16:00
Session 3A: Turbine Nozzles Simulations
Chair: Stefan aus der Wiesche
16:00
20 mins
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Experimental Characterization of Nozzle Flow Expansions of Siloxane MM for ORC Turbines Applications
Giorgia Cammi, Camilla Cecilia Conti, Andrea Spinelli, Fabio Cozzi, Alberto Guardone
Abstract: This paper reports extensive experimental results characterizing the supersonic expansion of organic vapor MM (hexamethyldisiloxane, C6H18OSi2) in conditions representative of Organic Rankine Cycle (ORC) turbines operating conditions, in the close proximity of the liquid-vapor saturation curve.
Experiments were conducted on the Test Rig for Organic VApors (TROVA), at the Laboratory of Compressible fluid-dynamics for Renewable Energy Application (CREA Lab) of Politecnico di Milano. Two different planar nozzles were tested, featuring an exit Mach number of 1.5 and 1.6. Nozzle flow expansions were characterized by measuring total pressure, total temperature, static pressures along the nozzle axis and by performing schlieren visualizations. A wide range of inlet conditions was explored in order to systematically span the thermodynamic region included between the saturation curve and the critical temperature. This is indeed the typical operating region of ORC turbines. It was verified that the expansion is influenced by total inlet conditions because of the non-ideal nature of the flow.
Collected data were analysed with the purpose of assessing the influence of the following three parameters on pressure ratios along the nozzle axis: total temperature TT, total pressure PT and total compressibility factor ZT. It was investigated whether parameter ZT alone is sufficient to assess the level of non-ideality of a nozzle flow and thus, to characterize the expansion and predict pressure ratio and Mach number profiles.
The nozzle with exit Mach number equal to 1.5 was also tested in a previous experimental campaign [1] with siloxane fluid MDM (octamethyltrisiloxane, C8H24O2Si3). It was thus possible to carry out a comparison of two different organic vapors flowing in the same nozzle and sharing the same total reduced conditions. As expected, given that the two compounds belong to the same family and exhibit comparable molar masses and molecular complexities, the difference between the measured pressure ratios was around 1% for all considered cases. Moreover, ratios measured in the supersonic portion of the nozzle were always slightly higher for MDM with respect to MM. This is qualitatively consistent with predictions made by 1D nozzle theory coupled with the Van der Waals equation of state, since MDM molecular structure is slightly more complex than MM.
The present experimental investigation provides important validation data for the improvement of state-of-the-art thermodynamic models and of design tools for siloxane fluids in general and for siloxane MM in particular.
[1] A. Spinelli, G. Cammi, S. Gallarini, M. Zocca, F. Cozzi, P. Gaetani, V. Dossena and A. Guardone: “Experimental evidence of non-ideal compressible effects in expanding flow of a high molecular complexity vapor”. In: Experiments in Fluids 59:126 (2018), https://doi.org/10.1007/s00348-018-2578-0
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16:20
20 mins
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Analysis of Transonic Nozzle Loss Generation in Organic Rankine Cycle (ORC) Turbines
Tao Chen, Peter Newton, Miles Robertson, Ricardo Martinez-Botas
Abstract: The Organic Rankine Cycle (ORC) is a technology facilitating the generation of useful mechanical power, from a wide variety of low-temperature heat sources. The specific application considered in this paper is heavy-duty vehicles, as part of a secondary (or bottoming) cycle. A single stage high-expansion-ratio turbine is a suitable type of expander for this application, the nozzle would be transonic given the high expansion ratios. This paper analyses the loss generation mechanisms within the transonic nozzle, with a view towards improving the design of high-pressure ratio turbine expanders for an increasing stage efficiency.
This paper simulates the transonic flow of R1233zd(E) in a linear nozzle blade cascade by means of Computational Fluid Dynamics (CFD). The cascade is designed to achieve a uniform outlet flow Mach number of 2. An initial analysis uses irreversible entropy generation rate to study and explain the loss generating mechanisms in the nozzle under on- and off-design conditions, and the entropy loss coefficient is used to quantitively analyse the loss distribution. The loss generated in the expansion process takes 28.2 % of the overall loss, and the loss in the region after expansion takes the remaining 71.8 %.
The impact of real-gas properties is then considered by comparing with an equivalent air cascade. The loss generated in the expansion process for the R1233zd(E) cascade is 19.7 % larger than that for air due to a larger non-ideal expansion loss. The loss generated in the region after the expansion process for the R1233zd(E) cascade is 11.4 % lower than that for air due to a lower loss caused by trailing-edge wave and trailing-edge wake. The overall loss is 4.4 % lower in the R1233zd(E) cascade; conversely, the total pressure loss coefficient is 13.8 % larger in the R1233zd(E) cascade than that in the air cascade. This is explained by an analysis of Gibbs entropy formula for a real-gas which shows that the stagnation pressure loss is attributed to both an entropy gain and a real gas effect.
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16:40
20 mins
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Method of Characteristics-based Design and Numerical Simulation of a Micro-ORC Supersonic Turbine Nozzle Ring
Marta Zocca, Antti Uusitalo, Teemu Turunen-Saaresti, Alberto Guardone
Abstract: A waste heat recovery micro-ORC test rig was recently built at the Laboratory of Fluid Dynamics, Lappeenranta University of Technology. The system uses the exhaust gases from a Diesel engine as heat source and linear siloxane MDM (Octamethyltrisiloxane C8H24O2Si3) as working fluid. The prime mover of the ORC is a hermetic high-speed turbo-generator-feed pump, in which the working fluid also acts as lubricant. The system has proven capability to convert low-grade heat to electricity.
In this work, the design of a new turbine nozzle ring for the turbogenerator unit is presented. The turbine is of the radial inflow type, and it is characterized by a highly supersonic flow at stator outlet. For the design of the supersonic portion of the blade passages, a design method based on the method of characteristics is devised, which treats the diverging portion of the blade passage as a planar asymmetric nozzle with a curved mean line. Numerical simulations are performed for the complete stator geometry to assess the behaviour of the flow in design and off-design conditions. Both the design of the stator blades and numerical simulations are performed using accurate and well-established thermodynamic models, which account for the non-ideal behaviour of the working fluid in the range of expected operating conditions.
Results presented in this work provide new guidelines for the design of supersonic stators of ORC turbines. The proposed design method for supersonic planar asymmetric nozzles proved to be suitable to the design of nozzle rings for supersonic radial turbines. In addition to the application presented in this work, the method could potentially be adopted to define baseline geometries for blade optimization problems. Numerical simulations of complete stator geometries give insightful information about the performance of the stator both at the design point and in off-design conditions.
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17:00
20 mins
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Deviation Angle in a Turbine Nozzle Cascade with Convergent Meridional Shape of Flow Path
Dmytro Maksiuta, Leonid Moroz, Maksym Burlaka, Vasileios Pastrikakis
Abstract: Usually, deviation angle for standard turbine cascades with subsonic flow is considered to be insignificant or isn’t taken into account at all while performing 1D/2D simulations.
The existing loss models evaluate deviation angle for subsonic flow taking into account the Mach number and the blade outflow gauging angle. However, the meridional shape of the flow path is often neglected.
Two turbines for which experimental data are available were considered in the article: axial (NASA Energy Efficient Engine (EEE)) and radial (NASA CR-3514). Both presented turbines have convergent meridional shape of the flow path (Figure 1). According to the Craig-Cox and Ainley-Mathieson loss models, deviation angle in the stator cascade for considered flow parameters and blade designs should not exceed 0.2-0.3 degrees. However, experimental reports have shown the deviation angle of approximately 2 degrees.
Conducted CFD simulations of the considered turbines with different meridional shapes have shown that convergence of the flow path meridional shape has significant effect on the cascade deviation angle. It was found out that such effect takes place in both axial and radial turbines.
Based on the analysis of the three-dimensional flow in turbine nozzle cascade a technique for calculating the deviation angle, depending on the meridional shape of the turbine flow path was proposed.
In this article the developed technique is described. Using it, the new calculation of the cascade deviation angle was performed. Obtained calculation results have met good agreement with experimental data.
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