Species Transport problem

skumar112 · April 6, 2017, 09:52

Hello I am trying to simulate combustion of a jet (hydrogen + O2) PREMIXED USING THE SPECIES TRANSPORT MODEL. I am however getting issues with this and have got divergence. I have attached a screenshot of the screen as it blows up any help would be appreciated. If you require any additional information I will be pleased to include

LuckyTran · April 6, 2017, 11:02

Something's clearly wrong with the setup. Honestly, it could be anything.

I would start with running the mesh quality check and then check your initial conditions and boundary conditions.

skumar112 · April 6, 2017, 14:24

im sure the mesh is ok as i made it myself in icem, (and ive run the check mesh and fluent likes the mesh).

I think it might be to do with the boundary conditions as ive never used the species transport model before and might have done something wrong. i have a jet that sprays into ambient air, in the jet inlet I have a mass fraction H2 0.66 O2 0.33, for the ambient walls(pressure outlets) and inlet(mass flow inlet) I have mass fraction of O2 0.23, and at the inlet i have mass fractions for all O2,H2,H2O as 0 i assumed this as theres no backflow at the outlet its a pressure outlet.

LuckyTran · April 6, 2017, 14:29

Make sure all your mass fractions add up to 1.

You can't specify all the mass fractions to 0 at the outlet. There should have been an error message to prevent this. You can't guarantee there will never be backflow. A pressure outlet is not a "there cannot be outflow" constraint. You expect there to be no outflow, but that doesn't mean there cannot be. Is there a reversed flow warning during simulation? There's only 3 iterations, it should not be hard to find.

What about your initial conditions?

skumar112 · April 6, 2017, 14:44

there was no reversed flow warning, I assumed it was ok as i found it on a tutoiral which does a very similar simulation to the one im running

https://www.google.co.uk/url?sa=t&rc...Oj-ov3ma4HSEzw

LuckyTran · April 6, 2017, 15:21

First a question and then let me clarify.

You mentioned air, is N2 or something part of your mixture?

Your mass fractions must sum up to 1 no matter what or you will get a floating point error. You can set H2, O2, and H2O to 0 but that means there must be something else with a mass fraction of 1.

Next, if all the BC's are okay. Then check the initial condition.

skumar112 · April 7, 2017, 04:53

I think i have managed to fix the problem and it was the outlet boundary condition ... thankyou for your help

April 6, 2017, 15:21		#6
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,672 Rep Power: 65	First a question and then let me clarify. You mentioned air, is N2 or something part of your mixture? Your mass fractions must sum up to 1 no matter what or you will get a floating point error. You can set H2, O2, and H2O to 0 but that means there must be something else with a mass fraction of 1. Next, if all the BC's are okay. Then check the initial condition. Last edited by LuckyTran; April 7, 2017 at 15:39.

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April 6, 2017, 11:02		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,672 Rep Power: 65	Something's clearly wrong with the setup. Honestly, it could be anything. I would start with running the mesh quality check and then check your initial conditions and boundary conditions.

April 6, 2017, 14:24		#3
skumar112 Member sunil kumar Join Date: May 2016 Posts: 80 Rep Power: 9	im sure the mesh is ok as i made it myself in icem, (and ive run the check mesh and fluent likes the mesh). I think it might be to do with the boundary conditions as ive never used the species transport model before and might have done something wrong. i have a jet that sprays into ambient air, in the jet inlet I have a mass fraction H2 0.66 O2 0.33, for the ambient walls(pressure outlets) and inlet(mass flow inlet) I have mass fraction of O2 0.23, and at the inlet i have mass fractions for all O2,H2,H2O as 0 i assumed this as theres no backflow at the outlet its a pressure outlet.

April 6, 2017, 14:29		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,672 Rep Power: 65	Make sure all your mass fractions add up to 1. You can't specify all the mass fractions to 0 at the outlet. There should have been an error message to prevent this. You can't guarantee there will never be backflow. A pressure outlet is not a "there cannot be outflow" constraint. You expect there to be no outflow, but that doesn't mean there cannot be. Is there a reversed flow warning during simulation? There's only 3 iterations, it should not be hard to find. What about your initial conditions?

April 6, 2017, 14:44		#5
skumar112 Member sunil kumar Join Date: May 2016 Posts: 80 Rep Power: 9	there was no reversed flow warning, I assumed it was ok as i found it on a tutoiral which does a very similar simulation to the one im running https://www.google.co.uk/url?sa=t&rc...Oj-ov3ma4HSEzw

April 7, 2017, 04:53		#7
skumar112 Member sunil kumar Join Date: May 2016 Posts: 80 Rep Power: 9	I think i have managed to fix the problem and it was the outlet boundary condition ... thankyou for your help