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10:30   Session 10: Turbine design II
Chair: Teemu Turunen-Saaresti
10:30
20 mins 		System-search UNSTEADY RANS SIMULATION OF THE OFF-DESIGN OPERATION OF A HIGH EXPANSION RATIO ORC RADIAL TURBINE
Enrico Rinaldi, Rene Pecnik, Piero Colonna
Abstract: The design of Organic Rankine cycle (ORC) turbines is a challenging task due to the complex thermodynamic behavior of the working fluid, the typical high expansion ratio which leads to a highly supersonic flow, the flow unsteadiness, and the continuous shift of operation between on- and off-design conditions. Computational fluid dynamic (CFD) simulations provide useful insights on the turbine flow which help the design, analysis and optimization process. Steady state CFD computations are nowadays common practice in the design of ORC turbines. However, the inherent unsteadiness of the problem requires time resolved simulations to capture a number of phenomena otherwise ignored, e.g., shock/shock and shock wave/boundary layer interactions, which are expected to dominate the flow evolution. This paper presents a numerical investigation of the off-design operation of a single stage high expansion ratio (>100) ORC radial turbine. Two-dimensional unsteady Reynolds-averaged Navier-Stokes simulations are presented to highlight the main flow characteristics and to study the stator/rotor interaction in terms of time dependent turbine performance parameters and blade loads. An in-house flow solver was used, which accounts for the non-ideal behavior of the fluid via look-up tables generated using a multiparameter equation of state model. The code was previously validated for applications relevant for this study. The analysis shows unique insights on the unsteady flow field in a supersonic ORC turbine and represents the first step toward a new component design approach based on non-stationary flow characteristics.
10:50
20 mins 		System-search NUMERICAL STUDY OF ORGANIC RANKINE CYCLE RADIAL-INFLOW TURBINES FOR HEAVY-DUTY DIESEL ENGINE COOLANT HEAT RECOVERY
Lei Zhang, Weilin Zhuge, Yangjun Zhang, Jie Peng
Abstract: Low rotating speed radial-inflow turbines are promising for the organic Rankine cycle systems in the small power size applications such as heavy-duty trucks and passenger cars waste heat recovery. Small mass flow rate and low rotating speed will lead to the turbine design specific speed lower than the optimal range of 0.34 to 0.72. Until now, few literatures reported the performance characteristics and loss mechanisms of low specific speed ORC radial-inflow turbines. The present study makes a RANS simulation of the low specific speed of 0.28 radial-inflow turbine using R245fa as working fluid to evaluate its performance characteristics and investigate the loss mechanisms. An optimal specific speed of 0.47 radial-inflow turbine in the same nominal operating condition and working fluid is also simulated for comparative analysis. The CFD results indicate that the low specific speed turbine nominal efficiency decreases 1.7% compared with the optimal one. And furthermore the off-design efficiency decreases are in the range of 2.6% to 0.6% from low to high pressure ratio conditions. The effect of specific speed on the efficiency characteristics mainly lies on rotor tip clearance and passage losses. The low specific speed turbine shows larger tip clearance losses but smaller passage losses compared with the optimal specific speed turbine. Flow field analysis of rotor entropy generation indicates that for the low specific speed turbine, the entropy generation caused by the tip clearance flow is much larger, and the tip clearance loss is mainly located on the suction surface in the inducer to midchord region, but on the pressure surface in the exducer region.
11:10
20 mins 		System-search FLUID-DYNAMICS OF THE ORC RADIAL OUTFLOW TURBINE
Claudio Spadacini, Lorenzo Centemeri, Dario Rizzi, Massimiliano Sanvito, Aldo Serafino
Abstract: It is well documented that axial turbines and radial inflow turbines have traditionally been the selected solutions for ORC, both with an overhung configuration. In the last years a different turbine technology for ORC has been developed, engineered, manufactured and tested by Exergy: the radial outflow turbine. In order to better understand its potential and limits, the present study has the purpose of conducting a fluid-dynamic study of the ORC radial outflow turbine. To pursue this aim, here firstly a summary description of the radial outflow turbine and of its features is given, by means of mechanical and thermodynamic fundamentals. Secondly, moving from the hypothesis of direct coupling with generator, boundary conditions for a 2 MW case are chosen and a radial outflow turbine is studied, focusing on fluid-dynamic design: after a preliminary mean line study a CFD simulation of the machine is performed. The described analysis includes also a comparison with an axial ORC turbine with the overhung configuration directed coupled with a generator: this approach could allow to valuate fluid-dynamic losses in both technologies and can explain the reason why the radial outflow turbine shows a higher efficiency than the axial overhung one in many ORC applications.
11:30
20 mins 		System-search NON-IDEAL COMPRESSIBLE-FLUID DYNAMICS SIMULATIONS WITH SU2: NUMERICAL ASSESSMENT OF NOZZLE AND BLADE FLOWS FOR ORGANIC RANKINE CYCLE APPLICATIONS
Giulio Gori, Alberto Guardone, Salvatore Vitale, Adam Head, Matteo Pini, Piero Colonna
Abstract: The growing interests towards Organic Rankine Cycle (ORC) turbo-machinery calls for reliable and well-established simulation and design tools, including Computational Fluid Dynamics (CFD) software, accounting for non-ideal thermodynamic behaviour in close proximity to the liquid-vapour saturation curve and critical point, as well as two-phase properties. SU2, an open-source CFD solver originally developed at Stanford University, Palacios et al. (2013, 2014), was recently extended to deal with non-ideal thermodynamics, including state-of-the-art multi-parameter equations of state implemented in the FluidProp library, Colonna and van der Stelt (2004), and it is now in the process of becoming a reliable simulation tool for academic and industrial research on ORC machinery. The investigation of SU2 performances in connection with the numerical simulation of steady nozzle and turbine flows of interest for ORC applications are provided. Numerical simulations refer to both inviscid and viscous flow, with diverse thermodynamic (ideal gas, Van der Waal gas, Peng- obinson Stryjek-Vera, Span-Wagner multi-parameter equation of state) and turbulence (Spalart-Allmaras, SST-k) models. Considered geometries include straight axis planar nozzle, and a typical ORC turbine blade passage.
11:50
20 mins 		System-search 3D FLUID DYNAMIC ANALYSIS OF A HIGH LOADED CENTRIFUGAL ROTOR FOR MINI ORC POWER SYSTEMS
Salvatore Vitale, Matteo Pini, Antonio Ghidoni, Piero Colonna
Abstract: Organic Rankine Cycle (ORC) power systems are a well-established technology for the conversion of thermal energy sources in the small-to-medium power range. In the last few years, efforts have been devoted to the development of mini ORC (mORC) power systems (5- 30 kWe) for waste heat recovery from truck engines, or distributed conversion of concentrated solar radiation. In these high-temperature applications the expander is arguably the most critical component. Due to the high expansion ratio, turbo-expanders are typically preferred. Recently, a multi-stage radial-outflow turbine (ROT) configuration for ORC power systems has been studied. However, even if the authors preliminarily demonstrated that ROT may allow for compact and efficient expanders [1], some research questions are still open. Notably, the key point is the fluid dynamic design of the first stages, which are subject to severe flow conditions (very high flow deflection, low aspect ratio of the blades and with high tip clearance losses). This work thus proposes a novel design methodology for centrifugal cascades, specifically targeted to the first rotor of the mORC centrifugal turbine described in Ref. [1]. Blades are initially designed using a novel in-house Turbomachinery Blade Modeler (BM), then performance is verified by means of 3D CFD simulation on unstructured grids using the solver SU2, recently extended also in-house to treat non ideal compressible fluid flows [2]. Results show that traditional blade design rules for axial cascades are not directly extendable to centrifugal profiles and new design guidelines are needed. Moreover, the 3D performance of the cascade has also been investigated by taking into account tip clearance and secondary loss mechanisms. Finally, an accurate comparison with the mean-line code predictions is provided. As expected, the outcome of the study reveals moderate discrepancy between the CFD results and the mean-line code. This suggests that in case of non-conventional machines a more tight integration of design tools of increasing fidelity may be convenient.
12:10
20 mins 		System-search AUTOMATIC DESIGN OF ORC TURBINE PROFILES BY USING EVOLUTIONARY ALGORITHMS
Pablo Rodriguez-Fernandez, Giacomo Persico
Abstract: In this paper, an automated design tool for Organic Rankine Cycle (ORC) turbines is presented. Supersonic flows and real-gas effects featuring ORC turbines complicate significantly their aerodynamic design, which may benefit significantly from the application of systematic optimization methods. This study proposes a complete method to perform shape optimization of ORC turbine blades, constructed as a combination of a generalized geometrical parametrization technique, a high-fidelity Computational Fluid Dynamic (CFD) solver (including real gas and turbulence models) and an evolutionary algorithm. As a result, a non-intrusive tool, with no need for gradients definition, is developed. The high computational burden typical of evolutionary methods is here tackled by the use of a surrogate-based optimization strategy, for which a Gaussian model is applied. % Application to ORC turbines has been proved to be successful, resulting in a comprehensive method for a very wide range of applications. In particular, the present optimization scheme has been applied to the re-design of the supersonic nozzle of an axial-flow turbine. In this design exercise very strong shocks are generated in the rear blade suction side and shock-boundary layer interaction mechanisms occur. Optimization aiming at a more uniform flow at the blade outlet section is shown to minimize the shock losses, resulting in a significant improvement in the nozzle efficiency. The optimal configuration determined with the present design tool is also successfully validated against the outcome of a previous optimization performed with a gradient-based method, demonstrating the reliability and the potential of the design methodology here proposed.