Learning Forum

[Noelly]

Hello,

I am running a Fluent simuation on a rectangular channel with 1 inlet 1 outlet

I am using the species transport model, I have a mixture of methane + air, I left all the default values for the reaction (1 step reaction)

I have the following for my inlet setup

 

And for the wall around it, I left the default values for the momentum and species tabs

After 500 iterations, my residuals look like this

Do the oscillating residuals necessarily mean that my solution is not converged or can it be an indication that my simulation is by nature not steady state? If the latter is the reason, what would be an appropriate way to evaluate convergence?

My second question has to do with patching. Initially when I tried to run this, I was not getting any reaction (when looking at a contour of the species for instance, I was not observing much of a change).

Looking online, I saw that people were suggesting patching a high temperature (2500K) at the initialization step. After that I observed signs of a reaction

So is patching just attaching a temperature in the cells initially to start the reaction, or is this temperature sustained throughout the iterations as well? Is it possible that the patching step I added has something to do with my oscillating residuals?

Thanks

[Essence]

Hello,

Which combustion model are you using in the Species Transport? You need to use EDM for one-step methane+air reaction. I also see that mesh is not fine enough at the region where the reaction takes place. Please improve the mesh. The mole fraction of oxygen is 0.4, which should be 0.21 logically. How did you calculate the Turbulent Intensity? 17% is a bit high value.

Please refer to the tutorial:

https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v211/en/flu_tg/flu_tg_magnus.html

If you are unable to access the link, follow this Forum discussion https://forum.ansys.com/forums/topic/using-help-with-links/#latest

[Essence]

 

Hello,

Adding to my reply, I could also not see anything which holds a flame in place. This may move the flame around.

 

[Noelly]

Hello,

 

Thanks for your answers and apologies for the late reply, I am really not familiar with the physics of combustion and wanted to review a few things before asking for some more help.

So

My mole fractions from before are wrong but I was wondering where the 21% came from. So for air it would be 1/(1+3.76) = 0.21

But shouldn’t the mole fraction for the case where I have CH4 as well be calculated as

2/( 1 + 2 + 2*3.76) = 0.19 for O2

1/( 1 + 2 + 2*3.76) = 0.095 for CH4

?

 

I fixed the turbulent intensity using this formula

It is now about 14 %

 

Regarding the flame, I asked on the learning hub before which model I should be using and they said that since my actual system does not have a flame, I could model it with the species transport model.

[Noelly]

Regarding the model, I am using the Finite-Rate Kinetics (no TCI) for now. The final process to model will potentially be such that the turbulence time-scale are faster compared to the chemistry so from section 7.1.2.1 I thought I'd be using this model

[Essence]

Hello,

Regarding the mole fractions, 0.21 is the mole fraction of oxygen in the air and not the air itself. Rest 0.79 is occupied by nitrogen and other small amount of gases which is often ignored. The calculations you have shown will be applicable if you have the mixed reactants of methane and air at the inlet. Generally, in combustion systems there is dedicated fuel injection system and oxidizer system where you would need to input mole fraction of 1 for methane and 0.21 for oxygen respectively. And adjusting the flow rates to achieve the required air-fuel ratio.

But, as you are using mixed flow of reactants at single inlet, please use the calculated mole fractions of ch4 and o2 at the inlet. The Damkohler number for methane combustion is high (reaction is very fast and thus controlled by turbulent mixing). And as you are using only one step reaction, you should use EDM instead of FR.

[Noelly]

Thank you!

[Noelly]

Hello,

I still have an issue with my simulation oscillating

So here is my geometry, two slender channels connected by small holes, the air+methane mixture enters from the bottom channel and exits from the top

Reynolds number is very low (about 40) so I selected laminar for the viscous model and the only option available for species transport was Finite rate/no TCI

I removed n2 because in the input box from boundary conditions it was occupying a spot of another substance part of the reaction, but other than this I left the other parameters and properties the same

It’s an application where there is much more CH4 than air so I used a 1% fraction

(the reference values are the default ones, they only play a role for postprocessing, correct?)

So here is my setup, is there something that is obviously amiss with my inputs and do I need to activate gravity and thermal diffusion or is it a matter of altering the algorithms used for the solution?

 

Thank you

 

[Essence]

Hello,

Why are you using laminar flow? Methane-air one step reaction needs turbulence to get properly mixed. You also need to have nitrogen in the selected species, as a "last species". Use High Order Term Relaxation. Select inlet diffusion, thermal diffusion and full multicomponent diffusion. Try using transient approach.

[Noelly]

 

Hello,

I was using the laminar flow because my flow rate is such that the Reynolds number is very low… so I selected it from the flow perspective, I did not know that I am supposed to specify turbulent flow for the sake of the reaction even when the Reynolds number is in the low 50.

I tried the suggested solutions but they did not really change the issues I was facing so I decided to move to the 2-step reaction. The results look sensible, the residuals have converged and there is a temperature rise in the channel.

There is one last issue I’m trying to understand though: 

In this simulation I am using a low methane concentration (from a volume fraction of 0.003, I get a mass fraction of 0.001667 which I use as input). From the mass flow rate input and the LHV, I should get a heat of reaction of 0.729 W and a dT of about 20 K but I am getting 2.559 W with dT of almost 70 K.

The simulation seems to be self consistent in the sense that when I use the 2.55 W to calculate dT, I indeed end up with 70 K. But I am not sure where the additional heat from 0.729 to 2.55 W comes from. Is there something that I am missing? Is there a way to get the actual mass of ch4 in the flow domain instead of the fraction? Please find attached Fluent input/results as well as my calculation check.

 

[Essence]

Hello,

The enthalpy value in the heat balance sheet, are you using the value from the Fluent material database?

[Noelly]

Hi,

I am not... I was using the values from a thermodynamics textbook. How can I check the values of the enthalpies of formation and enthalpies of reaction in the database?

[Essence]

Heat of Reaction is found in Fluxes (in images shared by you). Standard State Enthalpy can be found in the Material properties of the fluid mixture.

[Noelly]

Thanks for the clarification. So I double-checked the database and the LHV is about the same as I got in the calculation, so I am not sure what is the cause for the discrepancy... is it possible that it has to do with how Fluent interprets my input for the species?