DPM water injection

Alessandro_Lamberti · May 23, 2023, 10:42

Hi everyone, i'm trying to simulate water droplet injection inside an air ejector. I created an appropriate surface on ICEM, and i want to inject water droplets from that surface. Once i've created the DPM injection in Fluent, it asks me to give both velocity and total flow rate. How they are connected to each other? In particular, how is this total flow rate defined? Because i want to inject 0.46 kg/s mass flow of water, but i don't if this total flow rate is exactly what i want to inject.

Thanks.

alainislas · May 24, 2023, 06:27

Hi Alessandro Lamberti

Be cautious with velocity and mass flow rate, these are independent parameters. Keep in mind that in DPM, the mass flow rate is NOT a continuous mass flow rate (i.e., m_dot = rho*A*velocity is not valid). In DPM, the mass flow rate is discrete, and has nothing to do with the velocity that you input in the setup panel.

For the DPM mass flow rate calculation, you can refer to any of my old posts in the Ansys Fluids Forum

https://forum.ansys.com/forums/topic...ion-speed-m-s/

https://forum.ansys.com/forums/topic...tal-flow-rate/

https://forum.ansys.com/forums/topic...les-injection/

https://forum.ansys.com/forums/topic/regarding-dpm-2/

Alessandro_Lamberti · May 24, 2023, 06:34

Thank you alainislas, as i can see you have defined the massflowrate as the number of particles injected per second multiplied by the mass of the single
particle. This is ok using a transient simulation, but what is the number of particles injected per second if i use a steady state simulation? Is it the number of cells of my surface? (Because i'm using a surface injection type and i have like 900 cells in that surface).

alainislas · May 25, 2023, 06:16

For surface injection, typically Fluent injects one parcel per cell face. If your surface has 900 then it will inject 900 parcels. However this change if you are injecting a PSD with various diameters. It will also change if your parcel release method is other but "standard". Please consider that, for steady tracking, a stream of identical particles is tracked through the domain. This stream initially has a mass flow rate associated with it, although it may change during the tracking due to chemical reactions if enabled (evaporation, devolatilization, combustion, etc...). In contrast, when unsteady tracking is used the particles are represented by parcels that correspond to an ensemble of identical particles. Parcles do not have an associated mass flow rate, but rather have a specified mass (at the time of injection into the domain).

May 23, 2023, 10:42	DPM water injection	#1
Alessandro_Lamberti New Member Italy Join Date: Apr 2022 Posts: 5 Rep Power: 4	Hi everyone, i'm trying to simulate water droplet injection inside an air ejector. I created an appropriate surface on ICEM, and i want to inject water droplets from that surface. Once i've created the DPM injection in Fluent, it asks me to give both velocity and total flow rate. How they are connected to each other? In particular, how is this total flow rate defined? Because i want to inject 0.46 kg/s mass flow of water, but i don't if this total flow rate is exactly what i want to inject. Thanks.

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May 24, 2023, 06:27		#2
alainislas Senior Member Alain Islas Join Date: Nov 2019 Location: Mexico Posts: 142 Rep Power: 7	Hi Alessandro Lamberti Be cautious with velocity and mass flow rate, these are independent parameters. Keep in mind that in DPM, the mass flow rate is NOT a continuous mass flow rate (i.e., m_dot = rhoAvelocity is not valid). In DPM, the mass flow rate is discrete, and has nothing to do with the velocity that you input in the setup panel. For the DPM mass flow rate calculation, you can refer to any of my old posts in the Ansys Fluids Forum https://forum.ansys.com/forums/topic...ion-speed-m-s/ https://forum.ansys.com/forums/topic...tal-flow-rate/ https://forum.ansys.com/forums/topic...les-injection/ https://forum.ansys.com/forums/topic/regarding-dpm-2/

May 24, 2023, 06:34		#3
Alessandro_Lamberti New Member Italy Join Date: Apr 2022 Posts: 5 Rep Power: 4	Thank you alainislas, as i can see you have defined the massflowrate as the number of particles injected per second multiplied by the mass of the single particle. This is ok using a transient simulation, but what is the number of particles injected per second if i use a steady state simulation? Is it the number of cells of my surface? (Because i'm using a surface injection type and i have like 900 cells in that surface).

May 25, 2023, 06:16		#4
alainislas Senior Member Alain Islas Join Date: Nov 2019 Location: Mexico Posts: 142 Rep Power: 7	For surface injection, typically Fluent injects one parcel per cell face. If your surface has 900 then it will inject 900 parcels. However this change if you are injecting a PSD with various diameters. It will also change if your parcel release method is other but "standard". Please consider that, for steady tracking, a stream of identical particles is tracked through the domain. This stream initially has a mass flow rate associated with it, although it may change during the tracking due to chemical reactions if enabled (evaporation, devolatilization, combustion, etc...). In contrast, when unsteady tracking is used the particles are represented by parcels that correspond to an ensemble of identical particles. Parcles do not have an associated mass flow rate, but rather have a specified mass (at the time of injection into the domain).