Research Papers

Experimental Study of Periodic Free Stream Unsteadiness Effects on Discrete Hole Film Cooling in Two Geometries

[+] Author and Article Information
Daniel D. Borup

Department of Mechanical Engineering,
Stanford University,
Stanford, CA 94305
e-mail: borup@stanford.edu

Danyang Fan, Christopher J. Elkins, John K. Eaton

Department of Mechanical Engineering,
Stanford University,
Stanford, CA 94305

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received October 9, 2018; final manuscript received October 22, 2018; published online January 21, 2019. Editor: Kenneth Hall.

J. Turbomach 141(6), 061006 (Jan 21, 2019) (10 pages) Paper No: TURBO-18-1284; doi: 10.1115/1.4041866 History: Received October 09, 2018; Revised October 22, 2018

Discrete hole film cooling is widely employed to protect turbine blades and vanes from hot combustion gases entering the high-pressure turbine stage. Accurate prediction of the heat transfer near film cooling holes is critical, and high-fidelity experimental data sets are needed for validation of new computational models. Relatively few studies have examined the effects of periodic main flow unsteadiness resulting from the interaction of turbine blades and vanes, with a particular lack of data for shaped hole configurations. Periodic unsteadiness was generated in the main flow over a laidback, fan-shaped cooling hole at a Strouhal number (St = fD/U) of 0.014 by an airfoil oscillating in pitch. Magnetic resonance imaging (MRI) with water as the working fluid was used to obtain full-field, phase-resolved velocity and scalar concentration data. Operating conditions consisted of a hole Reynolds number of 2900, channel Reynolds number of 25, 000, and blowing ratio of unity. Both mean and phase-resolved data are compared to the previous measurements for the same hole geometry with steady main flow. Under unsteady freestream conditions, the flow separation pattern inside the hole was observed to change from an asymmetric separation bubble to two symmetric bubbles. The periodic unsteadiness was characterized by alternating periods of slow main flow, which allowed the coolant to penetrate into the freestream along the centerplane, and fast, hole-impinging main flow, which deflected coolant toward the laidback wall and caused ejection of coolant from the hole away from the centerplane. Mean adiabatic surface effectiveness was reduced up to 23% inside the hole, while mean laterally averaged effectiveness outside the hole fell 28–36% over the entire measurement domain. A brief comparison to a round jet with and without unsteadiness is included; for the round jet, no disturbance was observed inside the hole, and some fluctuations directed coolant toward the wall, which increased mean film cooling effectiveness. The combined velocity and concentration data for both cases are suitable for quantitative validation of computational fluid dynamics predictions for film cooling flows with periodic freestream unsteadiness.

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Grahic Jump Location
Fig. 1

Solid model rendering (with front wall removed) of unsteadiness generation and test sections of shaped hole channel. Airfoil shown at maximum angle of attack.

Grahic Jump Location
Fig. 2

Geometry of the laidback fan-shaped film cooling hole. Side view (a) shows initial hole height and angle, laidback wall angle, (X, Y, Z) origin (left), and S–N coordinate origin (right). Top view (b) shows initial hole width and exit footprint.

Grahic Jump Location
Fig. 3

Time profiles of streamwise and wall-normal velocity in reference ROI. Time is normalized by cycle period.

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Fig. 4

Flow upstream of hole at two key phases: t/T = 0.20 (a)and t/T=0.70 (b). Color contours and vectors indicate streamwise and in-plane velocities, respectively. The data shown are truncated in the spanwise and wall-normal directions to highlight flow near the hole. The reference vector at bottom left shows uy/Vmain=0.5.

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Fig. 5

Phase-resolved streamwise and wall-normal velocities around hole in spanwise centerplane. Each row shows a different time point in the periodic freestream oscillation cycle: t/T = 0 (a), 0.25 (b), 0.5 (c), and 0.75 (d). Indicators highlight changing streamwise flow near laidback wall (lower arrows), boundary layer thickness (brackets at right), and vertical velocities in hole (circles), as well as positive wall-normal velocities above hole AT t/T = 0.5 (dashed rectangle) and fast main flow reaching the hole exit plane at t/T = 0.75 (upper arrow in 5d).

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Fig. 6

Mean flow inside laidback fan hole in the steady (a) and unsteady (b) cases. An asymmetric separation bubble is visible under steady conditions, while two separation regions are evident in the mean unsteady data.

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Fig. 7

Two-view coolant streak isosurfaces of C = 0.1. The first row shows the steady case (a) while subsequent rows show the periodic case at four time points: t/T = 0, 0.25, 0.5, and 0.75 (b)–(e).

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Fig. 8

Color contours of normalized concentration. Within each row, the leftmost contour is in a near-wall plane at Y/D = 0.15, while the remaining contours are in three Streamwise planes at X/D = 8, 16, and 24. The top row shows steady case data (a). The middle and bottom rows show two phases in the periodic case: t/T = 0 (b) and t/T = 0.5 (c). Points with C < 0.05 have been blanked.

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Fig. 9

Adiabatic effectiveness contours inside laidback fan hole. η¯ Values are reported for the steady (a) and periodic (b) freestream cases. The RMS values of periodic fluctuations in η are also shown in (c).

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Fig. 10

Color contours of mean surface effectiveness, η¯, for the steady case (a) and periodic freestream case (b), with RMS of periodic fluctuations (c). Approximate footprint of hole is indicated by black region.

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Fig. 11

Laterally averaged surface effectiveness, 〈η¯〉. Averaging performed over Z/D=±3 range. Solid/dashed curves show 〈η¯〉 for the shaped hole steady and periodic cases, respectively. Gray shading indicates range of periodic fluctuations. Dot-dashed and dotted lines show 〈η¯〉 for the round hole steady and unsteady cases, respectively.

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Fig. 12

Profiles of maximum concentration by streamwise location. Results are computed from C¯ in steady flow (X's), C¯ in periodic flow (triangles), and peak c across all phases of periodic flow (circles).



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