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.

  • $\begingroup$ So show your calculations for conduction convection and radiation. $\endgroup$
    – Solar Mike
    Feb 23, 2022 at 4:09
  • $\begingroup$ Are you sure your frozen water tubing has any gaps at all after expansion? $\endgroup$ Feb 23, 2022 at 13:39
  • $\begingroup$ 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 $\endgroup$
    – Jonathan
    Feb 23, 2022 at 18:20
  • $\begingroup$ $$d_{tube} =10mm$$ $$L_{tube}=100mm$$ 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? $\endgroup$
    – Jonathan
    Feb 24, 2022 at 0:30

1 Answer 1


You mixed up w/mk and w/m^2k. Air properties are 0.03 w/mk and 5w/m^2k, last one also depends on speed - be it wind or convection. The property with 'm' is about bulk conductivity. You need to divide it by thickness, usually linearly depend on thickness. While 'm^2' property is about interfaces usually. Or ready items, where thickness is accounted for or irrelevant. Air gap heat conductivity is almost the same, be it 1m or 1cm. 1cm to 1mm there is some effect. And below 1mm it can be considered closer to a bulk material. And below 100nm it can be considered closer to a vacuum, but on the other hand phonons statt to play a role.


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