The Lewis number (Le) is a dimensionless number defined as the ratio of thermal diffusivity to mass diffusivity. It is used to characterize fluid flows where there is simultaneous heat and mass transfer.
It is defined as
where is the thermal diffusivity and is the mass diffusivity, is the thermal conductivity, the mixture-averaged diffusion coefficient, and the specific heat capacity at constant pressure.
It is named after Warren K. Lewis (1882–1975), who was the first head of the Chemical Engineering Department at MIT. Some workers in the field of combustion assume (incorrectly) that the Lewis number was named for Bernard Lewis (1899–1993), who for many years was a major figure in the field of combustion research.
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- W. K. Lewis: The Evaporation of a Liquid Into a Gas In: Transactions of the American Society of Mechanical Engineers, No. 1849, 1922, p. 325-340.
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