Single step and multiple step reaction

[AmirrezaHBSH]

Hello, your help would be highly appreciated. I am working on multiple steps and single step combustion. I used the same reactions that are discussed in Reference Guide in section 5.2.13, eq. (5.62) and eq. (5.63) which are the infinite rate reaction of Methane and Propane.

I have two cases with multiple steps reaction and single step reaction using same approach that is mentioned in eq. (5.64). I assumed that from pyrolysis reaction, the light gas consists of 40% Methane and 60% Propane mass fraction. My input files were attached.

Multiple.docx
Single.docx

Using both approaches, should we expect identical results for the heat release rate, or would there be differences? My heat release rate plot is attached.

[drjfloyd]

In a perfect world yes. However, your computer is not a perfect world. In one case you are tracking two seperate fuel gasses, and in the other case you are tracking a single effective species. All numerical transport schemes have numerical error. By changing the number of species you are going to change the details of how that error manifests. In other words tracking one vs. two species is going to result in very slight differences in concentration. Since you are reacting those species, those slight concentration differences will also mean slight differences in heat release. Difference in heat release will cause differences in the flow field which will result in differences in gas tranposrt resulting in more differences in concentration. Over many time steps those very small differences can add up and become noticeable. Your results are not surprising. The two cases are essentially identical at the start but over time there are slight differences in the result.