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Non-premixed/Partial Premix combustion Operating pressure

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Old   June 5, 2013, 21:59
Default Non-premixed/Partial Premix combustion Operating pressure
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Hi all, I would like to find out if the Operating Pressure value within the Non-premixed combustion settings is meant to approximate the actual combustion chamber pressure, so that solution will be more accurate?
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Old   June 6, 2013, 03:23
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Hi,
Are you running your combustion at atmospheric pressure or high pressure? If the pressure is high then you have to change the operating pressure to that value.

If you are using a conventional fuel or any fuel i would suggest you try to check whether your flame speed is being accurately predicted by CFD since pressure also influences the flame speed. That is if you are running at high pressure.

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Old   June 6, 2013, 03:52
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Thanks for the reply, Jimmy.
Yes, the combustion is occurring at high pressure. You meant check the flame speed in Fluent correspond to theoretical value? Can advise how to do that?

Also, would like to confirm if it is alright to set the Operating Pressure under the "Boundary Condition" independently to 0 Pa (i.e. need not follow that of the pressure in the partial premixed dialog box)
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Old   June 7, 2013, 14:02
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Hello,
It depends on the combustion model you are using. My approach was use the theoretical relations for the flame speed and input the flame speed value obtained from measurements and then used that to compute the pre exponential factor. That was for the finite rate chemistry model and my fuel was propane and the reaction was order.

Can you explain in more details the nature of your fuel and air inlet? Are they separate or not? Is any open to the atmosphere or not? If yes then it is easy. If no, then what is the measured pressure at that point? That is the value you give and mostly it is given as the gauge pressure. So if you measure absolute pressure u need to enter the gauge pressure and the software would determine the absolute pressure. Maybe tell me more if you are not clear with my response.

Good luck!
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Old   June 18, 2013, 23:19
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Fuel is injected ans simulated using Discrete Phase model. You can think of it as gas turbine combustion conditions, i.e. combustion occuring at high pressure, before it is expelled to atmosphere.
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Old   June 19, 2013, 05:02
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Hi,
My first response is then right. You have to set the operating pressure to the pressure at which you want to run your combustion. Using DPM is okay but make you are sure about the type of nozzle you have and whether the existing ones in Fluent are representative of your injector. In my case i used injection files that were hooked to the DPM panel because none of the implemented nozzles in Fluent could do for me. The droplets size distribution was Rosin-Rammler and it was based on measurements. I would suggest you first do cold flow simulation to ensure that your fuel is being injected properly as well as the continuous flow. When this is all fine you can then think about switching to hot flow with combustion.

Hope this helps.
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Old   June 19, 2013, 21:21
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Thanks Jimmy, the info you provided are of great help.

While we are on this topic. Would like to clarify another 2 doubts of mine

1) There are instances where I ran cold flow simulation first for about 800-1000 steps (i.e. without turning on the pdf and premixc equations under Solution Controls). After which I turned them on and also DPM on. There is no combustion occuring. However, if straight away run with all equations on and DPM starts injecting after the first 30 iteration (and every 30 iterations thereafter), combustion is possible. Why is that so? If the combustion model won't burn does is mean there is a chance it won't burn experimentally as well?

2) Combustion is not possible unless injected droplets' size are less than 9e-5m in diameter. Is that the case in real life?
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