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A Multi-Level Design Space-Based Enhanced Cordier Diagram for Radial Turbocompressors

Year: 2025
Type: dissertation
Abstract: Radial turbocompressor are an important kind of machine for providing pressurized gas. They are used in many energy conversion processes using gases such as air, nitrogen, or hydrogen. Due to increasing demand for efficient and downsized machines, the development process of radial turbocompressors is increasingly complex. High duty requirements necessitate the simultaneous consideration of both structural and energetic design objectives. The development process of a radial turbocompressor is based on the definition of a machine characteristic by identifying a combination of impeller diameter and rotational velocity. Based on this selection, the principal dimensions and energetic properties of the radial turbocompressor are computed. A common way of defining the machine's characteristics is the employment of the Cordier diagram. It contains the Cordier line, which provides a connection between the non-dimensional numbers specific speed and specific diameter. For a given duty, both can be used to compute the rotational velocity for a given impeller diameter or vice versa. In this thesis, it is shown that the Cordier line is no longer suitable to define machine characteristics for modern radial turbocompressors. To obtain a valid machine design definition, the work provided by the impeller must match the required aerodynamic work. The iterative matching process will move the design in the Cordier diagram. It is further demonstrated that the Cordier diagram is not able to indicate which machine characteristic is more beneficial for the required application. As the machine design is usually fixed for design optimization workflows, a potentially better machine characteristic may not be identified. In this thesis, an enhanced Cordier diagram is proposed which is able to overcome the shortcomings of the original Cordier diagram. It is capable of re-enabling the intended use of the Cordier line by providing machine characteristics which result in feasible machine designs, and expanding its possibilities by providing information regarding the thermodynamic and structural mechanic properties of the future machine. The enhanced Cordier diagram is based on data of numerous radial turbocompressors which were analyzed using computer simulations. To provide information at the very beginning of the design process, the enhanced Cordier diagram must process only information that is available at this point. The operational characteristics of radial turbocompressors are — however — heavily influenced by the geometry design, which is defined at a later stage. To enable the filtering of these geometry design-based influences, and thus the identification of the machine design-based influenced, a multi-level design space was introduced. Using this design space, a multi-level design of experiments was conducted. In a first level, 50 machine designs were created with randomly sampled machine design parameters. For each of these machine designs, a geometry design of experiments was conducted by analyzing 50 geometry design. Thus, a total of 2500 compressor variants were considered. For each compressor variant, the speed line was computed using computational fluid dynamics (CFD) and the structural load was determined by finite element analysis (FEA). To associate this geometry design space-based information to the machine design space, optimizations were conducted for each machine design using surrogate modeling-based optimizations. For consideration of the structural load, this procedure is not feasible, as the structural mechanic result parameters like von Mises stress or deformation are predominantly influenced by the geometry design space. Instead, a property called Structural Load Intensity is proposed. It is based on the elastic strain energy observed in the overall impeller. These energy-based result parameters are predominantly influenced by the machine design and thus suitable for the enhanced Cordier diagram. The Structural Load Intensity is defined in a way that it correlates with both the von Mises stress and the deformation of all geometry design of the specific machine design. The final properties of the enhanced Cordier diagram are the achievable efficiency, the achievable speed line width, the structural load intensity, and other, associated parameters. All are described by surrogate models which employ machine design properties as input parameters. The enhanced Cordier diagram was created by implementing these surrogate models into different applications. The enhanced Cordier diagram may create duty-specific Cordier lines, which directly replicate the intention of the original Cordier line. Here, the Cordier lines are computed for a specific duty and a based on feasible machine designs, which are computed on-the-fly. Along the duty-specific Cordier lines, properties like the achievable efficiency may be approximated. Thus, the most beneficial location of the Cordier line can directly be identified. Another application of the enhanced Cordier diagram used optimization algorithms to directly probe the surrogate models to identify the most suitable design for a given set of constraints. Again, the machine design will directly be computed during the process. Additionally, the enhanced Cordier diagram is able to provide an approximated meridional contour and speed line, both associated with the achievable efficiency of the machine design. In conclusion, the enhanced Cordier diagram is a tool that is based on the operational characteristics of 2500 compressor variants in the form of carefully designed surrogate models. The predesigned applications allow for a more streamlined machine design process and provide viable information, which allows making knowledge-based decisions at the very being of the design process of radial turbocompressors.
DOI: 10.25643/dbt.66980
Language: English
Open Access status: green
OpenAlex ID: W7082983343
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