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XiFoam lean-premixed combustion instabilities

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Old   May 16, 2014, 07:52
Default XiFoam lean-premixed combustion instabilities
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remi
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Hello everyone,


As a new OF user, trying to run simulations about vortex driven combustion instabilities in a lean premixed combustor, I came across some problems and questions:

I chose to use XiFoam (2D-LES) with the pitzDaily case, in order to see whether I could obtain close results to experimental ones that can be found in: Combustion in a Turbulent Mixing Layer Formed at a Rearward-Facing Step, Robert W. Pitz and John W. Daily. My main problem is to get a sustained combustion when the equivalence ratio is set as the experimental one (ER=0.57, propane fuel), for a Reynolds number of 15000 and 22000. I thought that the model for ignition in OpenFOAM was the reason of the “flame blow-off”, so I downloaded swak4foam in order to set my own initial flame front, setting the b field from 1 to 0 with a gradient, as can be seen in the following document on fig15: Numerical Investigation of the Aerodynamic Influence of Combustion on the Vortex Breakdown With OpenFOAM ,Carlos Trejo Velasco.

Only, this doesn’t work well for me ( The b field stays fixed in time after I fixed it with funkySetFields). Has anyone ever set up an initial flame front with this method in XiFoam, is there any better way? I find dealing with the “strength” parameter of the OF combustion model quite frustrating…

Do you think the XiFoam model is the one I should use to study lean-combustion instabilities (due to vortex shedding)? I have looked on the forum to see whether it had already been done, without great success. What other solver could do the job? ReactingFoam? With XiFoam, I have troubles reaching strong instabilities, and it can never be done at small equivalence ratio because of flame blow-off.


I hope I was not too confusing with my explanations. Just in case, here is a summary:


  1. Is XiFoam ok for my project of studying vortex driven combustion instabilities?
  2. Is there some parameters to change in order for the combustion to happen at low ER?
  3. Would reactingFoam be of use too?
  4. How to set up the ignition properly using the b field in XiFoam, or another field inreactingFoam?


Thanks for your time and help,


Remi
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Old   October 18, 2014, 05:27
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Jim KIT
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Dear remi,

1)XiFoam is the right solver for studying instabilities.
4)to Ignite the system I preffer to put the flame with setField as a thin flame front somewhere near flame holder and to set the tempreature befor the flame equal to Inlet tempreatuer and after the flame to adiabatic flame tempreature of propann
2) at lower equivalence ratio is the laminar flame speed lower. and I think you have to get blow off. In this case try to decrease the inlet massflow, or increase the equivalence ratio to see if youre able to get a sustained combustion.

so and I have some question:
a) what do you mean by 2D_LES?
b) Is the system adiabatic?

If you already solved your problem, I would be happy to finde out your solution.

Jim
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Old   October 18, 2014, 06:29
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Dear Jim, thanks a lot for your reply.

I am still studying combustion instabilities, with XiFoam and sometimes reactingFoam.
I successfully observed blow-off as I decreased the equivalence ratio, my only concern is that I had to manually decrease it by changing thermophysicalProperties and the parameter in combustionInstabilities. Problem is, if I change the equivalence ratio at t=1s for example, all the domain suddenly becomes at a different equivalence ratio, not only the new fluid coming through the inlet.

I'll try setFields, it seems to be the thing to do here for ignition.

About your questions,
a) I faked the 2D by taking one cell in z direction. So it's technically a 3D calculation using LES, but with only 1 cell in the z direction I like to call it 2D !

b) I'm using a fixedValue BC for the walls, but I don't know about thermal exchanges actually... Should it be obvious when looking at the simulation?

For combustion instabilities, the acoustic of the flow, and hence the boundary conditions at inlet/outlet are really important. I am trying to figure out how the waveTransmissive BC works, especially the lInf parameter, that is a "measure" of how far the farfield is. I havn't looked into the code yet.

Do you know what kind of BC should be applied for a work involving acoustic instabilities ?

Best,

Remi
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Old   October 18, 2014, 08:43
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Jim KIT
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Hello Remi,

what are you using for
a)"laminarFlameSpeedCorrelation"@combustionPropert ies?
b)"mixture propertise"@thermophysicalProperties


I think the equivalence ratio would be used for calculating laminar flame speed and adiabatic flame temperature and I'm wondering how do you see the change in equivalence ratio in post-processing.

Jim
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Old   October 18, 2014, 09:44
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Hello Jim,

a) For the laminar flame speed, I am using Gulders correlation.
b) The mixture is homogeneous. I havn't the files on my laptop so I can't tell you precisely, but I think I didn't change it from the PitzDaily tutorial case.

When changing the equivalence ratio parameter in @combustionProperties, the flame temperature will not change, but the recirculation zone may be different.
So for my simulations, I have to change the Cp values inside @thermophysicalProperties every time I have to change the equivalence ratio. (am I right to do so?)
During the post-processing, I don't "see" directly the equivalence ratio, as the mixture is homogeneous.

I have tried the inhomogeneous mixture, hoping to be able to specify another inlet consisting of only air (instability control can be made through second air jet), but I think this is not the way to do it.

I have some questions:
1) What mixture type would you recommend?
2) I know equivalence ratio fluctuations can be related to combustion instabilities, is there a way to do that?(in XiFoam)

Remi
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combustion instabilities, lean premixed, vortex, xifoam

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