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16:20   Session 6: Turbine design I
Chair: Matteo Pini
16:20
20 mins 		System-search EXPERIMENTAL OBSERVATION OF NON-IDEAL NOZZLE FLOWS OF SILOXANE VAPOR MDM
Andrea Spinelli, Alberto Guardone, Fabio Cozzi, Margherita Carmine, Renata Cheli, Marta Zocca, Paolo Gaetani, Vincenzo Dossena
Abstract: ABSTRACT The first experimental results from the Test-Rig for Organic Vapours (TROVA) at Politecnico di Milano are reported. The TROVA (Test Rig for Organic Vapors) was designed and built at Politecnico di Milano [1] in collaboration with Turboden s.r.l., for investigating the non-ideal compressible-fluid dynamics of organic vapours in exemplary ORC turbine passages. Expansion flows of different organic fluids can be investigated in the TROVA by independent measurement of pressure, temperature, and velocity. The facility implements a Rankine cycle (either subcritical or supercritical) where the expansion process takes place within a straight axis convergent-divergent nozzle, which is the simplest geometry representative of a supersonic ORC turbine blade passage [2]. The test rig can also accommodate linear blade cascades, as it is foreseen for future research. In order to reduce the required input thermal power, a batch operating mode was selected for the plant. Different working fluids can be tested, with adjustable operating conditions up to maximum temperature and pressure of 400 °C and 50 bar. The first experimental observation of non-ideal nozzle flows are presented for the expansion of siloxane fluid MDM (C8H24O2Si3, octamethyltrisiloxane, CAS 107-51-7) in conditions that are typical to ORC operations and also at different off-design operating regimes. Reported data include static pressure measurements along the nozzle axis and total pressure – total temperature measurements in the settling chamber. A double-passage Schlieren technique is applied to visualize the flow field in the nozzle throat and divergent section and to determine the position of shock waves within the flow field. Experimental results are compared to predictions obtained from the quasi-one-dimensional expansion theory using state-of-the-art thermodynamic models of the operating fluid. Three-dimensional numerical simulations of the flow field are carried out to support the interpretation of the experimental results. REFERENCES [1] A. Spinelli, M. Pini, V. Dossena, P. Gaetani, F. Casella, 2013. “Design, Simulation, and Construction of a Test Rig for Organic Vapours”. ASME Journal of Engineering for Gas Turbines and Power, Vol. 135, 042303. [2] A. Guardone, A. Spinelli, V. Dossena, 2013. “Influence of Molecular Complexity on Nozzle Design for an Organic Vapor Wind Tunnel”. ASME Journal of Engineering for Gas Turbines and Power, Vol. 135, 042307. [3] A. Spinelli, V. Dossena, P. Gaetani, C. Osnaghi, D. Colombo, “Design of a Test Rig for Organic Vapours”. In Proceedings of ASME Turbo Expo 2010, June 14-18, 2010, Glasgow – UK – GT2010-22959.
16:40
20 mins 		System-search WAVE SPEED MEASUREMENTS IN NON-IDEAL COMPRESSIBLE FLOWS USING THE FLEXIBLE ASYMMETRIC SHOCK TUBE (FAST)
Tiemo Mathijssen, Mauro Gallo, Emiliano Casati, Alberto Guardone, Piero Colonna
Abstract: Non-ideal compressible fluid dynamics (NICFD) are defined as compressible fluid flows occurring in the dense vapour, dense vapour-liquid equilibrium or supercritical thermodynamic region. This type of flow can occur in expanders of organic Rankine cycle power plants. In order to study NICFD, a Ludwieg tube-type facility has been designed and constructed at Delft University of Technology. A large variety of fluids can be employed in the facility, but for this study D6 siloxane is chosen as working fluid due to its high thermal stability and the possibility of encountering non-classical gasdynamic phenomena. This compound belongs to the siloxane class, which are also used as working fluids in ORC power systems. Gasdynamic experiments within the NICFD region are presented from which the wave speed and speed of sound can be inferred using the time-of-flight technique. These data can be used to improve and validate thermodynamic models.
17:00
20 mins 		System-search SCALING OF GAS TURBINE FROM AIR TO REFRIGERANTS FOR ORGANIC RANKINE CYCLE (ORC) USING SIMILARITY CONCEPT
Choon Seng Wong, Susan Krumdieck
Abstract: Organic Rankine Cycle (ORC) could be used to generate power from low temperature heat sources or improve overall cycle efficiency in waste heat applications with minimal environmental pollution. The design and development of an ORC turbine, however, is a complex and costly engineering problem. The common refrigerants for an ORC application exhibit non-ideal gas behaviour and some unfavourable characteristics, such as flammability and toxicity. These characteristics further increase the complexity of the design and laboratory testing process of a turbine. Similitude, or similarity concept, is an essential concept in turbomachinery to allow the designer to scale a turbine design to different sizes or different working fluids without repeating the whole design and development process. Similarity concept allows the testing of a turbo-machine in a simple air test bench instead of a full scale ORC test bench. The concept can be further applied to adapt an existing gas turbine as an ORC turbine using different working fluids. This paper aims to scale an industrial gas turbine to different working fluids, other than the fluid the turbine was originally designed for. Three different approaches using the similarity concept were applied on the turbine performance data using compressed air to generate the performance curve for two refrigerants, namely R134a and R245fa. The scaled performance curves derived from the air performance data were compared to the performance map generated using 3D computational fluid dynamics (CFD) analysis tools for R134a and R245fa. The three approaches were compared in term of the accuracy of the performance estimation, and the most feasible approach was selected. The result shows that complete similarity cannot be achieved using two turbo-machines with different working fluids, even at the best efficiency point for particular expansion ratio. Constant Δh0s/a012 is imposed to achieve similarity, but the volumetric ratio is varying using different working fluids due to the variation of sound speed. The differences in the fluid properties and the expansion ratio lead to the deviation in turbine performance parameters, velocity diagram, turbine’s exit swirl angle, and entropy generation. The use of Δh0s/a012 further limits the application of the gas turbine for refrigerants with heavier molecular weight to a pressure ratio less than the designed pressure ratio using air. The specific speed at the best efficiency point with different expansion ratio was shifted to a higher value if higher expansion ratio was imposed. A correction chart for R245fa was attempted to estimate the turbine’s performance at higher expansion ratio as a function of volumetric ratio.
17:20
20 mins 		System-search FAST DESIGN METHODOLOGY FOR SUPERSONIC ROTOR BLADES WITH DENSE GAS EFFECTS
Elio Bufi, Paola Cinnella, Benoît Obert
Abstract: In Organic Rankine Cycle turbines where a large work output is required from a turbine in a single stage, it is necessary to use high pressure ratios across the nozzle blades, thus producing supersonic velocities at the rotor inlet unless very high rotational speeds are used. Supersonic flow in the rotor can lead to significant losses unless a careful design of the rotor blades is adopted to avoid focusing of the characteristic lines. Moreover, the design of Organic Rankine Cycle (ORC) expanders requires numerical models taking into account the dense gas effects the working fluid undergoes in given ranges of operating conditions. This work describes a fast 2-D design methodology based on the method of characteristics (MOC) for rotor blade vanes of supersonic axial ORC impulse expanders. The MOC is generalized to gases governed by complex equations of state to fully take into account dense gas behavior. The fluid thermodynamic behavior is described by highly accurate multiparameter equations of state based on Helmholtz free energy. Several working fluids are considered, including R245fa, Novec649, R449, RE347mcc. The designs generated by the generalized MOC are compared with those obtained under the classical perfect gas model. Finally, CFD simulations of both isolated rotor blades and a full turbine stage are carried out to assess the performance of the designs using the ANSYS CFX solver. The nozzle blades are also designed by means of an extended method of characteristics previously developed by some of the present authors.
17:40
20 mins 		System-search DEVELOPMENT OF A TURBO-GENERATOR FOR ORC SYSTEM WITH TWIN RADIAL TURBINES AND GAS FOIL BEARINGS
Young Min Yang, Byung-Sik Park, Si Woo Lee, Dong Hyun Lee
Abstract: The interest in ORC plant is increasing steadily over recent years in terms of the energy and the environment costs. But the capital cost and maintenance cost of an ORC plant are the main obstacles in the wide spread in the global market. To overcome them, it is necessary to decrease the manufacturing and maintenance cost, and increase the turbine efficiency and the system availability. Korea Institute of Energy Research (KIER) and Jinsol Turbomachinery have jointly developed a novel turbo-generator applicable for the low temperature heat sources to meet those needs. The turbo-generator developed is almost maintenance free, highly efficient and completely hermetic. A high speed permanent magnet synchronous generator (PMSG) was applied to get the high efficiency. Radial turbines were directly coupled with the PMSG without gear box to reduce the power transfer loss and system cost. And the rotor shaft was supported by gas foil bearings to increase the system availability through the non-contacting bearings. Twin radial turbines were assembled to the rotor shaft at the both ends in the way of face-to-face to cancel out the axial load caused by the pressure difference. This configuration was helpful to apply a gas foil bearing as a thrust bearing despite of its low load capacity. A gas foil bearings is the simple and cheap solution for the rotor support system and the completely hermetic turbo-generator. The high efficiency of the radial turbine was acquired by the real gas modelled turbine design with optimum specific speed. The developed turbo-generator was integrated with the 100kWe ORC plant installed at KIER and it showed that the turbine efficiency was more than 85% with the temperature difference, 70℃ between the heat source and the heat sink as the result of the performance test.
18:00
20 mins 		System-search EFFICIENCY CORRELATIONS FOR AXIAL TURBINES IN ORC FIELD
Marco Astolfi, Ennio Macchi
Abstract: This work aims at defining a set of general correlations for the estimation of axial-flow turbine efficiency in Organic Rankine Cycle (ORC) field. A dedicated numerical tool is used for the optimization of several hundreds of turbines and the results are presented in specific parameters (SP, V_r and Ns) according to similarity rules. The analysis is carried out for single, two and three stages turbines. For each case a correlation of efficiency at optimal rotational speed is calibrated in function of the equivalent single stage SP and the total isentropic V_r. Three sensitivity analyses are proposed in order to highlight the effects of each single parameter on stage efficiency. Finally, the model is validated on four turbines comparing the results from the correlation with those attainable with a dedicated numerical tool for the turbine design. REFERENCES [1] Smith, M.H. A simple correlation of turbine effciency. : Journal of Royal Aeronautical Society, 1965. p. 467. Vol. 69. Cc [2] Baljè, O.E., Binsley, R.L. Axial turbine performace evaluation: part B - optimization with and without constraints. : ASME Journal of Engineering for Power. pp. 349-360. [3] Craig, H.R.M., Cox, H.J.A. Performance estimation of axial flow turbines. : Proceedings of he institution of mechanical engineers. pp. 407-423. Vol. 185 32/71. [4] Dixon, S. L., Eng., B. Fluid Mechanics, Thermodynamics of urbomachinery, Fifth Edition. s.l. : Eselvier, 1998.