Biot Number

The Biot Number (Bi) is a dimensionless quantity used in heat transfer calculations, comparing the heat transfer resistance inside of an object to the heat transfer resistance across the boundary layer of the object. It is defined as the product of the convective heat transfer coefficient (h) and the characteristic length of the object (L_c), divided by the thermal conductivity of the object (k).

Typically, the characteristic length is defined as the volume of the object divided by its surface area. In practical use, if the Biot Number is much smaller than 1, it indicates that the rate of heat conduction within the object is fast compared to the rate of convective heat transfer across the object's boundary, leading to a uniform temperature distribution within the object. This condition is often termed 'lumped system analysis'. Conversely, if the Biot Number is large, significant temperature gradients may exist within the object.

It is commonly used in engineering and thermodynamics when designing systems for cooling or heating, such as heat sinks, radiators, or when analyzing the rate of temperature change within a solid object subjected to a fluid environment.