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Old   July 25, 2012, 18:36
Default Unsteady Combustion
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Hello,

I'm having a bit of trouble successfully running a hydrogen-air transient combustion case. The basic geometry is a cylinder with a smaller, short concentric cylinder. Oxygen is introduced at the larger cylinder's cross-section while hydrogen is introduced inside of the smaller cylinder and free to flow out. Both are mass flow inlets. I am able to run a steady state simulation and achieve combustion but when I switch over to a transient regime, the mixture never ignites and remains at the species inlet temperature.

I've tried increasing the temperature of both species as to induce ignition but no luck. I've also tried timesteps ranging from 1e-05 to 1e-03 and no luck.


Some solver specifics:
  • Species transport and reactions combustion model (Volumetric, Finite Rate/Eddy Dissipation
  • k-e realizable turbulence model
  • I've used both PISO and SIMPLE pressure-velocity coupling schemes
  • All spatial discretization entries are first order upwind
Any help would be greatly appreciated. As I said, steady state works fine with the same solver settings (if applicable). Switching over to transient results in no ignition.
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Old   July 25, 2012, 23:05
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Unsteady combustion is really awful. Please use spark model for ignition on a point and also try to patch a small amount of species (reactants and products) around the point. I always use it.
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Old   July 26, 2012, 01:53
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Quote:
Originally Posted by eng_s_sadeghi View Post
Unsteady combustion is really awful. Please use spark model for ignition on a point and also try to patch a small amount of species (reactants and products) around the point. I always use it.
Thanks, at least I know I'm not the only one who has had trouble with transient combustion. Is this a known problem in FLUENT? I tried a spark earlier today but that didn't do anything, it was merely an increase in temperature. Are there any spark parameters you recommend (spark energy, diameter, etc)?
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Old   July 26, 2012, 03:05
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You should increase the spark energy to get a temperature of about 2000K at the point. Also there should be at least a small amount of both reactants and products. Don't forget products should also be around the spark point.
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Old   July 26, 2012, 16:33
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Alright thanks, I'll give it a try.
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Old   July 30, 2012, 18:17
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Quote:
Originally Posted by eng_s_sadeghi View Post
You should increase the spark energy to get a temperature of about 2000K at the point. Also there should be at least a small amount of both reactants and products. Don't forget products should also be around the spark point.
Thanks for the help. I was able to get my simulation to combust. Originally, I was using the hydrogen-air mixture template and only included oxygen and hydrogen in the computational domain. When I allowed water to flow into the domain along with the oxygen, the mixture successfully combusted. This was not the case for a steady simulation: the combustion occurred without water present.
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Old   April 23, 2014, 04:35
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hi NormalVector,i have met the same problem to use the spark ignited,
I am able to run a steady state simulation and achieve combustion but when I switch over to a transient regime, the mixture never ignites and remains at the species inlet temperature,
i see you have solved the problem,so i am very desired if you could help me
thanks very much
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Old   April 23, 2014, 12:52
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See my post above, try adding a small amount of products into your domain.
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Old   April 23, 2014, 13:26
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See my post above, try adding a small amount of products into your domain.
thanks for your help,i have do it as you said above, rigion a zone around the spark point,and add 0.01productsand some reactants,but it doesnot work,are there other parameters should notice??? desire for your reply
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Old   April 23, 2014, 13:58
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The only other thing I would suggest is to initialize your entire domain with a small amount of product. That fixed my problem so if that does not work then I don't know what is wrong with your simulation.
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Old   April 24, 2014, 03:23
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The only other thing I would suggest is to initialize your entire domain with a small amount of product. That fixed my problem so if that does not work then I don't know what is wrong with your simulation.
thank you i will give it try again
i give the spark energy 2500j and my inlet is 0.05ch4 0.2 o2 ;
i wonder if my inlet boundary conditions set is suited
i am very glad to see you reply
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