Auxiliary keyword “EXCI” is messes up energy balance in plasma models in Chemkin

    • SaiRa

      I am using the Plasma PSR model and whenever I use the auxiliary keyword "EXCI" in my chemistry file for specifying the electon energy loss for the electron impact reactions, the energy balance gets messed up. Is there another keyword that needs to be mentioned?  

      I ran the following test:

      Plasma PSR model, Solve Gas Energy Equation and Transient Solver. End Time = 0.1 microsec, Residence Time = 1 hour, Temperature = 300 K, Electron Temperature = 1 eV,  Pressure = 200 Torr, Volume = 5 cm3 and default electron momentum-transfer cross section 0 cm2.

      Gas Composition is E (Electron, 1.0E-10) and N2 (Nitrogen molecule, 1.0), I only include one reaction in the chemistry file for electron impact ionization of N2 (rate obtained from bolsig+)

      I have attached the equation I use to check the energy balance, H_initial is at time = 0, and H_final is at 20 nanoseconds. When the auxiliary keyword "EXCI" is present, the error is 47% whereas when it is absent the error is just 0.5%.

      Am I missing something?



    • Judy Cooper
      Ansys Employee

      Hi SaiRa:

      What is the reaction being modelled?

      EXCI (electronic excitation ) is always used in conjunction with TDEP/E/ to specify the energy loss to electrons during the excitation process. Excitation is when an electron extracts energy from the surroundings and attains a higher orbit around the nucleus of an atom without any other chemical change occurring.

      E + CL2 => E + CL2 6.3477E-06 -5.3987E-01 1.3920E+05
      TDEP/E/ !electronic excitation
      EXCI/ 9.25/

      The EXCI keyword overrides the usual calculation of energy loss from the change in enthalpy determined by the reaction description and the thermodynamic data of the reactants and products. Without the TDEP/E/ keyword, the presence of EXCI might cause problems because there is no flag to indicate that there is an electron temperature dependence, so you might get both energies (delta H products-reactants + excitation energy) attributed to the same reaction.

      If you are using both keywords, then the imbalance may result from an imbalance in electrons in the reaction. Excitation requires electron balance.

      Below are some example reaction from the Chemkin Cl plasma PSR tutorial:

      E + CL2 => E + CL2 2.5141E-02 -1.4443E+00 1.6650E+04
      TDEP/E/ !vibrational excitation
      EXCI/ 0.07/
      E + CL2 => CL- + CL 5.8901E-09 -2.5619E-01 1.5834E+04
      TDEP/E/ !dissociative attachment
      E + CL2 => 2CL + E 1.5356E-06 -3.4642E-01 7.0850E+04
      TDEP/E/ !dissociation
      E + CL2 => E + CL2 6.3477E-06 -5.3987E-01 1.3920E+05
      TDEP/E/ !electronic excitation
      EXCI/ 9.25/
      E + CL2 => CL2+ + 2E 1.1227E-04 -6.0067E-01 1.8070E+05
      TDEP/E/ !ionization
      E + CL- => CL + 2E 3.1197E-06 -2.8757E-01 7.2058E+04
      TDEP/E/ !detachment
      E + CL => E + CL* 1.2363E-05 -6.1356E-01 1.3297E+05
      TDEP/E/ !4s excitation
      E + CL => E + CL 1.2363E-05 -6.1356E-01 1.3297E+05
      TDEP/E/ !4s excitation energy loss
      EXCI/ 9.55/
      E + CL => E + CL 3.4801E-06 -6.6984E-01 1.4944E+05
      TDEP/E/ !5s excitation
      E + CL => E + CL 1.9354E-06 -7.3244E-01 1.7005E+05
      TDEP/E/ !6s excitation
      E + CL => E + CL 9.4444E-05 -7.3093E-01 1.5413E+05
      TDEP/E/ !3d excitation
      E + CL => E + CL 7.6428E-05 -7.7209E-01 1.6452E+05
      TDEP/E/ !4d excitation
      E + CL => E + CL 4.7440E-05 -7.8620E-01 1.6810E+05
      TDEP/E/ !5d excitation
      E + CL => E + CL 1.2295E-04 -8.0133E-01 1.4248E+05
      TDEP/E/ !4p excitation
      E + CL => E + CL 4.9579E-05 -8.4151E-01 1.5872E+05
      TDEP/E/ !5p excitation
      E + CL => CL+ + 2E 2.3736E-04 -7.0894E-01 1.8374E+05
      TDEP/E/ !ionization
      E + CL* => CL+ + 2E 2.6471E-05 -4.3906E-01 6.3670E+04
      TDEP/E/ !Cl* ionization

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