Biot number
From Free net encyclopedia
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.
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Definition
The Biot number is defined as:
- <math>\mathit{Bi} = \frac{h L_C}{\lambda_b}</math>
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<math>
\mathit{L_C} = \frac{V_{body}}{A_{surface}}. </math>
- λb = Thermal conductivity of the body
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Applications
Values of the Biot number larger than 1 imply that the heat conduction inside the body is slower than at its surface, and temperature gradients are non-negligible inside it.
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Heat transfer analog
An analogous version of the Biot number (usually called the "mass transfer Biot number", or <math>\mathit{Bi}_m</math>) is also used in mass diffusion processes:
- <math>\mathit{Bi}_m=\frac{h_m L}{D_{AB}}</math>
where:
- h - overall mass transfer coefficient
- LC - characteristic length
- DAB - mass diffusivity.de:Biot-Zahl