Lewis number

Physical key figure
Surname	Lewis number
Formula symbol	${\displaystyle {\mathit {Le}}}$
dimension	dimensionless
definition	${\displaystyle {\mathit {Le}}={\frac {a}{D}}}$
${\displaystyle a}$ Thermal diffusivity ${\displaystyle D}$ Diffusion coefficient
Named after	Warren Lewis
scope of application	thermal diffusion

The Lewis number (after Warren Lewis ) is a dimensionless number in physics . ${\displaystyle {\mathit {Le}}}$

In the case of heat and mass transfer , it represents the ratio of heat conduction to diffusion , expressed as the quotient of thermal diffusivity and diffusion coefficient ${\displaystyle a}$ ${\displaystyle D:}$

${\displaystyle {\mathit {Le}}={\frac {a}{D}}={\frac {\lambda }{D\cdot c_{\mathrm {p} }\cdot \rho }}}$

The Lewis number relates the thickness of the thermal boundary layer to the concentration boundary layer. According to the above equation, the thermal diffusivity can be calculated from the thermal conductivity , the isobaric specific heat capacity and the density of the fluid . ${\displaystyle \lambda }$ ${\displaystyle c_{\mathrm {p} }}$ ${\displaystyle \rho }$

By expanding the dynamic viscosity , the Lewis number can also be represented as the quotient of the Schmidt number and the Prandtl number : ${\displaystyle \eta }$ ${\displaystyle {\mathit {Sc}}}$ ${\displaystyle {\mathit {Pr}}}$

${\displaystyle {\mathit {Le}}={\frac {\mathit {Sc}}{\mathit {Pr}}}={\frac {\eta }{\rho \cdot D}}\cdot {\frac {\lambda }{\eta \cdot c_{\mathrm {p} }}}}$

Individual evidence

1. ^ WK Lewis: The Evaporation of a Liquid Into a Gas In: Transactions of the American Society of Mechanical Engineers , No. 1849, 1922, pp. 325-340.
2. ^ A. Klinkenberg, HH Mooy: Dimensionless Groups in Fluid Friction, Heat, and Material Transfer In: Chemical Engineering Progress , Volume 44, No. 1, 1948, pp. 17-36.
3. ^ Josef Kunes: Dimensionless Physical Quantities in Science and Engineering . Elsevier, 2012, ISBN 0-12-391458-2 , pp. 254 ( limited preview in Google Book search).
4. ↑ tec-science: Lewis number. In: tec-science. May 9, 2020, accessed on June 25, 2020 (German).