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How can the ‘number of particles’ in a parcel can be less than 1 ?

    • pmjsjs
      Subscriber

      I'm running a unsteady 2d axisymmetry DPM model. I'm employing a surface injection. A mass flow rate of 0.0166 kg/s, with particle size of 500microns, and density 3850 kg/cu.m, is used. The surface of injection is spread across 12 cells. And the timestep size is 1e-5 s. When I calculated the number of particles in a parcel, I got a fractional value (~0.05). 

      Even though this is a numerical value, how can this be less than 1. Some question which I have are mentioned below;

      1. How will the equations be solved computationally? 
      2. The new injections what I see at every timestep, is it corresponding to the parcel or particle?
      3. Another general ambiguity is? Why ANSYS fluent interchanges the term 'parcel' and 'particles'. Please find the attached example images below. The first one was obtained whilst loading the loading data file, whereas the second obtained via tui command 'report > particle-summary'. Which one of these is the actual number of particles?

      Image 1 - obtained whilst loading data file.

      Image 2 obtained using tui command. 

    • Rob
      Forum Moderator

      A particle is what we track (diameter & material density) but the parcel (some number of particles, can be fractional values) is what arrives at the other end when it hits/reacts/whatever with something. 

      The terms were mis-used back in the very early days of Fluent's DPM model so are incorrectly used by nearly everyone. In the example, we're tracking 1212 parcels, and you have a total of 9.39e-4 particles in the domain. At each time step if the injection is active and it's a surface injection you're adding 12 parcels. 

    • pmjsjs
      Subscriber
      Hi Rob, 
       
      Many thanks for the response. 
       
      Can I ask if you can explain on your response a bit more please, I still dont have the clarity on the parcel and particle concepts, and especially with fractional number of particles.  
       
      Also, can you please explain,
       
      1. If the particles are tracked, how are the equations solved for number of particles which is less than unity?
      2. You mentioned the terms were mis-used from the early days of Fluent's DPM model. Does this inconsistent in terminology still continue? 

      Thanks in advance. 

    • Rob
      Forum Moderator

      In simple terms, we track a parcel that comprises some (or part of) particles. The parcel trajectory is determined from all the drag laws etc for a particle of the diameter & density seen in the plot. When source terms (evaporation, erosion etc) are calculated we use the parcel mass to determine the effect. It's termed "parcel theory" and should be covered in some of the theory books: I did write up a good explanation in the Multiphase training course but that's on the Learning Hub and can't be shared here. 

      The terminology has become very ingrained, and given the TUI reports show trapped/escaped/etc particles as opposed to parcels it's not going to be untangled any time soon. 

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