Aerothermodynamic design of 10 kW radial inflow turbine for an organic flashing cycle using low-enthalpy resources

Abstract

Organic Rankine Cycles (ORC) have become a viable electric microgeneration framework option for the use of low enthalpy energy resources (90 °C–200 °C) such as renewable thermal sources, or residual heat from industrial processes. One of the critical components that affect the performance and cost of the system is the turbine; hence, its selection becomes relevant when studying these systems. From this approach, this paper presents the design of a 10 kW ’inflow’ radial turbine, operating at 30,000 rpm and using Isopentane as a working fluid for a low enthalpy ORC capable of harnessing the great potential of low and medium enthalpy geothermal resources available in Mexico. The turbine design is based on a theoretical one-dimensional model, from which the basic geometric dimensions of the impeller and the nozzle wheel are obtained. Subsequently, a numerical CFD model was performed to analyze the flow distribution using NUMECA’s Turbofine module with a Balwin-Lomax turbulence model, an HOH mesh topology, and a Runge-Kutta temporal discretization in order to validate the theoretical model results and to identify areas of entropy increase, pressure drops, and turbulent effects in the flow path at different mass flow rates (0.662 kg/s - 0.701 kg/s) and total pressure values ranging from 399 kPa to 438 kPa. Additionally, a structural analysis is conducted, evaluating the turbine at 30% overspeed (40,000 rpm), and considering 6061-T6 aluminum as the impeller manufacturing material, in order to obtain deformation zones, as well as maximum allowable stresses. The structural analysis of the impeller, subject to centrifugal force developed by operational conditions, shows acceptable values of stress (first main stress 46% lower than the elastic limit), which ensures reliable turbine operation. Finally, this paper presents the results of the machining of the radial impeller with the intent of knowing the necessary resources for its production, and of being able to propose medium and large scale manufacturing frameworks.