A dimensionless number, Fo, used in unsteady-state heat and mass transfer calculations that characterize the connection between the rate of change of temperature, the physical properties, and the dimensions of the body defined as:
where α is the thermal diffusivity, t is time, and l is the length through which conduction takes place. The Fourier number is used together with the Biot number to solve unsteady-state heat transfer problems. In the case where the Biot number is less than 0.1, the time can be calculated from:
where To is the initial temperature and T is the temperature at the centre of the body. The analogous form of the Fourier number used for mass transfer is given by:
(p. 157) where D is the diffusivity. The Fourier number is named after the French physicist and mathematician Jean Baptiste Joseph Fourier (1768–1830).