Multi-layer insulation (MLI) is thermal insulation composed of multiple layers of thin sheets and is often used on spacecraft and cryogenics. Also referred to as superinsulation, MLI is one of the main items of the spacecraft thermal design, primarily intended to reduce heat loss by thermal radiation. In its basic form, it does not appreciably insulate against other thermal losses such as heat conduction or convection. It is therefore commonly used on satellites and other applications in vacuum where conduction and convection are much less significant and radiation dominates. MLI gives many satellites and other space probes the appearance of being covered with gold foil which is the effect of the amber-coloured Kapton layer deposited over the silver Aluminized mylar. For non-spacecraft applications, MLI works only as part of a vacuum insulation system. For use in cryogenics, wrapped MLI can be installed inside the annulus of vacuum jacketed pipes. MLI may also be combined with advanced vacuum insulation for use in high temperature applications.
Function and design
The principle behind MLI is radiation balance. For example, consider a square meter of a surface in outer space, held at a fixed temperature of 300 K (27 °C; 80 °F), with an emissivity of 1, facing away from the sun or other heat sources. From the Stefan–Boltzmann law, this surface will radiate about 460 W. Now imagine placing a thin (but opaque) layer 1 cm (0.4 in) away from the plate, also with an emissivity of 1. This new layer will cool until it is radiating 230 W from each side, at which point the net heat flows are balanced. The new layer receives 460 W from the original plate. This layer also radiates 460 W in total; half is radiated back to the original plate, and half to space. The original surface still radiates 460 W, but gets 230 W back from the new layer, for a net loss of 230 W. Overall, the radiation losses from the surface are reduced by half by adding the additional layer.
More layers can be added to reduce the loss further. The blanket can be further improved by making the outside surfaces highly reflective to thermal radiation, which reduces both absorption and emission. The performance of a layer stack can be quantified in terms of its overall heat transfer coefficient U, which defines the radiative heat flow rate Q between two parallel surfaces with a temperature difference Δ T {\displaystyle \Delta T} and area A as
Q = U A Δ T . {\displaystyle Q=UA\Delta T.}
Theoretically, the heat transfer coefficient between two layers with emissivities ϵ 1 {\displaystyle \epsilon _{1}} and ϵ 2 {\displaystyle \epsilon _{2}} , at absolute temperatures T 1 {\displaystyle T_{1}} and T 2 {\displaystyle T_{2}} under vacuum, is
U = σ ( T 1 2 + T 2 2 ) ( T 1 + T 2 ) 1 1 / ϵ 1 + 1 / ϵ 2 − 1 , {\displaystyle U=\sigma (T_{1}^{2}+T_{2}^{2})(T_{1}+T_{2}){\frac {1}{1/\epsilon _{1}+1/\epsilon _{2}-1}},}
where σ = 5.67 × 10 − 8 {\displaystyle \sigma =5.67\times 10^{-8}} Wm−2K−4 is the Stefan-Boltzmann constant. If the temperature difference is not too large ( | Δ T | << ( T 1 + T 2 ) / 2 {\displaystyle |\Delta T|<<(T_{1}+T_{2})/2} , then a stack of N of layers, all with the same emissivity ϵ {\displaystyle \epsilon } on both sides, will have an overall heat transfer coefficient
U = 4 σ T 3 1 ( N − 1 ) ( 2 / ϵ − 1 ) , {\displaystyle U=4\sigma T^{3}{\frac {1}{(N-1)(2/\epsilon -1)}},}
where T = ( T 1 + T 2 ) / 2 {\displaystyle T=(T_{1}+T_{2})/2} is the average temperature of the layers. Clearly, increasing the number of layers and decreasing the emissivity both lower the heat transfer coefficient, which is equivalent to a higher insulation value. In space, where the apparent outside temperature could be 3 K (cosmic background radiation), the exact U value is different.
… excerpt ends here. Continue reading the full article.
