The thermal conductivity of thermal insulation should be the sum of the following transport mechanisms:
- conduction through the solid
- conduction through the gas (collisons between molecules in a brownian transport)
- a coupling term between the solid and gas conduction
This mysterious coupling term was needed to “explain” why the thermal conductivity drops more than the thermal conductivity of the gas after evacuation to a good vacuum (10e-1torr)
The coupling term was observed in four cases:
- evacuation of glass wool
- evacuation of perlite
- evacuation of a bed of glass beads
- evacuation of Aerogel
In all these cases, the coupling term was needed to explain the decrease of thermal conductivity due to evacuation. In the case of glass beads, the decrease is about 6 times the thermal conductivity of air. A bed of glass beads can be very permeable and natural convection is not excluded as explanation. However, in the other cases, natural convection is not obvious.
The coupling term is explained as a kind of short circuit like shown in the following graph.
There should be an extra heat transport system in the gas (on top of the normal gas conduction) in the neighbourhoud of the point contact, shown by the blue arrows. Today, nobody seems to be able to explain the nature of this transport. But I know a bright physicist at the KU Leuven in Belgium and I surely will consult him about this.
According to Richard Feynman, turbulence is the most important unsolved problem of classical physics and maybe the coupling term will be the second most.