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June 19, 2020, 06:52 
Air inlet BC for airblast DPM model

#1 
New Member
Join Date: Nov 2019
Posts: 3
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I have a twofluid atomizatinon nozzle of the airblast/airassist type as per this picture: https://imgur.com/xgoG975
The airflow is sonic (supply pressure is over 4 bar) and the ratio of air mass flow to water mass flow is about 100. Given the high air velocity, I guess the nozzle is "airassist" rather than "airblast", but that's probably not too important. I would like to use the builtin atomization model in Fluent but I'm not sure how to model the air intake. My thoughts so far:
The air is actually swirling a bit due to the shape of the passage inside the nozzle, but I'm leaving that complexity out for now. Thanks for any tips! 

June 19, 2020, 07:04 
Atomizer

#2 
Senior Member

Air flow has to be compressible if Ma is more than 0.3. And it does affect DPM.
1. If the nozzle geometry is simulated, then atomizer model is not required. You can use VOFtoDPM transition model. 2. If you want to use inbuilt atomizer models, then the actual geometry of the nozzle is not important from the DPM view point. However, you need to include its effects on the boundary conditions for air inlet. So, if gas is swirling because of the nozzle, then you must provide that swirl to the air. 3. Incompressible air only if Ma < 0.3. Rest of the setup could be same as either 1 or 2. Since the flow channels for air and liquid are different, you can calculate velocities based on mass flow rate and compressibility. Then you can calculate relative velocity. Users also use this as a tuning parameter to match data with the experiments.
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Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority. 

June 19, 2020, 10:53 

#3  
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Join Date: Nov 2019
Posts: 3
Rep Power: 2 
Thanks for the quick reply.
Quote:
Regarding the air velocity calculation, I know how mass flow changes with pressure difference across the nozzle from the manufacturer's catalogue. It turns out this is a linear function  doubling the pressure doubles the flowrate. Am I right that this indicates choked flow? I tried using these formulas: https://en.wikipedia.org/wiki/Orific...pressible_flow and I managed to tune the discharge coefficient to match the catalogue data reasonably well. However, I'm still unsure how to use the fact that Ma=1 to get the air velocity. The speed of sound at nozle exit still depends on temperature, which I don't have, or do I? Just a remark: VOFtoDPM (let alone pure VOF) is too expensive for this simulation so that's something I already ruled out. 

June 19, 2020, 11:01 
Air Flow

#4 
Senior Member

If you are not interested in using VOFtoDPM, then it does not matter how complex the geometry of the nozzle is. All that matter is the annular area where the air comes out. Its not a simplification, there are no assumptions. You just have to apply correct velocity vector. You don't need to know exact temperature, just an order of magnitude, which you can get from the application of the nozzle. At STP, air velocity higher than 100 m/s implies compressible flow. If it is lesser than that, then keep it incompressible.
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Regards, Vinerm PM to be used if and only if you do not want something to be shared publicly. PM is considered to be of the least priority. 

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