Reactor & Stoichiometric mixture

[Giovanni Padovani]

Good Afternoon,
I'm running Cantera on Matlab, to simulate engine combution.
I have two different question:
1) What's the difference beetwen Reactor and IdealGasReactor. There are some differences in the results?
2) I noticed that carrying out the simulation of stoichiometric combustion remains oxygen at the end of the test (I have attached the Matlab figure). Using gri30 mechanism also with other type of chemical kinetics mechanism, at the end of the test i remain some oxygen mass fraction at stoichiometric condition. I'd like to know if it is a known issue with cantera, or if it is my mistakes. I also use the composition which is in the tutorial of cantera but i've the same result.

T = 900; %K
p = 90.e+5; %Pa
phi = 1;

gas = importPhase('gri30.xml')
X = 'CH4:1, O2:2, N2:7.52';
set (gas, 'T', T, 'P', p, 'X', X)
R = Reactor(gas);
net = ReactorNet(R);

t = 0;
dt = 1.e-5;
tf = 1.e-1;
value = tf/dt;

for z = 1:value
    t = t+dt;
    advance(net,t)
    oxy(z) = massFraction(gas, 'O2');
    methane(z) = massFraction(gas, 'CH4');
    time(z) = t;
end

figure(1), hold on
plot(time, oxy)
plot(time methane, 'r')
legend('O2', 'CH4')
xlabel('Time [s]')
ylabel('Mass Fraction')
set(gcf, 'position', [100 100 550 450])


Thanks a lot for your response.

My apologies, for my bad english

[Steven DeCaluwe]

Hi Giovanni,

How much oxygen remains?   While the thermodynamics will generally favor the combustion products, at equilibrium I would imagine you would have *some* small amount of oxygen left. 

I'm not too familiar with the numbers, but imagine that this equilibrium amount if O2 would be pretty small. Anyway, just a thought. Can you provide more info about how much oxygen remains?

Cheers,

Steven 

Sent from my iPhone

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<massFraction.jpg>

[Giovanni Padovani]

Hi steve, thanks for your reply

how initial condition I set the stoichiometric condition for the methane CH4:1, O2:2, N2:7.52, that corrisponde at this molar fraction:
O2: 0.190114,
CH4: 0.095057,
N2:  0.714829.

At the end of the simulation, i have this molar fraction:
O2: 0.007323;
CH4:  6.30943e-13;
N2:  0.714829.

So how you can see, I have too much oxygen at the end of the test. I tried to make long simulation, but the oxygen molar fraction doesn't goes down.

Best regard
Giovanni

[Bryan W. Weber]

Dear Giovanni,

Those appear to be mass fractions at the final condition; for mole fractions, I get

O2: 0.00623297

CH4: 1.2457E-12

N2: 0.700564

So I can reproduce your results, and I'm using the Python interface. I get the same results using the equilibrate method of the Solution class, so I expect its some left over from the equilibrium, as Steven suggested.

Bryan

[Nick Curtis]

Also, in regard to your first question the IdealGasReactor explicitly uses the Temperature as a state variable and thus avoids a potentially problematic Newton iteration to determine the state of the gas from the internal energy and volume (or enthalpy and pressure).  In general, you should use the IdealGasReactor when possible
Nick

[Giovanni Padovani]

Hi Bryan,
I'm sorry, probably i confused the mass fraction with the molar fraction. My apologies.
I don't understand what do you mean with your sentence: "I get the same results using the equilibrate method of the Solution class, so I expect its some left over from the equilibrium". There's too much oxygen at the beginning of the test?
Also I calculated the oxygen number of mole by stoichiometry equations: 1 CH4 + 2 O2 => 1 CO2 + 1 H2O. 1 mole of fuel and 2 mole of oxygen, is that right?


Thank you very much for your reply
Best Regards

Giovanni

[Giovanni Padovani]

Hi Nick,
Ok I understand, thanks a lot!!!

Best regard
Giovanni