16:30
Session 3C: Turbomachinery (1)
Chair: Giacomo Persico
16:30
20 mins
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Advanced materials for the impeller in an orc radial microturbine
Isaias Hernandez Carrillo, Christopher Wood, Hao Liu
Abstract: For distributed generation with low-temperature sources, the micro organic Rankine cycle is an exceptional option. However, an abundance of work remains to be done to minimise its high cost; such a problem can be associated to the expander that represents up to 70% of the total capital cost. The design of a lean radial microturbine is proposed, which targets reduction of production costs using simplification strategies e.g. alternative materials for the impeller since low-temperatures allow the possibility of using polymers. The candidate materials are a composite (PEEK-GF30) and a thermoplastic (ABS); aluminium is used as reference. The study is developed in five stages, namely, heat-mass balance (H&MB), mean-line turbine design, 3D blading, fluid-structure interaction (FSI) and prototyping. A gross power and efficiency of 1.5kW and 70% are respectively targeted. R245fa is selected as working fluid. An impeller diameter of 49mm and a rotational speed of 36,000rpm results from the mean-line design. The structural reliability is assessed with an FSI analysis, the fluid modelling delivers consistent results with the mean-line design, except for the turbine efficiency, which is forecasted to be in the range 69-76%. Three situations are evaluated: full load operation, rotor blocked and 27% over-speed. Additionally, three materials are evaluated; therefore, nine scenarios are compared. The factor of safety is used as a unique parameter for comparison. The analysis confirmed that in the worst situation (over-speed), PEEK-GF30 is structurally 11% stronger than Aluminium whereas ABS is 40% weaker than Aluminium and both materials are sufficiently strong to be substitutes. Due to the superior performance of PEEK-GF30 and the fact that ABS is considerably inexpensive; both alternative materials are selected for prototyping using automated machining and additive manufacturing respectively.
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16:50
20 mins
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A revised TESLA turbine concept for ORC applications
Giampaolo Manfrida, Leonardo Pacini, Lorenzo Talluri
Abstract: The TESLA turbine is an original expander working on the principle of torque transmission by wall shear stress. The principle – demonstrated for air expanders at lab scale, and developed at commercial level as a pump - has some attractive features when applied to ORC expanders: it is suitable for handling limited flow rates (as is the case for machines in the range from 500W to 5 kW), it can be developed to a reasonable size (rotor of 0,1 to 0,3 m diameters), and with possible rotational speeds (from 1000 to 8000 rpm). The original concept has been revisited, improving the stator layout (which is typically responsible for a poor performance) and developing a modular design philosophy allowing to cover a wide power range, and a perfectly sealed operation, as well as other fluid dynamics improvements. The flow model was developed using complete real fluid assumptions, and includes several new concepts such as bladed channels for the stator and rotor disk profiling, and detailed treatment of losses.
Detailed design sketches are presented and preliminary results discussed and evaluated.
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17:10
20 mins
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Comparison of steady and unsteady rans CFD simulation of a supersonic ORC turbine
Benoit Obert, Paola Cinnella
Abstract: This article presents computational fluid dynamics (CFD) simulations of a supersonic flow inside an Organic Rankine Cycle turbine. The expander considered in this work is a single stage axial turbine using hexamethyldisiloxane (MM) as working fluid. The thermodynamic properties of the working fluid are modelled by a Helmoltz free energy based equation of state in both the design and simulation steps to accurately account for the non-ideal nature of the fluid under the considered conditions. The high expansion ratio of the turbine leads to a supersonic flow in the stator and rotor blade passages. The design of the stator and rotor blade shapes is carried out by means of the generalized method of characteristics. The CFD code used in this work is the commercial ANSYS CFX solver and the simulations are based on the two-dimensional Reynolds-Averaged Navier-Stokes (RANS) equations supplemented by a k-omega turbulence model. Results provided by steady mixing plane simulations are compared to unsteady sliding mesh simulations to understand the implications of stator/rotor flow interaction on blade loading, torque, entropy generation and flow structure, at nominal and partial load.
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