convergence problems by species transport
 

Hi, I hope any one can help me

I am simulating a free jet injection of hydrogen in two points in co-flow with air in a cylindrical mixing tube. First I run the simulation just with air and it has a good convergence. When I activate the option of species transport(using a Kinetic Mechanism from Chemkin), all the residuals go down except the residual of the Continuity Eqn. More than that it goes up and stays around 5e+01:eek:. I all ready tried changing the mesh(for a finer one) but it doesnt show any improvement :confused:.

Thank you

Felipe

hi felipb

i think u need to give more details for ur prb

why u use the Kinetic Mechanism from Chemkin if u focus to simulate only the jet dynamycs ?
i think also u dont need to make several changes in ur mesh if u say that they gave u a ggod convergence in ur first step

I am using a chemkin kinetic mechanism because I want to later on simulate the ignition by activating the volumetric reactions, but first I wanted to check for the mixing process(profile), thats why right now I am simulating without reactions.
On the order side if I change the composition of the fuel inlet (hydrogen) to for example O2 or N2. The residuals start going down but very slowly, thats why I thought in making the mesh finer to maybe improve simulation of the diffusion process of H2 but I increase its density in almost factor 3 and I dont see any change. By the way im simulating 3D with symmetry boundaries in a quarter of a cylinder(just a quarter because the of the h2 injection position).

I had the same problem with continuity. I played with the under-relaxation factors and it worked. decrease all the under relaxation factors to 0.2 for example. Try that and wait until it converges.

Mohammad

Thanks!!!! mahmohammad I am going to try that.

it did work
 

For the people who saw my thread, is now solved:

I improved the mesh quality; in some areas the Aspect Ratio wasnt that good, (my geometry wasnt that simple). I increase a little bit the amount of cells close to injections points. I used the Pressure based solver with the coupled algorithm and I reduced the courant number to 40 at the beginning and later to 100 to increase the convergence speed. I also reduce the under-relaxation factors for the eqn of the species I was simulating from the default one to a value of 0.8 . The user manual of fluent does not recommend to use under-relaxation factors below 0.5.

Thanks Mohammad for your help

good luck to the others

Felipe

Felipe

I happy that your problem is solved.
FYI, in some cases I had to start with underrelaxation factor of 0.01. You have to decrease it when there is sudden change in a variable. And is very common in reaction and species problems.

Mohammad

Ok, I will take it in account, because reactions is my direction =) ... =S.
Thanks a lot again

Felipe Bolanos

hi ...
would u plz help me?
I want to simulate mixing of Two fluids.
but by using species transport model, how can i allocate constant value to properties of each component?


best regard

Hi
first using DEFINE/MATERIAL determine the properties of components, as usual. we simply do this at normal problems too.
Then using CELL ZONE CONDITION/EDIT/MIXTURE SPECIES/ EDIT (in front of the material name: mixture template) you will choose your mixture component from the list.
For determining the properties of mixture you will find it from the paper you work on it. Maybe it uses the averaging method defined in the help.
or it can use one of the fluids , ...
Calculate them or let the fluent calculate by itself.
Check every part of HELP to clear for yourself.
Good Luck

I tried to decrease the under relaxation factor less than 0.5 and then run the simulation. But the continuity residual does not decreases instead makes a sine curve. Any recommendations?