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Numerical Analysis of Partial Admission in Axial Turbines
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

HTML clipboard Numerical analysis of partial admission in axial turbines is performed in this work. Geometrical details of an existing two stage turbine facility with low reaction blades is used for this purpose. For validation of the numerical results, experimental measurements of one partial admission configuration at design point was used. The partial admission turbine with single blockage had unsymmetrical shape; therefore the full annulus of the turbine had to be modeled numerically.

The numerical grid included the full annulus geometry together with the disc gaps and rotor shrouds. Importance of various parameters in accurate modeling of the unsteady flow field of partial admission turbines was assessed. Two simpler models were selected to study the effect of accurate modeling of radial distribution of flow parameters. In the first numerical model, the computational grid was two dimensional and the radial distribution of flow parameters was neglected. The second case was three-dimensional and full blades’ span height was modeled but the leakage flows at disc cavity and rotor shroud were neglected. Detailed validation of the results from various computational models with the experimental data showed that modeling of the leakage flow at disc cavities and rotor shroud of partial admission turbines has substantial importance in accuracy of numerical computations. Comparison of the results from two computational models with varying inlet extension showed that modeling of the inlet cone has considerable importance in accuracy of results but with increased computational cost.

Partial admission turbine with admission degree of  ε = 0.524 in one blocked arc and two opposing blocked arcs were tested. Results showed that blocking the inlet annulus in one single arc produce better overall efficiency compared to the two blocked arc model. Effect of varying axial gap distance between the first stage stator and rotor rows was also tested numerically for the partial admission turbine with admission degree of  ε = 0.726. Results showed higher efficiency for the reduced axial gap model.

Computations showed that the main flow leave the blade path down to the disc cavity and re-enter into the flow channel downstream the blockage, this flow would pass the rotor with very low efficiency. First stage rotor blades are subject to large unsteady forces due to the non-uniform inlet flow. Plotting the unsteady forces of first stage rotor blades for partial admission turbine with single blockage showed that the blades experience large changes in magnitude and direction while traveling along the circumference. Unsteady forces of first stage rotor blades were plotted in frequency domain using Fourier transform. The largest amplitudes caused by partial admission were at first and second multiples of rotational frequency due to the existence of single blockage and change in the force direction.

Results obtained from the numerical computations showed that the discs have nonuniform pressure distribution especially in the first stage of partial admission turbines. The axial force of the first rotor wheel was considerably higher when the axial gap distance was reduced between the first stage stator and rotor rows. The commercial codes used in this work are ANSYS ICEM-CFD 11.0 as mesh generator and FLUENT 6.3 as flow solver.

Place, publisher, year, edition, pages
Stockholm: KTH , 2010. , 114 p.
Series
Trita-KRV, ISSN 1100-7990 ; 2010:02
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-12856ISBN: 978-91-7415-390-3 (print)OAI: oai:DiVA.org:kth-12856DiVA: diva2:319327
Public defence
2010-05-21, Sal M2, Brinellvägen 64, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC20100622Available from: 2010-05-17 Created: 2010-05-17 Last updated: 2010-06-22Bibliographically approved
List of papers
1. Effects of multiblocking and axial gap distance on performance of partial admission turbines: A numerical analysis
Open this publication in new window or tab >>Effects of multiblocking and axial gap distance on performance of partial admission turbines: A numerical analysis
2011 (English)In: Journal of turbomachinery, ISSN 0889-504X, E-ISSN 1528-8900, Vol. 133, no 3, 031028- p.Article in journal (Refereed) Published
Abstract [en]

In this paper, the effects of axial gap distance between the first stage stator and rotor blades and multiblocking on aerodynamics and performance of partial admission turbines are analyzed numerically. The selected test case is a two stage axial steam turbine with low reaction blades operating with compressed air. The multiblocking effect is studied by blocking the inlet annulus of the turbine in a single arc and in two opposing blocked arcs, each having the same admission degree. The effect of axial gap distance between the first stage stator and rotor blades is studied while varying the axial gap by 20% compared with the design gap distance. Finally, full admission turbine is modeled numerically for comparison. Performance of various computational cases showed that the first stage efficiency of the two stage partial admission turbine with double blockage was better than that of the single blockage turbine; however, the extra mixing losses of the double blockage turbine caused the efficiency to deteriorate in the downstream stage. It was shown that the two stage partial admission turbine with smaller axial gap than the design value had better efficiency of the first stage due to lower main flow and leakage flow interactions; however, the efficiency at the second stage decreased faster compared with the other cases. Numerical computations showed that the parameters, which increased the axial force of the first stage rotor wheel for the partial admission turbine, were longer blocked arc, single blocked arc, and reduced axial gap distance between the first stage stator and rotor blades.

Keyword
Aerodynamics, Cold storage, Compressed air, Leakage (fluid), Marine engines, Numerical analysis, Rotors, Stators, Steam turbines
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13590 (URN)10.1115/1.4002415 (DOI)000287810000003 ()2-s2.0-79952036294 (Scopus ID)
Note

QC 20100622

Available from: 2010-06-22 Created: 2010-06-21 Last updated: 2017-12-12Bibliographically approved
2. Unsteady Forces of Rotor Blades in Full and Partial Admission Turbines
Open this publication in new window or tab >>Unsteady Forces of Rotor Blades in Full and Partial Admission Turbines
2011 (English)In: Journal of turbomachinery, ISSN 0889-504X, E-ISSN 1528-8900, Vol. 133, no 4, 041017-1-041017-12 p.Article in journal (Refereed) Published
Abstract [en]

A numerical and experimental study of partial admission in a low reaction two-stage axial air test turbine is performed in this paper. In order to model one part load configuration, corresponding to zero flow in one of the admission arcs, the inlet was blocked at one segmental arc, at the leading edge of the first stage guide vanes. Due to the unsymmetrical geometry, the full annulus of the turbine was modeled numerically. The computational domain contained the shroud and disk cavities. The full admission turbine configuration was also modeled for reference comparisons. Computed unsteady forces of the first stage rotor blades showed cyclic change both in magnitude and direction while moving around the circumference. Unsteady forces of first stage rotor blades were plotted in the frequency domain using Fourier analysis. The largest amplitudes caused by partial admission were at first and second multiples of rotational frequency due to the existence of single blockage and change in the force direction. Unsteady forces of rotating blades in a partial admission turbine could cause unexpected failures in operation; therefore, knowledge about the frequency content of the unsteady force vector and the related amplitudes is vital to the design process of partial admission turbine blades. The pressure plots showed that the nonuniformity in the static pressure field decreases considerably downstream of the second stage's stator row, while the nonuniformity in the dynamic pressure field is still large. The numerical results between the first stage's stator and rotor rows showed that the leakage flow leaves the blade path down into the disk cavity in the admitted sector and re-enters downstream of the blocked channel. This process compensates for the sudden pressure drop downstream of the blockage but reduces the momentum of the main flow.

Keyword
STEAM-TURBINE
Identifiers
urn:nbn:se:kth:diva-13598 (URN)10.1115/1.4002408 (DOI)000289956200017 ()2-s2.0-79955137210 (Scopus ID)
Note
QC 20100622 Uppdaterad från accepted till published (20110520).Available from: 2010-06-22 Created: 2010-06-22 Last updated: 2017-12-12Bibliographically approved
3. Numerical Study of Unsteady Flow Phenomena in a Partial Admission Axial Steam Turbine
Open this publication in new window or tab >>Numerical Study of Unsteady Flow Phenomena in a Partial Admission Axial Steam Turbine
2008 (English)In: Proceedings of ASME Turbo EXPO 2008, New York: AMER SOC MECHANICAL ENGINEERS , 2008, 713-722 p.Conference paper, Published paper (Refereed)
Abstract [en]

This paper presents a numerical investigation of unsteady flow phenomena in a two-stage partial admission axial steam turbine. Results from unsteady three-dimensional computations are analyzed and compared with the available experimental data. Partial admission in the present study is introduced into the model by blocking only one segmental arc of the inlet guide vanes. Blocking only one segment (which corresponds to the experimental setup) makes the model unsymmetrical; therefore it is necessary to model the whole annulus of the turbine. The first stage rotor blades experience large static pressure change on their surface while passing the blocked channel. The effect of blockage on the rotor blades' surface pressure can be seen few passages around the blocked channel. Strong changes of the blades' surface pressure impose large unsteady forces on the blades of first stage rotor row.

The circumferential static pressure plots at different cross sections along the domain indicate how the non-uniformity propagates in the domain. A peak pressure drop is seen at the cross section downstream of the first stage stator row. At further downstream cross sections, the static pressure becomes more evenly distributed. Entropy generation is higher behind the blockage due to the strong mixing and other loss mechanisms involved with partial admission. Analysis of the entropy plots at different cross sections indicates that the peak entropy moves in a tangential direction while traveling to the downstream stages. Comparisons of the unsteady three-dimensional numerical results and the experimental measurement data show good agreement in tendency. However some differences are seen in the absolute values especially behind the blockage.

Place, publisher, year, edition, pages
New York: AMER SOC MECHANICAL ENGINEERS, 2008
Keyword
Absolute values, Axial steam turbine, Cross section, Entropy generation, Experimental data, Experimental measurements, Experimental setup, Inlet guide vane, Loss mechanisms, Nonuniformity, Numerical investigations, Numerical results, Numerical studies, Peak pressure, Rotor blades, Static pressure, Strong-mixing, Surface pressures, Tangential directions, Three-dimensional computations, Two stage, Unsymmetrical, Aerodynamics, Engines, Entropy, Fluid dynamics, Marine engines, Pressure effects, Three dimensional, Turbomachine blades, Unsteady flow
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-13605 (URN)10.1115/GT2008-50538 (DOI)000262646400070 ()2-s2.0-69949159808 (Scopus ID)978-0-7918-4315-4 (ISBN)
Conference
53rd ASME Turbo Expo 2008 Berlin, GERMANY, JUN 09-13, 2008
Note
QC 20100622Available from: 2010-06-22 Created: 2010-06-22 Last updated: 2011-09-07Bibliographically approved
4. Numerical Investigation of Partial Admission Phenomena at Midspan of an Axial SteamTurbine
Open this publication in new window or tab >>Numerical Investigation of Partial Admission Phenomena at Midspan of an Axial SteamTurbine
2007 (English)In: Proceedings of 7th European Conference on Turbomachinery,Fluid Dynamics and Thermodynamics, 2007Conference paper, Published paper (Refereed)
Identifiers
urn:nbn:se:kth:diva-13607 (URN)2-s2.0-84925340846 (Scopus ID)
Note
QC20100622Available from: 2010-06-22 Created: 2010-06-22 Last updated: 2010-06-22Bibliographically approved

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