# Biot Number

Written by Jerry Ratzlaff on . Posted in Dimensionless Numbers

## Biot Number

Biot Number ( $$Bi$$ ) (dimensionless number) is the ratio of internal thermal resistance of solid to fluid thermal resistance.  This is used for heat transfer between fluids and solids.

### Biot Number FORMULA

$$biot \; number \;=\; \frac { heat \; transfer \; coefficient \;\;x\;\; characteristic \; length } { thermal \; conductivity }$$

$$Bi=\frac{hl_c}{k}$$

$$Bi=\frac{h_D l_c}{D}$$

Where:

$$Bi$$ = biot number

$$c_d$$ = diffusion coefficient

$$c_m$$ = mass transfer coefficient

$$h$$ = heat transfer coefficient

$$k$$ = thermal conductivity of the body

$$l_c$$ = characteristic length