operation. Because of the non-ideal gas properties of the sCO2 fluid above and near the critical point, the use of the traditional design code for an sCO2 compressor must be verified with the actual testing of the design and possible modification of the design code. However, there are few industrial application
level sCO2 compressor rigs for the experimental research.
In this paper, a 2MW rated sCO2 compressor rig is designed with some test articles considered — test articles include: (1) the compressor impeller, (2) the diffuser, and (3) the volute. This paper will review the final design of the compressor including the AERO and structural design of the sCO2 impeller, as well as a review of the bearings, shaft seal and plans for the instrumentation as required to quantify the performance of the compressor.
SARI (Shanghai Advanced Research Institute) is a multi-disciplinary scientific research institute based in Shanghai, China. SARI is developing a world class sCO2 turbomachinery testing facility. Concepts NREC LLC, known for its turbomachinery software and turbomachinery engineering services, is collaborating with SARI in the development of the compressor rig for SARI’s laboratory testing.

Author: Mark R. Anderson, Concepts NREC

Abstract

The “Smith Chart” has been recognized as an indispensable technique when applied to the initial design of axial compressors and turbines. The Smith Chart offers a simple method to locate the region of optimum efficiency which is achievable as a function of flow and work coefficient. The result is a targeted flow state represented by the velocity triangles that result from these coefficients.

The process was originally developed, and is best documented, for axial turbines¹. Over the years, several publications, of similar methods for axial compressors have been put forward. The author presented one such work² which made significant use of optimization to develop an improved Smith chart for moderate Mach number compressor designs. In the current work, these results are expanded to both low Mach number (basically incompressible) to high-speed transonic cases as well. Similar to the previous work, the effort makes extensive use of optimization to systematically explore the optimum 2D profile shapes for a wide range of target flow and work coefficients. The method uses an FNS quasi-3D CFD solver, coupled to an efficiently parameterized geometry generator, combined with an automated optimization process. The process was applied independently to dozens of flow and work coefficient points to generate comprehensive maps of performance.

Results are shown for three different relative inflow Mach numbers: 0.2, 0.75, and 1.1. The maps are displayed in classic Smith chart format of islands of stage efficiency as a function of the flow and work coefficient. Specifically, the results are for axial compressor stages of 50% reaction, the theoretical ideal reaction for 2D flow. The results and the implications over varying Mach numbers are discussed. Also included is an expanded discussion of the range and accuracy of various meanline modeling methods, along with their ability to determine the optimum design condition.

Authors: Edward P. Childs, Dimitri Deserranno, Akshay Bagi, Concepts NREC

Abstract

The application of Surrogate-Based Optimization (SBO) to the industrial design process for a radial compressor with two operating points is described. The design specification includes two operating points at mass flow rates differing by a factor of three, and efficiency and pressure ratio targets for each point. The base case, while roughly sized from 1D analysis, fails to achieve the pressure ratio targets. In this paper, the optimization focusses on correcting the two speed-line map of total to static pressure ratio vs. mass flow rate. “Smart parameterization”, combining independent and dependent geometric parameters, and yielding reasonable geometries for most input combinations, coupled with efficient SBO, with separate models for response surface modeling and failure prediction, yields a design achieving the targets in just 57 CFD runs. FINE/Turbo [1] is used as the CFD analysis code and FINE/Design3D [2] and MINAMO [3] as the multi-objective optimizer.

Authors: Carlos Felipe F. Favaretto, Mark R. Anderson, Shuo Li, Leon Hu

Abstract

Recirculating casing treatment is a feature frequently adopted in automotive turbocharger compressor design to extend its operating range. In the low flow region of the map, surge margin can be enhanced by bleeding a fraction of the main flow from the impeller casing back to the stage inlet. In the high flow region, choke margin can be improved by bleeding flow from the stage inlet to a location downstream of the impeller throat. The benefits of recirculating casing treatment are more prominent at higher rotational speeds and are usually accompanied by an associated penalty in efficiency, which may or may not be acceptable. The challenge posed to the designers is to create a recirculating casing cavity configuration which enlarges the map width, while minimizing its impact on efficiency.

In the lack of specialized meanline tools, early stages of recirculating casing cavity design are mostly reliant on results from expensive CFD runs or test data from existing designs, if available. This paper presents the development of a meanline model suitable for preliminary design. The model was compared with CFD results for an automotive turbocharger compressor. The authors believe the proposed model can be a valuable tool for reducing development costs in the preliminary design stages of recirculating casing treatment configurations.

Author: Mark R. Anderson, Concepts NREC

Abstract

Preliminary design of a turbomachinery stage usually begins with target flow states, represented by a velocity triangle. Guidance for these target velocities comes from a variety of sources, which include some physics-based rules but, more often than not, have a significant empirical basis.

The best-known guidelines for axial designs are the so-called “Smith Charts”. This approach was documented in detail by Smith [1] and quickly became a preferred approach for initial design of axial turbines stages. The method was based on a significant set of test data for various turbine designs, corrected for complicating factors, such as various clearance gaps and so on. Similar, but less well-documented methods, have been developed for axial compressor design. Despite the widespread use of the Smith chart method, these approaches have a very limited substantiation and no clear pedigree. Most attempts to develop Smith-like charts for compressor design guidance are based on fairly simplified models that carry significant unknowns, particularly at high loading.

This paper details the development of new Smith charts for axial compressor designs, by making extensive use of optimization techniques. Profile designs are optimized for a range of target flow coefficients and work coefficients. The result is a performance map, representing the maximum possible performance for a given set of coefficient values. The analysis was limited to 2D profile design (quasi-3D analysis) and a reaction of 50%, the theoretical ideal reaction rate for 2D flow.

Using the newly developed maps of maximum performance potential, the accuracy of various modeling methods is examined, along with their ability to determine the optimum design condition. Improvements to profile loss methods, based on the new data, are suggested.

Author: Mark R. Anderson, Concepts NREC

Abstract

The non-linear fluid properties near the critical point have received very significant consideration in terms of their effect on CO2 compressors. Less well documented, is the potential for even more non-linear behavior when the inflow conditions to the compressor shift toward the liquid side of the saturation curve.
Depending on the cycle conditions and heat sources, moderate to strong variation in inflow conditions to the compressor can be expected in many cycles and the effect on compressor performance must be considered.
Very strong variation in density and specific heat ratio come into play near the critical point which strongly impacts the compressor design. Near the liquid saturation line, the nature of the fluid properties changes and the speed of sound becomes the dominant nonlinear variation. This variation has significant potential to affect the performance of the compressor at off-design cycle conditions.
In this work, both 1D and 3D numerical and analytical methods are used to demonstrate highly unexpected behavior of the fluid crossing the liquid side of the saturation dome. These effects include choking flows at Mach numbers significantly less than unity as well as shock-like behavior at Mach numbers well below the sonic point.

Authors: Francis A. Di Bella, P.E., Concepts NREC and James Pasch, PhD, Sandia National Laboratories

Abstract

There is considerable enthusiasm in the utility power industry over the very high efficiencies that are attainable with supercritical CO2 systems. The high efficiencies are achieved with turbocompressor units that have been identified to be almost an order of magnitude smaller in size compared to their comparably powered steam turbine systems. However, the size of the high and low temperature recuperator heat exchangers in a recompression-type SCO2 cycle, with an advancement of a reheat turbine, shown in Figure 1a, requires effectiveness to be as high as 95-98%, to achieve the high efficiencies that have been promoted for the SCO2 cycle. It would seem contrary to the stated goals of the SCO2 power plant to achieve the desired reduction in size of the turbine-compressor but still require very large and expensive, high pressure recuperator heat exchangers. This paper presents some analyses to support the hypothesis that the goal of high cycle efficiency and reduced size for SCO2 System Power Generation Systems can be
aided by “trading” improved turbomachinery efficiency for regenerator effectiveness. It is further asserted that turbomachinery efficiency can be improved by utilizing aero designs that utilize the Additive Manufacturing’s Direct Metal Laser Sintering capabilities to form homogeneous, high strength metals into aerodynamic, more efficient designs.

Authors:Francis A. Di Bella, P.E., Adam Weaver, Colin Osborne, Concepts NREC

Abstract

A chiller cycle analysis is presented that identifies the relative performance of refrigerants R134a and R1234ze for drop-in or new chiller applications. The study reveals that there is a 30% increase in the volumetric flow rate for new chillers using R1234ze, compared to R134a for the same chiller capacity. The C.O.P. is comparable for the two refrigerants on new systems but may likely be reduced when R134a is exchanged for R1234ze on existing systems. A comparison of the compressor size and speed is also presented and indicates the R1234ze chiller compressor is larger in size and operates at a lower speed (rpm) for the same chiller capacity. For a simple replacement of R134a refrigerant with R1234ze into existing chiller systems, the C.O.P. will be reduced by 16-18%.

Author: Eric M. Krivitsky, Concepts NREC

Abstract

Fixed-geometry centrifugal compressors have inherent limitations in the off-design operating range and performance that can be delivered. Variable inlet swirl, typically introduced with variable inlet guide vanes, has been a frequent tool implemented across many industries to extend the stable operating range and off-design efficiency. However, a review of available data in the literature reveals a large disparity in the effectiveness of variable inlet swirl for useful map movement. An ideal configuration responds to preswirl with a significant reduction in the mass flow at instability along with increased efficiency. Unfortunately, some cases reveal no movement in the surge line, with increases in inlet swirl only serving to reduce the stage efficiency.This study investigates underlying causes for the variation in sensitivity to inlet swirl amongst various compressor designs, represented by form parameters. An analytical model is established and then verified using CFD and selected test data to explore fundamental causes for decreased sensitivity to inlet swirl. Analytical and computational results suggest a new understanding of the governing parameters for inlet swirl sensitivity, and thus provide insight into the scenarios when inlet swirl is an effective tool for manipulation of performance characteristics. Synthesizing the underlying trends, guidelines are developed for design selections and operational situations when inlet swirl can be effectively implemented as a means to extend the stable operating range and off-design efficiency.

Authors: Mark R. Anderson, Peter L. Klein, Concepts NREC

Abstract

Many factors come into play in a successful turbomachinery design, but peak aerodynamic performance, maximum operating range, stress levels, and manufacturability are generally the dominant concerns. Depending on the application, other considerations such as acoustics, size, weight, and environmental issues can also come into the picture. The most effective designers take a holistic approach, which tries to incorporate all of these concerns at the beginning of the design process. One of the challenges facing aerodynamicists is a basic understanding of how these downstream issues can be accommodated in the initial design process. Typically, an aerodynamic engineer has minimal knowledge of manufacturing methods such as 5-axis machining. This paper will provide a basic understanding of the subsequent manufacturing impact of various decisions typically made during the preliminary and detailed aerodynamic design process. The paper will focus on radial turbomachinery, but much of the information provided is common for axial turbomachinery produced by 5-axis Computer Numerical Controlled (CNC) machining.

In the initial phase of the design, basic layout decisions generally dictate the overarching manufacturing process. For components produced with 5-axis machining, these would typically be: flank milled or point milled, open or covered impeller, and for covered impellers, integral or welded shroud. The relative costs of these processes are considered in this work along with first-order estimates of their typical impact on aerodynamic performance and stress levels. Once a general layout (and accompanying manufacturing process) is determined, other geometric parameters of the components drive secondary costs in the manufacturing process. The secondary parameters include main blade count, the presence of splitter blades, the blade length, thickness, curvatures, leading and trailing edge shapes, and fillet radii. The use of Computational Fluid Dynamics (CFD) and numerical optimization has driven even more choices into blade shapes, such as splitter blades with different shapes and offsets from the main blade, bowed blades, irregular blade patterns, and non-axisymmetric hub shapes. In many cases, the aerodynamicists and CFD analysts have pushed geometry further than manufacturing capabilities are ready to accept. These secondary costs, and the aerodynamic compromises needed to reduce them, are also considered here. This paper attempts to lay out the basic principles of cost-effective manufacturing, and how these can be considered as early in the design process as possible. Specific examples are considered, and quantitative information is provided which can help guide the designer from the beginning and avoid expensive reworks resulting from downstream revisions. This paper will provide a framework for collaboration between aerodynamic engineers and the manufacturing teams assigned to produce the parts they have designed.

authors: Shuo Li, Eric M. Krivitsky, Xuwen Qiu

Abstract

High pressure ratio, radial-inflow turbines typically experience supersonic expansion in the nozzle section. Accurate estimation of the flow conditions and velocity triangle at the nozzle outlet is of critical importance in correctly predicting the overall turbine performance. The meanline modeling of such a nozzle requires special attention, due to the significantly altered flow field downstream of the throat.

In this study, the flow field of a supersonic expansion nozzle is investigated, using a three-dimensional (3D) computational fluid dynamics (CFD) simulation calibrated with test data. Three different CFD configurations are explored: the nozzle alone, the nozzle plus rotor coupled with a mixing plane, and the nozzle plus rotor coupled with the nonlinear harmonic (NLH) method. These configurations are compared to each other to gauge the effect of the rotor and stator interaction and the potential for error in establishing the velocity triangles. The exit vane angle, number of vanes, and expansion ratio across the nozzle are systematically varied to provide the data as the base for nozzle modeling. Finally, a meanline method is proposed to calculate the pressure loss and flow deviation at the nozzle outlet and is compared with CFD results.

Authors: David Japikse, Eric M. Krivitsky, Concepts NREC

Abstract

The design of centrifugal stages with an impeller and a downstream diffuser has generally been based on the assumption of axisymmetric flow at the impeller discharge. Flow entering the diffuser has customarily been assumed to also be axisymmetric, at least on a time-averaged basis, while laying out the diffuser vanes, establishing the preferred incidence, and sizing the throat areas. Recent stage studies have shown that the flow is often not fully axisymmetric, that not all diffuser passages perform the same way even at the design or best efficiency points, and that the actual time-averaged diffuser inlet flow conditions (distortion) may be changing from one operating point to another.

In this study, comprehensive time-averaged (steady) results from one test rig with a large array of impeller exit pressure taps is examined. Supporting results from five other test rigs are reviewed to broaden the picture of possible flow states. Hypotheses, suitable for future evaluation, are given to begin the explanation of the actual flow states and to guide further research. The current status of CFD to understand these phenomena is discussed.

Authors: Ryan K. Lundgreen, Ohio State University, Kerry N. Oliphant, Concepts NREC, LLC, and Daniel Maynes and Steven E. Gorrell, Brigham Young University

Abstract

High suction performance pumps are able to operate stably at very low inlet pressures with significant cavitation in the domain. Typically these pumps are required to have small inlet blade angles in order to limit the adverse effects of backflow at the inducer leading edge. The implementation of a stability control device (SCD) can allow inducers with larger inlet blade angles to suppress backflow and operate at the low flow coefficients required for high suction performance pumps. This allows inducers with larger inlet blade angles to be included in the design space for high suction performance pumps. Computational fluid dynamics simulations were performed to investigate this new design space by analyzing the performance of inducers with inlet blade angles of 7º, 9º and 11º. The flow coefficient for these cases was ∅=0.07. The inducers with inlet blade angles of and were also analyzed at the same flow coefficient with a modified SCD geometry that limits the mass flow rate through the SCD. It was observed that both the overall stability and suction performance are very sensitive to the mass flow rate through the SCD channel.

Authors: Mark R. Anderson, Daryl L. Bonhaus, Concepts NREC

Abstract

Through-flow solvers have historically played a very prominent role in the design and analysis of axial turbomachinery. While three-dimensional, Full Navier-Stokes (FNS) CFD is taking an increasing larger role, quasi-3D through-flow methods are still widely used. Automated optimization techniques that search over a wide design space, involving many possible variables, are particularly suitable for the computationally efficient through-flow solver.

Pressure-based methods derived from CFD solution techniques have gradually replaced older streamline curvature methods, due to their ability to capture flow across a wide range of Mach numbers, particularly the transonic and supersonic regimes. The through-flow approach allows for the solution of the three-dimensional problem with the computational efficiency of a two-dimensional solution. Since the losses are explicitly calculated through empirically based models, the need for detailed grid resolution to capture tiny flow entities (such as wakes and boundary layers) is also greatly reduced. The combined savings can result in computational costs as much as two orders of magnitude lower than full 3D CFD methods.

A state-of-the-art through-flow solver has several features that are crucial in the design process. One of these is the ability to run in both a design and an analysis mode. Also important, is the ability to generate solutions where critical components are solved using 3D FNS, while others are run using a through-flow method. Other desirable features in a through-flow solver are: an advanced equation of state, injection and extraction ability, the handling of arbitrary (non-axial) shapes, and a link to a capable geometry generation engine.

Through-flow solvers represent a unique mix of higher order numerical methods (increasingly CFD-based) coupled with empirically derived models (generally meanline based). The combination of these two methods in one solver creates a particularly challenging programming problem. This paper details the techniques required to effectively generate through-flow solutions. Special attention is given to an improved off-design loss model for compressors, as well as a transonic loss model needed for high-speed compressor and turbine flows. Validation with recognized test data along with corresponding 3D FNS CFD results are presented.

Author: Francis A. DiBella, P. E., Concepts NREC

Abstract

This presentation will discuss the results of the feasibility analysis of a Brayton cycle-based, supercritical CO₂ system that recovers waste heat from an MT30 gas turbine used in marine applications. The analysis also included the use of thermoelectric generator (TEG) devices that are one of several direct energy conversion methods known to be applicable to waste heat recovery. The analysis was conducted by Concepts NREC, in collaboration with the Maine Maritime Academy and their principal consultant, Thermoelectric Power Systems, LLC. The feasibility analysis was conducted under Navy SBIR Proposal Number N103-229-0533, entitled “Gas Turbine Engine Exhaust Waste Heat Recovery Shipboard Module Development”. The objective of the project was to improve the energy efficiency of the MT30 prime-mover power system for the Navy and other commercial vessels. The performance goal for the energy recovery system was to improve the fuel economy of the prime mover by 20% when significantly part-loaded.

Authors: Francis A. DiBella, Concepts NREC with Jang-Keun Cho, Jong-Seo Kwon, and Hui-gyun Jeong, POSCO Plantec

Abstract

POSCO Plantec is completing the commissioning tests for a 250 KWe Organic Rankine Cycle (ORC) system in Angang, South Korea. The heat source for this system is condensing steam from an industrial heat source. The ORC system uses an integrated, high-speed axial turbine, manufactured and packaged by Concepts NREC of White River Junction, Vermont, and a permanent magnet generator manufactured by Dresser-Rand Synchrony. The multi-stage axial turbine is directly coupled to the 20,000 rpm generator using magnetic bearings, which eliminates the cost and operating losses associated with shaft seals, bearing lubrication and a gearbox. The generator is evaporatively cooled by the same working fluid used in the ORC cycle, adding to the simplicity and efficiency of the unit. The output voltage of the generator is 440 Vac at 60 Hz, although the power conditioning electrical system can range from 380 to 480 Vac at 50 or 60 Hz. This technical paper reviews the thermodynamic cycle state points and mechanical and electrical design features of the ORC package, including the turbine generator unit and the Balance of Plant equipment. The paper concludes with a summary of the testing conducted to date.

Author: Francis A. DiBella, Concepts NREC

Abstract

The fuel efficiency of small gas turbines can be improved by 25% by using a modular Turbine Generator Unit (TGU) in an Organic Rankine Cycle (ORC) system to recover exhaust gas waste heat. This paper describes a high-speed, 20,000 rpm TGU that can operate completely sealed in the organic working fluid. The TGU is rated for 330 kWe, and consists of a close-coupled turbine and high-speed, permanent magnet generator, with magnetic bearings, in a hermetically sealed package. The TGU is an integral unit, without a shaft seal or lubrication required for its magnetic bearings. The turbine gas path employs a modular cartridge design, allowing for cost-effective optimization and modification for a wide range of operating conditions and working fluids. This modular design results in a compact, robust, and highly-efficient system, with reduced maintenance and operating costs when compared to traditional, open- architecture ORC systems. The instantaneous operating speed of the TGU can be changed to best match any part-load transients required—and the speed control helps to maintain the highest power recovery from the exhaust gas, while providing a constant generator frequency and voltage. A single 300 TGU rated for 330 kWe can recover the exhaust from a 1.25 to 1.75 MWe gas turbine, depending on exhaust temperature and flow rate. This paper describes the TGU’s mechanical and electrical design in more detail, and provides results from several case studies of gas turbine applications using the 300 ORC turbine generator.

Authors: Mark R. Anderson, Daryl L. Bonhaus, Concepts NREC

Abstract

A validation study of a variety of compressible flow turbomachinery cases is presented with comparisons to test data using OpenFOAM. OpenFOAM is open-source code consisting of various solvers and computational libraries focused on CFD. The study used a particular solver version with a density based approach that was derived from the “extended” branch of OpenFOAM. The example cases all consisted of single blade row designs at steady state and were run fully viscous (unless noted otherwise) with various turbulence models.

The results showed a definite superiority of the density based solver over other OpenFOAM solvers in a test suite of simplified cases as well as in more complex examples in actual turbomachinery designs. A typical Laval nozzle case and transonic bump case are presented demonstrating the basic ability of the solver to capture shocks and to handle transonic flow in general. Actual turbomachinery applications consisted of a two-dimensional transonic compressor cascade, a moderately supersonic two-dimensional turbine cascade, two radial compressor cases, and a radial inflow turbine.

The results showed the solver to be very capable of capturing pressure distributions and, most importantly, aerodynamic loss through the machines. The ability of the solver to accurately model performance in a wide range of different designs and across the entire performance map was demonstrated. Detailed comparisons to highly regarded test data are shown.

Special examination was made of the computational costs of the solver which were quite high with run times coming in at about 10 times longer than other commercial compressible flow solvers. Several acceleration methods are discussed which significantly improved run time performance.

Authors: Francis DiBella, Concepts NREC with Prof. Patrick Lorenz, Maine Maritime Academy

Abstract

An oscillating water-air column (OWC) is one of the most technically viable options for converting wave energy into useful electric power. The OWC system uses the wave energy to “push or pull” air through an air turbine, as illustrated simply in Fig. 1. The turbine is typically a bidirectional turbine, such as a Wells turbine, or an advanced Dennis-Auld turbine. The energy conversion from the water column to pneumatic power in an OWC system is affected by sinusoidal transients of volume flow rate and OWC chamber pressure through the turbine. The recovery of wave energy is also affected by the buoyancy of the floating OWC and its relative motion between the turbine and the wave front.
A new math model of an OWC system has been developed by Concepts NREC (CN) that is based on Lagrangian Dynamics and can provide more insight into what parameters most affect the recovery of energy from water waves. The sketch shown in Fig. 2 depicts the model of an OWC with distances in relation to the sea-bed floor taken to be the inertial reference. The variable X1 represents the massless distance of the water wave front from the inertial reference which imparts a non-conservative force Ft into the OWC chamber, thus exciting the entrapped air. The variable X2 represents the distance of the OWC mass from the inertial reference and includes a non-conservative force, Fb, that is associated with the buoyancy of the OWC mass caused by the motion of the OWC as it responds to the non-conservative force, Ft. The variable X3 represents the distance of the virtual joint connecting the spring constant and the damping coef. The power extraction by the turbine and the air cavity within the OWC are thus modeled using a damping coef., C [Lbf/(ft/s)], and a spring constant, Ks [Lbf/ft] derived as a function of the OWC geometry. The primary objective for the analysis is to determine the amount of work extracted from the OWC system via the damping system, with damping constant C as a function of the OWC size and the wave period and amplitude, and to discern how the recovery of the energy from the wave may be improved upon by designing the OWC features when the incident wave climate changes.
A secondary objective of the analysis was to compare the results from the Lagrangian Analysis with two other thermo-fluids models of an OWC system that were previously derived by Concepts NREC for a fixed OWC system. Ultimately, a solution derived from Lagrangian Dynamics would provide a greater insight into improving the capture of wave energy from a climate of waves with different frequency and amplitudes. This paper will provide the details of the solution of the Lagrangian Dynamics Solution and how they may be applied to an OWC design. The paper also provides a summary review of earlier computer models of an OWC that have been prepared by Concepts NREC.

Authors: Eric Kravitsky, Concepts NREC with Masashi Yamamoto

Abstract

Commercially available turbocharged internal combustion engines require robust system performance to maintain driveline power output capability. As in-service runtime increases, the accumulation of wear or deposits can adversely affect component performance levels. In a worst-case scenario, the component performance degradation leads to a vicious loop of declining system performance. Endurance testing of a heavy-duty diesel engine revealed performance deterioration over time. Oil deposits, resulting from oil mist associated with the closed crankcase ventilation loop, were observed on the turbocharger compressor and were tied to the deterioration. Cleaning of the compressor recovered initial performance for a short period of time. A different model of turbocharger, when substituted for the original, did not show the same degradation in output. This paper presents a study into the responsible mechanisms for performance deterioration and the compressor redesign that successfully avoided these issues. The results of numerical and physical investigations aimed at mitigating the system impact are discussed, starting with an overview of the observed engine and compressor performance decline, and an outline of potential areas of performance sensitivity to oil accumulation. Ultimately, the degradation of system performance was tied to the system response of deteriorating compressor characteristics. A compressor was redesigned and endurance tested to verify that the adjustment of engine-compressor matching and compressor characteristics results in improved robustness to oil-mist related performance degradation.

Authors: Xuwen Qiu, Carl F. Frederiksson, Nicholas C. Baines, Markus Backlund

Abstract

One of the more visible tasks when designing a turbocharger is the optimized design for a compressor and a turbine. The ultimate measure of a successful turbocharger design, however, is how well it works with a specific engine at various operating conditions. Final design decisions must be based on the engine-turbocharger system as a whole, rather than only on the individual component performance.

This paper describes the effort to develop an integrated design system which allows the user to design and optimize a turbocharger on a system level. With the basic engine parameters specified, along with simple models for other commonly used components, such as the Exhaust Gas Recirculation (EGR), wastegate, and intercooler, the program may be linked to two powerful meanline programs that can handle the fast iterations of the design and analysis of a compressor and a turbine. The output is either a new compressor or turbine that best matches the operation of the engine or the performance of an existing turbocharger at a specific engine operating condition.

A case study is presented where the program is applied in a real-life design situation to fit a new turbocharger for a large locomotive 18-cylinder diesel engine. The tool is extensively used in guiding the selection of the turbocharger and in the simulation of the overall system performance. The test data from the new design show close agreement with the simulation results, as well as an improvement over the original design.

Authors: Carl F. Fredriksson, Xuwen Qiu, Nick C. Baines, Markus Müller, Nils Brinkert, Cornel Gutmann

Abstract

Twin entry turbines are widely used in turbocharging as a means of using the exhaust pulse energy of multi-cylinder engines. For modern engines where high levels of EGR are required, an asymmetric twin-entry turbine has been shown to have considerable advantages. Such turbines require a more developed approach to analysis and design than usual. A meanline model for a radial inflow turbine with twin-entry scroll has been developed. Different total pressures and total temperatures may be specified at each entry. Each volute passage is solved separately from the inlet to the splitter location, where the static pressures of both passages are assumed to be the same. From the volute splitter to the rotor inlet, the two streams mix into one uniform flow following conservation laws of continuity, momentum and energy. Experiments have been conducted on a test stand with a radial turbine with an asymmetric twin-entry scroll, where the inlet conditions can be varied independently for each entry. The test results are compared with the model prediction. A good accuracy of prediction is achieved with a realistic set of modeling coefficients. In the future, insights gained from test data and CFD analysis will be used to develop further the volute mixing model and include explicit partial admission losses in the rotor.

Authors: Xuwen Qiu Eric M. Krivitzky, Concepts NREC and Peter Bollweg, Daimler

Abstract

The requirements for higher fuel economy and better diesel and gasoline engines demand a wider range in turbocharger compressor operation. Ported shroud compressor housing is one of the most commonly used techniques for compressor map width enhancement. Although the general mechanism of such a flow feature is well understood, there are no readily available design tools to guide the engineers at the preliminary design stage. Designers have had to rely on three-dimensional (3D) CFD tools to sort out many design variables, but these tools can be prohibitively expensive.
This paper explains how to develop a ported shroud compressor model on top of a commercial meanline compressor design program. The model considers some basic parameters, such as bleed location and geometry, which drive the recirculation or bypass flow through the bleed channel. The effects of the secondary flow on the compressor performance, such as pressure rise, efficiency, and stall and choke margins are also analyzed. The model prediction is validated with CFD simulation and test data.

Authors: Eric Krivitzky, Louis Larosiliere, Concepts NREC

Abstract

The prevalence of the turbocharged engines in passenger car applications has been increasing rapidly, particularly in the past decade or so. the tightening of emissions requirements and goals for engine downsizing has significantly increased the demand on turbocharger compressor stable operating ranges and efficiency levels, for both diesel and gasoline engines. the operability limits of single-stage turbocharger and centrifugal compressors are being severely tested, and customization of some key aerodynamic technologies is required to meet the challenge. This paper addresses some critical design challenges in wide-operability, single-stage turbocharger compressors for advanced automotive diesel engine applications. Starting with a few technological ground rules, a brief exploration of the design space for two notional advanced engine operating lines is presented. This serves to highlight basic correspondence between fundamental design choices and operability trends at the skeletal sizing level, while also considering the primary focus for detailed design customization of the 3D blading and flow path. A critical assessment of two key technology areas for addressing the aero design challenges is provided by way of a direct comparative example. Impeller passage diffusion has long been identified as an area for improving stage performance. It is shown that though a departure from the classic ruled-element blading approach used for most turbocharger centrifugal compressors, blade bowing and other techniques for controlling the impeller diffusion can improve the wide-range operability, albeit at the expense of manufacturing complexity. Similarly, the tailoring of a ported-shroud style casing treatment is presented as a necessary and integral design feature for tackling the performance goals of tomorrow. RANS CFD results are used to gain insight into the underlying flow field structure and help establish a framework for future enhancements.

It is essential to have a turbocharger compressor performance map that is sufficiently  broad so that the engine can function over its entire speed and throttle range without being constrained by compressor operability limits. Operability herein is qualitatively defined in terms of useable efficiency flow range. That is, mass flow range with relatively low levels of efficiency may not be useable for a specific application. The demarcation of a choke boundary need not be explicitly stated, since most turbocharger compressors seldom operate in choke. A more rational and quantitative measure of operability will be outlined later.

This paper addresses some key aerodynamic design challenges in wide-operability, single-stage turbocharger compressors for advanced automotive diesel engine applications. Starting with a few technological ground rules, a brief exploration for the design space for two notional advanced engine operating lines is presented. This serves to highlight basic correspondence between design choices and operability trends, while considering the primary technology areas which must be customized. A critical assessment of two key technology areas for successfully meeting the aero design challenges is provided by way of a direct comparative example. Finally, a framework for future enhancements is offered.

Author: Francis A. DiBella, P. E. , Concepts NREC

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Authors: David Japikse, Colin Osborne, Peter Klein, William Pope, Eric Krivitzky, Concepts NREC

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Author: Nicholas C. Baines, Ph.D., Concepts NREC

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Authors: Xuwen Qiu, David Japikse, Jinhui Zhao, Mark R. Anderson, Concepts NREC

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Authors: Nathan O. Packard, Brigham Young University, Dr. David Japikse, ConceptsNREC, R. Daniel Maynes, Brigham Young University, Steven E. Gorrell, Brigham Young University

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Authors: Carl Fredriksson, Nick Baines, Concepts NREC

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Authors: Xuwen Qiu, Mark Anderson, David Japikse, Concepts NREC

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