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Research Papers

Delineating Loss Sources Within a Linear Cascade With Upstream Cavity and Purge Flow

[+] Author and Article Information
Maxime Fiore

Safran Aircraft Engine, CFD Team, Rond point
René Ravaux, 77550 Moissy-Cramayel,
CERFACS,
Toulouse, France
e-mail: fiore@cerfacs.fr

Nicolas Gourdain

ISAE-Supaero,
Department of Aerodynamics,
Energetics and Propulsion, 10 avenue Edouard
Belin, 31055 Toulouse, France
e-mail: nicolas.gourdain@isae-supaero.fr

Jean-François Boussuge

CFD Team,
CERFACS,
42 avenue Gaspard Coriolis,
31057 Toulouse, France
e-mail: boussuge@cerfacs.fr

Eric Lippinois

Safran Aircraft Engine,
Rond point René Ravaux,
77550 Moissy-Cramayel, France
e-mail: eric.lippinois@safrangroup.com

1Corresponding author.

Manuscript received October 16, 2018; final manuscript received April 30, 2019; published online June 12, 2019. Assoc. Editor: Graham Pullan.

J. Turbomach 141(9), 091008 (Jun 12, 2019) (13 pages) Paper No: TURBO-18-1292; doi: 10.1115/1.4043660 History: Received October 16, 2018; Accepted April 30, 2019

Purge air is injected in cavities at the hub of axial turbines to prevent hot mainstream gas ingestion into interstage gaps. This process induces additional losses for the turbine due to an interaction between the purge and mainstream flow. This paper investigates the flow in a low-speed linear cascade rig with upstream hub cavity at a Reynolds number commonly observed in modern low-pressure turbine stages by the use of numerical simulation. Numerical predictions are validated by comparing against experimental data available. Three different purge mass flow rates are tested using three different rim seal geometries. Numerical simulations are performed using a large-eddy simulation (LES) solver on structured grids. An investigation of the different mechanisms associated with the turbine flow including cavity and purge air is intended through this simplified configuration. The underlying mechanisms of loss are tracked using an entropy formulation. Once described for a baseline case, the influence of purge flow and rim seal geometry on flow mechanisms and loss generation is described with the emphasis to obtain design parameters for losses reduction. The study quantifies loss generation due to the boundary layer on wetted surfaces and secondary vortices developing in the passage. The analysis shows different paths by which the purge flow and rim seal geometry can change loss generation including a modification of the shear layer between purge and mainstream, interaction with secondary vortices, and a modification of the flow behavior close to hub compared with a smooth configuration. The study shows the influence of purge flow rate and swirl on the strengthening of secondary vortices in the passage and the ability of axial overlapping rim seal to delay the development of secondary vortices compared with simple axial gaps.

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Figures

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Fig. 1

Cascade rig. Adapted from Schuler et al. [32].

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Fig. 2

Simulation domain including the boundary conditions

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Fig. 3

Leading edge grid refinement at midspan including Cartesian coordinates (x, y, z) and local body-fitted coordinates for wall-resolved requirements (s+, n+, r+)

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Fig. 4

Midspan averaged grid dimension at the wall, configuration A05

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Fig. 5

Pressure coefficient and domain of fluctuation at 4% span height, configuration A05

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Fig. 6

Pressure coefficient around blade at midspan, configuration A05

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Fig. 7

Total pressure loss coefficient downstream of the blade, configuration A0

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Fig. 8

Total pressure loss coefficient downstream of the blade, configuration A05 including the pressure loss coefficient for the standard grid used during the study (60 Mcells) and the refined grid (110 Mcells)

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Fig. 9

Total pressure loss coefficient downstream of the blade, configuration A1

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Fig. 10

Example of a simulation domain discretized in axial subvolumes Vi (black lines). For a simple configuration where only the hub boundary layer is considered, Vi can be split in a subvolume associated with the hub boundary layer (dark grey lines with dots) and a remaining domain out of boundary layers that is simply the subvolume Vi minus the subvolume associated with the hub boundary layer (light grey lines with squares).

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Fig. 11

Viscous entropy production along the domain S(visc, tot), within hub S(visc, hub), shroud boundary layers S(visc, shroud), and in hub-shroud boundary layers S(visc, b.l.) with the restriction to ∂us/∂r contribution that is the hub/shroud wall-normal velocity gradient, configuration A05

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Fig. 12

Streaklines obtained from friction vectors at the shroud, configuration A05

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Fig. 13

Density modes related to horseshoe vortex process at the shroud, configuration A05: 1, horseshoe vortices; 2, suction side leg; 3, pressure side leg

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Fig. 14

Density spectrum from DMD based on shroud skin temporal signal, configuration A05

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Fig. 15

Streak lines on blade suction and pressure side, configuration A05: 1, quasi 2D boundary layer; 2, shroud passage vortex; 3, hub passage vortex; 4, corner vortices; 5, detachment line; 6, separation bubble; 7, re-attachment line; 8, adverse pressure separation

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Fig. 16

Streak lines at hub

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Fig. 17

Three-dimensional DMD modes related to the first harmonics of Kelvin–Helmholtz instability, configuration A05

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Fig. 18

Total temperature distribution on the hub surface (a) at the rim seal interface and (b) density modes related to Kelvin–Helmholtz instability (K-H) at the hub: 1, K-H vortices; 2, suction side leg; 3, pressure side leg

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Fig. 19

Two-dimensional DMD modes related to Kelvin–Helmholtz instability base (a) on a meridional plane extraction face to blade leading edge and (b) at the center of the passage, configuration A05

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Fig. 20

Horseshoe vortex and interaction with cavity flow based on an iso Q-criterion Q = 106 colored by streamwise vorticity, configuration A05: 1, Kelvin–Helmholtz rolling vortices; 2, pressure side leg

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Fig. 21

Density spectrum from DMD based on hub skin temporal signal, configuration A05

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Fig. 22

Viscous entropy production at a constant distance from the blade wall (y+ ≃ 30), configuration A05

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Fig. 23

Viscous entropy production along the domain S(visc, tot), within the blade boundary layer S(visc, ∂us/∂c, blade), where ∂us/∂c contribution is the blade wall-normal velocity gradient, configuration A05

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Fig. 24

Sight from downstream to upstream of secondary flow in the passage obtained using iso Q-criterion Q = 106 colored by vorticity for the configuration A05. The shroud is omitted

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Fig. 25

Viscous entropy production along the domain S(visc, tot) and contribution out of the hub, shroud, and blade boundary layers where secondary vortices are alleged to produce entropy, configuration A05

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Fig. 26

Nondimensionalized total pressure and streamwise vorticity 25% chord length downstream of the blade, configuration A05: 1, boundary layer; 2, corner vortex; 3, passage vortex; 4, trailing shed vortices

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Fig. 27

Integrated pressure coefficient downstream of the blade for the different rim seal geometries depending on the purge flow rate supplied

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Fig. 28

Entropy generation along the simulation domain for configuration A05 and A1

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Fig. 29

Entropy generation at hub boundary layer along the simulation domain for configuration A05 and A1

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Fig. 30

Entropy generation at blade boundary layer along the simulation domain for configuration A05 and A1

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Fig. 31

Entropy generation due to secondary losses along the simulation domain for configuration A05 and A1

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Fig. 32

Radial cut close to the hub colored by the nondimensional radial velocity for (a) axial A and (b) single overlapping B geometry: 1, cavity flow blowing; 2, main annulus ingestion

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Fig. 33

Radial cut close to the hub colored by the nondimensional temperature for (a) axial A and (b) single overlapping geometry B

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Fig. 34

Entropy generation along the simulation domain for A (axial), B (simple), and D (double overlapping) rim seals

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Fig. 35

Entropy generation at hub boundary layer along the simulation domain for A (axial), B (simple), and D (double overlapping) rim seals

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Fig. 36

Entropy generation at blade boundary layer along the simulation domain for A (axial), B (simple), and D (double overlapping) rim seals

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Fig. 37

Entropy generation due to secondary losses along the simulation domain for A (axial), B (simple), and D (double overlapping) rim seals

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