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Research Papers: Jets, Wakes, and Impingment Cooling

An Experimental Study of Mist/Air Film Cooling With Fan-Shaped Holes on an Extended Flat Plate—Part 1: Heat Transfer

[+] Author and Article Information
Reda Ragab, Ting Wang

Energy Conversion and Conservation Center,
University of New Orleans,
New Orleans 70148, LA

Contributed by the Heat Transfer Division of ASME for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received May 19, 2016; final manuscript received June 21, 2017; published online December 12, 2017. Assoc. Editor: Gennady Ziskind.

J. Heat Transfer 140(4), 042201 (Dec 12, 2017) (12 pages) Paper No: HT-16-1282; doi: 10.1115/1.4037641 History: Received May 19, 2016; Revised June 21, 2017

Motivated by the need to further improve film cooling in terms of both cooling effectiveness and coolant coverage area, the mist/air film cooling scheme is investigated through experiments using fan-shaped holes over an extended downstream length in this study. Both an existing wind tunnel and test facility, used in previous work, have been retrofitted. The first modification was extending the length of the flat plate test section to cover longer distances downstream of the injection holes, up to X/D = 100, in order to investigate whether mist cooling can be harnessed farther downstream where single-phase film cooling is not effective. X represents the axial distance downstream of the cooling hole of diameter D. The second modification was to incorporate fan-shaped (diffusion) holes which are proven to have a higher film cooling efficiency, than cylindrical holes. The objective is to investigate whether mist can further enhance the film cooling performance of the already highly effective fan-shaped holes. A phase Doppler particle analyzer (PDPA) system is employed to measure the droplet size, velocity, and turbulence information. An infrared camera and thermocouples are both used for temperature measurements. Part I is focused on the heat transfer result on the wall. The results show that, at low blowing ratios when the film is attached to the surface, the enhancement of the mist film cooling effectiveness, compared to the air-only case, on the centerline of the hole ranges from 40% in the near hole region to over 170% at X/D = 100. Due to the diffusive nature of the fan-shaped hole, the laterally averaged enhancement is on par with that on the centerline. The significant enhancement over the extended downstream distance from X/D = 40–100 is attributed to the evaporation time needed to evaporate all of the droplets. Each droplet acts as a cooling sink and flies over a distance before it completely vaporizes. This “distributed cooling” characteristic allows the water droplets to extend the cooling effects farther downstream from the injection location. At higher blowing ratios, when the cooling film is lifted off from the surface, the cooling enhancement drops below 40%. Although the enhancement in the near hole region X/D < 40 is about 20% lower than that achieved by using the cylindrical holes, the magnitudes of the mist adiabatic film cooling effectiveness using fan-shaped holes are still much higher than those of the cylindrical holes. Part II of this study is focused on analyzing the two-phase droplet multiphase flow behavior to explain the fundamental physics involved in the mist film cooling.

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Figures

Grahic Jump Location
Fig. 5

Cylindrical hole cooling effectiveness and mist enhancement (Eq. (2)) for M = 0.6 (cases 1 and 2): (a) centerline data, (b) spanwise-averaged data, and (c) zoomed-in view of (a) close to the hole

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Fig. 4

Contour of cylindrical-hole cooling effectiveness (a) case 1 (M = 0.6, air-only film) and (b) case 2 (M = 0.6, mist film)

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Fig. 3

Thermocouples layout (a) base test section (b) extension

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Fig. 2

Coolant hole geometry (a) cylindrical hole (b) fan-shaped hole

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Fig. 1

Schematic of mist cooling experiment facility and instrumentation [32]

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Fig. 6

Contour of cylindrical-hole cooling effectiveness: (a) case 3 (M = 1.0, air-only film) and (b) case 4 (M = 1.0, mist/air film)

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Fig. 7

Cylindrical-hole cooling effectiveness and mist enhancement (Eq. (2)) for M = 1.0 (cases 3 and 4): (a) centerline data and (b) spanwise-averaged data

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Fig. 8

Contour of cylindrical-hole cooling effectiveness: (a) case 5(M = 1.4, air-only film) and (b) case 6 (M = 1.4, mist/air film)

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Fig. 9

Cylindrical-hole cooling effectiveness and mist enhancement (Eq. (2)) for M = 1.4 (cases 5 and 6): (a) centerline data and (b) spanwise-averaged data

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Fig. 10

Contour of cooling effectiveness for M = 0.6 for fan-shaped (diffusion) holes: (a) case 7, air-only film and (b) case 8, mist/air film

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Fig. 11

Cooling effectiveness and mist enhancement (Eq. (2)) for M = 0.6 for fan-shaped (diffusion) holes: (a) centerline data and (b) spanwise-averaged data

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Fig. 12

Centerline cooling effectiveness enhancement produced by fan-shaped (diffusion) holes versus cylindrical holes for M = 0.6: (a) air-only case (Eq. (3)) and (b) mist case (Eq. (4))

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Fig. 14

Contour of cooling effectiveness for M = 1.0 for fan-shaped (diffusion) holes: (a) case 9, air-only film and (b) case 10, mist/air film

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Fig. 15

Cooling effectiveness and mist enhancement (Eq. (2)) for M = 1.0 for fan-shaped (diffusion) holes: (a) centerline data and (b) spanwise-averaged data

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Fig. 16

Centerline cooling effectiveness enhancement produced by fan-shaped (diffusion) holes versus cylindrical holes for M = 1.0: (a) air-only case (Eq. (3)) and (b) air/mist (Eq. (4))

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Fig. 17

Comparison of mist film cooling enhancement (Eq. (2)) over air-only cases between fan-shaped (diffusion) holes (in Fig. 15) and cylindrical holes (in Fig. 7) with M = 1.0: (a) centerline data and (b) spanwise-averaged data

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Fig. 18

Contour of cooling effectiveness for M = 1.4 for fan-shaped (diffusion) holes: (a)case 11, air-only film (b) case 12, mist/air film

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Fig. 13

Comparison of mist film cooling enhancement (Eq. 2) over air-only cases between fan-shaped (diffusion) holes (in Fig. 11) and cylindrical holes (in Fig. 5) with M = 0.6: (a) centerline data and (b) spanwise-averaged data

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Fig. 19

Cooling effectiveness and mist enhancement (Eq. (2)) for M = 1.4 for fan-shaped (diffusion) holes: (a) centerline data and (b) spanwise-averaged data

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Fig. 20

Centerline cooling effectiveness enhancement produced by fan-shaped (diffusion) holes versus cylindrical holes for M = 1.4: (a) air-only case (Eq. (3)) and (b) air/mist (Eq. (4))

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Fig. 21

Comparison of mist film cooling enhancement (Eq. (2)) over air-only cases between fan-shaped (diffusion) holes (in Fig. 19) and cylindrical holes (in Fig. 9) with M = 1.4: (a) centerline data and (b) spanwise-averaged data

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