What is the Biot Number?

The Biot number (Bi) is a dimensionless quantity used in heat transfer calculations. It provides a simple ratio between the internal thermal resistance of a body and the external thermal resistance at its surface (convection). In thermodynamic engineering, understanding the Biot number is crucial for determining whether the temperature within a body can be treated as uniform or if significant temperature gradients exist.

The Biot Number Formula

The formula for calculating the Biot number is quite straightforward:

Bi = (h × Lc) / k

• h: Convective heat transfer coefficient (W/m²·K).
• Lc: Characteristic length (m), often calculated as the volume of the body divided by its surface area.
• k: Thermal conductivity of the material (W/m·K).

How to Use This Calculator

Using our Biot number calculator online is simple. Enter the heat transfer coefficient (h) provided by your environment (fluid flow), the characteristic length of your object (L), and the thermal conductivity of your material (k). Once you hit calculate, the tool will provide the Bi value and a basic interpretation of the result.

Why is the Biot Number Important?

In transient heat conduction, the value of the Biot number determines the validity of the Lumped System Analysis. If Bi < 0.1, it is generally accepted that the internal conduction resistance is negligible compared to the surface convection resistance. This means the temperature of the object is nearly uniform throughout its volume at any given time, simplifying complex differential equations into basic algebraic ones.

Frequently Asked Questions

What does it mean if Bi is greater than 0.1?

If the Biot number exceeds 0.1, the internal temperature gradients are significant. In such cases, the Lumped System Analysis is no longer accurate, and you must use Heisler charts or numerical methods to find temperature distributions.

What are the units of the Biot number?

The Biot number is dimensionless. This means that as long as your input units are consistent (SI units like meters, Watts, and Kelvin), the resulting number will have no unit.