# mass flow rate in DPM steady tracking

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 June 1, 2015, 09:26 mass flow rate in DPM steady tracking #1 New Member   Join Date: Jul 2013 Posts: 28 Rep Power: 5 hi all i want to inject a single particle (mass = 0.001 kg) in a flow, and track the particle using steady DPM. but in the 'Set Injection Properties' dialog box, it asks for the flow rate (kg/sec)..so how do i decide this flow rate... should it be (1) just the mass of the 1 particle, like 0.001 kg/s or (2) particle area * density * flow velocity at the injection point if option (1) then the problem is that i want to inject only 1 particle, so using 0.001 kg/s will mean something else..isn't it? and if option (2) then how do I know the velocity at the injection point? do i have to run the simulation without DPM first, get the flow velocity at injection point, and then use it to calculate the mass flow rate. i am just a bit confused about this flow rate thing.. if anyone has any idea, please share... thanks a lot..

 June 1, 2015, 09:58 #2 Senior Member   Join Date: Nov 2013 Posts: 943 Rep Power: 13 That flow rate is relevant for postprocessing of your results. If you have two sources of particles, and one source gives 0.1 kg/s and the other 0.3 kg/s, you put those numbers and Fluent will do the relevant postprocessing, while it (at minimum) only needs to calculate two trajectories. It does not influence the trajectory calculation in any way, at least not when the DPM and flow solution are uncoupled. In your case, you don't need this kind of postprocessing, so you can put any number you want. Make it 1 kg/s, and you are done.

June 1, 2015, 10:14
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hi thanks for such a quick reply

Quote:
 Originally Posted by pakk It does not influence the trajectory calculation in any way, at least not when the DPM and flow solution are uncoupled.
actually i have enabled "interaction with the continuous phase" in the DPM dialog box. so will the flow rate value affect the trajectory?

Quote:
 In your case, you don't need this kind of postprocessing, so you can put any number you want. Make it 1 kg/s, and you are done.
so if i use 1 kg/s, then will it inject just 1 particle or 1 kg/0.001 kg = 1000 particles...how is fluent knowing how many particles to inject..i just want to inject 1 particle..

so what does that flow rate actually mean:
is it the rate at which the particle is moving in the flow
or
is it the rate at which the particle is injected into the flow

sorry for asking so many questions,
i just want to know it properly.

thanks so much for ur quick reply

June 1, 2015, 10:27
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Quote:
 Originally Posted by Roule hi thanks for such a quick reply actually i have enabled "interaction with the continuous phase" in the DPM dialog box. so will the flow rate value affect the trajectory?
I believe that in that means that the particle will influence the flow, and the flow will influence the particle. If you disable it, the flow will still influence the particle, but not the other way around. Disabling it will make your simulation much, much faster. So only enable it when you are sure it will influence the flow. If you inject a single particle of 1 gram, it will not influence the flow significantly, so disable it.

Quote:
 so if i use 1 kg/s, then will it inject just 1 particle or 1 kg/0.001 kg = 1000 particles...how is fluent knowing how many particles to inject..i just want to inject 1 particle..
If you select 1 kg/s in a steady simulation, Fluent will still inject 1 particle. Like I said, the "1 kg/s" is just a label.

Quote:
 so what does that flow rate actually mean: is it the rate at which the particle is moving in the flow or is it the rate at which the particle is injected into the flow
It means neither. It means the flow rate that your calculation particle represents. If you put "1 kg/s", and your particle has a mass of 1 gram, then Fluent knows: "If I ever have to do any postprocessing from this trajectory, I can imagine that 1000 particles follow this trajectory every second."

But, as I said, you are not going to do that kind of postprocessing. So in your case it is just a label that will not be used for anything. You can enter your age, your phone number of your favourite Reynolds number, it does not matter.

June 1, 2015, 10:44
thanks a lot :)
#5
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Quote:
 Disabling it will make your simulation much, much faster. So only enable it when you are sure it will influence the flow. If you inject a single particle of 1 gram, it will not influence the flow significantly, so disable it.
Ya, u r correct..my particle is too small to affect the flow...i will just disable it to speed up the iterations..thanks

Quote:
 If you select 1 kg/s in a steady simulation, Fluent will still inject 1 particle. Like I said, the "1 kg/s" is just a label.
i see, thanks for clarifying again.

Quote:
 You can enter ..... your phone number of your favourite Reynolds number,
okay, funny, i will ask my fav reynolds number abt her phone number

and thanks so much for helping me out, now i am clear about the problem i had..thanks so much...

and can i ask one more favor from u,
since u have already been so helpful.....i asked another question in the forum yesterday but didnt get any proper reply, it is also about/related to DPM only..

if u dont mind, can u see that question too, and give some suggestions...i feel u may know what to do..here is the link to the question:

how to simulate particle jumping out from a water flume cavity

sorry for asking so many questions...
thanks so much again

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