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Research Papers

Experimental Investigation of Turbine Stage Flow Field and Performance at Varying Cavity Purge Rates and Operating Speeds

[+] Author and Article Information
Johan Dahlqvist

KTH Royal Institute of Technology,
Stockholm SE-100 44, Sweden
e-mail: jdahlq@kth.se

Jens Fridh

KTH Royal Institute of Technology,
Stockholm SE-100 44, Sweden

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received August 22, 2017; final manuscript received October 26, 2017; published online December 20, 2017. Editor: Kenneth Hall.

J. Turbomach 140(3), 031001 (Dec 20, 2017) (10 pages) Paper No: TURBO-17-1129; doi: 10.1115/1.4038468 History: Received August 22, 2017; Revised October 26, 2017

The aspect of hub cavity purge has been investigated in a high-pressure axial low-reaction turbine stage. The cavity purge is an important part of the secondary air system, used to isolate the cavities below the hub level from the hot main annulus flow. A full-scale cold-flow experimental rig featuring a rotating stage was used in the investigation, quantifying main annulus flow field impact with respect to purge flow rate as it was injected upstream of the rotor. Five operating speeds were investigated of which three with respect to purge flow, namely, a high loading design case, and two high-speed points encompassing the peak efficiency. At each of these operating speeds, the amount of purge flow was varied from 0% to 2%. Observing the effect of the purge rate on measurement plane averaged parameters, a minor flow angle decrease and Mach number increase is seen for the low speed case, while maintaining near constant values for the higher operating speeds. The prominent effect due to purge is seen in the efficiency, showing a linear sensitivity to purge of 1.3%-points for every 1% of added purge flow for the investigated speeds. While spatial average values of flow angle and Mach number are essentially unaffected by purge injection, important spanwise variations are observed and highlighted. The secondary flow structure is strengthened in the hub region, leading to a generally increased over-turning and lowered flow velocity. Meanwhile, the added volume flow through the rotor leads to higher outlet flow velocities visible at higher span, with associated decreased turning. A radial efficiency distribution is utilized, showing negative impact through span heights from 15% to 70%. Pitchwise variation of investigated flow parameters is significantly influenced by purge flow, making this a parameter to include for instance when evaluating benefits of stator clocking positions.

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Figures

Grahic Jump Location
Fig. 1

Cross section of the investigated stage with indicated measurement points and detail of the seal

Grahic Jump Location
Fig. 2

Close up of pneumatic probe head with incorporated thermocouple

Grahic Jump Location
Fig. 3

Stage efficiency of investigated operating points, normalized to a maximum of 1 at νtot-stat=0.55 with 0% purge

Grahic Jump Location
Fig. 4

Rotor inlet flow field over one vane pitch in terms of Mach number (top) and relative flow angle (bottom) (measurement plane 2) at νtot-stat 0.43 without the influence of purge flow

Grahic Jump Location
Fig. 5

Rotor outlet flow field over one vane pitch in terms of Mach number (top) and relative flow angle (bottom) (measurement plane 3) at νtot-stat 0.43 without the influence of purge flow

Grahic Jump Location
Fig. 6

Rotor inlet spanwise Mach number (top) and relative flow angle (bottom) at zero-purge for speeds νtot-stat 0.3–0.75

Grahic Jump Location
Fig. 7

Rotor outlet spanwise Mach number (top), relative flow angle (center), and normalized rotor efficiency according to Eq.(4) (bottom) at zero-purge for speeds νtot-stat 0.3–0.75

Grahic Jump Location
Fig. 8

Spanwise variation of rotor outlet Mach number (left), indication of the pitchwise Mach number variation at each span location (right) for investigated speeds of νtot-stat 0.43–0.65 (from top to bottom)

Grahic Jump Location
Fig. 9

Rotor outlet flow field over one vane pitch in terms of Mach number (top) and relative flow angle (bottom) (measurement plane 3) at νtot-stat 0.43 with 2% of purge flow

Grahic Jump Location
Fig. 10

Spanwise variation of rotor outlet relative flow angle (left), and rotor efficiency (right) for investigated speeds of νtot-stat 0.43–0.65 (from top to bottom)

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