10:30
Session 6C: Turbomachinery (3)
Chair: Apostolos Pesiridis
10:30
20 mins
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Preliminary verification of the open-source CFD solver SU2 for radial-inflow turbine applications
Joshua Keep, Salvatore Vitale, Matteo Pini, Matteo Burigana
Abstract: CFD simulations are a key design and analysis tool within the design process for turbomachinery. In order for the designer to utilise these simulations to their maximum potential, such simulations need to adequately predict machine overall performance and to model underlying flow physics as closely as possible.
Within the domain of conventional turbomachinery (eg. gas and steam turbines) there exists an acceptable degree of confidence in commercial CFD solvers for these applications due to close correlations with performance predictions, and control of relevant modelling features for these applications.
ORC turbomachinery is a developing field with consequently fewer applications. At this stage, the relative importance of modelling flow features is not fully understood. As a starting point it is necessary for codes used to model ORC turbomachinery to incorporate several additional features over conventional machinery including the ability to:
- Model Compressible flow, including transonic and highly supersonic flows
- Remain stable over large density range with large spatial gradients
- Incorporate fluid models for highly non-ideal fluids
With the uncertainty in modelling requirements, it can be beneficial for the designer to utilise an open source solver in order to have the freedom to complete relevant modifications. For the present paper, the open source compressible flow solver SU2 will be used to simulate an ORC turbine.
In this paper a simulation of an ORC turbine using the SU2 open source CFD solver will be presented. The purpose of such a simulation is to assess and validate the present performance data against experiment and simulation data from established CFD codes.
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10:50
20 mins
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Large multistage axial turbines
Roberto Bini, Davide Colombo
Abstract: Though multistage axial turbines are being widely used since many decades for water steam expansion or in gas turbines, their use in ORCs has traditionally found limitations as a consequence of the choice of designing turbines with cantilever arrangements. The cantilever arrangement is mainly related to the advantages of having simple shaft bearings design, only one rotating seal system in contact with the working fluid, a more compact turbine casing, only one side of the turbine casing occupied with the bearing and sealing systems (leaving the other side free to host the outlet duct for working fluid expanded vapour which often features very large volumetric flow rates), ease of access for ordinary and extraordinary maintenance. Hence cantilever arrangement involves definitely lower turbine costs, both in terms of Capex and Opex and is practically the only solution adopted by the industrial suppliers of ORC turbines.
By adopting the cantilever arrangement the installation of several turbine discs hosting several expansion stages, traditionally installed one close to the other, would increase the overhung mass of the turbine rotor and decrease the vibration natural frequencies of the system, yielding some mechanical criticalities and, at the end, limiting the possibility of installing, practically, more than 2-3 stages with this kind of arrangement.
On the other hand, however, modern ORC turbines require to increase number of stages to better exploit the expansion enthalpy drop, in order to increase turbine efficiency (both in on-design and in off-design conditions) and/or to allow to adopt a low rotating speed (even 1,500 rpm) for a direct connection with synchronous generator.
The paper will describe an innovative solution that has allowed to increase the number of stages from the traditional 2-3 to at least 5 without leaving the effective cantilever arrangement, keeping the concept of ‘rigid’ rotor (i.e. operating below the first flexional natural frequency), with very low measured vibrations.
Operating data of a first 9 MW 5 stage ORC turbine put in operation in 2016 will also be presented.
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11:10
20 mins
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Performance assessment of a standard radial turbine as turbo expander for an adapted solar concentration ORC
Michael Deligant, Quentin Danel, Farid Bakir
Abstract: Organic Rankine cycles are one of the available solutions for converting low grade heat source into electrical power. However the development of plants tends to be very expansive due to the specific design of the expander. Usually, the input parameters for designing an ORC plant are the temperature and power of the heat and cold source. They lead to the selection of a working fluid, pressure and temperature. The expander is then designed based on the required operating parameters. Using standard turbine easily available on the market and with well known performances would allow to reduce the development and manufacturing cost. However, the ORC would have to be adapted to make the expander work in its best conditions. For a solar concentrated heat source, the temperature and power can be adapted by adjusting the concentration factor and the panel total area.
In this paper, a given gas turbine is considered to be used as the expander of the ORC. Knowing the turbine's performances with air, the optimal operating parameters (pressure, temperature, flow rate and rotational speed) of the ORC with different fluids are sought based on similitude rules. The adaptation aims to maintain same density evolution, same inlet speed triangle and same inlet Mach number with the working fluid as with air. The performance maps of the turbine are then computed with CFD simulations and showed a maximum isentropic efficiency of 80\%. Finally, the methodology is summarized and the potential applications are discussed.
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11:30
20 mins
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Optimal aerodynamic design of a transonic centrifugal turbine stage for Organic Rankine Cycle applications
Giacomo Persico, Vincenzo Dossena, Paolo Gaetani
Abstract: This paper presents the results of the application of a shape-optimization technique to the design of the stator and the rotor of a centrifugal turbine conceived for Organic Rankine Cycle (ORC) applications. Centrifugal turbines have the potential to compete with axial or radial-inflow turbines in a relevant range of applications, and are now receiving scientific as well as industrial recognition. However, the non-conventional character of the centrifugal turbine layout, combined with the typical effects induced by the use of organic fluids, leads to challenging design difficulties. For this reason, the design of optimal blades for centrifugal ORC turbines demands the application of high-fidelity computational tools.
In this work, the optimal aerodynamic design is achieved by applying a non-intrusive, gradient-free, CFD-based method implemented in the in-house software FORMA (Fluid-dynamic OptimizeR for turboMachinery Aerofoils), specifically developed for the shape optimization of turbomachinery profiles.
FORMA was applied to optimize the shape of the stator and the rotor of a transonic centrifugal turbine stage, which exhibits a significant radial effect, high aerodynamic loading, and severe non-ideal gas effects. The optimization of the single blade rows allows improving considerably the stage performance, with respect to a baseline geometric configuration constructed with classical aerodynamic methods. Furthermore, time-resolved simulations of the coupled stator-rotor configuration shows that the optimization allows to reduce considerably the unsteady stator-rotor interaction and, thus, the aerodynamic forcing acting on the blades.
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11:50
20 mins
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Unsteady simulation of quasi-periodic flows in Organic Rankine Cycle cascades using a Harmonic Balance method
Antonio Rubino, Matteo Pini, Piero Colonna
Abstract: Currently, turbomachinery design optimization methodologies are mainly restricted to steady state approaches, due to the high
computational cost associated with time-accurate shape optimization algorithms. However, the possibility to include unsteady
effects in turbomachinery optimization can significantly increase the level of accuracy of the design predictions, leading to a more
realistic representation of the actual performance and ultimately to a substantial increase in operating efficiency. Unsteady effects
are particularly relevant in Organic Rankine Cycle turbines. A trade-off between high-fidelity time-accurate unsteady simulations
of the flow solution and computational cost is therefore needed at design level. In this paper, a first application of the harmonic
balance method to non-ideal compressible flows is presented. The methodology allows to solve the unsteady flow equations for
a set of specified frequencies only, with significant computational time savings. An algorithm is proposed for non uniform time
sampling in order to resolve frequencies that do not need to be integral multiple of one fundamental harmonic. This enables
the solution of quasi-periodically forced non-linear flow problems, in combination with complex fluid models based on accurate
equations of state. The method is applied to the unsteady analysis of a supersonic Organic Rankine Cycle stator with quasi-periodic
inlet operating conditions, showing about one order magnitude lower computational cost compared to time-accurate simulations.
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12:10
20 mins
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Non-ideal fish-tail shocks in ORC turbine cascades
Davide Vimercati, Giulio Gori, Andrea Spinelli, Alberto Guardone
Abstract: Non-ideal shock waves at the trailing edge of supersonic high-pressure turbine vanes for ORC applications are studied numerically using the open-source SU2 solver coupled with mesh adaptation. Flow separation at the trailing edge of ORC turbine, where a supersonic Prandtl-Meyer expansion occurs, generates a limited region of separation between the supersonic flows on the pressure and suction sides of the blades.
The merging of these two supersonic regions results in the formation of compression waves that eventually form a characteristic shock pattern comprising two oblique shock waves, called fish-tail shocks.
The present investigation follow the study of Andrew Wheeler et al.~(NID 2016), where the authors focused on non-ideal compressible-fluid effects on the flow turning angle resulting from the Prandtl-Meyer expansion, which influences the shape and size of the downstream recirculating region.
Here, the downstream process of coalescence into oblique shock waves is studied numerically to determine whether non-ideal shock waves can be observed in typical ORC operation.
Non-ideal shock waves are possible only if the fundamental derivative of gasdynamics is less than unity and are characterised by the increase of the flow Mach number across the shock wave.
The accuracy of the numerical tool is first assessed against a experimental results over a simplified backward-facing step geometry.
Then, numerical simulations are carried out to determine the occurrence of non-ideal oblique shocks in conditions typical to ORC applications.
Experimental results regard supersonic expanding flows of siloxane fluid MDM (Octamethyltrisiloxane, C8H24O2Si3) around a 90° corner. The Mach number is around 1.1 and the compressibility factor Z \sim0.75.
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