APPLICATION OF DIGITAL TWIN CONCEPT FOR SUPERCRITICAL CO2 OFF-DESIGN PERFORMANCE AND OPERATION ANALYSES

This is an excerpt from a technical paper, presented at the ASME Turbo Expo 2020 online conference and written by Leonid Moroz, Maksym Burlaka, Tishun Zhang, and Olga Altukhova. Follow the link at the end of the post to read the full study! 

Introduction

The attempts to simulate transient and steady-state sCO2 cycles off-design performance were performed by numerous authors [1], [2], [3], [4], and [5]. Some of them studied the dynamic behavior of regulators, some studied different control strategies or off-design behavior in different scenarios, which definitely has certain utility in the development of the reliable technology of sCO2 cycle simulation. Nevertheless, they used rather simplified models of components, especially turbomachinery and heat exchangers, which are of crucial importance to correctly simulate cycle performance.

The authors of this paper attempted to apply the digital twin concept to a simulation of off-design and part-load modes of the sCO2 bottoming cycle considering real machine characteristics and performance, which nobody tried to apply in this area.

On IGTC Japan 2015, SoftInWay Inc. has published a paper “Evaluation of Gas Turbine Exhaust Heat Recovery Utilizing Composite Supercritical CO2 Cycle”. The paper considered combinations of different bottoming sCO2 cycles for a specific middle power gas turbine. It mainly studied the advantages of different types of sCO2 cycles to increase the power production utilizing GTU waste heat.

The present paper is a further study based on that so the Cycle 2 [6] from that previous paper was selected as the sCO2 bottoming PGU layout in the present paper for subsequent analysis. The cycle is a combination of recompression cycle and simple cycle which offers 16.13 MW as output. GE LM6000-PH DLE gas turbine, was used as the heat source for bottoming PGU. According to GE official brochure [7], the GE LM6000 offers 40 MW to over 50 MW with up to 42% efficiency and 99% fleet reliability in a flexible, compact package design for utility, industrial and oil and gas applications. GE LM6000-PH DLE provides 53.26 MW output with exhaust temperature at 471 ℃ and exhaust flow at 138.8 kg/s. (This information came from GE products specification from 2015. It appears that GE continuously modifying the parameters of its turbines along with the naming of different modifications. Therefore, today’s parameters and configuration names might be slightly different comparing to 2015) Exhaust gas pressure was assumed to be 0.15 MPa. These parameters were taken to analyze the bottoming PGU and are presented below in TABLE 1.

SELECTED SET OF GE LM6000-PH DLE PARAMETERS
TABLE 1: SELECTED SET OF GE LM6000-PH DLE PARAMETERS

The digital twin (DT) concept is the developing technology that allows simulation of object behavior during its life cycle or in specified time due to changing ambient conditions, for example. The DT is applicable for performance tuning, digital machine building, healthcare, smart cities, etc [8] that allows decreasing the time and costs of development and optimize the object on the developing stage. GE has raised DT concepts for power plants to continually improves its ability to model and track the state of the plants [9].

In the context of this paper, DT is a simulation system comprised of physicist-based models organized in a special algorithmic structure that allows simulating the behavior of sCO2 PGU under alternating ambient conditions and grid demands.

The DT in this study was created utilizing AxSTREAM® Platform, which includes multiple software tools. The following software tools were utilized in this study: AxCYCLE™ was used to perform cycle thermodynamic calculation; solution generator in AxSTREAM® helped with finding possible machine geometry with given boundary conditions when performing preliminary design for compressors and turbines at design point; parameters and performance of turbomachinery including mass flow rate, pressure, power, efficiencies, etc. were calculated by Meanline/Streamline solver in AxSTREAM® for design and off-design conditions; AxSTREAM NET™ is a 1D system modeling solver and it was introduced here to simulate performance of heat exchangers (HEX) and pressure drop in the pipes involved in the cycle; AxSTREAM ION™ was used to integrate all modules and tools together in one simulation system. Read More

Modern Approach to Liquid Rocket Engine Development for Microsatellite Launchers

[:en]Microsatellites have been carried to space as secondary payloads aboard larger launchers for many years. However, this secondary payload method does not offer the specificity required for modern day demands of increasingly sophisticated small satellites which have unique orbital and launch-time requirements. Furthermore, to remain competitive the launch cost must be as low as $7000/kg. The question of paramount importance today is how to design both the liquid rocket engine turbopump and the entire engine to reduce the duration and cost of development.

The system design approach applied to rocket engine design is one of the potential ways for development duration reduction. The development of the design system which reduces the duration of development along with performance optimization is described herein.

The engineering system for preliminary engine design needs to integrate a variety of tools for design/simulation of each specific component or subsystem of the turbopump including thermodynamic simulation of the engine in a single iterative process.

The process flowchart, developed by SoftInWay, Inc., integrates all design and analysis processes and is presented in the picture below.

Execution Process Flow Chart
Execution Process Flow Chart

The preliminary layout of the turbopump was automatically generated in CAD tool (Block 11). The developed sketch was utilized in the algorithm for mass/inertia parameters determination, secondary flow system dimensions generations, and for the visualization of the turbopump configuration. The layout was automatically refined at every iteration. Read More

[:en]The Simultaneous Simplicity and Complexity of Supersonic Turbines and their Modern Application[:]

[:en]Supersonic axial turbines have attracted interest in the industry since the 1950s due to the high power they  provide, allowing a reduction in the number of low-pressure stages, and thus leading to lighter turbines as well as lower manufacturing and operational costs. Due to these valuable features, supersonic axial turbines are currently widely used in different power generation and mechanical drive fields such as rocket engine turbopumps [1, 2, 3, 4], control stages in high pressure multi-stage steam turbines, standalone single stage and 2-row velocity compound steam turbines [5, 6], ORC turbo-generator including geothermal binary power stations [7, 8, 9, 10], turbochargers for large diesel engines [11] and other applications. Therefore it is not forgotten, but instead a very important field in turbomachinery when highest specific power, compactness, low weight, low cost and ease of maintenance are dominant requirements. Especially nowadays, when development of small capacity reusable low-cost rocket launchers, compact and powerful waste heat recovery (WHR) units in the automotive industry, distributed power generation, and other fields are in extreme demand.

Meanline Results of Supersonic Turbine in AxSTREAM
Meanline Results of Supersonic Turbine in AxSTREAM

Typically, supersonic turbine consists of supersonic nozzles with a subsonic inlet and one or two rows of rotating blades. The turbine usually has partial arc admission. The total flow could go through either a single partial arc or several ones. The latter is typical for a steam turbine control stage or standalone applications. The inlet manifold or nozzles chests, as well as exhaust duct, are critical parts of the turbine as well. Due to the very frequent application of partial admission, it is not possible to implement any significant reaction degree. Thus, this kind of turbine is almost always an impulse type. However, some reaction degree could still be applied to full admission turbines. The influence of  the rotor blades profile designed for high reaction degree on rotor-stator supersonic interaction and turbine performance is not well studied at the moment.

Read More

What parallels exist between traditional Gas Turbines with SCO2 turbine of the future?

[:en]At the beginning of my studying of the peculiarities of supercritical CO2 (S-CO2) cycle I was wondering: why do scientists involved in this area state that highest temperature limit for the cycle is about 650-700 ˚C. In turn, the inlet temperature in the first stages of gas turbines handles the temperatures about 900 ˚C without cooling at similar pressure levels as for supercritical CO2 Turbines. As a result the following question rose in my mind – why the temperature magnitudes of 900 ˚C are not achievable in S-CO2 turbines?

As a next step, some investigations were performed with the aim to reveal the essence of such a temperature limit. Eventually the result was quite obvious but rather interesting. The density of S-CO2 is significantly higher than the density of combustion products at the same pressure and temperature magnitudes. This fact means that stresses at static vanes and rotating blades are significantly higher than in gas turbines vanes and blades at the same conditions. Therefore the maximum allowable temperature for S-CO2 turbine will be respectively less with the same high temperature material. However, you might say that there is another way to solve the problem with stresses, namely, increasing the chords of blades, leading edge thickness, trailing edge thickness, fillets etc. This approach would lead to such blades shape and turbine cascade configuration that their aerodynamic quality becomes very low so the gain in efficiency at cycle level will be leveled.

Interested in learning more about our research, and how using the AxSTREAM turbomachinery platform, we were able to study these phenomena?

Contact us for a chat! [:cn]At the beginning of my studying of the peculiarities of supercritical CO2 (S-CO2) cycle I was wondering: why do scientists involved in this area state that highest temperature limit for the cycle is about 650-700 ˚C. In turn, the inlet temperature in the first stages of gas turbines handles the temperatures about 900 ˚C without cooling at similar pressure levels as for supercritical CO2 Turbines. As a result the following question rose in my mind – why the temperature magnitudes of 900 ˚C are not achievable in S-CO2 turbines?

As a next step, some investigations were performed with the aim to reveal the essence of such a temperature limit. Eventually the result was quite obvious but rather interesting. The density of S-CO2 is significantly higher than the density of combustion products at the same pressure and temperature magnitudes. This fact means that stresses at static vanes and rotating blades are significantly higher than in gas turbines vanes and blades at the same conditions. Therefore the maximum allowable temperature for S-CO2 turbine will be respectively less with the same high temperature material. However, you might say that there is another way to solve the problem with stresses, namely, increasing the chords of blades, leading edge thickness, trailing edge thickness, fillets etc. This approach would lead to such blades shape and turbine cascade configuration that their aerodynamic quality becomes very low so the gain in efficiency at cycle level will be leveled.

Interested in learning more about our research, and how using the AxSTREAM turbomachinery platform, we were able to study these phenomena?

Contact us for a chat! [:]