Lower Flammability Limit in Cantera

[Chris Cloney]

Hello,

I have a little experience with running Cantera for freely propagating flames; however, i would like to investigate the lower flammability limit.

Can someone advise what Cantera will do when I decrease methane gas concentration for example below 0.4? Will it still give a solution or will it somehow indicate that the flame will not propagate?

I have read in the group that the domain must be quite wide as the flame thickness will increase. Are there any other considerations to keep in mind (I am typically looking at methane near atmospheric conditions).

Thanks,

Chris

[Santosh Shanbhogue]

Hi Chris,

The flammability limit of a gas is an empirical quantity, determined by whether it can propagate through a tube of a certain length (I think it is 50m or so). 

You will never consistently reproduce the equivalence ratio or mole fraction at which this occurs using a 1D model, as it is setup dependent.

Second, as you get close to the flammability limit, limit phenomenon like radiation become increasingly important

A concentration (mole fraction) of 0.4 is way too high anyways (above the rich flammability limit). Did you mean 0.04? This would correspond to an equivalence ratio of 0.4, which again is way too low

I would expect it to be in the vicinity of phi=0.6 for a 1atm, 300K mixture, and depends on the setup (a bit higher for jet flames, a bit lower for swirling flames)

All the best,

Santosh

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[Santosh Shanbhogue]

Correction - The tube is 1.5m in length and dia 51mm for the flammability test

Regards

Santosh

On Tuesday, January 3, 2017 at 2:36:33 PM UTC-5, Santosh Shanbhogue wrote:

Hi Chris,

The flammability limit of a gas is an empirical quantity, determined by whether it can propagate through a tube of a certain length (I think it is 50m or so). 

You will never consistently reproduce the equivalence ratio or mole fraction at which this occurs using a 1D model, as it is setup dependent.

Second, as you get close to the flammability limit, limit phenomenon like radiation become increasingly important

A concentration (mole fraction) of 0.4 is way too high anyways (above the rich flammability limit). Did you mean 0.04? This would correspond to an equivalence ratio of 0.4, which again is way too low

I would expect it to be in the vicinity of phi=0.6 for a 1atm, 300K mixture, and depends on the setup (a bit higher for jet flames, a bit lower for swirling flames)

All the best,

Santosh

On Tue, Jan 3, 2017 at 2:12 PM Chris Cloney <chris.cloney.organization@gmail.com> wrote:

Hello,

I have a little experience with running Cantera for freely propagating flames; however, i would like to investigate the lower flammability limit.

Can someone advise what Cantera will do when I decrease methane gas concentration for example below 0.4? Will it still give a solution or will it somehow indicate that the flame will not propagate?

I have read in the group that the domain must be quite wide as the flame thickness will increase. Are there any other considerations to keep in mind (I am typically looking at methane near atmospheric conditions).

Thanks,

Chris

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[Chris Cloney]

Hi Santosh,

Thank you so much for your response. You are correct, I meant equivalence ratio and the value should be higher (around 0.5 or about 5% mol fraction is a typically quote value I think?).

You make good point that the flammability limit is in the end an empirical quantity and dependent on the setup. 

I guess the question is what is the value for a truly "free flame" (i.e., an infinitely wide flame) and can Cantera reproduce that? Also, does the LFL converge on some value as the flame is made wider and wider (indicating that there is a converged "free flame" answer)?

I understand that other phenomena such as radiation may play the dominant role at the limiting case and resolving these are needed to get the quantitative value. However, would Cantera produce a value at all (although not the accurate one)? 

If anyone has any papers/references for the use of Cantera in this manner it would be very helpful for my research.

Thanks Santosh and anyone else who can help!

Chris

On Tuesday, January 3, 2017 at 3:43:09 PM UTC-4, Santosh Shanbhogue wrote:

Correction - The tube is 1.5m in length and dia 51mm for the flammability test

Regards

Santosh

On Tuesday, January 3, 2017 at 2:36:33 PM UTC-5, Santosh Shanbhogue wrote:

Hi Chris,

The flammability limit of a gas is an empirical quantity, determined by whether it can propagate through a tube of a certain length (I think it is 50m or so). 

You will never consistently reproduce the equivalence ratio or mole fraction at which this occurs using a 1D model, as it is setup dependent.

Second, as you get close to the flammability limit, limit phenomenon like radiation become increasingly important

A concentration (mole fraction) of 0.4 is way too high anyways (above the rich flammability limit). Did you mean 0.04? This would correspond to an equivalence ratio of 0.4, which again is way too low

I would expect it to be in the vicinity of phi=0.6 for a 1atm, 300K mixture, and depends on the setup (a bit higher for jet flames, a bit lower for swirling flames)

All the best,

Santosh

On Tue, Jan 3, 2017 at 2:12 PM Chris Cloney <chris.cloney...@gmail.com> wrote:

Hello,

I have a little experience with running Cantera for freely propagating flames; however, i would like to investigate the lower flammability limit.

Can someone advise what Cantera will do when I decrease methane gas concentration for example below 0.4? Will it still give a solution or will it somehow indicate that the flame will not propagate?

I have read in the group that the domain must be quite wide as the flame thickness will increase. Are there any other considerations to keep in mind (I am typically looking at methane near atmospheric conditions).

Thanks,

Chris

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[Santosh Shanbhogue]

Chris,

Cantera can easily provide a flame speed for a methane flame with phi=0.5. If you can't find the script for how to do this in Cantera, let me know and I'll point you to it

Regarding your question about the value for an infinitely wide flame, I have trouble internalizing it as it corresponds to a Damkohler number of 0 and thus zero reaction rate...

Maybe someone else can comment on the latter

All the best

Santosh

[Ray Speth]

Chris,

For a truly adiabatic flame, i.e. in the absence of any heat loss term (which is what the freely propagating flame model in Cantera normally simulates) there is technically no lower flammability limit -- the flame speed just becomes infinitesimally small, and the flame becomes infinitely thick. Of course, this makes things difficult for the numerical solvers, so you're likely to run into issues there if you try to simulate a flame where the equivalence ratio is too low.

Cantera does have a rudimentary radiation model available, but I'm not sure it would work well for identifying a flammability limit. The way the boundary conditions for the freely propagating flame are applied, if conditions were such that the flame should blow out, I think the solver would fail, but this failure is probably indistinguishable from solver failures that could occur under other conditions that are not indicative of the mixture being outside the flammability limits.

Regards,

Ray

[Chris Cloney]

Thanks Santosh, 

I have a working script but I have just not ran it lower than 0.6 yet.

Ray, 

Thanks for the response. This is what I thought might happen (and was kind of hoping against!). 

However, I was thinking that it is possible that at some flame thickness heat diffusion from conduction might "outweigh" reaction and the flame blow out. In this case my current Cantera model (no radiation) might be able to produce a lower limit (although radiation may be needed for it to be quantitatively correct).

It seems like I will have to look more into the literature at some of the fundamentals to see what approaches can be used, if any. Flammability limits is supposed to be the last part of my thesis (starting this part in a few months). I am just trying to figure out if it can be tackled in a few chapters or if it is like a whole separate thesis (in which case I may need to adjust my scope a bit).

Any suggestions or thoughts from others would be tremendously helpful. I will report back my findings once I look through the literature more, but it will not be for a few months!

Best,

Chris

[Eduardo Gris]

Hi Chris Clooney,

I found this article that runs numerical simulations for one-dimensional, nonadiabatic flames near lean flammability.

I tried to implement his methodology but I could't get much further. I have difficulties to present the data (peak temperature or flame speed) in function of time.

Maybe this could help both of us evaluate lower flammability limits.

Source:

On the flammability limit and heat loss in flames with detailed chemistry

K. N. Lakshmisha P. J. Paul H. S. Mukunda (1991)

https://doi.org/10.1016/S0082-0784(06)80288-2

Thanks.