Heat Transfer Between Two Surfaces 1

[winchengr]

Is there a simple way to estimate the difference in heat transfer for two cases.
1. Two steel surfaces in contact
2. An small air gap between the two steel surfaces

Would this be approx. the diff in thermal conductivity of air vs. steel?

 

[ko99]

1. Steel surfaces in "contact" typically only touch each other at about 1-2% of the total contact area. The rest of the area is microscopic gaps filled with air. Actual heat transfer depends on surface conditions and contact force. One calculation method is shown here:

http://www.electronics-cooling.com/Resources/EC_Articles/MAY97/article3.htm

2. An air gap is a little easier to calculate if you assume the gap is thin enough to neglect natural convection. Thermal loss (C/W) due to conduction through the air = gap / (conductivity * area)
gap and area in meters, and air conductivity ~0.026 W/m-C at 30C.
Heat transfer by radiation may be negligible, but can be checked here:

http://www.efunda.com/formulae/heat_transfer/radiation/calc_2bodies_enclosure.cfm

Warning: The formulas are simplified and 1-dimensional. Among other things, they assume uniform temperature on the surface and uniform gap.


ko (

http://www.ecooling.biz)

 

[JStephen]

"Actual heat transfer depends on surface conditions and contact force."

I have noticed when a pan of water is starting to boil on the stove, you can push it down onto the burners (electric elements) and it immediately starts to boil better/faster. IE, contact force makes a noticable difference there.

 

[ko99]

Hey, I just found an online calculator. It uses the equations in the article I referred to above. It's at

http://www.coolingzone.com/Mag/Interface.html

ko (

http://www.ecooling.biz)