heat transfer from extended surfaces: conduction-convection systems (fins)

1

What you Dare to Dream ...

Dare to Analyse !!!

J.G. Pharoah214 – McLaughlin Hall

533-6579

Department of Mechanical EngineeringQueen's University Kingston, Ontario,Canada, K7L 3N6

Mech 346: Heat TransferChapter 3:Heat Transfer from Extended

Surfaces: Conduction-Convection Systems


Dare to Analyse !!!

2


Dare to Analyse !!!


Dare to Analyse !!!

HEATED SURFACE Convectionto fluid

Radiation

Conduction

contactresistance

3


Dare to Analyse !!!

L W

H

dx


Dare to Analyse !!!

4


Dare to Analyse !!!


Dare to Analyse !!!

5


Dare to Analyse !!!


Dare to Analyse !!!

Additional Resource Text

• http://web.mit.edu/lienhard/www/ahtt.html

6


Dare to Analyse !!!


Dare to Analyse !!!

Variable h• Small differences in temperature distribution result in large changes in Q

•The fin becomes less effective as the length increases

•Base gradient is best to visualize fin performance.

7


Dare to Analyse !!!

Same Geometry, Different k

• Flux in now proportional to k dT/dx base gradient misleading in isolation

•As the conductivity decreases, less and less of the fin is working

•Look at effectiveness and efficiency

•Careful of Bi


Dare to Analyse !!!

•In this case, at least, effectiveness is misleading

•K=1 has an effectiveness of over 6 and yet it is very wasteful. Based on the previous figure a proper design of this fin would be about 15% of this length.

8


Dare to Analyse !!!

•Conductivities below 1 (for this geometry convection coefficient) are extremely suspect with a 1D analysis

heat transfer from extended surfaces: conduction-convection systems (fins)

Documents