Learning Forum

[NimaNZ]

Hi everyone

I want to adopt the CO2 gas in a specific volume within a larger domain by patching the value for CO2. When I use FLUENT's database for CO there is CO2 in the list but I don't want other materials to be solved cause it makes simulation longer. How can I only adopt one single gas in the species transport model? (note that the main gas in the domain is Air in my model and I am willing to model the rate of decay of CO2 from a volume to the main domain)

I'd appreciate any help.

Kind regards,

Nima

[Rob]

You want to mix air and CO2 ? You just need two species in the mixture taking care that the bulk species is last in the list.

[NimaNZ]

Thank you for your response. I have a room and the domain around it. I want say 10000ppm CO2 just inside the room by patching the amount at the room volume and keep the air outside without CO2. All mixtures in Species Transport are including about 5 materials, I'm just interested in CO2.

[Rob]

Just change the mixture contents. We often run models with a main species and a tracer. Just make sure you get the diffusion coefficient right.

[NimaNZ]

Could you please provide more details on what you're referring to? By main species and a tracer you mean you use species transport or not? I just apply one of the mixtures and put all other materials as zero and just give value to CO2 at the room volume. It's working but as it's solving transport equations for all other four materials, it increases the computational cost. That's why I want to reduce the number of materials in the mixture.

[Rob]

Yes, species transport. We may have water and water-tracer in a model or air and pollutant if we're looking at a plume dispersion. Edit the mixture template to remove species you don't want. Setting fractions as zero is wasting compute resource. Have a look in the tutorials (under Help) one is bound to cover combustion, just ignore the reactions bit to see how to edit the mixture.

[NimaNZ]

First of all, thanks a million for your help and patience, it is really appreciated and helpful. I finally managed to remove the species as following I thought I should add air to be the last species otherwise it takes CO2 itself as the last species, is that alright?
Then because there is no reaction I set the species transport as follows I am not quite sure if this setting will lead to solving the following transport equation or not? as I couldn't find it in the theory guide!
You mentioned selecting the proper "diffusion coefficient", is it different from the Schmidt number we specify in the viscous model?
As my model is wind dominant I don't need to apply gravity in the simulations, is it necessary for species transport to add gravity?

[Rob]

That looks about right. Diffusivity is set in the species mixture panel, and is separate from the turbulence setting. Chances are the system is sufficiently turbulent that turbulence will dominate mixing.
Gravity is only important if the CO2 is going to move downwards relative to the air in the initial mixing. Otherwise it'll not make much difference: I'd turn it on to avoid awkward questions later on.

[NimaNZ]

Thank you so much . I really appreciate your help.