Methane combustion using built in Arrhenius equation in ANSYS CFX
 

Good day all,

My main aim is to model methane steam reforming using heat source in presence of Catalyst. For time being I am modelling methane combustion by defining my own reaction using ANSYS CFX 18.

My first problem is reaction occurs all the time even if I set activation temperature to be 3000K. Whereas in domain I assign maximum temperature of 1000K. Is there any default value which is negating my set activation temperature?

Regards

Not certain about your question.

Regardless of the value set for activation temperature, the reaction is always evaluated. The result of the reaction rate is what matters. If the rate is zero, no products are formed, and not reactants are consumed.

How did you set up the maximum temperature for the domain?

What combustion model have you selected?

I thought reaction should occur only if temperature is above activation temperature like of catalyst.

I have changed mixing rate so for zero mixing rate reaction does not proceed.

I haven't applied zero reaction rate, but reaction should occur for both +/- values which I have done. Just reaction direction is reversed by changing sign

Temperature of wall and inlet are assigned same value so domain temperature is close to that.

I have used eddy dissipation model with no limiter

The file attached is for zero reaction rate. Reaction is occurring even then.

I think there is confusion on what the options mean:

For the Arrhenius reaction rate type:

Rate = A * T^Beta * exp (-T_activation/T)

where

A is the pre-exponential
B is the temperature exponent
T_activation is effectively E_activation / R_constant

as you can see, it is evaluated everywhere.


Now on the combustion model being used. You have selected a reaction mechanism for Methane, you have set up the details of the chemical reaction; however, at the domain level you have explicitly indicated to use mixing rate by selecting Eddy Dissipation w/o any chemical reaction considerations, i.e. the setting at the reaction definition are being ignored.

Are you trying to model Finite Rate Chemistry, or turbulent flow combustion mixing model? My advice is to check the documentation for the differences between the options, and proceed as you see fit.

so Rate = A * T^Beta if T_activation is equal to zero. Fair enough.
However, when I changed the Rate value to zero (both for my case and ANSYS tutorial) the reaction is still taking place. I have edited and attached the file to previous post.
Eddy dissipation model is more robust than Finite Chemistry. I have tried both anyway.

Details of chemical reaction are set at 'Reaction' tab in outline tree. The 'material' tab takes these values when user define 'material'. Then in Domain setting, when fluid is selected, that 'material' is selected comprising mixture of fluids (e.g. methane, CO, CO2,O2). Mixing rate is one of many option when @eddy dissipation model' is selected but there is none there about activation T or reaction rate. These are defned in 'Reaction' tab.

Are you sure , this is what activation temperature mean?"I thought reaction should occur only if temperature is above activation temperature like of catalyst."
because I have seen activation temperature 26,000 k
Can someone explain what activation temperature is?

Can someone explain what activation temperature is?

https://lmgtfy.app/?q=activation+temperature

Or read the CFX documentation for CFX's specific application of the concept.