performance of pyramidal fin arrays

8/23/2019 Performance of Pyramidal Fin Arrays

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Performance of

Pyramidal Fin ArraysYannickCormier

April 12th,2013


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Outline

Background

Objectives

Experimental Procedure and Testing

Calculation Method

Heat Transfer and Pressure Drop Results

Conclusion

2


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Background Heat Exchanger

Important parameters of an heat exchanger

Heat transfer Pressure drop across the fin array Dimensions

Background 3

Global Heat Transfer, Cooler Design, http://www.ghtthx.com/Design.aspx , Consulted March2013
http://www.ghtthx.com/Design.aspxhttp://www.ghtthx.com/Design.aspxhttp://www.ghtthx.com/Design.aspxhttp://www.ghtthx.com/Design.aspx


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Background Straight Cut

Braytons Wire Mesh Heat Exchanger (WMHE) with

straight cut fins

Background 4


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Background Pyramidal Fins

Manufactured using cold spray

Pure aluminium Good thermal conductivity

Melting point at 933 K

Stainless steel 304L Melting point at 1673 K

Background 5


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Background - Nomenclature

The nomenclature used to characterize the fin array tested

12x12x0.035x1.3 AlFin per inch (x axis)

Z

X

Fin per inch (z axis)

Wire diameter in inches Fin height in millimeters

Fin material

Background 6


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Background - Nomenclature

Background 7


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Objectives

Characterize the heat transfer and pressure dropperformances of the pyramidal fins

Compare these performances with traditional fins (straight

cut currently used at Brayton Energy Canada)

Help the design process by providing accurate empirical

correlations

Objectives 8


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Experimental Procedure

Fin Sample

2" x 2" Sample 2" x 4" Sample

PureAluminium Stainless Steel304L PureAluminium Stainless Steel304L

Thermal

Conductance &Total Pressure

Drop

Fin Pressure

Drop

Friction Factor

Experimental Procedure and Testing 9


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Experimental Procedure Pressure Drop

Total Pressure Drop for a 2" x 2" Sample

Fin Pressure Drop on 2" x 4" Sample

10Experimental Procedure and Testing


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Testing Apparatus



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Testing Apparatus Accuracy

Comparison with Kays and London experiment for an

unfinned surface (flat plate) Maximum of 20% of error on the Colburn factor

0 1 2 3 4 5 6 7 8 9 100.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

ExperimentalResults

Kays andLondonExperimentalResults

ReD

StPr2/3



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Calculations Method Heat Transfer

Reynolds

Number

Hydraulic Diameter

Heat Transfer

Coefficient

Log Mean Temperature

Difference

Calculation Method 13


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Calculations Method Heat Transfer

Thermal

Conductance

Heat Flux

Colburn FactorNusselt Number

Calculation Method 14


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Heat Transfer (HT) Results

Traditional straight cut fins are presently used at Brayton

Energy Canada

The theory of bank of tubes is similar to the pyramidal fintested in the point of view of their discontinuity

Pyramidal FinsStraight Cut Fins Bank of Tubes

Heat Transfer and Pressure Drop Results 15


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0 1 2 3 4 5 6 7 8 9 100

2

4

6

8

10

12

14

1618

20

12x12x0.035x1.35Al

ReD

Nu

HT Results- Bank of Tubes

ukauskas separated data in three regimes: Laminar (0 < < 1000) Sub-critical ( 500 < < 200 000) Critical ( > 200 000)

Sub-critical



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HT Results Comparison with traditional fin

0 1 2 3 4 5 6 7 8 9 100

50

100

150

200

250

300

350

400

24x24x0.014x1.4 Al

Straight Cut Al

ReD

h(W/m2K)

Sub-critical regime


Laminar regime


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HT Results Comparison with traditional fin

Higher convective heat transfer coefficient due to theincrease in turbulence

Pyramidal Fins Straight Cut Fins



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19

0 1 2 3 4 5 6 7 8 9 100.0

0.5

1.0

1.5

2.0

2.5

Unfinned Surface

12x12x0.035x1.8 Al

16x16x0.028x2.2 Al

24x24x0.014x1.4 Al

ReD

UA(W/K)

0 1 2 3 4 5 6 7 8 9 100.0

0.5

1.0

1.5

2.0

2.5

ReD

UA(W/K)

Fin density effect on the aluminium samples

Higher fin density results in better thermalconductance

HT Results Aluminium Fins

Heat Transfer and Pressure Drop Results


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HT Results Stainless Steel Fins

Fin density effect on the stainless steel samples Same trend as aluminium samples


0 1 2 3 4 5 6 7 8 9 100.0

0.5

1.0

1.5

2.0

2.5

Unfinned Surface12x12x0.035x2.0SS16x16x0.028x1.5SS24x24x0.014x1.1SS

ReD

UA(W/K)

0 1 2 3 4 5 6 7 8 9 100.0

0.5

1.0

1.5

2.0

2.5

ReD

UA(W/K)


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Height effect on the aluminium samples


0 1 2 3 4 5 6 7 8 9 100.0

0.5

1.0

1.5

2.0

2.5

Unfinned Surface12x12x0.035x1.8 Al

12x12x0.035x2.4 Al

12x12x0.035x1.3 Al

ReD

UA(W/K)

0 1 2 3 4 5 6 7 8 9 100.0

0.5

1.0

1.5

2.0

2.5

ReD

UA(W/K)


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Compromise between the heat transfer convective

coefficient (h) and the total area (Atot)


0 1 2 3 4 5 6 7 8 9 100

100

200

300

400

500

600

700

Unfinned Surface

12x12x0.035x1.8Al

12x12x0.035x2.4Al

12x12x0.035x1.3Al

ReD

h(W/m2K)

0 1 2 3 4 5 6 7 8 9 100

100

200

300

400

500

600

700

ReD

h(W/m2K)


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Compromise between the heat transfer convective

coefficient (h) and the total area (Atot)


ReD = 500

ReD = 1500


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HT Results Stainless Steel Fins

Height effect on the stainless steel samples


0 1 2 3 4 5 6 7 8 9 100.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Unfinned Surface

12x12x0.035x1.0 SS

12x12x0.035x1.4 SS

12x12x0.035x1.9 SS

12x12x0.035x2.0 SS

ReD

UA(W/K)

0 500 1000 1500 2000 2500 30000.0

0.5

1.0

1.5

2.0

ReD

UA(W/K)

0 1 2 3 4 5 6 7 8 9 100.0

0.5

1.0

1.5

2.0

ReD

UA(W/K)

0 1 2 3 4 5 6 7 8 9 100.0

0.5

1.0

1.5

2.0

ReD

UA(W/K)

0 1 2 3 4 5 6 7 8 9 100.0

0.5

1.0

1.5

2.0

ReD

UA(W/K)

0 1 2 3 4 5 6 7 8 9 100.0

0.5

1.0

1.5

2.0

ReD

UA(W/K)


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HT Results Aluminium vs. Stainless Steel

Aluminium is more efficient in terms of heat transfer for

the same fin geometry

0 1 2 3 4 5 6 7 8 9 100.0

0.5

1.0

1.5

2.0

2.5

Unfinned Surface

12x12x0.035x1.3 Al

12x12x0.035x1.4 SS

ReD

UA(W/K)


Pressure Drop (PD) Results


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Pressure Drop (PD) Results Comparison with traditional fin

Similar total pressure drop even if the thermal conductance

for the pyramidal fin is higher


0 2 4 6 8 10 12 14 160

1000

2000

3000

4000

5000

6000

7000

24X24X0.014X1.4 Al

Straight Cut Al

ReD

DifferentialPressure(Pa)


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PD Results Total Aluminium

Better thermal conductance results in higher pressure

drop


0 500 1000 1500 2000 2500 30000

1000

2000

3000

4000

5000

6000

Unfinned Surface

12x12x0.035x1.8 Al

16x16x0.028x2.2 Al

24X24X0.014X1.4 Al

ReD

DifferentialPressure(P

a)

0 500 1000 1500 2000 2500 30000

1000

2000

3000

4000

5000

6000

ReD



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0 500 1000 1500 2000 2500 30000

1000

2000

3000

4000

5000

6000

7000

Unfinned Surface

12x12x0.035x1.8 Al

12x12x0.035x2.4 Al

12x12x0.035x1.3 Al

ReD


PD Results Total Aluminium


Height effect on the aluminium samples

0 500 1000 1500 2000 2500 30000

1000

2000

3000

4000

5000

6000

7000

ReD



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PD Results Friction Factor

Useful design tool Similar to the Moody chart

Laminar drop at low Reynolds number Close to constant friction factor in the turbulence

region


0 2 4 6 8 10 12 14 160.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

12x12x0.035x1.3 Al

16x16x0.028x1.4 Al

24x24x0.014x1.8 Al

ReD

DarcyFrictionFactor(f)


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Conclusion

The pyramidal fins outperform traditional straight cut

fins at the same fin density while having the samepressure drop

For the pyramidal fins, higher thermal conductanceresults in higher pressure drop, which is expected

Fin density increases the thermal conductance andthe pressure drop

Fin height influence depends on the compromisebetween the total area and the convective heat

transfer coefficient

Conclusion 30


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Future Work

Need an efficiency index that includes the thermal

performance and the pressure drop effects

Obtain samples at same height with different fin

densities

Conclusion 31

performance of pyramidal fin arrays

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