Design and Analysis of Centrifugal Compressors for Air Systems

Abstract

The Air Generation department at Airbus Operations GmbH has the aim to create optimization tools for the performance and sizing of air generation systems, such as the environmental control system. These type of tools will support the development of novel systems for next-generation aircraft, whose designs are driven by environmental and energy efficiency requirements. In support of this goal, this thesis focuses on the centrifugal compressor, one of the key components in aircraft air systems. The work concentrates on the development of a preliminary design and performance prediction tool that is well suited to the broad design space and future integration into a system analysis. The objective is to enable fast and robust design and performance analysis, with support for vaneless, vaned and variable vaned diffuser configurations. The method is designed to be broadly applicable, minimizing the reliance on empirical correlations commonly found in existing models, and accessible to users without extensive experience with turbomachinery design.

The methodology is based on mean-line analysis, with component models implemented in Python to ensure computational efficiency and user accessibility. While the impeller and volute are modeled using established formulations, the diffuser model is developed with a novel structure for the vaned configuration that solves the flow conditions in the channel passage using a strong interaction model for the viscid and inviscid zones. The geometric design of the vaned diffuser uses a throat area matching approach relative to the impeller in order to maximize performance without sacrificing operating range. To ensure the accuracy of the predicted impeller exit conditions, the mixing losses are computed using a two-zone model to estimate the wake area fraction.

The model is validated against six different compressor designs from open literature, four of which include both vaneless and vaned diffuser configurations, resulting in a total of ten validation cases. The validation is based on comparison of the experimental data from open literature and the model’s prediction of the performance maps for total to total pressure ratio and total to total efficiency. The model shows overall good agreement with the experimental data, with a consistent marginal underprediction of the total to total efficiency of about 1-3%.

The design and analysis tool (DACT-A) is sufficiently accurate for its purpose and has a broad applicability. However, the model is expected to have a margin of error and higher fidelity flow simulations are necessary if a higher degree of accuracy is required for the flow analysis. The validation shows good agreement for the computed losses and operating range of the vaned diffuser, but the question remains whether the flow characteristics accurately represent reality and further research and validation in this area is recommended. Furthermore, it was found that adequate flow analysis in the diffuser requires highly accurate prediction of impeller exit conditions, leading to the conclusion that, for vaned diffuser configurations, the impeller model must also possess sufficient fidelity to ensure good results.