Biot number
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The Biot number (Bi) is a dimensionless number used in unsteady-state (or transient) heat transfer calculations. It is named after the French physicist Jean-Baptiste Biot (1774-1862), and relates the heat transfer resistance inside and at the surface of a body.
[edit] Definition
The Biot number is defined as:
where:
- h = overall heat transfer coefficient
- LC = characteristic length, which is commonly defined as the volume of the body divided by the surface area of the body, such that
- kb = Thermal conductivity of the body
The physical significance of Biot number can be fairly understood by assuming the heat flow from a hot liquid in an cylindrical pipe (steel) to the surroundings. The heat flow experiences two resistances. One of the resistances is given by the wall of the cylindrical pipe and other by the air present near the surface of the cylindrical pipe. In this case the resistance given by air is more than the one given by the wall of the pipe. Hence Biot number is less than one. Imagine, now, that the cylindrical pipe is made of wood, which will resist the heat flow more than the air. So, here Biot number is more than one.
[edit] Applications
Values of the Biot number larger than 0.1 imply that the heat conduction inside the body is slower than at its surface, and temperature gradients are non-negligible inside it.
[edit] Mass transfer analogue
An analogous version of the Biot number (usually called the "mass transfer Biot number", or Bim) is also used in mass diffusion processes:
where:
- h - overall mass transfer coefficient
- LC - characteristic length
- DAB - mass diffusivity.
