A dimensionless number, Ki, used in heat and mass transfer used to relate drying conditions of materials. It is expressed as the ratio of the external to internal heat or mass transfer intensity:
where q is the heat flux, L is a characteristic length of the body, k is the thermal conductivity, ΔT is the temperature difference, G is the mass velocity, D is the diffusivity, ρ is the density, and n is the specific mass content. They are named after Russian materials scientist Victor Kirpichev (1845–1913).