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Unsteady Forces of Rotor Blades in Full and Partial Admission Turbines
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.ORCID iD: 0000-0002-1033-9601
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Heat and Power Technology.
2011 (English)In: Journal of turbomachinery, ISSN 0889-504X, 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.

Place, publisher, year, edition, pages
2011. Vol. 133, no 4, 041017-1-041017-12 p.
Keyword [en]
STEAM-TURBINE
Identifiers
URN: urn:nbn:se:kth:diva-13598DOI: 10.1115/1.4002408ISI: 000289956200017Scopus ID: 2-s2.0-79955137210OAI: oai:DiVA.org:kth-13598DiVA: diva2:326155
Note
QC 20100622 Uppdaterad från accepted till published (20110520).Available from: 2010-06-22 Created: 2010-06-22 Last updated: 2012-03-20Bibliographically approved
In thesis
1. Numerical Analysis of Partial Admission in Axial Turbines
Open this publication in new window or tab >>Numerical Analysis of Partial Admission in Axial Turbines
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:nbn:se:kth:diva-12856 (URN)978-91-7415-390-3 (ISBN)
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

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