# Biot Number

Written by Jerry Ratzlaff. Posted in Dimensionless Numbers

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

## FORMULA

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

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

Where:

$$Bi$$ = biot number

$$h$$ = film coefficient or heat transfer coefficient or convective heat transfer coefficient

$$l_c$$ = characteristic length

$$k$$ = thermal conductivity of the body

$$c_m$$ = mass transfer coefficient

$$c_d$$ = diffusion coefficient