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Old   July 20, 2009, 23:51
Exclamation Combustion model - Fluid Material
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George Thomas
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Hi,

I'm simulating combustion in an SI engine using BVM model. I tried using the 'Methane Air FLL STP and NO PDF' as the fluid mixture for all the boundaries of my model (inlet, outlet, cylinder). This fuel mixture is used in the combustion tutorials in CFX. However, I'm not getting the results I expected, like the pressure predicted is considerably less than my mathematical model results.

Is it advisable to use the predefined fluid mixutres in similar kind of simulation?

In my mathematical model I have an air-fuel ratio. Can I employ the same thing in CFX?

Do I need to develop my own mixture to simulate the gasoline engine combustion? Which could be a better material for the fuel?

Please help me with the above queries.
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Old   July 21, 2009, 12:58
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What is the fuel of engine you intend to model? Is the fuel methane or gasoline?
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Old   July 21, 2009, 22:40
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Hi,

I wish to use gasoline as the fuel for the CFD model. I do not know how to specify reactions then, and also the air-fuel ratio that I used in mathematical model.

In the case of 'Methane Air FLL STP and NO PDF' the reactions and fuel properties were predefined, and so it was easy to implement in the model. Unfortunately no much support is available from my Uni in this area as it is kind of new work here.
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Old   July 22, 2009, 17:45
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Hi,

Any one having experience in modeling combustion in gasoline engines, please help regarding specifying fluid material suitable for spark ignition model.

Can I use any default 'gas phase combustion model'?

Or do I need to introduce a new material? I just want to use normal gasoline as fuel.

I have an inlet port and an exhaust port. Do I need to specify air and fuel inputs to the domain separately as done in the combustor tutorial in CFX? If so, could you advice me how to do this in my case?
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Old   July 22, 2009, 18:21
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for engine simulations without coupling external kinetic code (for example chemkin) you will need to use reduced species but well defined fuel mixture for example isoctane and the chemistry involved depends on how many species you want to take into account. gasoline has hundreds of species and you cant sufficiently model it without some assumptions

I have not used the flamalet library so I cant comment if you can go towards that route

the only recommendation i can say to you is to start collecting/studying papers regarding IC engine simulations and look into if you will need to couple a kinetics code in your model.

perhaps the code from convergecfd is a better solution for you as its specifically created for IC engine simulations
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Old   July 22, 2009, 19:36
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Thanks George,

I'm not well experienced in this area as I have started the project recently only. I'm doing this work as a part of my masters thesis. I do not have access to any software package except Ansys CFX at the moment, and the computing facility available for me is very limited. We do not have any faculty with experience in similar area in this Uni for a discussion or guidance and so I have to depend on myself.

I'm not looking for a highly accurate and full fledged combustion simulation as it is beyond the scope of a maters thesis and the work has to fit withing my time frame. However, I will definitely try some more journal articles.

As I can use only Ansys CFX now, I cannot think of developing codes in external tools and coupling it with CFX. I may have to rely on CFX itself to do my job.

I know combustion simulation is tricky, but I want to crack it. Unfortunately only one tutorial simulation is there in the CFX help regarding combustion modeling and that too not discussing BVM model which I want to use here.

Please give your suggestions on this.
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Old   July 22, 2009, 19:54
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glenn might be a better person to ask for his opinion to be honest; I only performed 1D engine models as a part for my phd.
my concern is how it is possible to do a thesis without a supervisor that can help you on the subject. maybe you are trying to chew more than what you can eat ?

well my problem approach would be:
forget about gasoline but concentrate on isoctane; find the chemistry involved and the kinetic rates required for cfd.
learn how to use the ignition model in cfx.
learn how to use moving meshes
start with 2d and if physics are right then go to 3d
do a cold flow analysis first - do you have any data to compare? how will you validate your results?

perhaps contacting ansys and asking them for an example is the best way for you to get started if you dont have any other source of information
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Old   July 22, 2009, 20:28
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Thanks George<br><br>I have already successfully implemented moving mesh and BVM combustion model with ignition regardless of my limitations.<br><br>However, the problems now I'm facing are,<br><br>I used gas phase combustion as implemented in one of the tutorial in CFX help. I do not know if that's the right way to do it. <br><br>I used Methane Air FLL STP and NO PDF as the mixture, and the result obtained from simulation gave very low pressure predictions than I obtained from mathematical model I developed. I do not know if I have to modify the default mixture available in the flamelet library.<br><br>Expecting your comments on this.<br><br><br>
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Old   July 22, 2009, 20:42
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can you say that you can trust the mathematical model 100% how can you tell its more accurate than the CFD model?
do you have experimental data for cross checking?

swirl in the chamber and the turbulence model can have significant impact.

why you need to use the BVM combustion model? the only way you can use this model efficiently is if you have access to cfx rif or someone with access can make the particular fuel library for you.
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Old   July 22, 2009, 20:58
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Hi George,

I do not have experimental data for verifying my results, and its true that I cannot assure my mathematical model is accurate.

I preferred SST turbulence model over k-eps as the latter has problem with boundary flows and flows with separations. My supervisor also suggested to go for SST model.

I do not have access to CFX RIF. We do not have the license for RIF as the version we use in Uni is the education version of Ansys and my project is not funded to buy a license. Unfortunately this area of research is not even there in the research strength of the Uni. I took this project as a challenge.

I'm respecting your comments on my issue as rarely I get chance to speak to someone who has experience in similar area. Thank you for advising me on this.
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Old   July 22, 2009, 21:05
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do check the part in the manual which discusses which combustion model is more appropriate in solver modeling guide>combustion modeling> using combustion models

in this instance I'd think that you will use something in terms of the combined eddy/finite rate chemistry depending on your chemical reactions

let us know how you get on
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Old   July 22, 2009, 22:58
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Hi,

Quote:
I preferred SST turbulence model over k-eps as the latter has problem with boundary flows and flows with separations.
k-e has no problems with boundary layer flows. Yes, k-e has problems with separations but so does SST. 95% of engine simulations have been done with k-e so you will need a better reason than that to change. Use k-e until you have firm evidence it is inadequate.

You may also like to have a look at my prehistoric PhD thesis. It used CFX4 to model the aerodynamics of an IC engine. No combustion modelling, just the air flows. That was more than enough for a CFD model 10 years ago. http://hdl.handle.net/2100/248

From memory (it has been a long time since I looked at combustion in IC engines) the most common approach is using eddy breakup models. These models can account for swirl and tumble which, as ckleanth says is vital for model IC engines.

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Old   July 22, 2009, 23:19
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Hi Glenn,

I have tried using k-eps model couple of times, and my simulation finished with error at times. Later I shifted to SST to give a try, and my simulation went well. So, I assumed SST has a hand over k-eps. May be that was an early judgment; I would have given more trials with k-eps. In most of the journals also I found k-eps is most widely used from a long time.

Thank you for referring your thesis. In fact I had gone through it several times (even now) and it helped me to understand IC engine modeling, especially gas flow through valves and swirl motion inside cylinder.

I would try the eddy break up models. I was clinging to BVM model since a long time as it had a spark ignition option coming with the model. May be its time for me to think about my modeling problem in a different way. I think CFX is quite rarely used for IC engine combustion modeling.

Thanks Glenn and George, for your valuable suggestions.
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Old   July 22, 2009, 23:51
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Hi,

Star-CD is by far the market leader in IC engine modelling. Fluent also does quite a bit and AVL's FIRE used to be common but is not so common now. Just about nobody in IC engines use CFX. Except me and ckleanth I guess.

But in recent releases of CFX, especially V12, there is lots of stuff specifically aimed at IC engine modelling. I think CFX has made a big effort to close the gap in IC engine modelling with the V12 release.

The result of this is that you should get your school to put V12 on. No point working with an out-of-date tool, you will just do months of work for no reason.

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Old   July 23, 2009, 00:44
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Hi Glenn,

If you have already experienced V12,

does it have a new combustion model for handling IC engine combustion problems? (like BVM introduced in V11)

does it have more tutorials on combustion (especially on IC engines) modeling?(V11 had only 1 tutorial)

Would you recommend me to go on with BVM? Do you know if anyone tried it successfully?

I heard my Dept. is planning to buy V12, but don't know when this is going to happen. I have hardly 1 month to finish my work, and I know its not going to be a cakewalk for me.
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Old   July 23, 2009, 00:58
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Hi,

I have not done IC engine modelling in V12. My PhD was in CFX4 and afterwards I continued doing related IC aerodynamics work in CFX5 but never combustion modelling. So I can't really comment about the combustion models.

If you only have 1 month to model IC engine combustion and you are new to CFD then you either are a genius or have taken on an impossible task. I recommend you simplify your topic to something which will actually be possible.

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Old   July 23, 2009, 01:22
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Hi,

I have started this project in Spring '08, and so it is almost 1 year.

However, no experienced person is available in the area of CFD based IC engine modeling in my Uni for guiding me and no good computing facilities available. So I was doing self learning right from modeling engine geometry using Ansys DM, meshing using ICEM, and simulation using CFX. These works consumed my 1 year.

Once I implemented BVM model I thought its done. Now I understand it is not even the beginning!!!

As you suggested I may should think about stripping down complexities in my model to make it work in 1 month time.
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Old   July 23, 2009, 02:59
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I hope you have one more month for modeling and not writing up

in this instance forget isoctane, do methane or butane combustion - these reaction rates are already available in the library, if you cant use the combustion/ignition model use an energy source to represent ignition

what I'm concerned about is the data you have to compare the cfd results with. back your assumptions up and be proud of what you have achieved as cfx (especially v11) is the worst software to have in regards to required user input for IC simulations
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Old   July 23, 2009, 03:22
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Hi George,

I have already done some part of my writeup. To move further I need results from CFD analysis. So I can invest (I have to) some time for trying simulation in CFX.

Since I do not have experimental results I have to rely on the mathematical model results I obtained. I generated this model in spreadsheet. (I know this result may not be accurate, but for the time being I do not have anything else to compare)

I have already modeled some simulations with moving boundaries with another concept which I call domain source, where I use air ideal gas and explicitly supply energy to the domain. This model worked fine and I got results. (However, it is not combustion)

I'm planning to compare results in terms of P-V plots, T-crankangle plots, Velocity and turbulence plots. I'm not worried about combustion after-burn products and other similar complicated things.

By energy source, I assume you are referring to use source point to supply energy. I think I would try EDM or FRC now, not BVM.
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Old   July 24, 2009, 04:13
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Hi,

I have started shifting to EDM for combustion modeling. I have a few doubts if you could help me to clarify.

In BVM model when I use a default fuel mixture I can initiate the combustion reaction with the help of spark ignition. I wonder how this is done in case of EDM. I'm using Methane Air WD1 NO PDF. (I know EDM is easier compared to BVM and is widely used for quite some time now)

Another doubt, not specific to the topic, but would like to hear your comment - I am modeling air-standard cycle as well for this particular engine model. I wish to run 2 cycles of numerical simulation instead of just 1 (I think the same is done by Glenn in his thesis). I'm running my simulation in cluster. I want to take the cycle-1 end-result as the initial assumption for cycle-2. Can anyone help me how to do this in cluster environment (do not have parallel license for CFX, SSH secure shell)
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