Advancing Jet Engine Design
An SwRI-led NPSS consortium helps design next-generation propulsion systems.
David L. Ransom, P.E., is manager of the Propulsion and Energy Machinery Section in the Fluids and Machinery Engineering Department of SwRI’s Mechanical Engineering Division. At SwRI, Ransom specializes in the design, analysis, and testing of machinery systems for the aerospace and energy industries.
The NPSS consortium membership comprises leading engine manufacturers and airframe companies that design and build today’s advanced aircraft.
Engineers use the NPSS software as a design tool for developing jet engines.
NPSS is file-based and typically runs through the system command window (above and below).
Air travel has been a safe and largely accepted part of our worldwide transportation system for several decades. For the most part, we take it for granted and probably give little consideration to the technologies involved in the various systems such as the airframe, the engines, or the cabin air supply and conditioning machinery. We may pay even less attention to the increased level of difficulty in designing high-performance military aircraft. Our main concern is usually regarding our on-time arrival, the availability of Wi-Fi on the flight, and how we can get that aisle seat so we can move about the cabin freely when we want.
Fortunately, engineers pay great attention to all of the systems required to achieve reliable air transportation and national air defense systems. Designing and building these complex systems requires powerful software tools in our modern information age. We need software that allows for rapid evaluation of various design options early in the design of a new aircraft. There are important decisions to be made regarding the power of the engine, the configuration of the engine, and how the engine will be integrated into the airframe. We also need software that improves communication of key performance parameters between the manufacturers of engines and airframes. Engineers in Southwest Research Institute's Mechanical Engineering Division are helping to advance the state of the art through a lead role in the Numerical Propulsion Systems Simulation (NPSS®) consortium and its engine design software package.
What is NPSS?
In 1991, engineers at NASA's Glenn Research Center (GRC) created NPSS to take advantage of that era's emerging parallel computing technologies and scientific computing advances. The idea was to develop an engine simulation that could serve as a virtual test stand for detailed development of specific jet engine components, such as the compressor. The concept is referred to as "zooming," in which a detailed, three-dimensional fluid dynamics model is coupled to an engine performance model to create a "computational test cell." Today, engineers use the NPSS software to develop engine performance models and integrate them into vehicle system models. Primary application areas include thermodynamic system analyses for jet engines and rockets, but NPSS can also be used for other energy systems including power-generating gas and steam turbines. NPSS also supports industry standardization for model sharing and integration. The software has other industrial fluid/thermal applications such as multi-phase heat transfer systems, refrigeration cycles, variations of common power cycles, and vehicle emission analyses.
By the time NASA's development role ended in 2007 with Version 1.6.5, the NPSS software had many major industrial aerospace companies as devoted users. Engine and airframe manufacturers alike found success in sharing engine models and studying combined engine-vehicle performance. However, the industrial user base wanted further development with more features and improved functionality. NASA GRC transferred control of code maintenance and development to an industrial consortium organized and operated by the NPSS members and managed by an organization local to NASA GRC. For the first few years, NPSS members performed much of the administrative work, but the governing board later decided to look for a technical organization to manage the consortium. SwRI officially assumed this role May 1, 2013.
SwRI and NPSS
As consortium manager, SwRI is responsible for planning the future of NPSS, addressing users' current needs, and managing the multiple projects under development. The governing board of the consortium provides direction, and SwRI proposes project plans to meet them. Some of those projects include developing a new capability for handling multiple fluid types, such as jet fuels and refrigerants, improving the capability of the solver for difficult transient, or fast-changing, problems, developing a graphical user interface (GUI), and developing reliable interfaces between NPSS and other engineering design tools.
How NPSS works
To develop a model in the NPSS environment, an engineer first specifies the type and order of engine components (referred to as "elements") and provides the technical data that describes their individual performance. NPSS has a library of thermodynamics databases and standard elements used in engine cycle models. Once the models are defined through input files, simulations are launched using a system-command window. Most users select a programming text editor that supports language-specific highlighting, coloring, and auto-complete features.
Basic elements of the jet engine are specified in a user-defined input file based on the generic elements already included in NPSS, such as "Compressor," "FuelStart," and "Burner." The engineer selects the appropriate elements and then assigns to each the known physical parameters needed to solve the problem. The elements are then connected through further software commands made by the engineer.
In addition to its standard library of elements, NPSS allows engineers to define new elements or modify or customize the NPSS-provided elements. This powerful feature provides significant flexibility in the capabilities of the elements of an NPSS model.
NPSS has a library of thermodynamic databases and standard elements used in engine cycle models.
Problem setup and solution
Once a model is developed, the engineer sets up a problem and defines the solution goals and constraints. Although NPSS is organized to use various input files, there are no formal rules regarding their number, type, or organization. In the simplest models, such as a small gas turbine with a single compressor and turbine stage used for auxiliary power, for example, the engineer can define an entire simulation in a single file. For larger, more complicated systems, such as a multi-stage compressor or turbine system, the engineer can use a variety of file types and link them to form a simulation.
An important aspect of defining a problem is how one specifies the settings for solving it. NPSS is unique in that it enables the engineer to use the same model to solve a multitude of problems. NPSS has a built-in solver that drives the model to a valid solution. The solver receives a run command and then iteratively adjusts the values of the specified independent variable in order to satisfy the dependent conditions.
Suppose an engineer wants to know the fuel flow rate required to achieve a specific burner temperature. Fuel flow is identified as an independent variable and some user-defined limits are placed on the solver. Burner temperature, meanwhile, is a dependent variable, with a model parameter for monitoring and targeting temperature. The solver determines the fuel flow rate associated with each temperature value. Solving the problem requires only a simple command for NPSS to complete the solution.
Many more options are available for controlling a simulation, and the solver can handle many independent and dependent pairs in a single solution. In fact, multiple solvers can be used to improve overall solution performance for models with subsystem assemblies. Solutions can be derived in three different modes. The design mode determines the performance characteristics needed to meet the design objective; the off-design mode determines how the selected design will perform away from the design point. Finally, the transient mode evaluates the system's response to time-dependent conditions, such as changing power levels. Once the solution sequence is complete, the output data can be sent straight to the screen or to an output file.
Data can be imported into other plotting tools to generate graphs as needed, and an engineer can easily update the plots with new data. Other data output formats can be defined, such as an overall performance summary, or a series of messages to monitor progress.
NPSS serves as a virtual test stand for developing specific jet engine components, such as the combustor shown here.
Current development efforts
SwRI is continuing to develop the NPSS core program, the graphical user interface, and also new elements, models, and interfaces. The core work will add more thermodynamic capabilities as well as additional features to the transient solver. GUI development is scheduled for completion in May 2016. Work on the models, elements, and interfaces is focused on upgrading models for new users and streamlining the process for interfacing NPSS to other engineering tools.
Outside the consortium, SwRI is leveraging NPSS to explore other technology areas such as a power-generation application that uses NPSS to study component performance and degradation in power-generating gas turbines. Energy companies are interested in developing NPSS models of their commercial gas turbine engine systems to support their condition-based maintenance programs. SwRI engineers are working with industry to determine the accuracy expected of such models and how they can be used to convert some measured gas turbine performance parameters (pressure, temperature, power, and fuel flow) into decision-making material regarding the health of the individual subsystems such as the compressor, the combustor, or the turbine.
The team is also investigating using NPSS in real-time simulations, such as hardware-in-the-loop simulations, in which an NPSS model simulates the response of the engine system to conditions experienced in a test environment. This is important for developing engine control systems because the transient response of the engine can influence the ability of the control system to perform properly. In such an experimental setup, an actual engine control system, complete with processor, data systems, and actuators would be coupled to a sensor and actuator system that represents the engine response. The NPSS model would be used to determine how the actual engine would respond to the sensed inputs, thus driving the engine response actuators. Testing the control system in this manner is much more affordable, flexible, and lower risk when compared to testing with an actual engine.
NPSS in the future
NPSS will continue to be relevant to the aerospace community for the foreseeable future, as evidenced by the continued financial support from existing consortium members and the recent addition of a new member. NPSS- demonstrated capabilities for studying coupled system performance will continue to be crucial as the aerospace industry trends toward more coupled power-propulsion and thermal systems.
Because NPSS is essentially a fluid-thermal modeling environment, it can model many variations of power cycles, including supercritical CO2-based cycles, which is of major interest to the U.S. Department of Energy.
The NPSS consortium continues to develop new features based on member feedback, and SwRI's management of the consortium provides full-time professional project management, product development, and user support.
Questions about this article? Contact Ransom at (210) 522-5281 or firstname.lastname@example.org.