# How much a 0.1 mm air gap between 2 materials will affect heat transfer rate?

I have a disc with drilled holes with a 0.1 mm tolerance. The plate is 6061 T6 Aluminum and I want to insert a polyethylene tube with water in it that is frozen. Assuming some tubes are in full contact with the Aluminum and some have a 0.1 mm air gap. How much will this affect the defrosting time.

Heat transfer coefficient

• K_aluminum = 170 W/m.K
• K_polyethylene = 0.5 W/m.K
• K_air = 2.5 - 10 W/m.K

I have chosen the low range of the air heat transfer coefficient as the air between the tube and aluminum would be relatively still so I assume convection effect is smaller.

further assumptions are that the Aluminum stays at 35C.

my calculations show negligible effect of <1% but my intuition is that the effect is larger as there is no direct contact and air is known as a good insulator.

• So show your calculations for conduction convection and radiation. Feb 23, 2022 at 4:09
• Are you sure your frozen water tubing has any gaps at all after expansion? Feb 23, 2022 at 13:39
• I assume expansion will compensate for the gaps, but it will happen in different times for a tube with a gap compared to one in full contact Feb 23, 2022 at 18:20
• $$d_{tube} =10mm$$ $$L_{tube}=100mm$$ For a case with full contact between tube and metal and assuming metal in constant temperature (p denoting plastic): $$R_1 = \frac{\Delta x_p}{(K_p*A_p)}$$ For the case of an air gap $$R_2 = \frac{\Delta x_p}{(K_p*A_p)} + \frac{1}{(h_{air}*A_p)}$$ $$R{_1 \approx 1.6*10^{-7} [K/W]}$$ $$R_2\approx 1.5*10^{-5} [K/W]$$ I don't understand how why there is no component of length in the convection equation. Wouldn't a bigger air gap act differently than a small one? Feb 24, 2022 at 0:30