I am trying to understand the following figure of laminar premixed flames:
The Lewis number is defined as $Le = \frac{\alpha_{mix}}{D_{fuel}}$ where $\alpha_{mix}$ is the thermal diffusivity of the mixture and $D_{fuel}$ is the mass diffusivity of the fuel.
The propane fuel has larger molecular weight than air and methane has lower molecular weight compared to air.
It is not clear to me why mass diffusivity increases as the flame gets richer in the case for propane (Lewis number drops) but the converse is true for methane. I know it has to do with the difference in molecular weight, but why does diffusivity increase for propane if you introduce more propane into the system but decrease in the case of methane? I am therefore not sure why Le decreases in case of propane but increases in the case of methane as the mixture gets richer.
Maybe the thermal diffusivity is changing?
