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

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

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Heat Transfer
Prandtl number is the ratio of

Momentum diffusivity to thermal diffusivity
Momentum diffusivity to mass diffusivity
Thermal diffusivity to mass diffusivity
Thermal diffusivity to momentum diffusivity

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