Number of Biot

The number of Biot (Bi) is a Nombre without dimension used in calculations of transfer of heat in transitional stage. It compares resistances to the transfer of heat inside and the surface of a body.

It is defined by: \ mathit {Bi} = \ frac {H L_C} {\ lambda_b} where:

  • H is the total coefficient of Transfer of heat
  • LC is a characteristic length, usually definite like the volume of the body divided by its surface
\ mathit {L_C} = \ frac {V_ {body}} {A_ {surface}}.
  • λb is the thermal Conductivité body

Values of the number of Biot higher than 1 means than the conduction of heat inside the body is slower than on its surface, and than the gradients of temperature are significant within the body. If the number of Biot of a system is small in front of 1 (one will often use Bi<0,1), that means that internal resistance is negligible, and thus that the temperature can be regarded as uniform inside the body.

This number draws its name from the French physicist Jean-Baptiste Biot (1774-1862).

Another version of the number of Biot (called " commonly; number of Biot of transfer of masse" , \ mathit {Bi} _m) is also used to describe the processes of diffusion of mass:

\ mathit {Bi} _m= \ frac {h_m L} {D_ {AB}}

where:

  • H is the total coefficient of Transfert of mass
  • LC - a characteristic length
  • DAB - the coefficient of diffusivity of mass.

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