1

INVESTIGATION INTO USING LIQUID CRYSTAL THERMOGRAPHY FOR

MEASURING HEAT TRANSFER COEFFICIENTS AND WALL

TEMPERATURE PROFILES AT INLETS AND UNDERDEVELOPED REGIONS

Department of Mechanical and Aeronautical Engineering,University of Pretoria, South Africa,

Japie van der Westhuizen, Jaco Dirker and Josua P Meyer

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Presentation Outline

1. Introduction

2. Experimental Facility

3. Test Setup

4. Experimental Procedure

5. Data Capturing & Reduction

6. Initial Results

7. Conclusion

8. Questions

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1. Introduction

• Inlet and underdeveloped region heat transfer data lacking

• Proposed experimental setup to address this issue

• Local as well as average Nusselt numbers obtainable

• Inlet regions heat transfer is dominant in short heat exchangers

• Local wall temperatures needs to be measured

• Implementation of Liquid Crystal Thermography (LCT)

• Methodology is developed for using LCT in tube in tube heat exchangers

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1. Introduction - Entrance lengths

Air Water OilPr ≈0.7 ≈7 >100

4.6 46.2 >6600.66 0.66 0.66

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1. Introduction - Purpose

• Obtain a better understanding of heat transfer at entrance regions

and underdeveloped flow regions

• Employing Liquid Crystal Thermography (LCT) as primary surface

measurement technique

• Develop a test methodology for using LCT in water or clear fluid

based heat exchangers

• Calculate local heat transfer coefficients at inlet and

underdeveloped regions in heat exchangers

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1. Introduction – Liquid Crystal Thermography

• Liquid Crystal thermography is the technique of using Thermochromic Liquid Crystals (TLC’s) for measuring temperature

• TLC’s change their reflected colour as a function of it’s own temperature

• Accuracy is correlated as 0.1% of the TLC’s bandwidth

Copper tube coated with TLC

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2. Experimental Facility

• Outline of Experimental Facility:

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3. Test Setup

• The Test section consists of three concentric tubes:

• Tj is the bulk fluid temperature in the Annulus• Tw is the wall temperature of the inner pipe, measured using LCT• Ti is the bulk fluid temperature of the inner tube, measured using

thermocouples

Annulus

Inner

Measuring rod

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Measuring rod

Copper Inner Tube (Tube 2)

Perspex Annular Tube (Tube 1)

TLC coated area

3. Test setup - Cooled Annulus (CA) Configuration

Annulus

280 mm

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Measuring rod

Copper Inner Tube (Tube 2)

Perspex Annular Tube (Tube 1)

TLC coated area

3. Test setup - Heated Annulus (HA) Configuration

Annulus

280 mm

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3. Test Setup – Mechanical Imaging Mechanism

Camera with LED lighting

Rollers

Rotating cage

Stepper motor

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4. Experimental Procedure• All measurements taken at steady state conditions (± 0.2 °C over 3 min)

• Wait until steady state was reached

• Thermocouple readings are taken continually during digital imaging

• Photos are taken in “rings”:

• Rig rotates camera around the heat exchanger

• Stops at 10 pre-set increments around circumference to take photos

• Continues until a full rotation is completed

• 7 rings covers an axial length of 280mm

• Process is repeated until the whole regions is imaged

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4. Experimental Procedure

Switch pumps on to correct

flow rates and set inlet temp

Wait for steady state conditions

Start logging thermocouple

data

Image single ring of inner

tube

Move camera to correct axial

position

Stop thermocouple

logging

No

Last ring?

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4. Experimental Procedure – Operating Conditions• Operating conditions for experimental heat exchanger:

Cooled annular cases Heated annular cases

CA 1 CA 2 CA 3 CA 4 CA 5 HA 1 HA 2 HA 3 HA 4

1000 2500 5000 10 000 13 800 1000 2500 5000 8000

41.8 42.9 43.5 42.7 43.4 20.8 20.5 20.5 20.2

3600 3600 3600 3600 3600 3700 3700 3700 3700

21.2 21 20.7 20.6 20.3 41.1 41.6 41.8 41.8

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5. Data Capturing & Reduction

• Original Photo:

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5. Data Capturing & Reduction

• Read into computerised script and crop:

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5. Data Capturing & Reduction

• Select the edges of the inner tube and extract data region:

40mm

10mm

1.5mmSamplingarea

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5. Data Capturing & Reduction

• Take RGB values from the pixels inside the data region• Convert RGB to HSV colour system• Use Calibration curve to convert hue angle from pixel into

temperature:

20

25

30

35

40

0.0 0.2 0.4 0.6

Tem

pera

ture

[°C

]

Normalized hue angle [-]

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5. Surface temperature (LCT) measurements

• Liquid Crystal Thermography is implemented in order to obtain local wall temperature distributions on the inner tube’s outer wall:

180° 27.5 27.1 25.8 30.3 29.1 29.5 27.7‐144° 27.5 25.1 25.2 29.4 28.3 27.7 27.0‐108° 27.4 25.0 24.7 28.7 27.3 27.5 27.1‐72° 27.5 25.1 24.7 28.1 26.2 26.3 25.9‐36° 25.2 27.2 25.8 25.1 25.40° 25.1 27.2 25.8 25.3 25.3

36° 24.6 27.2 26.0 25.9 25.472° 27.3 25.3 24.3 28.0 27.0 27.1 26.3

108° 27.3 24.9 24.8 30.4 28.5 29.3 27.9144° 27.5 25.3 24.9 32.4 30.9 30.9 29.7180° 27.5 27.1 25.8 30.3 29.1 29.5 27.7

Tw

Rad

ial p

ositi

on [°

]

0

i,0i,1i,2i,3i,4i,ji,j+1i,n

1234jj+1n

w,1 w,2 w,...

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5. Measuring Rod Measurements

• Thermocouple stations are used to construct an axial temperature profile for the inner tube fluid:

20.5

21.0

21.5

22.0

22.5

23.0

0 200 400 600 800 1000

T i[°

C]

Axial position from annulus inlet in mm

Inner tube temperature profile

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5. Measuring Rod Measurements

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6. Heat transfer Coefficient Results

Local heat transfer coefficients :

0

2000

4000

6000

8000

10000

12000

0 100 200 300

h a[W

/m2 K

]

Axial position from inlet [mm]

Re1000Re2500Re5000Re10000Re13800u1

Rei ≈ 3670CA 1 (Re ≈ 1 000)CA 2 (Re ≈ 2 500)CA 3 (Re ≈ 5 000)CA 4 (Re ≈ 10 000)CA 5 (Re ≈ 13 800)

0

1000

2000

3000

4000

5000

0 100 200 300

h a[W

/m2 K

]

Axial position from inlet [mm]

Re1000Re2500Re5000Re8000u1

HA 1 (Rea ≈ 1 000)HA 2 (Rea ≈ 2 500)HA 3 (Rea ≈ 5 000)HA 4 (Rea ≈ 8 600)

Rei ≈ 3740

Cooled annulus: Heated annulus:

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6. Heat transfer coefficient Results

Averaged heat transfer coefficients at the inlet compared with the literature:

0

1000

2000

3000

4000

5000

6000

7000

0 5000 10000 15000

h a[W

/m2 K

]

Reynolds number

Current study1 CVMcAdamsDittus and BoelterGnielinski (0.28m)Gnielinski (1m)Dirker and Meyer

Averaged280mm CVMc AdamsDittus and BoelterGnielinski (0.28m)Gnielinski (1m)Dirker and Meyer

0

500

1000

1500

2000

2500

3000

3500

4000

0 5000 10000

h a[W

/m2 K

]

Reynolds number

s

Cooled annulus: Heated annulus:

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7. Conclusion

• Promising signs of using Liquid Crystal Thermography for directly measuring surface temperature inside a water-based tube-in-tube heat exchanger.

• This data may be used to determine local heat transfer coefficients at the inlet and underdeveloped regions of different types of heat exchangers.

• Inlet and underdeveloped regions amounts for a big portion of the total heat transfer in the case of a short heat exchangers, as seen when compared with the literature.

• It may further be used as temperature visualization in an array of different type of heat exchangers including micro channel research.

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8. Questions

[1]J. Dirker, J.P. Meyer, "Convection Heat Transfer in Concentric Annuli". Experimental Heat and Mass Transfer, 17 (2004) 19-29.[2]V. Gnielinski, "Heat transfer coefficients for turbulent flow in concentric annular ducts". Heat Transfer Engineering, 30 (2009) 431-436.[3]Y.A. Cengel, Heat and Mass Transfer. 3 (2006), Mc Graw Hill,. New York[4]G. Maranzana, I. Perry, D. Maillet, "Mini- and micro-channels: influence of axial conduction in the walls". International Journal of Heat and Mass Transfer, 47 (2004) . 3993-4013[5]C. Camci,, "Introduction to Liquid crystal thermography and a brief review of past studies".[6]T.M Tam, H.K.Tam, A.J Ghajar, "Heat Transfer Measurements for a Horizontal Micro-Tube Using Liquid Crystal Thermography". International Symposium on Heat Transfer and Energy Conservation. 4 (2012)[7]C.R Smith, D.R.Sabatino, T.J Praisner, "Temperature sensing with thermochromic liquid crystals". Experiments in Fluids, (2001) 190-201.

Acknowledgements:The Funding obtained from the NRF, TESP, University of Stellenbosch/ University of Pretoria, SANERI/SANDENI, CSIR, EEDSM Hub and NAC is acknowledged and duly appreciated.

References