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Research Papers

Effect of the Stator Hub Configuration and Stage Design Parameters on Aerodynamic Loss in Axial Compressors

[+] Author and Article Information
Sungho Yoon

GE Global Research,
Munich 85748, Germany
e-mail: yoons@ge.com

Rudolf Selmeier

GE Aviation,
Munich 85748, Germany

Patricia Cargill

GE Aviation,
Cincinnati, OH 45069

Peter Wood

GE Aviation,
Cincinnati, OH 45069

In this comparison, the difference in radius is neglected. To be precise, the seal clearance is located at a smaller radius than the hub flow path. Therefore, even for the same clearance, the effective contribution of the shroud clearance is smaller than that of the hub clearance in a cantilevered stator.

This is not true since there is added energy due to rotating surfaces. However, this impact of rotating surfaces will be discussed in the section, Work due to Rotating Surfaces.

Denton assumed that the driving pressure for the leakage flow in a shrouded turbine rotor is the difference between the upstream total pressure based on the axial velocity and the downstream static pressure. However, in this analysis, the static pressure difference was used to estimate the driving pressure for compressor.

To be precise, the discharge coefficient would be different for each configuration. However, a constant value is used for this study to simplify the problem.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received December 11, 2014; final manuscript received January 8, 2015; published online February 10, 2015. Editor: Ronald Bunker.

J. Turbomach 137(9), 091001 (Sep 01, 2015) (10 pages) Paper No: TURBO-14-1315; doi: 10.1115/1.4029598 History: Received December 11, 2014; Revised January 08, 2015; Online February 10, 2015

The choice of the stator hub configuration (i.e., cantilevered versus shrouded) is an important design decision in the preliminary design stage of an axial compressor. Therefore, it is important to understand the effect of the stator hub configuration on the aerodynamic performance. In particular, the stator hub configuration fundamentally affects the leakage flow across the stator. The effect of the stator hub configuration on the leakage flow and its consequent aerodynamic mixing loss with the main flow within the stator row is systematically investigated in this study. In the first part of the paper, a simple model is formulated to estimate the leakage loss across the stator hub as a function of fundamental stage design parameters, such as the flow coefficient, the degree of reaction, and the work coefficient, in combination with some relevant geometric parameters including the clearance/span, the pitch-to-chord ratio, and the number of seals for the shrouded geometry. The model is exercised in order to understand the effect of each of these design parameters on the leakage loss. It is found that, for a given flow coefficient and work coefficient, the leakage loss across the stator is substantially influenced by the degree of reaction. When a cantilevered stator is compared with a shrouded stator with a single seal at the same clearance, it is shown that a shrouded configuration is generally favored as a higher degree of reaction is selected, whereas a cantilevered configuration is desirable for a lower degree of reaction. Further to this, it is demonstrated that, for shrouded stators, an additional aerodynamic benefit can be achieved by using multiple seals. The second part of the paper investigates the effect of the rotating surfaces. Traditionally, only the pressure loss has been considered for stators. However, the current advanced computational fluid dynamics (CFD) generally includes the leakage path with associated rotating surfaces, which impart energy to the flow. It is shown that the conventional loss coefficient, based on considering only the pressure loss, is misleading when hub leakage flows are modeled in detail, because there is energy addition due to the rotation of the hub or the shroud seals for the cantilevered stator and the shrouded stator, respectively. The calculation of the entropy generation across the stator is a better measure of relative performance when comparing two different stator hub configurations with detailed CFD.

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References

Figures

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Fig. 4

Driving pressure for the stator hub leakage flow

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Fig. 3

Isentropic stage efficiency over stator 3 and rotor 4 in a low speed four-stage compressor (Lange et al. [5])

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Fig. 2

Comparison of shrouded and cantilevered (unshrouded) stators at 50% reaction (Freeman [3])

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Fig. 1

Stator hub configuration: cantilevered (unshrouded) versus shrouded. (a) Cantilevered and (b) shrouded (Wisler [1]).

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Fig. 5

Velocity triangle across a repeating stage

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Fig. 7

Contours of normalized kinetic energy at the stator inlet, (V/U)2, when the flow coefficient (ϕ) is 0.5

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Fig. 6

Flow angles at the stator inlet and the stator exit for the flow coefficient (ϕ) of 0.5 and the work coefficient (ψ) of 0.4

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Fig. 11

Effect of the pitch-to-chord ratio on efficiency loss (Δη, %) at g/h = 0.01, as the degree of reaction is varied at a fixed flow coefficient and the work coefficient (ϕ = 0.5 and ψ = 0.4)

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Fig. 8

Contours of efficiency loss (Δη, %) due to the mixing of the stator leakage flow with the main flow for cantilevered and shrouded stators at g/h = 0.01. (a) Cantilevered, p/c = 0.6 and (b) shrouded with one seal.

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Fig. 9

Reduction in the efficiency loss (Δη, %) by employing multiple seals for a shrouded stator at g/h = 0.01. (a) Δη(one–two seals) and (b) Δη(one–three seals).

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Fig. 10

Effect of the number of seals on efficiency loss (Δη, %) at g/h = 0.01, as the degree of reaction is varied at a fixed flow coefficient and the work coefficient (ϕ = 0.5 and ψ = 0.4)

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Fig. 12

Pitchwise-averaged total pressure drop and total temperature rise across the stator (including the stator cavities). (a) Total pressure drop (psi) and (b) total temperature rise (R).

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Fig. 13

Pitchwise-averaged loss coefficients (ξ, %) across the stator (including the stator cavities). (a) Pressure drop based loss coefficient (%) and (b) entropy change based loss coefficient (%).

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