[Manu4CFD]

Hi,

Suppose I have a fluid domain with an inlet and outlet. I have made the domain to be in rectangular shape. I want to pass 400ppm of carbon dioxide along with air into the fluid domain. I am defining the air to be ideal gas. In the domain, there are few other sources generating carbon dioxide as well. After some time, the total amount of CO2 in the domain would be higher than the rate at which CO2 is being introduced into the domain. I want to simulate a scenario whereby I could measure the time taken to dilute the amount of carbon dioxide inside a room to a specific value, say 400ppm. To simulate this scenario, I believe the species transport equation can be used. But then I am not able to find any relevant material that explains how much of mass fraction value could be equal to concentration of CO2 (400ppm). In the general how can we relate the mass fraction to CO2 concentration in PPM(volume) in Ansys Fluent. I have found some online equations to convert mg/m3 to ppm, that I am stating below:


Concentration (ppm) = 24.45 x concentration (mg/m3) ÷ molecular weight.
For instance, if we have 1305 mg/m3 of CO2, then it values in ppm(v) is 725.

May I have any suggestions with respect to mass fraction and PPM in Ansys Fluent 13.0. It would be of great help.

[FliegenderZirkus]

ppm can represent a mass fraction, volume fraction of mole fraction, each time multiplied by 1e6. Often you differentiate between these by an extra letter, such as ppmm (= 1e6*mass fraction), ppmv etc. So the first thing you need to know is what your 400 ppm represents. If it's a mole fraction, then you can convert it to a mass fraction like this https://en.wikipedia.org/wiki/Mole_f...#Mass_fraction
Also note the difference between fractions and concentrations. Fractions are dimensionless, whereas concentrations typically divide the mass (or amount) of a species by mixture volume, hence they aren't dimensionless. In CFD you normally work with mass fractions or (less often) mole fractions. It's quite common to use the word concentration to talk about ppm too, but as far as I understand it it's strictly speaking incorrect.

[Manu4CFD]

Originally Posted by FliegenderZirkus

ppm can represent a mass fraction, volume fraction of mole fraction, each time multiplied by 1e6. Often you differentiate between these by an extra letter, such as ppmm (= 1e6*mass fraction), ppmv etc. So the first thing you need to know is what your 400 ppm represents. If it's a mole fraction, then you can convert it to a mass fraction like this https://en.wikipedia.org/wiki/Mole_f...#Mass_fraction
Also note the difference between fractions and concentrations. Fractions are dimensionless, whereas concentrations typically divide the mass (or amount) of a species by mixture volume, hence they aren't dimensionless. In CFD you normally work with mass fractions or (less often) mole fractions. It's quite common to use the word concentration to talk about ppm too, but as far as I understand it it's strictly speaking incorrect.

Thank you for your reply @FliegenderZirkus
I read that for ideal gases, mole fraction is same as volume fraction. Also, as per my online search I found that the composition of Carbon Dioxide (CO2) in dry air, by volume is around 0.0400%.
Hence, mole fraction of CO2 would be (0.0400/100 = 0.0004).
Expressing the Concentration of CO2 in ppm : 0.0004 * 10^6 = 400ppm

If in mass fraction, it would be mass fraction = 0.0004 * (molar mass of CO2 / average molar mass of mixture (dry air))
mass fraction = [0.0004 * ((44.01 g/mol )/ (28.97 g/mol))]= 6.07 * 10^-4

In Ansys fluent, if I need to enter mole fraction or mass fraction in the species tab for CO2 (400ppmv), then
For mole fraction, I need to enter 0.0004.
For mass fraction, I need to enter 0.0006.

I hope the calculations above are correct. Please let me know your thoughts on my interpretations.

[FliegenderZirkus]

Looks good to me

[Manu4CFD]

Originally Posted by FliegenderZirkus

Looks good to me

Thanks @FliegenderZirkus. I hope this thread becomes useful to others facing similar queries.