A composite slab has two layers of different materials with thermal conductivities k1 and k2. If each layer has the same thickness, the equivalent thermal conductivity of the slab would be

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BPSC AE Paper 5 (Mechanical) 25th March 2022 Official Paper
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Concept:

Thermal resistance:

  • Thermal resistance is defined as the ratio of the temperature difference between the two faces of a material to the rate of flow per unit area.

          

  • Its unit is K/W 
  • The concept of thermal resistance is used to solve composite layer problems.

For a solid plate, thermal resistance is given by:

Since the slabs are in contact with each other side by side. Therefore, the thermal resistances will be in series.

The equivalent thermal resistance to the flow of heat is given by,

Since 

Important Points

Thermal resistance for various shapes: 

Heat conduction through the plane wall

Heat conduction through a hollow cylinder

Heat conduction through the hollow sphere

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