Lewis number

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The Lewis number for a given species $k$ is

$Le_k \equiv \frac{\lambda}{\rho C_p D_k}$

Denoting $D_{th}= \lambda / \rho C_p$ the heat diffusivity coefficient the Lewis number can be expressed as

$Le_k = \frac{D_{th}}{ D_k}$

which is the ratio of the heat diffusion speed to the diffusion speed of species $k$ .

In many combustion models, all species are assumed to diffuse at the same speed and therefore $Le \equiv 1$