The realization of commercial mini organic Rankine cycle (ORC) power systems (tens of kW of power output) is currently pursued by means of various research and development activities. The application driving most of the efforts is the waste heat recovery from long-haul truck engines. Obtaining an efficient mini radial inflow turbine, arguably the most suitable type of expander for this application, is particularly challenging, given the small mass flow rate, and the occurrence of nonideal compressible fluid dynamic effects in the stator. Available design methods are currently based on guidelines and loss models developed mainly for turbochargers. The preliminary geometry is subsequently adapted by means of computational fluid-dynamic calculations with codes that are not validated in case of nonideal compressible flows of organic fluids. An experimental 10 kW mini-ORC radial inflow turbine will be realized and tested in the Propulsion and Power Laboratory of the Delft University of Technology, with the aim of providing measurement datasets for the validation of computational fluid dynamics (CFD) tools and the calibration of empirical loss models. The fluid dynamic design and characterization of this machine is reported here. Notably, the turbine is designed using a meanline model in which fluid-dynamic losses are estimated using semi-empirical correlations for conventional radial turbines. The resulting impeller geometry is then optimized using steady-state three-dimensional computational fluid dynamic models and surrogate-based optimization. Finally, a loss breakdown is performed and the results are compared against those obtained by three-dimensional unsteady fluid-dynamic calculations. The outcomes of the study indicate that the optimal layout of mini-ORC turbines significantly differs from that of radial-inflow turbines (RIT) utilized in more traditional applications, confirming the need for experimental campaigns to support the conception of new design practices.

No abstract

No abstract

Non-axisymmetric endwall profiling offers features to simultaneously mitigate aerodynamic losses and hot gas ingestion in axial turbines. The current paper presents an optimization methodology to generate a contoured surface integrated with real geometrical effects such as blade fillets and a rim seal channel with the aim of achieving higher efficiencies while reducing the hot gas ingestion. The contoured rotor platform is constructed using a B-spline surface clamped in the axial direction. On the azimuthal direction the surface is unclamped to allow geometrical continuity across the periodic boundaries. The endwall parametrization is used to optimize a rotor hub platform of a high-pressure turbine stage. A differential evolution optimizer is used to rank individuals in terms of efficiency. The single-objective optimization is set to maximize the aerodynamic efficiency and it is defined such that it accounts for the flow non-uniformity trough a mixed out averaging procedure. Engine representative conditions typical of a two-stage high pressure turbine are used as boundary conditions. Geometrical and aerodynamic constraints are set to guarantee a fair comparison among individuals and to meet engine requirements. Two surface parameterizations, which use a different number of design variables but share the same construction strategy, are presented to show the trade-off between the number of degrees of freedom and the aerodynamic improvement. Different purge flow conditions are considered to assess the robustness of the optimization results at off-design condition for relevant geometries. The aim of the paper is to show the advanced shape flexibility of the implemented parametrization for contoured platforms featuring technological effects such as blade fillet and rim seal channel. The work provides design guidelines to set-up engine-realistic constraints for endwall contour optimization of turbine stages.

Non-axisymmetric endwall profiling offers features to simultaneously mitigate aerodynamic losses and hot gas ingestion in axial turbines. The current paper presents an optimization methodology to generate a contoured surface integrated with real geometrical effects such as blade fillets and a rim seal channel with the aim of achieving higher efficiencies while reducing hot gas ingestion. The contoured rotor platform is constructed using a B-spline surface clamped in the axial direction. In the azimuthal direction, the surface is unclamped to allow geometrical continuity across the periodic boundaries. The endwall parametrization is used to optimize a rotor hub platform of a high-pressure turbine stage. A differential evolution optimizer is used to rank individuals in terms of efficiency. Engine representative conditions typical of a high-pressure turbine are used as boundary conditions. Geometrical and aerodynamic constraints are set to guarantee a fair comparison among individuals and to meet engine requirements. Two surface parameterizations, which use a different number of design variables but share the same construction strategy, are presented to show the trade-off between the number of degrees of freedom and the aerodynamic improvement. Different purge flow conditions are considered to assess the robustness of the optimization results at off-design conditions for relevant geometries. The paper shows the advanced shape flexibility of the implemented parametrization for contoured platforms featuring technological effects such as blade fillet and rim seal channel. The work provides design guidelines to set up engine-realistic constraints for endwall optimization of turbine stages.