Biot Number
Biot Number
The Biot number tells you whether a body heats or cools uniformly or with a steep internal gradient — key to choosing a transient method.
The Biot number is Bi = h·L/k. where h is the convective coefficient, L a characteristic length and k the conductivity.
Inputs
h := 200 W/(m^2·K)=200 kg/K/s^3
L := 0.02 m=20 mm
k := 50 W/(m·K)=50 kg·m/K/s^3
Form the conduction-to-convection ratio.
Bi := h·L/k=0.08
Results
Below about 0.1 the body is nearly uniform in temperature (lumped-capacitance applies); above it, internal gradients matter and a full conduction solution is needed.
Assumptions & limitations
- Characteristic length = volume/surface area.
- Transient conduction.
- Constant properties.