Combustion model on SI simulator in Chemkin

[ss00321]

The SI model uses the flame-sheet approximation for fuel combustion, which assumes complete combustion occurs instantaneously.

I would like to disable this option to use a detailed kinetic in the SI Simulator, similar to what is done in other reactor models, such as Batch or PFR.

However, I cannot find options such as “Burned Gas Chemistry: Finite-Rate”, “Enable Chemistry in Burned Zone”, or “Post-Flame Kinetics” to prevent the burned zone from using equilibrium.

Is it necessary to modify the model through programming to achieve this?

If so, could someone please show me how to implement it?

[jcooper]

Hi:

The Chemkin models are designed for simplified analyses of more complex cases. The premise of the zonal SR model is two distinct zones, an unburned zone and a burned zone, separated by a infinitely thin flame region. This is adequate for many of the combustion processes in SI engines.  Enabling detailed chemistry would "break" this assumption, because it would create a much thicker intermediate zone and invalidate the other assumptions inherent in the model. 

To model detailed chemistry in the same framework, I would consider creating a 3d model and using Fluent or Forte.

Regards,

Judy

[ss00321]

Thanks, Judy. I understand that. For now, could you please provide more information about how the SI model works and how to interpret the ROP results for the unburned, burned, and average zones? The information on this in the theory guide is quite limited.

I’ve noticed that the ROP values in the average zone do not correspond to the volumetric weighting of the unburned and burned zones. Therefore, I’m inferring that the model recalculates the average ROP at the average temperature.

In addition, when I run fuel blends — for example, CH₄–NH₃ — the flame-sheet approximation seems inconsistent, since I observe more CO than what the post-flame chemistry predicts. It almost looks like CO is being produced in the flame sheet itself. Could that be possible in this model, or is there another explanation for the higher CO observed compared to the post-flame chemistry?

[jcooper]

Hi:

The ROP values in the averaged zone are computed from the pressure, the temperature, and the species mass fractions of the averaged zone. They are not weighted values of the ROPs of the two zones. Most chemical activities should take place in the flame sheet. The SI model uses an equilibrium model together with the mass burned rate to compute the conversion/production rates of the major combustion product species. The ROPs therefore do not include all the specie conversion rates in the SI engine model. It is not possible to compute the predicted species production rates from the flame sheet using the solution of the SI engine model.

 

2. All the CO (and CO2, H2O, NO, ...) in the burned zone are formed in the flame sheet. If you are asking the reasons why you see more CO in the burned zone for a fuel mixture like CH4 + NH3 than a fuel mixture with only CH4, it could be that NH3 is also consuming O2 (or other chemical interactions between NOx, NxHy, and COx species), or CO is more stable than CO2 at high temperature. Note that both the pressure and the unburned gas temperature in an SI engine vary during the cycle while premixed flame calculations are at a fixed pressure and initial temperature.

 

I hope this helps.

 

Regards,

 

Judy