DEVELOPMENT OF A TURBOFAN PERFORMANCE MODEL USING A GENERIC SIMULATION TOOL

This paper describes the modeling of a turbofan engine using a general purpose object-oriented simulation tool. EcosimProTM is a tool that can be adapted to different fields through the creation of reusable modeling component libraries representing parts or equipment of a physical system. These libraries are developed using an object-oriented language. A flexible graphical user interface allows for quick implementation of new models and rapid analysis of results. The turbofan model developed is used for both steady state and transient performance simulation. Results are presented and compared to those produced by an industry-accepted model. The possibility to include the adaptivity feature of the model to measured values is also demonstrated. Finally, use of the model to study the frequency response of the engine is presented. INTRODUCTION When a new gas turbine engine is designed, an aerothermodynamic model is created to study its performance at design and off-design conditions. Predictions from this model are then used as input to other analysis codes, e.g. finite element thermo-mechanical stress analysis (FEA), computational fluid dynamics (CFD), etc. The results from these codes often lead to design modifications and improved or new component performance data. These changes must be fed back to the performance model and this procedure is repeated until the design criteria are met. This requires a flexible, modular environment that allows the user to make rapidly the necessary changes (i.e. incorporate a new compressor design) without affecting the rest of the model. Such models may also be used during the operational stage of the engine for monitoring and diagnostic purposes. Thus, a user-friendly interface is essential. Traditionally, these engine cycle models are written in FORTRAN, the preferred computer language for scientific programming, because of its robustness and speed of execution. However, the procedural nature of FORTRAN and its programming environment make it difficult to extend, maintain and re-use large codes. In order to support modular design, code re-use, extensibility and user-friendly interface one has to use a modern object-oriented computer language like C++, Java, Borland Delphi, etc. [1]. Currently, there are only a few commercially available gas turbine specific simulation tools based on object-oriented languages. One such tool is GasTurb [2]. Borland Delphi is used to create a user friendly interface through which the user can select a predefined gas turbine configuration. All the mathematical complexity is hidden from the user making this program suitable even for the less experienced user. Another example is NLR’s Gas Turbine Simulation Program, GSP [3]. It is also implemented in Borland Delphi but (compared to GasTurb) makes use of all the object-oriented features offered by it. The user can create different gas turbine configurations by arranging component icons (representing gas turbine engine components) in a model window using a graphical drag & drop interface. When new (non-standard) components are required for specific GSP users, these are supplied in the form of a custom component library; the creation of new components is the job of the developer and not of the user. Onyx is a Java-based object-oriented simulation framework developed by researchers at the University of Toledo [4, 5]. The user can create or modify components representing physical components in the gas turbine engine domain and then combine them either programmatically or visually (like in GSP) to form the required engine model. Higher order analysis methods (like CFD and FEA) can also be integrated within a component object. Support for distributed computing is provided through several common software distribution mechanisms (e.g. CORBA). Onyx was developed to investigate advanced software design techniques as part of NASA’s Numerical Propulsion System Simulation (NPSS) project [6]. The aim of NPSS is to reduce time and cost in developing new propulsion systems while increasing confidence and reducing risk in achieving a final design. This is possible due to its open and extensible architecture that fully exploits the capabilities of object-oriented programming. It consists of three layers: the interface layer (using a C++ like language) within which a command and a visual interface exist, the object layer containing the fundamental engineering specifics for propulsion systems and the appropriate support objects needed by propulsion systems such as access to geometry and legacy FORTRAN codes and lastly the computing layer for deploying the simulations. Currently, NPSS is only available to the partners involved in its development. Another option for developing an engine cycle model is to use a generic simulation tool like MATLAB-Simulink [7]. This is especially popular for some types of performance analysis