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Replacing n-heptane (chlamers mechanism) for ECN Spray Simulation

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Old   August 16, 2016, 14:56
Post Replacing n-heptane (chlamers mechanism) for ECN Spray Simulation
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Hi all,
I am using Chalmers n- heptane mechanism (http://www.tfd.chalmers.se/~valeri/MECH.html ) for ECN reacting spray validation.
I want to replace the mechanism with T. Lu's n-heptane mechanism (http://www.engr.uconn.edu/~tlu/mechs/mechs.htm). Except the mechanism part, the case setup is the same. The changes are as follows:
  1. Replaced therm.dat
  2. Replaced mech.dat
  3. Renamed C7H16 in liquid.dat to NC7H16 (C7H16 is not available in Lu's mechanism)
  4. Defined Parcel as NC7H16
  5. Selected fuel as NC7H16 and mass fraction =1
  6. In evaporating species selected NC7H16
  7. Selected NC7H16 in combustion tab
The simulation is getting initiated. Now the problem is except the species which I initialize (CO2, N2 , H2O and O2) other species are showing zero concentration whereas as in Chalmers mechanism, after the first time step, even though not significant a change in species concentration is observed.

What could be the possible reason? Is it a problem with mechanism ?

Any leads will be highly appreciated
Thank You
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Old   August 17, 2016, 03:42
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Hi,

can you post further information regarding the mechanisms you used?

Number of Elements:
Number of Species:
Number of Reactions:

Maybe they differ in size?
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Old   August 17, 2016, 06:29
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Quote:
Originally Posted by MFGT View Post
Hi,

can you post further information regarding the mechanisms you used?

Number of Elements:
Number of Species:
Number of Reactions:

Maybe they differ in size?
Number of Elements 4
Species 52
Reactions 48

Can you elaborate more on "differ in size"?
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Old   August 17, 2016, 06:59
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Hi,

i guess this is T. Lus mechanism. What about the other that you replaced?
I meant, if this Chalmers mechanism is more detailed (larger number of species and reactions), it might be that small reactions already occur, that are not present in the reduced mechanism of Lu.
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Old   August 17, 2016, 07:12
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Thank you MFGT for the reply

I've replaced Chalmers mechanism with T Lu's mechanism. Yes it is a highly reduced mechanism. I am using this for ECN Spray simulation not for combustion (though I tried Lu's mechanism for combustion also ; it gave me the same result- no combustion)

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Originally Posted by MFGT View Post
Hi,

i guess this is T. Lus mechanism. What about the other that you replaced?
I meant, if this Chalmers mechanism is more detailed (larger number of species and reactions), it might be that small reactions already occur, that are not present in the reduced mechanism of Lu.
Even if the mechanism is small, the species available for the selected mechanism (here Lu's Mechanism) should show some change right? its is consistently showing zero if I monitor species_mass.out .

Thank You
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Old   August 17, 2016, 07:43
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Quote:
Originally Posted by alenp07 View Post
I am using this for ECN Spray simulation not for combustion (though I tried Lu's mechanism for combustion also ; it gave me the same result- no combustion)
Do you have SAGE combustion activated or not?
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Old   August 17, 2016, 07:48
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SAGE is activated.
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Old   August 17, 2016, 16:47
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Hi alenp07,

I will be curious to know the magnitude of species concentrations with Chalmers mechanism. Are these hydrocarbons in the order of 1e-09~1e-10 Kg? If it is so, I will actually ignore those values, as they are comparably low when compared to ~10 mg of fuel injection. Different mechanism can lead to different pathways depending upon local temperature, and equivalence ratio as Tobias was referring to.

Anyhow, keep us posted about your findings.
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Last edited by SauravMitra; August 18, 2016 at 11:13.
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Old   August 18, 2016, 15:22
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Thank You Saurav for your response. I agree with your point but what I am trying to say is after certain time step, say for example 1.5 ms, the OH radical formation is significant. I can observe this in Chalmers but not in Lu's mechanism. Also in Lu's mechanism , I cannot even observe traces of OH radical.

Is this problem due to mechanism alone? or I am making a mistake in case setup?

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Old   August 22, 2016, 20:18
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Hi alenp07,

How long does this take to run ? Can you send me the setup for both ? I can have a look at it.

Thanks
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Old   August 23, 2016, 20:30
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Hi alenp07,

I noticed the mechanism of Lu might need special treatment.

If you open chem.inp for 52 species reduced mechanism, you will notice that the reaction rates are zero out there. You have to specify custom reaction rates via UDF route by using the CKWYP routine. Refer to section 1.4 in the UDF manual regarding this. If you do not take the UDF route, CONVERGE will parse the mechanism file as it is, but you will not get combustion out of it as the rates are zero.

Can you just double check for me, which reaction mechanism you are using ?

Thanks
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Old   August 24, 2016, 04:13
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Hello Saurav,
Run time is lower for Lu's mechanism. Yes I can share the case setup with you
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Old   August 24, 2016, 04:23
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Hi Saurav,
Since we were facing difficulty with Lu's mechanism we started the simulation with n-heptane mechanism which comes with CONVERGE. Why we decided to go with Lu's mechanism because a literature was available and lesser number of reactions will have some advantage with runtime.
At present we are using Chalmers mechanism.

What is your suggestion in this case. using Lu's mechanism for our study or we should continue with Chalmers mechanism?
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Old   August 24, 2016, 17:21
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Hi alenp07,
Chalmers mechanism will do a good job for ECN Spray reacting flow.

If you keep the oxygen concentration at 15% as specified here:
http://www.sandia.gov/ecn/cvdata/tar...ion/sprayA.php

you should be okay.

You can refer to this work of Convergent Science for ECN Spray (reacting flow) simulations.
http://proceedings.asmedigitalcollec...icleid=1721041

Thanks
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Old   August 25, 2016, 08:10
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Hi Saurav,

Thank you for the suggestions. I also felt that using Chalmers mechanism will give some reasonable results. I will keep you updated. Thank you once again
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