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Boundary Condition of a High Pressure Burner

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Old   April 13, 2014, 15:31
Default Boundary Condition of a High Pressure Burner
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Hi, all. I have a question about the computation of reactive flow of a small burner, which is used in the lab. The burner has two inlets, one for air and the other for fuel--CH4. The pressure in most part of the burner is 5atm. And the outlet is a small converging nozzle which will expand the gas to sonic state.

1. I have some confusion about boundary conditions to apply. I have already tried multiple combinations.
a). PressureInet and PressureOutlet, this could not reach the mass flow rate measured in the experiment.
b). MassFlowInlet and PressureOutlet, this could not reach the desired pressure
c). VelocityInlet and PressureOutlet, this would be the same as 2.
Any one have similar experience?

2. Because of converging nozzle, the flow is transferring form incompressible flow to compressible flow. Therefore I think I need to model the gas as ideal gas, which allows density variation. But this setting would give out the warning: Boundary mach number exceeds maximum limit on pressure outlet==0.98. However, when I set the gas to be incompressible ideal gas (maybe reasonable), there's no such warning. Could anybody know how to get rid of this??

I am finishing my thesis, and it's kind of urgent. Any advice and help is highly appreciated. Thank you and please help..
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Old   April 13, 2014, 17:17
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any help? please.
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Old   April 14, 2014, 03:22
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Up, help? Any body?
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Old   April 14, 2014, 14:13
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plz..........................................
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Old   April 14, 2014, 15:59
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Quote:
Originally Posted by WJXu View Post
Hi, all. I have a question about the computation of reactive flow of a small burner, which is used in the lab. The burner has two inlets, one for air and the other for fuel--CH4. The pressure in most part of the burner is 5atm. And the outlet is a small converging nozzle which will expand the gas to sonic state.

1. I have some confusion about boundary conditions to apply. I have already tried multiple combinations.
a). PressureInet and PressureOutlet, this could not reach the mass flow rate measured in the experiment.
b). MassFlowInlet and PressureOutlet, this could not reach the desired pressure
c). VelocityInlet and PressureOutlet, this would be the same as 2.
Any one have similar experience?

2. Because of converging nozzle, the flow is transferring form incompressible flow to compressible flow. Therefore I think I need to model the gas as ideal gas, which allows density variation. But this setting would give out the warning: Boundary mach number exceeds maximum limit on pressure outlet==0.98. However, when I set the gas to be incompressible ideal gas (maybe reasonable), there's no such warning. Could anybody know how to get rid of this??

I am finishing my thesis, and it's kind of urgent. Any advice and help is highly appreciated. Thank you and please help..
Try this: Set the inlet BCs to mass flow rate and set the pressure outlet to zero and also tick the target mass flow rate for outlet boundary. Although this way may over-specify the problem but smt it helps.
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Old   April 14, 2014, 21:08
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Originally Posted by AbbasRahimi View Post
Try this: Set the inlet BCs to mass flow rate and set the pressure outlet to zero and also tick the target mass flow rate for outlet boundary. Although this way may over-specify the problem but smt it helps.
Thanks a lot, I will try this
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Old   April 21, 2014, 10:53
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Quote:
Originally Posted by AbbasRahimi View Post
Try this: Set the inlet BCs to mass flow rate and set the pressure outlet to zero and also tick the target mass flow rate for outlet boundary. Although this way may over-specify the problem but smt it helps.
The solution provided by Abbas is quite good and that should work out.
You may also try with mass flow inlet, pressure outlet without target mass flow inlet using ideal gas law.
This may provide some problem with convergence.
Let me know if you require any other help
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Old   April 21, 2014, 15:09
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Hi WJXu,

Sorry in advance, I'm not bringing any solution to your problem. But I would like to know: what combustion model do you use?

I have to model furnace high pressure natural gas burners that work at velocities 300-500 m/s. I'm not actually modeling the burners, my model starts at the burner inlet where I impose a mass-flow-inlet in order to obtain the desired velocity. I'm not really caring about matching the pressure at the burner inlet, should I? The mass flow of the burner inlet is negligeable compared to the air mass flow in the furnace.
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Old   April 21, 2014, 19:25
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Originally Posted by macfly View Post
Hi WJXu,

Sorry in advance, I'm not bringing any solution to your problem. But I would like to know: what combustion model do you use?

I have to model furnace high pressure natural gas burners that work at velocities 300-500 m/s. I'm not actually modeling the burners, my model starts at the burner inlet where I impose a mass-flow-inlet in order to obtain the desired velocity. I'm not really caring about matching the pressure at the burner inlet, should I? The mass flow of the burner inlet is negligeable compared to the air mass flow in the furnace.
Hi?
I am comparing Eddy dissipation model and Finite rate/Eddy dissipation model both, which show similar results with some discrepancy about the location of highest temperature region. I am simulating CH4/Air combustion.

I don't know the model and I don't know what you are simulating. In my case, I believe the pressure is important, because this pressure is what measured in the experiment. And in my case, there is pressure variation.

Actually I think both of the two, pressure and mass flow rates are important. The mass flow rate determines the equivalence ratio which apparent affects the reactions. The pressure, especially in pressurized burner/furnace should also play a role in the reaction. Confusing...

Now what I did is use pressure inlet/pressure outlet boundary. The mass flow rate deviate a little from measurements, but still acceptable.
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Old   April 21, 2014, 19:28
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Quote:
Originally Posted by AbbasRahimi View Post
Try this: Set the inlet BCs to mass flow rate and set the pressure outlet to zero and also tick the target mass flow rate for outlet boundary. Although this way may over-specify the problem but smt it helps.
Thanks for you advice. The mass flow rate and pressure outlet boundary cannot maintain the pressure inside. This may be because of the pressure drop at the burner exit--a converging nozzle.
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