Heat Transfer
Arithmetic mean area can be used in heat transfer problem to calculate the heat flow by conduction through a cylinder which is

Thick walled
Having the value of Ao/Ai > 2
Thin walled having the value of Ao Ai/< 2
Both B and C

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Heat Transfer
If Prandtl number is greater than the Schmidt number, then the

concentration boundary layer lies inside the thermal boundary layer
thermal & concentration boundary layers are of equal thickness
none of the listed here
thermal boundary layer lies inside the concentration boundary layer

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