Abstract
The design of organic Rankine cycle (ORC) turbines often requires dealing with transonic flows due to the cycle efficiency requirements and the matching of the temperature profiles with heat sources and sinks, as well as the nature of organic fluids, often featuring high molecular weight. Consequently, the use of convergent–divergent turbine stators has been widely established as a solution in the published literature for use in both axial- and radial-inflow machines. With respect to the latter layout in particular, the available design guidelines are still limited. The present work shows the results of an investigation into a series of ORC radial-inflow convergent–divergent nozzles that differ with respect to the vane count and the designed metal angle of the outlet. These stators were designed by fitting the divergent portion of a sharp-edged minimum-length nozzle, designed by means of the method of characteristics (MoC) adapted to dense gases, into a radial-inflow turbine stator. The geometries were analysed by means of steady-state RANS CFD calculations, and the results were used to assess the influence of the design parameters on the nozzle losses and downstream flow field uniformity, showing that conflicting trends exist between optimum stator efficiency and optimum downstream flow field uniformity.
Highlights
In particular, pitch-wise pressure and flow angle distributions at mid-span computed in the coarsest mesh case, Figure
radial-inflow turbine (RIT) vanes have been designed by means of the with method presented the with respect to the finest mesh
Thistochoice allowed a trailing edge and rotor inlet has been kept constant for all and equal the value resolution enhancement of the expansion fans and shock waves, which appear to give the found in the mean-line design
Summary
Organic Rankine cycles (ORC) require an organic medium to operate through thermodynamic states determined by the matching of temperature profiles to heat sources and sinks, often resulting in large expansion ratios. 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
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