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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 \equiv \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 = 1

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