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 at the surface of a body to the resistance within the body. It helps determine whether a temperature gradient is significant inside an object during heating or cooling.

2. How Does the Calculator Work?

The calculator uses the Biot number equation:

Where:

• \( h \) — Heat transfer coefficient (W/m²K)
• \( L \) — Characteristic length (m)
• \( k \) — Thermal conductivity of the material (W/mK)

Explanation: The Biot number represents the ratio of internal thermal resistance to external thermal resistance. A small Bi (<0.1) suggests uniform temperature within the object, while large Bi indicates significant internal temperature gradients.

3. Importance of Biot Number

Details: The Biot number is crucial in transient heat transfer analysis, determining whether the lumped capacitance method can be used, and analyzing conduction-convection systems.

4. Using the Calculator

Tips: Enter heat transfer coefficient in W/m²K, characteristic length in meters, and thermal conductivity in W/mK. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is characteristic length?
A: It's the ratio of volume to surface area (V/A) for regular shapes. For a sphere it's radius/3, for a cylinder it's radius/2, and for a slab it's half thickness.

Q2: What does a high Biot number indicate?
A: High Bi (>0.1) means internal thermal resistance dominates, creating significant temperature gradients within the object.

Q3: When can lumped capacitance be used?
A: When Bi < 0.1, as temperature can be considered uniform throughout the object.

Q4: How does Biot number relate to Fourier number?
A: Fourier number relates to heat conduction over time, while Biot number relates to conduction vs convection at boundaries.

Q5: What are typical values for heat transfer coefficients?
A: Air (natural convection): 5-25 W/m²K; Air (forced): 10-200 W/m²K; Water: 500-10,000 W/m²K; Boiling water: 3,000-100,000 W/m²K.