Alessandro Mora

Unsteady aerodynamics of a low aspect ratio turbine stage: modeling issues and flow physics

In this paper the three-dimensional unsteady aerodynamics of a low aspect ratio, high pressure turbine stage is studied. Fully unsteady, three-dimensional numerical simulations are performed using the commercial code ANSYS-CFX The numerical model is critically evaluated against experimental data. Measurements were performed with a three-dimensional fast-response aerodynamic pressure probe in the closed-loop test rig operating in the Laboratorio di Fluidodinamica delle Macchine of the Politecnico di Milano (Italy). An analysis is first reported about the strategy to reduce the CPU and memory requirements while performing three-dimensional simulations of stator-rotor interaction in actual turbomachinery. What emerges as the best choice, at least for subsonic stages, is to simulate the unsteady behaviour of the rotor blade row alone by applying the stator outlet flow field as rotating inlet boundary condition. When measurements are available upstream of the rotor the best representation of the experimental results downstream of the stage is achieved. The agreement with the experiments achieved at the rotor exit makes the CFD simulation a key-tool for the comprehension and the interpretation of the physical mechanisms acting inside the rotor channel (often difficult to achieve using experiments only). Numerical investigations have been carried out by varying the incidence at the vane entrance. Different vane incidence angles lead to different size, position, and strength of secondary vortices coming out from the stator. The configuration is chosen is such a way to isolate the effects of the vortex-blade interaction. Results show that some general trends can be recognized in the vortex-blade interaction. The sense of rotation and the spanwise position of the incoming vortices play a crucial role on their interaction with the rotor vortices, thus determining both the time-mean and the time-resolved characteristics of the stage-exit secondary field.Copyright

Unsteady Aerodynamics of a Low Aspect Ratio Turbine Stage: Modeling Issues and Flow Physics

In this paper the three-dimensional unsteady aerodynamics of a low aspect ratio, high pressure turbine stage are studied. In particular, the results of fully unsteady three-dimensional numerical simulations, performed with ANSYS-CFX, are critically evaluated against experimental data. Measurements were carried out with a novel three-dimensional fast-response pressure probe in the closed-loop test rig of the Laboratorio di Fluidodinamica delle Macchine of the Politecnico di Milano. An analysis is first reported about the strategy to limit the CPU and memory requirements while performing three-dimensional simulations of blade row interaction when the rotor and stator blade numbers are prime to each other. What emerges as the best choice is to simulate the unsteady behavior of the rotor alone by applying the stator outlet flow field as a rotating inlet boundary condition (scaled on the rotor blade pitch). Thanks to the reliability of the numerical model, a detailed analysis of the physical mechanisms acting inside the rotor channel is performed. Two operating conditions at different vane incidence are considered, in a configuration where the effects of the vortex-blade interaction are highlighted. Different vane incidence angles lead to different size, position, and strength of secondary vortices coming out from the stator, thus promoting different interaction processes in the subsequent rotor channel. However some general trends can be recognized in the vortex-blade interaction: the sense of rotation and the spanwise position of the incoming vortices play a crucial role on the dynamics of the rotor vortices, determining both the time-mean and the time-resolved characteristics of the secondary field at the exit of the stage.

Impeller–Vaned Diffuser Interaction in a Centrifugal Compressor at the Best Efficiency Point

Centrifugal compressors find a huge number of applications in industry and in aero-engines; the detailed comprehension of the complex fluid-dynamic mechanisms occurring in these machines is crucial to improve their efficiency and their operating range. The paper presents a study on the impeller–vaned diffuser interaction in a high-performance compressor stage, in the frame of a wide experimental campaign devoted to the comprehension of unsteady flows in centrifugal compressor stages. The paper focuses on the best efficiency operating point. Data were collected in the impeller–vaned diffuser gap by applying a fast response probe. The impeller runs at 12500 RPM and the peripheral Mach number is 0.77. At first, data are reduced to highlight the main flow structures released by the impeller. CFD simulations — first verified against experimental data — were also performed to get detailed information of the flow field inside the impeller. The core of the paper is the discussion of the impeller–diffuser interaction. Results evidence the effect of the diffuser on the impeller in terms of static pressure and flow velocity. Moreover, the interaction process makes the average flow rate discharged by an impeller channel and the power exchange to be unsteady.Copyright

Flow Evolution in a One and a Half Axial Steam Turbine Stage Under Different Operating Conditions

In order to investigate in detail the performance of steam turbine stages the Low Speed Test Rig at Politecnico di Milano has been adapted. The setup consists of a one and an half turbine stage with an inlet guide vane. Two kind of experimental approaches are planned in the project: the first, denominated “performance”, has been carried out by the OGTL department of GE OilG this large amount of data will be used to validate the results of the CFD simulation carried out in the design stage. In this paper the preliminary findings of the steady flow-field will be presented as the basis for further analysis.© 2015 ASME

Impeller–Vaned Diffuser Interaction in a Centrifugal Compressor at Off Design Conditions

As centrifugal compressors find a huge number of applications in industry and in aero-engine, the detailed analysis and comprehension of the impeller–vaned diffuser interaction is of interest to improve the efficiency and the operating range. The paper presents the results of a wide experimental campaign devoted to the understanding of the impeller–diffuser interaction; in particular the paper focuses on the features occurring when the compressor works in off design conditions. Data were taken at 3 operating points (near surge, best efficiency and maximum flow rate point) in the impeller–vaned diffuser gap by a fast response probe; the single stage compressor runs at 12500 RPM giving peripheral Mach number of 0.77. At first, data are reduced to evidence the main impeller flow features which are compared at different flow rates. Furthermore time mean diffuser effects on the impeller are commented and finally the impeller–diffuser interaction is discussed. Results evidence the effect of the diffuser on the impeller, mainly in terms of static pressure and flow velocity, which have a strong dependence on the flow rate.Copyright