Learning Forum

[Nicholas Strack]

Okay, so I have a mixture template defined as 5 volumetric fluids, so2-liquid, so2-vapor, c2h3n-liquid, c2h3n-vapor, and air as last species. For the mixture multiphase I have defined 2 phases, liquid for the primary and vapor for the secondary. I have defined two mass phase interactions that takes phase 1 so2-liquid and transitions it to phase 2 so2-vapor. Likewise for the C2H3N. I have left air out of it as it is just part of the open environment. I have specified an evap/cond using the Lee model and have plugged in the saturation temperature for both situations. The geometry is as follows, it is a small 2D axisymmetric volume with a small opening to a free field of air at environmental pressure. The small volume is patched to 2.41e6 Pa and 368K. The mass fractions for the liquid and vapor component of so2 are 0.594 and 0.175. The mass fractions for the liquid and vapor component of c2h3n are 0.223 and 0.008. A vapor volume fraction is specified at 0.166. This is all executed through the patch functionality following initialization at atmospheric pressure and temperature with none of the aforementioned components defined making air mass fraction 1 for all volumes initially. Here is the problem, if I calculate this by hand, depending on my EOS choice I get between 15-20g of material in the small volume. Fluent says it is 0.8g. I have checked and triple checked material properties. I am using real-gas pengrobinson to define the density of the mixture template. All the critical point values and acentric factor are correct. Standard enthalpy has been checked for the vapors and set to zero for liquids. This model runs and the behavior looks right, but the amount of material in the volume seems wrong. 

 

Any ideas? Any help would be very much appreciated. I can provide screenshots or additional details just ask! 

[Ahmed Hussien]

Hi Nicholas, how did you calculate the mass by hand and in Fluent? Keep in mind that you are solving 2D axisymmetric which can be a little confusing when calculating the volume and mass. In your case, the Total Volume of a cylinder is (PI *r^2*L) and the mass should be Density*Total Volume. 

[Nicholas Strack]

 

 

Ahmed,

Firstly, thank you for the response!

My internal volume is 1.953E-5 m^3 and just looking at the so2-liquid component, which makes up 0.594 of the overall mixture mass and a density of ~1200, this gives me 13.9g. If I check the acetonitrile-liquid component, 0.228 and ~786 for mass fraction and density, I get 3.5g. I am taking mf*Density*volume. These values are also in rough agreement with some real values from a manufacturer.

I am a little perplexed by this problem. Side note, for the materials, the liquids are defined with constant density and the vapors are defined as real-gas-peng-robinson as I am working with temps and pressures near the critical point.

I would think Fluent is using the Peng-Robinson EOS to determine the density for the vapors and taking the constant density values listed above for the liquids and performing a similar calculation, but I cannot understand how it is coming up with a value so far off. 

Any additional ideas you can give me would be appreciated!

 

 

[Nicholas Strack]

Okay, so I figured this out and figured I could answer here for anyone else in the future who runs into this.

I had two key issues:

The first was lumping the liquid materials with the vapor materials in a single mixture. Create a liquid mixture and a vapor mixture to get around this. Do not put any vapor in the liquid mix, not even as a last species this was a secondary issue I created. 

The second issue was with the patching. I did not patch the volume fraction of air into the exterior volume thinking it being last species would make the difference. I was treating the liquid and vapor phases as needing to add to a value of 1 for mass fraction overall. Instead, it works where each phase individually adds to 1 for mass fraction. This is something that should be documented. I have not found anything in my searches. 

Feel free to reply here if you have any questions or want further details.