Definition Fourier number

the thermal fourier number, foh, defined conduction rate rate of thermal energy storage:

where:

α
=



k


c

p


ρ






{\displaystyle \alpha ={\tfrac {k}{c_{p}\rho }}}

thermal diffusivity (si units: m/s)
t characteristic time (s)
l length through conduction occurs (m)

for transient mass transfer diffusion, there analogous mass fourier number, fom, defined by:

where:

d diffusivity (m/s)
t characteristic timescale (s)
l length scale of interest (m)