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Research Papers

Improved Accuracy in Jet Impingement Heat Transfer Experiments Considering the Layer Thicknesses of a Triple Thermochromic Liquid Crystal Coating

[+] Author and Article Information
Alexandros Terzis

Group of Thermal Turbomachinery (GTT),
École Polytechnique Fédérale
de Lausanne (EPFL),
Lausanne CH-1015, Switzerland
e-mail: alexandros.terzis@me.com

Stavros Bontitsopoulos, Peter Ott

Group of Thermal Turbomachinery (GTT),
École Polytechnique Fédérale
de Lausanne (EPFL),
Lausanne CH-1015, Switzerland

Jens von Wolfersdorf

Institute of Aerospace Thermodynamics (ITLR),
University of Stuttgart,
Pfaffenwaldring 31,
Stuttgart D-70569, Germany

Anestis I. Kalfas

Laboratory of Fluid Mechanics
and Turbomachinery (LFMT),
Aristotle University of Thessaloniki (AUTH),
Thessaloniki GR-54124, Greece

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received May 13, 2015; final manuscript received September 16, 2015; published online November 3, 2015. Assoc. Editor: Cengiz Camci.

J. Turbomach 138(2), 021003 (Nov 03, 2015) (10 pages) Paper No: TURBO-15-1094; doi: 10.1115/1.4031786 History: Received May 13, 2015; Revised September 16, 2015

This paper examines the applicability of a triple layer of thermochromic liquid crystals (TLCs) for the determination of local heat transfer coefficients using the transient liquid crystal (LC) technique. The experiments were carried out in a narrow impingement channel, typically used for turbine blade cooling applications. Three types of narrow bandwidth LCs (1 °C range) of 35 °C, 38 °C, and 41 °C were individually painted on the target plate of the cooling cavity and the overall paint thickness was accurately determined with an integral coating thickness gauge. The 1D transient heat conduction equation is then implicitly solved for each individual TLC layer on its realistic depth on the painted surface. Local heat transfer coefficients are therefore calculated three times for the same location in the flow improving the measurement accuracy, especially at regions where the LC detection times are too short (stagnation points) or too long (wall-jet regions). The results indicate that if multiple LC layers are used and the paint thickness is not considered, the heat transfer coefficients can be significantly underestimated.

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Figures

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Fig. 1

Local Nusselt number distributions under a single round impingement jet. Adopted from Terzis [11].

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Fig. 2

Experimental setup: (a) 3D representation of the test facility and (b) schematic representation of the test model

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Fig. 3

Schematic representation of the different TLC layers

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Fig. 4

Hot gas temperature extraction for different thermocouples

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Fig. 5

Overall paint thickness measurements

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Fig. 6

LC calibration test rig

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Fig. 7

Color-play of the TLC during calibration procedure: (a) 34.65 °C, (b) 34.71 °C, (c) 34.80 °C, (d) 34.85 °C, (e) 34.91 °C, (f) 35.01 °C, (g) 35.10 °C, (h) 35.15 °C, (i) 35.21 °C, and (j) 35.27 °C

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Fig. 8

Green channel intensity calibration curves

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Fig. 9

Video frames after the initiation of the heating step with base intensity level removed: (a) t = 0 s, reference image, (b) t = 2.2 s, TLC35 at stagnation regions, (c) t = 4.5 s, TLC38 at stagnation regions, (d) t = 7.5 s, TLC41 at stagnation regions, and (e) t = 15 s

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Fig. 10

Normalized and filtered green intensity histories at two different locations in the flow domain: (a) stagnation points → short indication times → high heat transfer and (b) wall-jet regions → late indication times → low heat transfer

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Fig. 11

Time matrices at ReD = 37,500 for all TLC layers: (a) t for TLC35 and z = 16 μm, (b) t for TLC38 and z = 27 μm, and (c) t for TLC41 and z = 38 μm

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Fig. 12

Heat transfer coefficient (h/href) surface contours at ReD = 37,500 for all TLC layers. href = 350 W/(m2K): (a) TLC35 and z = 16 μm, (b) TLC38 and z = 27 μm, and (c) TLC41 and z = 38 μm.

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Fig. 13

Local heat transfer coefficients on the channel centerline (y = 0) for all TLC layers, ReD = 37,500

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Fig. 14

Effect of paint thickness on the evaluation of heat transfer coefficients for all examined experiments

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Fig. 15

Effect of paint thickness on the evaluation of heat transfer coefficients for all examined experiments

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Fig. 16

Individual uncertainty terms for all independent variables as a function of time. ReD = 27,500, z = 28 μm, and θ = 0.56.

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Fig. 17

Effect of coating thickness on the uncertainty term of z at the same θ = 0.46

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Fig. 18

Probability density functions of the overall heat transfer coefficient. h¯=255.7  ± 33.1 W/(m2 K).

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