Why the mass fraction nephogram is same？

[wangjinjing]

Hello，developers！
This is an case of methane combustion to generate CO2 and H2O. One problem I encountered is that the mass fraction nephogram of the reaction product CO2 and H2O is the same, and the amount generated is very small. Can you help me see why this phenomenon occurs? This is my case, thank you!
CH4-0210.mfx (21.2 KB)
usr_rates.f (4.2 KB)

[jeff.dietiker]

Please attach supporting plots to help describe your issue.

[cgw]

@jeff.dietiker It does look like something odd is going on here. There’s only one reaction,

  CH4 + 2*02 -> CO2 + 2*H2O

Setting up a monitor for average mass fraction of CO2 and H2O over the entire domain:

# Run type: NEW
# Monitor type: Arithmetic average over region, Volume =   0.13959536E-02
# "Time","x_g(3)","x_g(5)" 
    0.0000000    ,    0.0000000    ,    0.0000000    
   0.49600000E-03,   0.23029333E-02,   0.23029333E-02
   0.99600000E-03,   0.32448094E-02,   0.32448094E-02
   0.15360000E-02,   0.39408184E-02,   0.39408184E-02
   0.20410000E-02,   0.45089983E-02,   0.45089983E-02
   0.25160000E-02,   0.49502661E-02,   0.49502661E-02

Note that the numbers in columns 2 (CO2) and 3 (H2O) are identical. This doesn’t seem right. Mol. weight(CO2) = 44 and mw(H2O) = 18, the reaction produces 2 moles of H2O for each mole of CO2, so the mass fractions should be in the ratio 36/44, right?

[jeff.dietiker]

Several issues:

1. Mesh: Your cylinder is closed at both ends and the cylinder caps are located along the y-ymin and y=ymax planes. This conflicts with the inlet and outlet BCs. You need to increase the size of the cylinder so the two caps are outside of the domain.
2. BC: You have set a constant wall value for all gas species. You souls use “No-flux” bc. This is what is generating the same mass fraction. You reaction rates is basically zero (at least for the short duration I have run the simulation), so the reaction is not generating any CO2 nor H2O.

[wangjinjing]

@jeff.dietiker

set a constant wall value for all gas species

The first article has been modified as you said.
For the second one, do I need to modify these parameters in the diagram? I modified the two parameters marked in red in the figure, and found that the contour of the mass fraction of CO2 and H2O is still the same, but the values are different.
I would like to ask if there are any requirements for the setting of these values?Thank you！

[cgw]

Change the species equation type to “No-flux”, for all species.

[wangjinjing]

Thanks！How can I increase the rate of chemical reactions? Is there a problem with my code for this reaction rate?