Design for manufacturing and characterization of small-scale turbomachinery impellers

Organic Rankine cycle (ORC) power systems and heat pumps (HP) are suitable technologies for valorizing waste heat sources and producing electricity or useful heat. Expanders and compressors are key components of the performance and cost in these thermodynamic cycles. In small-scale power applications, the use of oil-free turbomachinery as expansion or compression technology is the subject of current research, as oil-free operation can further enhance the efficiency of next-generation cycles. However, a major barrier to the widespread use of turbomachinery in low-temperature systems is the lack of cost-effective solutions. Manufacturing-oriented design is an important aspect in the development of low-cost turbomachinery components.

This Ph.D. thesis investigates the challenges in the design for manufacturability and geometrical characterization of small-scale radial impellers for turbomachinery applications with a two-fold objective. The first goal is to develop suitable methods to integrate the manufacturability aspect in the preliminary design stage of radial impellers. The second is to establish appropriate methods for the geometrical characterization of these components. The developed methods enable to quantify the effect of time reduction measures on the manufacturing errors in the manufacturing process chain.

First, a reduced-order model is developed to predict the machining time of radial impellers. The model is verified against the results of two case studies, including one with a turbine impeller running on an organic working fluid. The results suggest that the proposed methodology allows the accurate prediction of machining tool paths lengths and manufacturing
time derived from the feature-based tool selection and associated machining parameters. A feature making it a suitable tool for the integration into the preliminary design phase of turbomachinery for machinability. Similarly to commercial CAM software, the developed tool does not guarantee that the impeller can be produced with the required geometrical accuracy. The process chain of a radial impeller for a heat pump application is designed and machined based on the predicted tool selection and machining parameters to demonstrate is feasibility.

Second, a suitable metrological approach is established to characterize these complex components to verify the manufacturing quality of the small-scale impellers. Volume scans using X-ray computed tomography offer high enough resolution to draw conclusions due to the compact dimensions of the inspected workpiece. However, these measurements are not yet traceable. A traceable method, in the present case tactile measurements via coordinate measuring machine (CMM), is exploited to ensure traceability for the inspection using X-ray computed tomography (CT). The traceability was guaranteed by applying the experimental substitution method following the ISO 15530-3 guidelines. The applicability
of this method is demonstrated for a case study of a radial compressor impeller in a heat pump set-up. The results indicate that the accessibility limitations of tactile measurements resulting from the complex geometry of the workpiece can be overcome by the volume inspection method, and thus the two inspection methods complement each other. The proposed methodology is crucial for facilitating further research on parts where the manufacturing performance is optimized to shorten machining times. It allows a characterization of the workpiece with lower uncertainty than the expected machining error.