14:20
Session 4C: Turbomachinery (2)
Chair: Hua Tian
14:20
20 mins
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Numerical sensitivity analysis for supercritical CO2 radial turbine performance and flow field
Alireza Ameli, Antti Uusitalo, Teemu Turunen-Saaresti, Jari Backman
Abstract: The predominant advantage of a supercritical CO2 (SCO2) Brayton cycle is it’s diminished compression work when compared to an ideal working fluid such as helium, due to low compressibility factor. For flows where the density approaches the critical density, molecular interactions get stronger and the ideal gas assumption is no longer appropriate.
As a means to investigate the accuracy of real gas models and sensitivity of the performance of a turbomachine on the numerical accuracy in the supercritical region, numerous unsteady simulations of a radial turbine have been performed. Real Gas Properties (RGP) table has been generated to overcome difficulties of instabilities in simulations in the supercritical region. Four Equation of States (EOS) models with different RGP table resolutions have been studied and results are compared with the experimental measurement performed at the Sandia National Laboratory. The studied unshrouded radial impeller has 11 blades while the nozzle has 10 vanes. The unsteady simulation results show the dependency of the predicted radial turbine performance on the RGP table resolution as well as on the implemented EOS models. In general, the results indicate that by using appropriate EOS model and a look up table with high resolution, the CFD can predict turbine performance with high accuracy while the use of some EOS and low resolution look up tables lead to significant deviations between the simulated and measured results. As a result of this study, an appropriate EOS model and sufficient resolution of the look-up table for turbines operating at supercritical region are suggested in this paper.
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14:40
20 mins
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Experimental observation of non-ideal expanding flows of Siloxane MDM vapor for ORC applications
Andrea Spinelli, Giorgia Cammi, Marta Zocca, Simone Gallarini, Fabio Cozzi, Paolo Gaetani, Vincenzo Dossena, Alberto Guardone
Abstract: Extensive experimental results characterizing the supersonic expansion of an organic vapor in non-ideal conditions are reported in this paper for the first time.
The collected data also allowed the assessment of the accuracy of Computational Fluid Dynamic (CFD) tools employed to predict the non-ideal behavior of such flows, including the consistency of thermodynamic models adopted.
The investigation has been carried out on the converging-diverging nozzle test section of the Test Rig for Organic VApors (TROVA), at the Laboratory of Compressible fluid-dynamics for Renewable Energy Application (CREA) of Politecnico di Milano. Supersonic nozzle flow has been chosen as the simplest one of significance for organic Rankine cycle (ORC) turbine channels.
The working fluid under scrutiny is Siloxane MDM, a widely employed compound for high temperature ORCs. MDM vapor expands through the TROVA nozzle at moderate non-ideal conditions in the close proximity of the vapor saturation curve. This is the region where ORC expanders typically operate, thus proving the relevance of the investigation for the ORC community. Indeed, detailed experimental data representative of typical ORC expansions were lacking in the open literature up to date.
Two different nozzle geometries, featuring exit Mach number of 2.0 and 1.5 respectively, have been tested, exploring a wide range of thermodynamic inlet conditions and diverse levels of non-ideality; from moderate non-ideal state, indicated by a compressibility factor Z = Pv/RT ≅ 0.80, to dilute gas conditions, Z ≥ 0.97.
Maximum operating total pressure and temperature are PT ≅ 5 bar and TT ≅ 250 °C.
The nozzle flow has been characterized in terms of total pressure, total temperature, static pressure at discrete locations along the nozzle axis, and schlieren imaging.
In contrast to the well known case of polytropic ideal gas, the vapor expansion through the nozzle is found to be dependent on the inlet conditions, thus proving the non-ideal character of the flow. This influence is found to be consistent with the one predicted by the quasi-1D theory coupled with simple non-ideal gas models.
Experimental data at the nozzle centerline have also been compared with those resulting from a two-dimensional viscous CFD calculation carried out using the SU2 software suite and the improved Peng Robinson Stryjek Vera (iPRSV) thermodynamic model. A very good accordance is found, demonstrating the high accuracy of the applied tools.
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15:00
20 mins
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Design optimization of a small scale high expansion ratio organic vapour turbo expander for automotive application
Qiyu Ying, Weilin Zhuge, Yangjun Zhang, Lei Zhang
Abstract: Global environmental concern and concept of energy conservation in heavy duty trucks nowadays has given rise to increasing attention on the automotive waste heat recovery applications based on Organic Rankine Cycle (ORC) system. However, the system efficiency is theoretically low as a result of comparatively small temperature difference between exhaust gas and atmosphere, which brings about increasing expansion ratio as a feasible method to improve system performance. However, turbo expander, which is the most significant component in the system, might generate huge losses due to supersonic flow caused by high expansion ratio, therefore a study on supersonic organic vapor turbo expander design is required.
In this paper, a single-stage radial-inflow organic vapor turbo expander with pressure-ratio up to 8 is preliminarily designed, its performance and internal flow are numerically studied. The performance is simulated with three dimensional computational fluid dynamic (CFD) method at operating conditions. Several geometry modifications are conducted in order to learn their influence on turbine performance. Shock waves and trailing edge losses are observed as main losses in the nozzles, in addition, flow separations are the main losses in the rotors. Turbine blade is accordingly optimized after the analysis, and the nozzle total pressure loss coefficient decrease clearly, meanwhile a rise of 3.4% on turbine efficiency can be obtained. Shock waves and reflected waves are suppressed, and a more uniform stream at nozzle outlet can be observed.
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15:20
20 mins
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Fluid-dynamic design and characterization of a high-speed mini-ORC turbine for laboratory experiments
Matteo Pini, Carlo De Servi, Matteo Burigana, Sebastian Bahamonde, Antonio Rubino, Salvatore Vitale, Piero Colonna
Abstract: High-speed mini-turbines are utilized to generate mechanical power in Organic Rankine Cycle turbogenerators of power capacity in the range 3-50 kW [1]. In the vast majority of cases, these machines are of radial-inflow type [2], due to their compactness, high power density, and capability to operate at high flow coefficients. The combined effect of high- pressure ratios and the low speed of sound of organic compounds leads to single-stage machines constituted by a supersonic radial stator vane and a transonic mixed-flow rotor. Moreover, their design is further complicated by the small dimensions, which makes tip- leakage flow and viscous effects proportionally more dissipative than in larger machines. Achieving high efficiencies is therefore particularly challenging and conventional design rules developed for turbochargers are of limited application for mini-ORC turbines. At the moment, optimal turbine shapes are obtained by means of non-validated tools applied at the various design stages and the predicted efficiency may be affected by large uncertainties.
To bridge this gap, a high-speed (~100 krpm) mini-ORC turbine will be constructed and tested at the Propulsion and Power Lab of TU-Delft with the aim to measure the actual performance of mini-ORC expanders and, evenly important, to create public datasets for i) validating CFD tools and ii) calibrating loss correlations for preliminary design. This paper describes the layout of the machine and the analysis of the flow-field within the flow passages by means of steady computational fluid-dynamic calculations. A loss breakdown is provided to gain insight of the principal loss mechanisms in mini-ORC turbines and first design guidelines are drawn on the basis of physical understanding.
REFERENCES
[1] P. Colonna et al., “Organic Rankine Cycle Power Systems: from the Concept to the Current Technology, Applications, and an Outlook to the Future”, Journal of Engineering for Gas Turbines and Power, 2015
[2] A. Wheeler, J. Ong, “A Study of the Three-Dimensional Unsteady Real Gas Flows within a Transonic ORC Turbine”, ASME Turbo-Expo 2016
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15:40
20 mins
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Robust optimization of ORC blade turbines under a low quantile constraint
Nassim Razaaly, Giacomo Persico, Pietro Marco Congedo
Abstract: Heat sources for ORC turbines typically feature variable energy sources such as WHR (Waste Heat Recovery) and solar energy. Advanced uncertainty quantification and robust optimization methodologies could be used during the ORC turbines design process in order to account for multiple uncertainties. This study presents an original robust shape optimization approach for ORC blade turbines, to overcome the limitation of a deterministic optimization that neglects the effect of uncertainties of operating conditions or design variables. Starting from a baseline blade, we search for an optimal shape that maximizes the 5% quantile of the expander isentropic efficiency, which is evaluated by means of an Euler 2D simulation. Real-gas effects are modeled through the use of a Peng-Robinson-Stryjek-Vera equation of state. The 5% quantile of the expander isentropic efficiency is estimated using a tail probability strategy: points are iteratively added on the failure branches in order to build a reliable metamodel from which a Monte-Carlo sampling method is used. In order to speed-up the optimization process, an additional Gaussian Process model is built to approximate the isentropic efficiency. The robustly optimized ORC turbine shape is finally compared to the initial configuration and the deterministic optimal shape.
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