1. What is the Biot Number?

The Biot number (Bi) is a dimensionless quantity used in heat transfer calculations. It compares the resistance to heat transfer inside a body (conduction) to the resistance at the surface of the body (convection).

2. How Does the Calculator Work?

The calculator uses the Biot number formula:

Where:

• \( h \) — Heat transfer coefficient (W/m²·K)
• \( L \) — Characteristic length (m)
• \( k \) — Thermal conductivity (W/m·K)

Explanation: The Biot number helps determine if temperature gradients inside a body are significant compared to the temperature difference between the surface and the surrounding fluid.

3. Importance of Biot Number

Details: The Biot number is crucial in transient heat transfer analysis. A small Biot number (Bi < 0.1) suggests uniform temperature distribution within the body, while a large Biot number indicates significant internal temperature gradients.

4. Using the Calculator

Tips: Enter heat transfer coefficient in W/m²·K, characteristic length in meters, and thermal conductivity in W/m·K. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is characteristic length?
A: Characteristic length is typically the volume of the object divided by its surface area (V/A). For a sphere it's radius/3, for a cylinder it's radius/2.

Q2: What does a high Biot number mean?
A: A high Bi (>0.1) indicates that conduction inside the body is limiting heat transfer more than convection at the surface.

Q3: What are typical values for heat transfer coefficients?
A: For air (natural convection) h ≈ 5-25 W/m²·K, for water h ≈ 50-1000 W/m²·K, depending on conditions.

Q4: How is Biot number used in lumped capacitance method?
A: Lumped capacitance method can be used when Bi < 0.1, assuming uniform temperature throughout the object.

Q5: What's the difference between Biot and Nusselt numbers?
A: Both are ratios of resistances, but Biot number compares internal to external resistance, while Nusselt number compares convective to conductive heat transfer.