Aerodynamic Design of Turbo-Expander Stages for Radial and Radial-Axial Machines for Energy Transition Applications

Radial turbo-expanders play a key role in energy transition applications for their flexibility, cost, and efficiency. Radial in-flow turbines, in fact, are capable to operate in many different working conditions, from high expansion ratios, low temperatures or even critical conditions. Furthermore, its robustness allows the machine to tolerate even small percentages of liquid, which makes this configuration necessary in processes where the working fluid can liquefy at some point of the thermodynamic cycle. The focus of this Ph.D. is to develop an entire aerodynamic design procedure for radial in-flow turbines for energy transition applications. The goal of this procedure is to design an efficient machine, meeting manufacturability, structural, and dynamical constraints. All the design steps are addressed, from the selection phase, where the rotational speed and the size of the machine are defined, to detailed aerodynamic investigation, where the performance of the turbo-expander is assessed. A preliminary design tool is developed for the generation of the turbo-expander stage geometry. The tool is integrated with the REFPROP database for the gas properties extraction. It is accurate, fast and robust, and it produces all the inputs necessary to further analyses. A mean-line tool based on an in-depth research in literature is developed to generate the performance curve at an early stage of the design, allows to estimate the behavior of the entire cycle the machine is part of, so that an accurate idea of the cycle behavior can be drawn to optimize all the components of the process. The tool is integrated with the REFPROP database to consider real gas behavior, and it is validated against experimental data based on a test rig realized by Baker Hughes. An extensive DOE campaign is carried out to generate general design rules to be directly implemented in the preliminary design tool. In this way, a feasible high efficiency design can be generated in short time in order to reduce the time-to-market. Such rules are validated against three case studies provided by Baker Hughes. Finally, a design procedure for a radial-axial machine for geothermal applications is described. This unconventional configuration combines the flexibility of a radial machine with the better performance of the axial stages, representing a good tradeoff between cost and efficiency. The design is performed using either in-house and commercial tools. A case study provided by Baker Hughes is investigated. Real gas CFD analyses are needed in this case to accurately predict the machine behavior due to the high expansion ratios realized by the working fluid. For renewable energy applications it is important to keep high levels of efficiency also at off-design conditions, due to the variability of the primary source.