Mixing between two fluids (species transport)
 

Hi guys, currently i m working on simulation regarding the mixing between fluid A and B from two different inlets...i m using species transport model...i know that the bulk species has to be at the last in the mixture template...My bulk species is fluid A.


Fluid A = pure water (inlet 1)
Fluid B = water with dye (inlet 2)

I have set the mass fraction of dye in inlet 1 as 0 whereas the mass fraction of dye in inlet 2 as 0.2...

Afterwards, when i output the mass fraction of dye in particular plane (located at further downstream of the mixing channel), the mass fraction of dye becomes nearly zero (10^-15)...i have expected that mass fraction will reduce but not equal to 0...is it i miss out something in the settings or this is normal results for species transport?

Even though i exchange the bulk species, the result still the same, the mass fraction of the water becomes nearly zero...

i really need some help in this case...

Can you check if you can use passive scalar model here or not? If it is applicable is more simpler. Just defining 1 passive scalar will solve your issue. We use this model for EGR mixing which is kind of similar to your problem.

Dear ashokac7, thank you for your quick response, do u mean the UDS (User-Defined Scalar) in boundary condition?change it from 0 to 1?

Our conventions may be different. I use Star-CCM+. But for dye with water I will use some constant passive scalar value while for pure water passive scalar will be zero. But please read some document for passive scalar and see if it is applicable to the case you are modelling.

What are the mass flow rate are you using for fluid A and B.?

Mr. ashokac7, both of the inlets have the same mass flow rate

Hi Hong,
I guess you need to let the transport equation of the species converges. I mean usually, even in steady solutions, it takes time for the species to propagate from the inlet. My suggestion is to patch the domain for the species (with a reasonable initial guess i.e. 0.1 ) and let the residuals drop below 10^-6 (if possible).

Dear e_cfd, what you mentioned here is absolutely right. I have found the root of the problem. The low concentration of species A at the output plane is due to insufficient time for the species to reach the plane from the inlet as i am using a very time step size. Hence, now i resolve the problem by using a larger time step size.

Thank you for your opinion and guidance, Mr.e_cfd!