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Modeling compression systems using parallel compressor theory has been used for the analysis of compression system operability since the 1960s. Parallel compressor models have been traditionally designed and used for the analysis of circumferential distortion effects as a means to evaluate the impact of various inlet flow field disturbances on compressor operation. This paper (the first of two) provides a review of the parallel compressor concept and discusses extensions to the original theory. These extensions include the incorporation of dynamic response, application to complex distortions, and the application to inlet swirl. Understanding these effects and the application of parallel compressor theory extensions is required to produce analytical models and computer simulations that can be used to enhance the development testing and the understanding of the response of gas turbine compression systems. Once a computer simulation has been constructed for a particular test article, it can be exercised and results compared against test results where distortion-generator devices (such as distortion screens) have been used, generally with favorable accuracy. The usefulness of the extended parallel compressor model is derived from its ease of use, simplicity, and ability for quick turn-around of results. It is often more desirable to have an analysis capability that is easy and quick to use than to have one that is extremely accurate, especially when understanding basic physics is of primary concern during a test operation. Extreme accuracy may require large amounts of computer resources and take days or weeks to compute a single performance point. While this may be acceptable for design, the limitations of high-fidelity simulations make them impractical to use due to the time constraints imposed by the pace of testing. Applying a timely analysis capability, using a parallel compressor simulation can provide a new physical understanding of the effects of complex distortion during the testing process when comparing the analytical and test results. This concept is presented in two companion papers: the first paper, Part 1, concentrates on the parallel compressor concepts, theory and limitations of the methodology while the second paper, Part 2 [1] presents applications of the approximate methods developed and compares results with experimental data.

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