The global temperature rise, which directly results from greenhouse gases emitted by burning fossil fuels, requires humanity to harness renewable energy sources at an increased rate. However, renewable energy sources are either highly intermittent, such as wind and solar radiation, or available at low temperatures leading to low efficiencies of current thermal conversion systems. Two exciting technologies that can alleviate the low thermal conversion efficiencies of power plants with low-temperature heat sources are supercritical carbon dioxide (s-CO2) and organic Rankine cycles (ORCs). Compared to conventional power cycles, ORCs and s-CO2 power cycles have different workingmedia (e.g., CO2 and hydrocarbons), such that the working fluid provides an additional degree of freedom to better adapt to low-grade heat sources. As a consequence, the power cycles have higher thermal efficiency and a more compact design. However, the main difficulty in designing highly efficient components of these nonconventional power plants lies in the fact that the heat exchangers and the turbines operate either with fluids of high molecular complexity or with fluids in highly non-ideal thermodynamic conditions. These complexitiesmake it challenging to accurately design efficient components with computational fluid dynamic (CFD) software that can reliably predict heat transfer and pressure losses in heat exchangers, and aerodynamic performance parameters in turbomachinery equipment.@en

Organic Rankine cycle (ORC) systems are a readily available technology to convert thermal energy from renewable- and waste heat sources into electricity. However, their thermal performance is relatively low due to the low temperature of the available heat sources, but more importantly, due to the low efficiency of the employed expander. Designing the turboexpander is exceptionally challenging, because the flow field is highly supersonic and unsteady, and since the expansion takes place in the highly non-ideal dense-vapor region. In this work, we perform unprecedented three-dimensional unsteady simulations of several high-expansion cantilever ORC turbines to highlight distinctive loss mechanisms. The simulations indicate strong unsteady effects in the rotor blade passage, as a result of unsteady propagating shock waves interacting with viscous wakes and boundary layers. Moreover, the flow field in the rotor blade passage is strongly affected by three-dimensional secondary flow features and a sharp expansion in the shroud region at the inlet of the rotor blade. These span-wise mechanisms and unsteady flow interactions introduce irreversible losses which must be taken into account for designing highly efficient ORC expanders.

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This paper presents a novel methodology for improving eddy viscosity models in predicting wall-bounded turbulent flows with strong gradients in the thermo-physical properties. Common turbulence models for solving the Reynolds-averaged Navier–Stokes equations do not correctly account for variations in transport properties, such as density and viscosity, which can cause substantial inaccuracies in predicting important quantities of interest, for example, heat transfer and drag. Based on the semi-locally scaled turbulent kinetic energy equation, introduced in [Pecnik and Patel, J. Fluid Mech. (2017), vol. 823, R1], we analytically derive a modification of the diffusion term of turbulent scalar equations. The modification has been applied to five common eddy viscosity turbulence models and tested for fully developed turbulent channels with isothermal walls that are volumetrically heated, either by a uniform heat source or viscous heating in supersonic flow conditions. The agreement with results obtained by direct numerical simulation shows that the modification significantly improves results of eddy viscosity models for fluids with variable transport properties.

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The stator vane of high-temperature Organic Rankine Cycle radial-inflow turbines operates under severe expansion ratios and the associated fluid-dynamic losses account for nearly two-third of the total losses generated inside the machine. The efficiency of the machine can strongly benefit from specialized high-fidelity design methods able to provide shapes attenuating shock wave formation. Shape optimization is certainly a viable option to deal with supersonic ORC stator design, but it is computationally expensive and often case specific. In this work, a robust method to approach the problem in a more systematic manner is documented. The methodology involves an optimization procedure encompassing the method of characteristics extended to non-ideal fluid flow for profiling the diverging part of the nozzle. The subsonic section and semi-bladed suction side are retrieved using a simple conformal geometrical transformation. The method is applied to the design a supersonic ORC stator working with Toluene vapors, for which two blade shapes were already available. The comparison of fluid-dynamic performance clearly indicates that the MoC-Based method is able to provide the best results with the lowest computational effort, and is suitable to be used to draw general design guidelines@en