59444 - A Reduced-Order Model for the Preliminary Design of Small-Scale Radial Inflow Turbines 

The worldwide interest in reducing greenhouse gas emissions along with increasingly stringent environmental legislations, are forcing the scientific community to develop more efficient renewable energy power plants. For this purpose, economically viable thermal power conversion systems represent attractive solutions across the range of vehicle applications. A promising technology can be identified in bottoming mini-Organic Rankine Cycle (ORC) power plants, intended to recover and convert into mechanical power the thermal power discarded as hot fluid streams from internal combustion engines.  In the framework of transport applications, the design methodology of small scale ORCs is affected by several technical challenges, since the system must be characterized by small size, small weight, and high efficiency. For this reason, turbines employment instead of volumetric expanders is nowadays being reconsidered. In particular, for small-power applications (3-50 kW), single-stage radial inflow turbines (RIT) are often considered, due to their compactness, high power density, and capability to operate at high flow coefficients. The design of this component depends on a large number of parameters and constraints, and it is often tailored to a particular thermodynamic cycle of reference.

This paper proposes a comprehensive reduced-order model for the preliminary design of radial inflow turbines operating with organic fluids. A steady-state 1D mean-line model of RITs is developed, which recursively exploits conservation equations together with geometrical relations and loss correlations tailored to RITs to carry out the turbine design. The mean-line model includes two different procedures to design the rotor, depending on the set of design variables chosen, and a strategy to handle chocking conditions, which allows to consider both convergent and convergent-divergent nozzle geometries. This feature makes the model independent on the thermodynamic cycle considered, which constraints the total inlet conditions and the required mass flow rate and pressure ratio. The methodology proposed can be exploited also in an indirect way, as an analysis tool capable to provide the RIT performance maps once a specific geometry is defined. The mean-line model is validated exploiting currently available experimental and numerical data in literature, which are based on several turbine geometries operating with various fluids. The results of the validation procedure demonstrate the effectiveness and robustness of the methodology proposed, which is intended to be used as a benchmark for the future design of small-scale RITs.