Heat Transfer from Extended Surfaces (Fins)

Heat Transfer from Extended Surfaces Extended surfaces may exist in many situations but are commonly used as fins to enhance heat transfer by increasing the surface area available for convection (and/or radiation).

Typical Fin Configurations 38

Fins in Heat Exchangers They have dense arrays of finned tubes or plates. The tubes may be flat (a) or circular (b, c) and the fins plate (a,b) or circular (c). Parallel plate exchangers may be finned or corrugated, single pass (d) or multiple pass (e). Widely used to achieve large heat rates per unit volume, particularly when one or both fluids is a gas. Characterized by large heat transfer surface areas per unit volume (>700 m2/m3), small flow passages, and laminar flow.

where

Percentage error of the two assumptions:

Example 1: A rectangular metal fin having width w = 0.05 m, thickness, t = 0.001m, thermal conductivity, k= 200 w/m.0c and base temperature, Tb= 40 0C exposed to convection to a medium at T∞=20 0C and heat transfer coefficient h = 20 w/m2.0c Assume that the fin is very long, Find the temperature of the fin at distance L = 0.05 m. and Estimate it is the total heat transfer rate?

m = a

Example 2: Aluminium fin of constant diameter 4 mm,, length L = 10 cm , and thermal conductivity, k= 237 w/m.0c exposed to convection to a medium at T∞ and heat transfer coefficient h = 12 w/m2.0c Find the percentage error in the heat transfer sate the infinitely length fin assumption is used instead of adiabatic fin tip assumption?

m = a

Fin Efficiency But Ideally: Afin ?????

See Table 3. 3 and Figures 3. 42 and 3 See Table 3.3 and Figures 3.42 and 3.43 for the efficiencies of common fin shapes

Estimate the fin efficiency ?

Fin Performance Fin effectiveness: Ratio of the fin heat transfer rate qf to the heat transfer rate that would exist without the fin ef should be as large as possible (at least >2) For a very long (infinite) fin

What does mean?

Overall Effectiveness