Unsteady Aerodynamics of Low-Pressure Steam Turbines Operating Under Low Volume Flow
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Nonsynchronous excitation under low volume operation is a major risk to the mechanical integrity of last stage moving blades (LSMBs) in low-pressure (LP) steam turbines. These vibrations are often induced by a rotating aerodynamic instability similar to rotating stall in compressors. Currently extensive validation of new blade designs is required to clarify whether they are subjected to the risk of not admissible blade vibration. Such tests are usually performed at the end of a blade development project. If resonance occurs a costly redesign is required, which may also lead to a reduction of performance. It is therefore of great interest to be able to predict correctly the unsteady flow phenomena and their effects. Detailed unsteady pressure measurements have been performed in a single stage model steam turbine operated with air under ventilation conditions. 3D computational fluid dynamics (CFD) has been applied to simulate the unsteady flow in the air model turbine. It has been shown that the simulation reproduces well the characteristics of the phenomena observed in the tests. This methodology has been transferred to more realistic steam turbine multistage environment. The numerical results have been validated with measurement data from a multistage model LP steam turbine operated with steam. Measurement and numerical simulation show agreement with respect to the global flow field, the number of stall cells and the intensity of the rotating excitation mechanism. Furthermore, the air model turbine and model steam turbine numerical and measurement results are compared. It is demonstrated that the air model turbine is a suitable vehicle to investigate the unsteady effects found in a steam turbine.Keywords:
Stall (fluid mechanics)
Axial Compressor
The utilization of axial-flow compressors (Fig. 9.1) in gas-turbine engines has been relatively recent. The history of this compressor type began following an era when centrifugal compressors (Fig. 9.2) were dominant. It was later confirmed, on an experimental basis, that axial-flow compressors can run much more efficiently. Earlier attempts to build multistage axial-flow compressors entailed running multistage axial-flow turbines in the reverse direction. As presented in Chapter 4, a compressor-stage reaction in this case will be negative, a situation that has its own performance-degradation effect. Today, carefully designed axial-flow compressor stages can very well have efficiencies in excess of 80%. A good part of this advancement is because of the standardization of thoughtfully devised compressor-cascade blading rules.
Axial Compressor
Centrifugal compressor
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Stall (fluid mechanics)
Axial Compressor
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Abstract Measurements of velocity fluctuations in stalled operation of an axial-flow compressor have demonstrated that stalling occurs for the most part in well-defined regions over the compressor annulus. These stalled regions rotate without changing shape in the direction of the blade rotation with a speed proportional to, but of smaller magnitude than, the rotor speed. Two principal types of propagating stall were observed, one with the stalled region or regions extending over part of the blade height, the other with a single stalled region over the full blade height.
Stall (fluid mechanics)
Axial Compressor
Annulus (botany)
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Stall (fluid mechanics)
Axial Compressor
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Rotating stall is a primary constraint for the performance of axial flow compressors. This paper establishes a necessary and sufficient condition for a quadratic feedback controller to locally stabilize the critical equilibrium of the uniform flow at the inception of rotating stall. The explicit condition obtained in this paper provides an effective synthesis tool for rotating stall control.
Stall (fluid mechanics)
Axial Compressor
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Effective active control of rotating stall in axial compressors requires detailed understanding of flow instabilities associated with this compressor regime. Newly designed miniature high frequency response total and static pressure probes as well as commercial thermoanemometric probes are suitable tools for this task. However, during the rotating stall cycle the probes are subjected to flow direction changes that are far larger than the range of probe incidence acceptance, and therefore probe data without a proper correction would misrepresent unsteady variations of flow parameters. A methodology, based on ensemble averaging, is proposed to circumvent this problem. In this approach the ensemble averaged signals acquired for various probe setting angles are segmented, and only the sections for probe setting angles close to the actual flow angle are used for signal recombination. The methodology was verified by excellent agreement between velocity distributions obtained from pressure probe data, and data measured with thermoanemometric probes. Vector plots of unsteady flow behavior during the rotating stall regime indicate reversed flow within the rotating stall cell that spreads over to adjacent rotor blade channels. Results of this study confirmed that the NASA Low Speed Axial Compressor (LSAC) while in a rotating stall regime at rotor design speed exhibits one stall cell that rotates at a speed equal to 50.6% of the rotor shaft speed.
Stall (fluid mechanics)
Axial Compressor
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This chapter contains sections titled: 8.1 Introduction, 8.2 Cascade tests, 8.3 The preliminary design of single-stage fans and compressors, 8.4 Prescribed-curvature compressor-blade design, 8.5 Performance prediction of axial-flow compressors, 8.6 The design and analysis of multi-stage axial compressors, 8.7 Compressor surge, 8.8 Axial-compressor stage stacking, 8.9 Alternative starting arrangements to reduce low-speed stalling, 8.10 Axial-radial compressors, 8.11 Transonic compressors and fans, 8.12 Improved compressor-blade geometries and flutter, 8.13 Axial-flow pump design, References, Problems
Axial Compressor
Reciprocating compressor
Diffuser (optics)
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Experimental studies are carried out at a low speed axial compressor with five different rotor/stator gaps. Analysis of the effect of axial spacing of two successive blade rows on the measured mean flow coefficient at stall inception and on the flow range of compressor under multi-cell rotating stall operating conditions proves that the stator can suppress the flow disturbance in the compressor and strengthen the stability of the compressor. Experimental data show that the stall flow coefficient decreases by reducing the axial spacing of successive blade rows. Moreover, by reducing the axial spacing, the stall pattern transition pace from multi-cell stall to single-cell stall can be shifted. And the compressor directly slips into single-cell stall at 21.0% C(subscript R) axial spacing. By analyzing the pressure fluctuation closed to the surge line, it can be known that there exists an eigenfrequency where the amplitude of the oscillating pressure suddenly and dramatically increases as the compressor runs close to the surge line and this pressure disturbance is relevant to the compressor instability.
Stall (fluid mechanics)
Axial Compressor
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The flow field and the distribution of the flow parameters in the rotating stall regime in a three stage axial flow compressor were obtained in detail using three-hole cylindrical probes containing fast response transducers in association with a digital data acquisition system and an ensemble averaging technique. An appreciable amount of experimental data are presented in this paper with a critical discussion on those.
Stall (fluid mechanics)
Axial Compressor
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Axial flow compressor is one of the most important parts of gas turbine units. Therefore, its design and performance prediction are very important. One-dimensional modeling is a simple, fast and accurate method for performance prediction of any type of compressors with different geometries. In this approach, inlet flow conditions and compressor geometry are known and by considering various compressor losses, velocity triangles at rotor, and stator inlets and outlets are determined, and then compressor performance characteristics are predicted. Numerous models have been developed theoretically and experimentally for estimating various types of compressor losses. In present work, performance characteristics of the axial-flow compressor are predicted based on one-dimensional modeling approach. In this work, models of Lieblein, Koch-Smith, Herrig, Johnsen-Bullock, Pollard-Gostelow, Aungier, Hunter-Cumpsty Reneau are implemented to consider compressor losses, incidence angles, deviation angles, stall and surge conditions. The model results are compared with experimental data to validate the model. This model can be used for various types of single stage axial-flow compressors with different geometries, as well as multistage axial-flow compressors.
Axial Compressor
Stall (fluid mechanics)
Centrifugal compressor
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