5th International Seminar on
ORC Power Systems
Athens Greece

 
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14:00   Session 2A: Turbines-Design aspects (2)
Chair: Giacomo Persico
14:00
20 mins
Geometry and Performance Assessment of Tesla Turbines for ORC
Lorenzo Talluri, Olivier Dumont, Giampaolo Manfrida, Vincent Lemort, Daniele Fiaschi
Abstract: The Tesla turbine - also known as friction, viscous or bladeless turbine - is a peculiar expander, which generates power through viscous entrainment. In the last years, it has gained a renewed appeal due to the rising of distributed power generation applications. Indeed, this expander is not suitable to large size power generation, but it could become a breakthrough technology in the low power ranges, due to its characteristics of low cost and reliability. The current study presents a design approach to the Tesla turbine, applied to organic working fluids (R1233zd(E), R245fa, R1234yf, n-Hexane). Three fundamental geometric parameters are identified (rotor channel width/inlet diameter ratio, rotor outlet/inlet diameter ratio, throat width ratio) and their effects on the performance are analyzed. The geometry of the turbine has been defined and the assessment of the performance potential is run, applying a 2D code for the viscous flow solution, considering real compressible fluid properties. For all the investigated working fluids, an efficiency higher than 60% has been achieved, with the defined geometry, under suitable thermo fluid-dynamic conditions.
14:20
20 mins
Design Guidelines for Supersonic Vanes Operating with Non-ideal Compressible Flows
Nitish Anand, Matteo Pini, Piero Colonna
Abstract: Supersonic stator vanes of high temperature mini-ORC machines operating at high-expansion ratios account for two-thirds of the total losses of the turbine. Reducing these losses can substantially improve the turbine fluid-dynamic performance and eventually the economic viability of the ORC system. Although these vanes have predominant impact on the overall turbine performance, there is no established procedure for their optimal design. The usual design methodology involves a choice of the conceptual design parameter (e.g. solidity, degree of divergence) based on existing, non-tailored, correlations and the use of method of characteristics and CFD-based analysis to obtain the detailed blade profile. Therefore, there is need of best practices that can enable the rapid design of efficient vanes all along the entire turbine design phase. To the authors’ knowledge, the only existing model that can be employed to define the degree of divergence, i.e. the ratio between the nozzle throat section and the nozzle outlet section, at preliminary design level is the one proposed in [1]. The model provides the optimum degree of nozzle divergence as function of the averaged downstream Mach number for best vane efficiency. The model was exclusively derived for supersonic axial cascades operating with perfect gases, therefore its application to the design of supersonic vanes operating with non-ideal compressible flows is debatable. This work stems from these considerations and is focused on investigating the accuracy of such model for complex-fluids. To achieve the above-mentioned objective, the vane geometry generation method documented in [2] is adapted for axial configuration. The design procedure requires the flow-angle, the degree of divergence and the pitch as input. The baseline geometry is obtained using the optimum degree of divergence given by the model in [1] and variants to the baseline blade shape are generated by changing the degree of divergence, in a pre-defined range. The fluid-dynamic performance of the different blades is calculated using the open-source RANS solver SU2. The flow equations are complemented by the one-equation Spalart-Allmaras turbulence model. The results of the work show that the empirical model [1] is not accurate for the design of vanes operating with complex-fluids and thus highlight the need of a new unified design guideline for this type of vanes. [1] M. Y. Deych and M. Troyanovskiy, “Investigation and calculation of Axial-turbine stages”, Foreign Technology Division, Report No.: FTD-MT-65-409. [2] Anand N, Vitale S, Pini M, Otero GJ, Pecnik R. “Design Methodology for Supersonic Radial Vanes Operating in Non-ideal Flow Conditions.” ASME. J. Eng. Gas Turbines Power. 2018;141(2):022601-022601-9. doi:10.1115/1.4040182.
14:40
20 mins
One Dimensional Design for Carbon Dioxide Axial Flow Turbine
Ningjian Peng, Enhua Wang
Abstract: Carbon dioxide power cycle has many advantages for future energy utilization. Turbine is a vital component in this system, and its efficiency has a great impact on system performance. In general, the efficiency of turbine expander is assumed to be a fixed value according to experience when analysing the performance of carbon dioxide power cycle. However, the thermodynamic properties of working fluid have a great influence on turbine efficiency, so design parameters are critical to ensure a high efficiency of carbon dioxide turbine. On the other hand, most of the carbon dioxide turbines for MW-class are radial turbines at present whereas axial turbines take advantages in terms of fluidity and simple structure compared with radial turbines. In this study, one dimensional design is carried out for a single-stage carbon dioxide axial-flow turbine by a mean-line design procedure, and geometric parameters of the flow passage and turbine efficiency are obtained. The results can provide a reference for the practical design of turbine expander of carbon dioxide power cycle system.
15:00
20 mins
Efficiency Correlations for off-design Performance Prediction of ORC Axial-flow Turbines
Roberto Pili, Nikolaos Siamisiis, Roberto Agromayor, Lars O. Nord, Christoph Wieland, Hartmut Spliethoff
Abstract: Organic Rankine Cycle (ORC) power systems are efficient and cost-effective to convert heat from low/medium temperature heat sources into electrical or mechanical power. Depending on the time behaviour of the heat and cold sources, the ORC can be operated close to the design point or at part-load, which is generally affected by lower efficiency with respect to the nominal point. Axial-flow turbines are the dominant type of expander for large-scale ORCs and their behaviour is crucial for the overall ORC performance. While correlations for efficiency estimation of the design of ORC axial-flow turbines are already available, only very few works have studied the off-design of this component. Two MATLAB mean-line tools of ORC axial-flow turbines, one for the design, AxialOpt, and one for the off-design, AxialOff, are here presented and used to develop part-load efficiency correlations based on simple thermodynamic quantities. The codes have been validated against data and measurements available in literature. A total of eight turbines from different fields of application have been designed and performance maps have been developed to define the efficiency based on the relative difference in specific enthalpy over the turbine and the relative outlet volume flow rate with respect to the nominal point. The coefficient of determination for the fitting was larger than 99%. A test case not included in the curve fitting was studied to prove the prediction capabilities of the proposed correlations. They could predict the part-load behaviour of the turbine with coefficients of determination above 90% for one, two and three stages. The results of the work can be used in ORC system analyses to estimate the turbine performance at part-load conditions prior to its design.
15:20
20 mins
Application of a Heat Resistant Plastic in a High-speed microturbine Designed for the Domestic ORC System
Grzegorz Żywica, Tomasz Z. Kaczmarczyk, Eugeniusz Ihnatowicz, Pawel Baginski, Artur Andrearczyk
Abstract: ORC (organic Rankine cycle) systems have gained popularity in recent years. They are used in many new economic sectors. This is due to numerous advantages of this technology as well as the possibility to adjust operation parameters and output power to an available heat source and the user’s demand. One example of a successful attempt to apply ORC systems are single-family houses, where there is a great demand for heat and electricity. The high price of small ORC systems is one of the main factors that hamper their development (the cost of a single kilowatt of installed power increases considerably with a decrease in the power rating of the system). This article discusses research works aimed at applying modern plastics to build a high-speed microturbine that is intended for use in a domestic ORC system. Because of the high production cost of the rotor disc, it was decided to make it from plastic. The nominal electric power of the microturbine is 1 kW and the maximum rotational speed of the rotor is 120,000 rpm. It turned out that some plastics can be usefully applied to the microturbine because they are able to withstand such operating conditions. Machining of these plastics is easier and faster as compared to conventional metallic alloys, which could decrease the overall production costs. Preliminary analyses covered several plastics that met the adopted criteria, which are related to the operating conditions of the ORC system. Selected plastics were subjected to further tests such as chemical compatibility and dimensional stability when exposed to the contact with the ORC system’s working medium. Then, for selected materials, strength calculations were carried out, taking into account the full geometry of the rotor disc. The purpose of these calculations was to determine maximum stresses and deformations. Based on the results of the analyses performed, several rotor discs were made of one carefully selected type of plastic. Already after the first tentative series of plastic blade systems was manufactured, it became clear that it is possible to significantly shorten the machining time and reduce production costs. The rotor disc made of plastic was then installed in the turbogenerator that has passed preliminary tests under laboratory conditions. The conducted research and its results could be of interest for those scientists and engineers who are looking for new material solutions that can enable shortening the production time and lowering the production costs of microturbines employed in ORC systems. The research carried out so far demonstrated that well-chosen heat resistant plastics can be successful in replacing constructional materials used to date.