# Fourier Number

Written by Jerry Ratzlaff on . Posted in Dimensionless Numbers

Fourier number, abbreviated as Fo, also called Fourier modulus, a dimensionless number, is the ratio of heat conduction rate to the rate of thermal energy storage in a solid.  This is mainly used in unsteady state heat transfer.

## Fourier number formula

 $$\large{ Fo = \frac{\alpha \; t}{l_c^2 } }$$

### Where:

$$\large{ Fo }$$ = Fourier number

$$\large{ l_c }$$ = characteristic length

$$\large{ \alpha }$$  (Greel symbol alpha) = thermal diffusivity

$$\large{ t }$$ = time