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Old   April 13, 2006, 14:07
Default Ignition
  #1
Mgtripple
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Hi Guys,

I am trying to simulate combustion in a nozzle. The gas is premixed Methane-Air mixture. I am using EDM with option Extinction Temperature. In reality the mixture is ignited at the end of nozzle.

I dont know how to model this ignition.

Is there any way to do this

Thanks

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Old   April 13, 2006, 15:25
Default Re: Ignition
  #2
Quest
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Have you tried once the simulation with the standard parameters? The standard parameters are very good for methane air combustion. A=4, B=0.5, T=900K, mixture limit and so on? Do you use a single step combustion? The flame normally ignite by itself.

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Old   April 14, 2006, 06:08
Default Re: Ignition
  #3
Mgtripple
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Hi,

It is a single step reaction with NO formation.

When I use T=900K mixture doesnt ignite. Excluding coefficient B, I am using default values for the reaction (Methane Oxygen WD1)

You can think of the nozzle two parts. From inlet fixed mass flow is coming and then velocity increases (upto 43 m/s) through 6 small holes. After holes comes the combustion volume.

Any help is appreciated,

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Old   April 19, 2006, 02:08
Default Re: Ignition
  #4
Joe
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extinction temperature means no reaction beyond 900K. If you use values of B larger than zero (what is required for premixed combustion) then there is region required with a extinction temperature larger than 900K to start the combustion.

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Old   April 21, 2006, 09:36
Default Re: Ignition
  #5
Mgtripple
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Hi,

The problem is I dont know how can I define that region that has temperature over 900 K and ignites the premixed gas.

I have tried adding very small amount of premixed gas which has temperature of 1000 K at the entrance of combustion region, however it did not ignite the gas.

Waiting for zour answer

Thanks
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Old   April 23, 2006, 15:09
Default Re: Ignition
  #6
matej
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hi,

you may try to do few things. First is to switch off the limiter. run for a while to see if its going to ignite and if so, switch the limiter back.

you may try to start also with some products at inlet and later change the inlet back to just fuel. This may work, as the B constant should limit the reaction with respect to the products -check the EBU equation in the manual. what can happen is you will have combustion where it should not be, but later with propper setting it should go where it should be.

(editing in cfx5solve would not allow you to change the inlet concentrations, but copying *.dir/ccl file elsewhere and changing the data and finaly using cfx5control to push the data back will work)

i've used EBU models years ago with cfx4 and other soft, but it should work the same today too.

good luck

matej
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