Research Papers: Heat and Mass Transfer

Dependence of Film Cooling Effectiveness on Three-Dimensional Printed Cooling Holes

[+] Author and Article Information
Paul Aghasi, Ephraim Gutmark

Department of Aerospace Engineering,
University of Cincinnati,
Cincinnati, OH 45221

David Munday

Research Associate Professor
Department of Aerospace Engineering,
University of Cincinnati,
Cincinnati, OH 45221

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received September 13, 2016; final manuscript received March 27, 2017; published online June 1, 2017. Assoc. Editor: Gennady Ziskind.

J. Heat Transfer 139(10), 102003 (Jun 01, 2017) (15 pages) Paper No: HT-16-1577; doi: 10.1115/1.4036509 History: Received September 13, 2016; Revised March 27, 2017

Film cooling effectiveness is closely dependent on the geometry of the hole emitting the cooling film. These holes are sometimes quite expensive to machine by traditional methods, so 3D printed test pieces have the potential to greatly reduce the cost of film cooling experiments. What is unknown is the degree to which parameters like layer resolution and the choice among 3D printing technologies influence the results of a film cooling test. A new flat-plate film cooling facility employing oxygen-sensitive paint (OSP) verified by gas sampling and the mass transfer analogy and measurements both by gas sampling and OSP is verified by comparing measurements by both gas sampling and OSP. The same facility is then used to characterize the film cooling effectiveness of a diffuser-shaped film cooling hole geometry. These diffuser holes are then produced by a variety of additive manufacturing (AM) technologies with different build layer thicknesses. The objective is to determine if cheaper manufacturing techniques afford usable and reliable results. The coolant gas used is CO2 yielding a density ratio (DR) of 1.5. Surface quality is characterized by an optical microscope that measures surface roughness. Test coupons with rougher surface topology generally showed delayed blow off and higher film cooling effectiveness at high blowing ratios (BR) compared to the geometries with lower measured surface roughness. At the present scale, none of the additively manufactured parts consistently matched the traditionally machined part, indicating that caution should be exercised in employing additively manufactured test pieces in film cooling work.

Copyright © 2017 by ASME
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Fig. 1

(a) Close-up view of the OSP painted aluminum CNC machined coupon as the representative for all tested coupons along with (b) 7-7-7 hole dimensions units are (mm) inches

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Fig. 2

Experimental arrangement: hatched lines show the flow path that can be used to feed the coolant plenum with CO2 or air

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Fig. 3

Detailed experimental dimensions with outlined axis convention. The dots represent gas sampling port locations. The test coupons are the parts manufactured by 3D printing and each contain a row of seven 7-7-7 cooling holes as well as the first row of gas sampling taps (D = 0.1 in, 2.54 mm).

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Fig. 4

AM build orientation for all test coupons: top surface was built at 60 deg build angle, while the cylindrical portion of the film hole interior was built at 90 deg build angle

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Fig. 5

Comparisons of OSP versus gas sampling at BR 1.0 for the middle effectiveness profiles. Gas sampling was used to produce in situ calibration curves for each test coupon.

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Fig. 6

Surface profilometry measurement location

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Fig. 7

Surface inclination with respect to build orientation presented by Kim and Oh [26]

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Fig. 8

Bare aluminum CNC machined 3D roughness plot

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Fig. 9

Surface topology for various test coupons after OSP application with scan side length of 0.5D

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Fig. 10

Round hole validation effort: (a) present study and (b) from Baldauf et al. [29]

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Fig. 11

Schematic of extracted OSP profile that is used for OSP versus gas sampling validation

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Fig. 12

Discharge coefficients for each test coupon for all BR's and coolant plenum pressure ratios, geometries sorted with the highest roughness first and decreasing on top figure

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Fig. 13

Comparison of various test coupons at BR = 1.5, I = 1.50

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Fig. 14

Comparison of various test coupons at BR = 2.0, I = 2.67

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Fig. 15

Comparison of various test coupons at BR = 3.5, I = 8.17

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Fig. 16

Span-averaged film cooling effectiveness for each test coupon at various BR's

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Fig. 17

Span-averaged effectiveness comparison for each BR across various test coupons

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Fig. 18

Area-averaged effectiveness for 0 < Y/D < 12

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Fig. 19

Area-averaged effectiveness of CNC aluminum coupon subtracted from area-averaged effectiveness of each coupon (geach2 gCNC)

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Fig. 20

Individual hole span averages for film effectiveness consistency assessment at BR = 2.5, outermost holes are omitted




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