McCarthy, P., Niedbalski, N., McCarthy, K., Walters, E. et al., "A First Principles Based Approach for Dynamic Modeling of Turbomachinery," SAE Int. J. Aerosp. 9(1):45-61, 2016, doi:10.4271/2016-01-1995.
As the cost and complexity of modern aircraft systems increases, emphasis has been placed on model-based design as a means for reducing development cost and optimizing performance. To facilitate this, an appropriate modeling environment is required that allows developers to rapidly explore a wider design space than can cost effectively be considered through hardware construction and testing. This wide design space can then yield solutions that are far more energy efficient than previous generation designs. In addition, non-intuitive cross-coupled subsystem behavior can also be explored to ensure integrated system stability prior to hardware fabrication and testing. In recent years, optimization of control strategies between coupled subsystems has necessitated the understanding of the integrated system dynamics. To this end, a dynamic vapor cycle modeling toolset known as the AFRL Transient Thermal Management and Optimization (ATTMO) toolset was developed to address two-phase flow systems. This toolset has been further expanded to include components typical of air cycle topologies. Current air-cycle modeling tools rely heavily on user supplied performance maps to predict turbomachinery output. This approach has a variety of limitations. First, at a conceptual design level, modifications to a specific component design requires the generation of a new map; thereby, limiting one’s ability to rapidly evaluate and optimize across a wide design space. Second, interpolation routines in map-based approaches fail to yield precise solutions near critical operating points, such as compressor stall lines, due to unknown data outside of the stall point. As a result, development of control strategies around the stall margin of a machine is difficult due to the model induced instabilities or incorrect predictions from interpolation around those operating points. Lastly, modeling startup and shutdown requires torque prediction near or at zero speed, which for map-based approaches is ill-defined. To address these limitations, first principles models of traditional turbomachinery components have been developed and will be discussed in this paper. Conservation of mass, energy, and momentum are applied to capture appropriate volume dynamics relevant to plant and controls engineers. Enthalpy based calculations derived from machine geometry and fluid flow conditions allow for design optimization of machine parameters while still maintaining accurate performance predictions. These approaches have been implemented in the open-source Simulink toolset, ATTMO. Based on user defined system architecture and associated design parameters, a time-domain simulation for an integrated system analyses can be formed.