Design Method and Performance Prediction for Radial-Inflow Turbines of High-Temperature Mini-Organic Rankine Cycle Power Systems

Abstract

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.

Flow deviation and critical choking in transonic turbine cascades operating with non-ideal compressible flows

“…Therefore, the design of ORC turbines has to be carried out to avoid critical choking in both design and off-design conditions. Furthermore, in mini-ORC turbines, the expansion ratio is so high that the flow is strongly supersonic and the machine might meet critical choking at partial load (De Servi et al, 2019).…”

Multirow Adjoint-Based Optimization of NICFD Turbomachinery Using a Computer-Aided Design-Based Parametrization

“…Gradient-free methods only require the evaluation of the cost function values, and they are widely used due to their robustness, simple integration with blackbox models, and ability to handle nonsmooth or even discontinuous optimization problems [11]. Over the years, gradient-free methods have been successfully applied for the aerodynamic design of turbines [6,12,13] and compressors [14,15]. However, these methods require a large number of function evaluations to converge to the optimum solution, especially when the number of design variables is large.…”

Preliminary Design of an Axial-Flow Turbine for Small-Scale Supercritical Organic Rankine Cycle

“…The blade height at the inlet of the turbine which is constrained by the associated operation pressure [38]. Therefore, some radial-outflow turbines [39] and radial-inflow turbines [40][41][42][43] were designed. The performance maps were strongly influenced by the pressure ratio, shroud-to-tip radius ratio for small-scale radial turbine [44].…”

“…The small-scale turbines mentioned previously are designed for ORCs with low-temperature sources, about 100 • C [32][33][34][35]42,43,47]. Most of them are radial turbines, and the pressures at the turbine inlet are not very high, 19.9 bar in [38], 0.392 bar in [39], 34.7 bar in [40], 18.1 bar in [41], 13.0 bar in [46], 7.9 bar in [45]. Such a low pressure is not suitable for a supercritical ORC.…”