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Unit of Pre Expoential Factor in Finite Chemistry Rate

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Old   November 28, 2022, 09:53
Question Unit of Pre Expoential Factor in Finite Chemistry Rate
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I am doing a Low Re(about 1000) laminar combustion simulation with combustion model of finite chemistry rate. However, the unit pre exponential factor is very strange (mm^459 s^-1 mol^153)

The original chemical equation is 41C2H6+112N2O==62CO2+20CO+112N2+113H2+10H2O

I found that this unit is related to the coefficient of reactants, like you can see the figure 1 when I changed the reactants as 1C2H6+1N2O but it makes the equlibrium no sense.

And also, the unit when both coefficient are 1 is still strange, as I know the unit should be kind of mol mm^-3 s^-1 (the pre expoential from the book is about 10^10 mol mm^-3 s^-1). Also, when there is finite chemstry rate in the combustion model (finite chemistry rate or eddy dissipation and finite chemstry rate), the reaction almost doesn't happen at all, and also there are some N2 in the product near the inlet.

I have no idea how to fix that
1. how to make the unit of pre exponential factor normal and the reaction run with finite chemtry rate combustion;
2. I tried to change the model to SST gamma turbulance model with onset Reynolds number 6000 + eddy dissipation combustion model, how accurate of this model for a low Re situation?

Thanks a lot!
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Last edited by zyb2629; November 28, 2022 at 15:14. Reason: typo of chemical equation
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Old   November 28, 2022, 15:10
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Quote:
Originally Posted by zyb2629 View Post
I am doing a Low Re(about 1000) laminar combustion simulation with combustion model of finite chemistry rate. However, the unit pre exponential factor is very strange (mm^459 s^-1 mol^153)

The original chemical equation is 42C2H6+112N2O==62CO2+20CO+112N2+113H2+10H2O

I found that this unit is related to the coefficient of reactants, like you can see the figure 1 when I changed the reactants as 1C2H6+1N2O but it makes the equlibrium no sense.

And also, the unit when both coefficient are 1 is still strange, as I know the unit should be kind of mol mm^-3 s^-1 (the pre expoential from the book is about 10^10 mol mm^-3 s^-1). Also, when there is finite chemstry rate in the combustion model (finite chemistry rate or eddy dissipation and finite chemstry rate), the reaction almost doesn't happen at all, and also there are some N2 in the product near the inlet.

I have no idea how to fix that
1. how to make the unit of pre exponential factor normal and the reaction run with finite chemtry rate combustion;
2. I tried to change the model to SST gamma turbulance model with onset Reynolds number 6000 + eddy dissipation combustion model, how accurate of this model for a low Re situation?

Thanks a lot!
Are you certain the chemical equation above is correct? Something is OFF, way OFF.

How much C is supposed to be?

Left side = 42 C2 --> 84 C
Right side = 62 CO2 + 20 CO --> 82 C

C is unbalanced.. Leave it with you to fix every element.
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Old   November 28, 2022, 15:16
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Quote:
Originally Posted by Opaque View Post
Are you certain the chemical equation above is correct? Something is OFF, way OFF.

How much C is supposed to be?

Left side = 42 C2 --> 84 C
Right side = 62 CO2 + 20 CO --> 82 C

C is unbalanced.. Leave it with you to fix every element.
Sorry, I checked my document and it's 41C2H6, it's a typo and the equilibrium is correct in the CFX setting
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