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How to increae diffusion in flow?

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Old   February 12, 2020, 12:10
Default How to increae diffusion in flow?
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Abdullah Arslan
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Good day,

Can we increase diffusion in a fluid flow, say by Lewis number (if there is any option)? My flow is laminar with thermal energy on. I don't want to change either density, heat capacity or thermal conductivity. I am using also real gas mixture from library.

The reason I want to increase it, just to see its effect without changing the 3 mentioned parameters.

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Old   February 12, 2020, 13:02
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Gert-Jan
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Diffusion is for molecules. So, if you have a massfaction equation for a component that you want to diffuse in the continuous phase, than you can add a kinematic diffusion coefficient in the tab Fluid Models/Component Models.
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Old   February 12, 2020, 16:06
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May I ask what you mean by diffusion?

Momentum diffusion is through molecular viscosity, mu

Thermal diffusion is through thermal conductivity, k

Mass diffusion is through mass diffusivity, density * D, where D is the kinematic Diffusivity

What are you trying to achieve?

Lewis number is the ratio of thermal diffusion to mass diffusion, i.e.

Le = [k/(rho Cp) ] / D

which relates the energy equation respect to the mass diffusion equations.

Similarly, you can use the Schmidt number as the ration between momentum diffusion, and mass diffusion, i.e.

Sc = mu / (rho D)

which diffusion are you trying to modify or study?

As you can see, there are 5 parameters so far:

Density, rho
Specific Heat, Cp
Thermal Conductivity, k
Dynamic Viscosity, mu
Kinematic Diffusivity, D

and you want to fix the first 3, you are only left with changing viscosity and kinematic diffusivity.

I would fix the viscosity, vary the Schmidt number and compute the corresponding input kinematic diffusivity,

D = Dynamic Viscosity / Density / Schmidt Number

Hope the above helps,
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