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PaSR + infinite reaction rate in reactingFoam --> no reactions occurring

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Old   November 15, 2012, 09:31
Default PaSR + infinite reaction rate in reactingFoam --> no reactions occurring
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Tatu Pinomaa
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Hello!

Background:
First of all I want to say I'm a noobie with OpenFOAM, and I'm using OF 2.1.1. My goal is to model hydrogen combustion in a high temperature as a one-step reaction, so that the combustion reaction will not be a pure 2H2 + O2 => 2H202, but will include dissociated molecules (e.g. O, OH). Therefore Arrhenius data, i.e. reaction rate coefficient A or activation energy Ea, will not be available.
Also, the flow is compressible, so it is important to make the chemistry model as computationally light as possible. Therefore, I will be using (at least for now) eddy-dissipation model with infinite chemical reaction rates, i.e. I assume that turbulent mixing is the bottleneck for the combustion.

Problem:
I am studying the case counterFlowFlame2D under reactingFoam solver, where the reaction rate is controlled by both the turbulent mixing and Arrhenius chemical kinetics (i.e. PaSR model). Now my goal is to simulate the combustion in this test case simply with the eddy-dissipation model, so that I switch off Arrhenius chemistry.

Q1: To use the simpler eddy-dissipation model without Arrhenius chemistry, should I merely change the reaction type from irreversibleArrheniusReaction to irreversibleinfiniteReaction in file constant/reactions? Are there other options I should consider for this case?

Q2: Why are there practically no reactions occurring when I use irreversibleinfiniteReaction? Shouldn't the infinite chemical reaction rate model increase the reaction rate, and not decrease it?

As you can see from the two figures (infinite reaction rate and Arrhenius chemistry) for time 0.3, the number of reaction products are practically non-existing for the infinite reaction rate. Also, for the case of infinite reaction rate, the maximum temperature doesn't exceed 800K, which is the inlet temperature of the oxygen and CH4.

Thanks in advance,

Tatu
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File Type: png infinite.png (23.2 KB, 15 views)
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Old   November 21, 2012, 06:02
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Tatu Pinomaa
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Quote:
Originally Posted by tatu View Post

Q1: To use the simpler eddy-dissipation model without Arrhenius chemistry, should I merely change the reaction type from irreversibleArrheniusReaction to irreversibleinfiniteReaction in file constant/reactions? Are there other options I should consider for this case?

Tatu
I tried to study the reactingFoam solver, and here's where I am now.

I figured out that by setting turbulentReaction to false, the reactive volume fraction (kappa in the PaSR model), that scales the fuel consumption rate RR(specieI) for each species i, will be automatically set to 1. By doing this, the reaction rate increases significantly, as it should.

I cant find any other dependencies on Arrhenius data in PaSR.C, than on lines 82-89 (useReactionRate == true), and lines 95-132 (turbulentReaction == true).
However, setting useReactionRate and turbulentReaction as false in constant/combustionProperties and then fiddling with the Arrhenius data, affects the reaction rate (increasing the activation temperature Ta in file constant/reactions decreases the reaction rate significantly).

Am I right that all the chemistry in the reactingFoam solver can be seen from PaSR.C?

Where exactly does the Arrhenius data come into play in the chemistry of reactingFoam solver (PaSR)?

And still, by setting reaction type to irreversibleinfiniteReaction from infiniteArrheniusReaction, the reaction rate decreases to almost zero. Why doesn't the reaction rate increase instead?

Cheers,

Tatu
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