[MFGT]

Hi,

i am using CONVERGE for Gasoline PFI and DI engines. I have always worked with SAGE, because it is pretty straight forward. All what is needed is the fuel within the engine and a suiting reaction mechanism and a Ignition source. However, i wanted to know if someone has used other combustion models for specific cases?

I noticed that especially with several cycles the WallTimes increase, even though i added the skip species.

Thanks for respones.
Tobias

[dprobst]

Yes, you can use simplified combustion models for gas PFI and Diesel. For gas PFI, the G equation model could be used, while for Diesel the shell/CTC could be used.
While these models might require tuning to match data, they can offer shorter runtimes.
Detailed chemistry, however, is a good choice when predictive results are required.

[MFGT]

Yes, at the moment i am completly on the predictive side, as no experimental data is available yet.

But i think i am running into another issue with the validity range of the mechanism. The SAE 2013-01-1098 paper is mentioning the range for the jia mechanism, which is 10-50 bar. However, i might have cases with even lower pressures at spark timing (3-4 bar).

What can i do to compensate this? Adding spark energy? In that paper mentioned above, a reduced mechanism especially for low pressure range has been used. But i dont want to try reducing a mechanism, because it would be a completly new field.

What do you think about the idea of implementing a mechanism database in the converge download area of your homepage?

[dprobst]

If the operating conditions are outside of the applicable range for the reduced mechanism, the results may not be satisfactory. Adding spark energy would not be a solution for a poorly performing mechanism.
I would suggest first running the baseline case using an appropriate mechanism and then assess the results against experimental data.

We do not currently have a mechanism library on the downloads page, but we may have something like this in the future. We do recommend mechanisms with our example cases, which are available for download. Thus, you can look at the example case that most closely matches your situation (SI, DI, etc...) for a recommended mechanism.

[MFGT]

I am using the Jia PRF / TRF mechanism.
When i compare the 0D Ignition delay times with the detailed mechanism from LLNL within a range of 1-11 bar, they seem not that far off.

How can i further check the performance of the mechanism?

edit:
Ive set up the same simulation with G-Eq. and default inputs. G-Eq. doesnt need a mech.dat and Temperature AMR, right?

[dprobst]

The performance of the mechanism can be assessed by running simulations and comparing to experimental data.

Geqn does not need Temp AMR. The combustion is determined by correlations for flame speed. You can look at our example case for SI8 Geqn to see recommendations.

[MFGT]

Hey Dan,

the G-Equation model performed a lot better for my current case (2000rpm - 2bar bmep). I took all default inputs from the Example case.

edit:
Sorry again, but i am running into problems with the G-Equation model.

During gas Exchange, i suddently run into the following issue (here is the log):

ncyc= 2212, time= 3.173915676e-002, crank= 3.808698811e+002, dt= 1.516968111e-006, time-step limit =dt_cfl by region 1
ncyc= 2213, time= 3.174066981e-002, crank= 3.808880377e+002, dt= 1.513049886e-006, time-step limit =dt_cfl by region 1
ncyc= 2214, time= 3.174217835e-002, crank= 3.809061402e+002, dt= 1.508547680e-006, time-step limit =dt_cfl by region 1
ncyc= 2214, time= 3.174142408e-002, crank= 3.808970890e+002, dt= 7.542738401e-007, time-step limited by max_residual_passive, recovering ......
ncyc= 2214, time= 3.174104694e-002, crank= 3.808925633e+002, dt= 3.771369201e-007, time-step limited by max_residual_passive, recovering ......
ncyc= 2214, time= 3.174085837e-002, crank= 3.808903005e+002, dt= 1.885684600e-007, time-step limited by max_residual_passive, recovering ......
ncyc= 2214, time= 3.174076409e-002, crank= 3.808891691e+002, dt= 9.428423002e-008, time-step limited by max_residual_passive, recovering ......
ncyc= 2214, time= 3.174071695e-002, crank= 3.808886034e+002, dt= 4.714211501e-008, time-step limited by max_residual_passive, recovering ......
ncyc= 2214, time= 3.174069338e-002, crank= 3.808883205e+002, dt= 2.357105750e-008, time-step limited by max_residual_passive, recovering ......
ncyc= 2214, time= 3.174068159e-002, crank= 3.808881791e+002, dt= 1.178552875e-008, time-step limited by max_residual_passive, recovering ......
ncyc= 2214, time= 3.174067981e-002, crank= 3.808881577e+002, dt= 1.000000000e-008, time-step limit =dt_min forced by max_residual_passive
ncyc= 2215, time= 3.174068981e-002, crank= 3.808882777e+002, dt= 1.000000000e-008, time-step limit =dt_min forced by dt_recover
ncyc= 2216, time= 3.174069981e-002, crank= 3.808883977e+002, dt= 1.000000000e-008, time-step limit =dt_min forced by dt_recover

if i change screen output, i see the following line:

passive G_EQN iteration= 1, residual= -1.#IND00e+000, jac= 0
max residual exceeded on passive

Question: Can i not run several cycles with G-Equation combustion model?
Or what else is causing this issue?
I am checking if the simulation can continue from 380°CA without combustion model.

[dprobst]

It looks like there is a problem with your case setup. G eqn uses passives to track the flame.
Please email your case setup to support@convergecfd.com for us to help resolve this issue.

[MFGT]

Thanks.
I´ve done that. Incuding a furher description and coressponding map files.

[MFGT]

Originally Posted by MFGT

Question: Can i not run several cycles with G-Equation combustion model?
Or what else is causing this issue?

i found the information. Figure 13.27 of the manual shows a sample g_eqn_init.in file. However, i am not sure how to use it exactly.

What is the "best" CA to reinitialize G?
Can i limit the G-Equation model to region0?

[dprobst]

You need to reinitialize G for multiple cycles. The SI8 Geqn example case shows how to do this. This example case is available for download.

The g_eqn_init file is referenced in combust.in:

"g_eqn_init.in" g_eqn_init_value Initial G-value.

G should be reinitialized at a appropriate crank angle after combustion is expected to be complete - such as 120 dATDC.

[MFGT]

During ignition, i ran into time-step limit =dt_min forced by dt_src_passive.
dt_min is 1e-8
Luckily, it can recover an get back to values in the range of 2e-6.

Is this supposed to happen? Whats the reason? I couldnt find any information about dt_src_passive.

edit:

Originally Posted by dprobst

You need to reinitialize G for multiple cycles. The SI8 Geqn example case shows how to do this. This example case is available for download.

The g_eqn_init file is referenced in combust.in:

"g_eqn_init.in" g_eqn_init_value Initial G-value.

G should be reinitialized at a appropriate crank angle after combustion is expected to be complete - such as 120 dATDC.

If i use the g_eqn_init.file, how is G initialized in the simulation domain at the beginning of the simulation? Shouldnt this be done under "Regions & Initialization"?
Or is CONVERGE receiving a value from the g_eqn_init.file?

[MFGT]

Dan helped me out here as well.

As help for other users, i had forgotten to add the G passive at the IN- and OUTFLOW boundaries. Of course this leads to a problem, if i flush my G passive out of the domain with multi-cycle

Btw, the initialization with the g_eqn_init.file worked fine.

[MFGT]

Another question regarding the combustion model, this time about SAGE.

What is the best practice to start and end times of SAGE?

In the engine sector cases, SAGE is activated 1°CA before SOI.
In the SI8 PFI case, SAGE is also activated 2°CA before SOI.

However, in the Premixed SI8 case, SAGE is activated 2°CA before IGN.

In my PFI and DI gasoline simulations, I have seen heat release even before my Spark Timing, especially in multi cycle simulations. There is no real temperature rise, so i dont think its auto-ignition.

[dprobst]

You should be sure SAGE is turned on 1 or 2 CA before you expect the earliest SOC. If you might expect auto ignition before spark, the SAGE start time should be advanced accordingly.
The end time is typically EVO but can be longer or shorter as necessary. The primary condition for keeping SAGE on after EOC is emissions.

[MFGT]

Yep, that makes sense. For now, i switch off SAGE before EVO and my skip_species timing.

[MFGT]

In a simulation with SAGE, i have a slower burn rate (and pressure rise) compared to measurement data. I guess its reasonable to tune SAGE with the help of the reaction multiplier then?

Or is this something you would rather not do?

[xieshengbai]

Originally Posted by MFGT

In a simulation with SAGE, i have a slower burn rate (and pressure rise) compared to measurement data. I guess its reasonable to tune SAGE with the help of the reaction multiplier then?

Or is this something you would rather not do?

Hi,

The lower burn rate can be resulted by many reasons. You may have underestimated turbulence, or the mechanism you used is not good, etc. I think you'd better first check your case setup and make sure everything is correct. You can also compare other properties such as TKE with the measurement data to figure out where the problem is.

Tuning the multiplier may help to get a correct burn rate, but it may also mask some other problems in your case. It is purely numerical. So you'd better at first make sure everything else is OK.

[MFGT]

Thanks for your feedback. The mechanism should be fine. I am simulating a 1500 WOT case and the mechanism has been tested for similar conditions in SAE 2013-01-1098.

I am trying to rerun it with new inputs, as i also have troubles to get the desired cylinder charge. Therefore i am using GT-Power pressure and temperature profile inputs.

[MFGT]

I figured out to not touch the SAGE multiplier. With optimized other inputs i was able to achieve better suiting conditions at IGN timing and the combustion was matched better.

Now, new question regarding the G-Eq. Combustion Model:
i use G-Eq. for idle/low load operating conditions. However, i came across a few situations where i had to retard the G source timing by up to 20CA to match pressure rise. Otherwise i had a way to high peak pressure and early pressure rise.

Also, i had to lower b1 down to 1.5 to match the combustion intensity. Now today i came across the dillution species mass fraction, which i had never adjusted and i obviously had lots of EGR in the above mentioned cases of low loads.

So would it be right to set it to e.g. 0.222 if my GT-Power model has an EGR ratio of 22.2%?