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Old   August 2, 2004, 03:38
Default Combustion
  #1
Sébastien ERNANDES
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Hi everybody,

I'm working on a Diesel combustion problem with EGR (Exhaust Gas Recirculation) and I've some problem to define correctly my scalars. The solver can't succeed the convergence of N2 equation when the injection begins (it seems that Star considers the nitrogen as a product of my chemical reaction, so the value of N2 concentration becomes near to zero...). Has anybody experience this problem? Has anybody found a good tutorial or web site on combustion with EGR?

Thanks a lot for your help.

Best regards, Sebastien.
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Old   August 2, 2004, 07:03
Default Re: Combustion
  #2
Birute
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Try to set N2 as internal, not transport in "Analysis controls"-"Solution controls"-"Equation behaviour"-"Additional scalars".
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Old   August 12, 2004, 19:29
Default Re: Combustion
  #3
Jo
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Which combustion model do you use, "Laminar-Turbulent EBU + Shell ignition" ? In that case, combustion modelling requires you take care in defining your recirculation products. The latter should be input as different scalar than the one used by the EBU model. And the EBU fixed mixture fraction for O2 should be corrected depending on the EGR rate. This could explain the algebraic unmatch for N2

I have another question, but for me please. Which law of evaporation do you use for your fuel ? I have observed that the droplet temperature is limited to Tcrit when the spray model is used even if the boiling model is off! It appears to be a bug. I am to switch to 3.2 but still using 3.15. Perhaps this bug has been corrected in 3.2.

I am working on EGR too but, with respect to the literature I consulted, it seems that few work on that has been published in multi-D Diesel computation.
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Old   August 13, 2004, 03:41
Default Re: Combustion
  #4
Sébastien ERNANDES
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Hi Jo,

I effectively use the 'Local Source' model with LATCT EBU + Shell ignition. I defined my scalars as follows : C12H26, MIX_FRAC, O2, CO2, H2O, N2, RAD1,2,3, CO2_r, H2O_r (the both latters scalars are residuals gases, not reaction products...). But my real problem is that I first tried to compute combustion without EGR, and when I turn off my chemical scheme the computation is OK, but when I turn it on the N2 scalar can't converge immediately from the BDC, 160 °CA before the start of my injection!!! I tried to solve this scalar via 'Internal' equation but the results are aberrant, so I let it in 'Transport'...

As for evaporation, the french hotline has said to me the boiling model on V3.15 is defective, and that the evaporation is mainly controlled by the definition of your nozzle and your injection table...

Do you think that a good method for combustion computation is to perform your calculation first of all without chemical scheme, then restart with the chemical scheme, maybe with a lower time step??

Sebastien
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Old   August 13, 2004, 09:06
Default Re: Combustion
  #5
Jo
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Is there any ground in your configuration to have chemical reaction before injection ? I am quite surprised about that because it should not.

For your question, as far as I am concerned in CFD, I think you can, provided you do NOT run a transient case. And that seems NOT to be your case. Be careful !

Thanks a lot for your information about the boiling model.

I read you use dodecane as fuel, do you use the properties laws provided by Star in Dropro.f and Drmast.f sample ? Some of them do not seem to be reliable in the range of temperature encountered in Diesel engine.

Do you use any engine model to define H2O_r and CO2_r ?

For your preceding question about EGR, one the few papers I found for now about that (perhaps you already know it):

M. Chan, S. Das, R.D. Reitz, Modeling multiple injection and EGR effects on Diesel engine emissions, SAE 972864

Read you soon.
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Old   August 13, 2004, 10:53
Default Re: Combustion
  #6
Sébastien ERNANDES
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Actually, I launch my calculation with two load steps in the 'transient module', the first from the BDC to 15°CA BTDC (load step 0.05°CA), and the second load step up to TDC with a lower time step (0.01°CA) in order to converge... and I can't activate my chemical scheme just for the second load step!! That explains why I've chemical reaction before injection... Whatever, with the help of Adapco hotline, it seems that my computations run successfully : verdict monday!!

What did you mean by "engine model" for the both scalars??

thanks for the SAE reference, I'll search it as soon as possible.

Best regards, Sebastien
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Old   August 13, 2004, 11:55
Default Re: Combustion
  #7
Jo
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It seems to be a wrong definition of what is fuel and what is oxidizer in the combustion model. You should not burn anything before injection starts !! I strongly suggest you to check in the EBU model, if not already done, that:

(i) the specified fuel is C12H26 scalar ; (ii)oxidizer is a fixed mass fraction of O2 scalar corresponding exactly to your initialization conditions and nothing else ; (iii) the products option is enabled with H2O and CO2 scalars defined as products and nothing else.

If you need to start combustion at a certain step, you can custom the start time of the Shell ignition model, but it does not explain your unphysical combustion in fresh gas without fuel!

What I called "engine model" is the correlation used to define the trapped gases (CO2_r and H2O_r initial conditions in the computation) which exist even with 0% EGR rate. If you have any correlation like this as well as thermodynamical properties (saturation pressure, density, viscosity, heat of vaporization... as function of T), I would be greatly interested.

Good luck for your new computation
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Old   August 13, 2004, 12:51
Default Re: Combustion
  #8
Sébastien ERNANDES
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I've any combustion process before my fuel injection. my only problem is that my N2 scalar is crazy : its concentration varies from 5.10^-3 to 0.77 during my computation, but the temperature or others scalars are OK... it seems that the problem was due to bad scalar boudaries.

Be sure that if my calculation with EGR succeed, I'll inform you quickly.

Thanks for your help.

Sebastien
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