Biot number

The Biot number (symbol: Bi, after Jean -Baptiste Biot ) is a dimensionless parameter of thermodynamics and fluid mechanics.

You will like the Fourier number for the calculation of heating and cooling processes used ( unsteady heat conduction ) and gives the heat transport through the surface of a body on the ratio of the heat ( conductive ) resistor body to the heat transfer resistance of the surrounding medium:

For a plane geometry applies:

With

• L = characteristic length of the solid body, such as the layer thickness, which must be heated,
• A = cross-sectional area of the solid
• ? s = thermal conductivity of the solid material (s = solid)
• α = (specific ) heat transfer coefficient of the flowing fluid.

So that the Biot number is formally formed equal to the Nusselt number, in which however instead of the specific thermal conductivity is used? S? L of the fluid, and L has a different meaning.

A large Biot number states that temperature variations within the solid body is greater than the boundary layer of the fluid, so that an improvement of the outer heat transfer, for example, by forced rather than free convection, the process is not significantly accelerated. Important This connection is for example in industrial freezing and thawing of foods.

The similarity theory is that the temperature fields of two geometrically similar structures are similar if their Biot numbers are the same, regardless of the scale.

• Code ( Fluid Mechanics )
• Code ( thermodynamics)