injecting VOC mass through virtual cells

Hi everyone,

I am trying to simulate a 1D diffusion scenario where the domain is a column comprised of 200 grid cells (domain is 1m*1m*20m with 200 cells in z-direction) and every grid cell has been initialized in a two-phase water/gas region. Then I connected a virtual cell to every actual cell of the domain and injected the required mass of VOC per each layer over a time period of one second (I need to have 1238.4 kg; therefore, I'm injecting 6.192 kg/s into 200 grid cells for only one second). The reason I'm using the virtual (or extra cell) is to avoid any issues with respect to the enthalpy of the injected fluid. Supposedly, I should have 1238.4 kg of the VOC after one second of injection over the entire model; however, when I look at my material balance in the output file after 1 sec, there is only 0.30830875 kg of the VOC injected. Why is that? why am I not able to inject the full mass specified into the model? I have attached all the files in case somebody would like to see the details. 

Thanks,

 


 

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  • Dear Alireza,

    Your input file does not match what your message says.  You do not have any mass sources- these would be specified in the GENER block and yours is empty.  You are specifying initial conditions through the INCON block.  To understand what is in your INCON block, I looked in the output file for the string "EQUATION OF STATE", then compared the following information to your INCON block.  For each element in your column, you have a phase code of 4, meaning 2-phase gas-aqueous conditions.  The first primary variable is pressure (101300 Pa), the second is mass fraction of VOC in the gas phase (0), the third is aqueous-phase saturation (0.2926), and the fourth is temperature (60 C).  In your virtual cell you specify phase code 7, for three-phase gas-aqueous-NAPL conditions.  The primary variables are P (101300), gas-phase saturation (1e-9), aqueous-phase saturation (1e-9), and temperature (60 C). 

    So to start, your column has no VOC and your virtual cell has almost entirely VOC in the NAPL phase.  After 1 second, some VOC transfer to the column has taken place.  If you want to specify introduction of VOC into your column from your virtual cell that is fine, but I don't think it will be happening at a constant mass flow rate, and you won't know ahead of time how long it will take for a give mass to be transferred.  Also, you probably will want to make the volume of your virtual cell huge (1E30 m3), so the properties there do not change.  If you want to specify a given mass rate, I recommend using the GENER block instead.  Perhaps you can find a similar example problem in the TMVOC User's Guide, and follow that procedure.

    Good luck, Christine

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      • Alireza
      • Alireza
      • 7 mths ago
      • Reported - view

      Christine Doughty 

      Thanks very much, Christine for taking the time to answer my question. It is really appreciated.

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  • Dear Alireza,

    one way to inject the desired amount of VOCs is, as suggested by Christine, to inject in each element using constant rate sources in GENER. You can avoid thermal effects by assigning an infinite specific heat to the column rock domain. You will lose the calculation of components and phases mass from BALLA.

    If you want to use the extra cell approach, you should assign the infinite specific heat and a tiny porosity to the extra cell rock domain. And of course a constant rate injection into the extra cell. I suggest also a big permeability to minimize the P increment in the extra cell.

    In this way almost all the injected mass should be transferred into the column elements at the initial T of extra cell. The mass balances from BALLA will tell you if you injected into the column the desired VOCs mass.

    Of course the injection will change the initial atmospheric P of column elements. Some perturbation of initial T is also possible because of phase appearance and mass transfer between phases, unless you assign an infinite specific heat to the column element too.

    Regards,

    Alfredo

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      • Alireza
      • Alireza
      • 7 mths ago
      • Reported - view

      Alfredo Battistelli 

      Thanks very much, Alfredo. I made the changes as you suggested and now I get exactly what I was trying to simulate. Thanks again for the help.

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