Design and CFD Analysis of a Radial-Inflow Turbine for Small Scale ORC Applications

Abstract

In recent years, Organic Rankine Cycle (ORC) technology has received growing interests, thanks to its high flexibility and to the capability to exploit energy sources at temperature levels difficult to be approached with conventional power cycles. These features allow exploiting renewable and renewable-equivalent energy sources, by either improving the energy conversion efficiency of existing plants or using waste heat from industrial process. As far as the expander is concerned, a high potential solution is represented by turbo-expanders, which allow reduction of plant clutter and complexity, so enhancing the potential impact on the diffusion of small power ORC-based plants. The present work concerns the design of a RadialInflow Turbine for a bottoming Organic Rankine Cycle in the tens of kW scale. Design boundary conditions are retrieved by a zero-dimensional model of a solar-assisted micro gas turbine in cogenerating mode. The design process is started by means of an in-house mean-line design code accounting for real gas properties. The code is used to carry out parametric analyses to investigate the design space for several working fluids encompassing different classes, namely refrigerants and siloxanes. The program is used to assess the effect of design variables and working fluid on the turbine performance and turbine design characteristics. Subsequently, the most promising design candidates are selected and three-dimensional first guess stator and rotor geometries are built on these preliminary designs. Stationary and rotating passages are then meshed and analyzed by means of RANS CFD based solution of the stator – rotor interaction.

Design Parameter Influence on Losses and Downstream Flow Field Uniformity in Supersonic ORC Radial-Inflow Turbine Stators

“…The stator design specifications, say inlet and outlet flow angles and vane height, are derived from one of the test cases investigated in the authors' previous work [3] that describes a RIT preliminary design performed by mean of an in-house mean-line code linked to CoolProp thermodynamic library [24]. More specifically, the chosen test case assumes R245fa as working fluid, which is undergoing a volumetric expansion ratio of 14 with upstream total temperature and pressure of 439 K and 28 bar, respectively.…”

“…Additionally, the high molecular weight often exhibited by organic fluids leads to low values of the speed of sound [1], resulting in turn in the onset of highly supersonic flows at stator discharge [2]. The authors [3] carried out an investigation of the radial-inflow turbine (RIT) design space using a mean-line model and parametric analyses, showing that, in order to achieve satisfactory efficiency with typical cycle configurations on the scale of tens of kilowatts, the stator discharge Mach numbers attain levels that commonly require convergent-divergent stators.…”