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thermal decomposion of H2O2, multiphase model + species model

    • Robert Hübner
      Subscriber

      Hello , 

      I am Robert from Germany and for my thesis, I simulate the thermal decomposition of hydrogen peroxide with ANSYS FLUENT. I have some questions about it and I hope you can help me. 

      1. In the decomposition chamber is pressure of 900 000 Pa before the H2O2 comes in. The outlet expands to atmosphere. How do I set this? I have set the pressure (operating conditions) to 900 000 Pa and the outlet gauge pressure is 0. 

       

      1. The H2O2 is admitted in liquid form in the chamber, where is p = 900 000 Pa and T = 800 K. I want to use the multiphase model (eulerian) and the species model (volumetric). There are 3 Phases (1. Air 2. H2O2-liquid 3. mixture: H2O2-gas, H2O, o2, n2). Mass transport: evap-cond from Phase 2 to Phase 3(H2O2-gas). Reaction: H2O2-gas to Products o2 and h2o. I have set the Arrhenius Rate and the correct stochiometric formula in the multiphase reaction card and species model. Boundary condition for the Inlet: Phase 2 volume fract = 1 and for the other phases 0. After Initializing, I have patch the volume fractions of phase2 and 3 to zero, so in the chamber is just air at the beginnig. When I start the simulation, the warning of zero gas phase reaction comes. Where is the failure? 

       

      1. The setting options with the dicrete phase model/injections are closer to the real inlet of H2O2 of the experiment I want to simulate. (droplets, cone, ...) Is it possible to combine this with multiphase and species? 

       I hope my description is understandable. If any Informations are missing, please ask.  

      Thank you! 

       

      Best regards  

      Robert 

    • Rob
      Ansys Employee

      OK, a few slight changes will be needed. 

      How is the chamber at 9 bar if the outlet vents to atmosphere? 

      What is the volume fraction of droplets? You may be better off with DPM so read up on the models. 

      Put all of the gas species into one phase, you can then react at your convenience. So, you'll have a liquid phase/spray and a gas phase containing the four species (or more than four if the reactions get complicated). 

    • Robert Hübner
      Subscriber

      I checked the report about the real experiment and the pressure in the chamber is not 9 bar. After opening the valve for the liquid H2O2 the pressure increases to 9 bar. The feeding pressure is 13 bar.

      I have changed the pressure in the operating conditions to the default value and the boundary condition for the inlet to gauge pressure = 13 bar.

      I have changed the phase number to 2, mixture/primary phase and H2O2-liquid/secondary phase.

      I still got the problem with the warning: zero number gas phase reaction and additionaly the solution does not converge (floating point).

    • Rob
      Ansys Employee

      Simplify the problem by turning off reactions, just model phase change and species. 

      What is the volume fraction of the droplets? 

    • Robert Hübner
      Subscriber

      Do you think I should exhibit the reactions in the multiphase model and just do the reaction via species model? 

      The volume fraction of the droplets is 1 at the inlet. I forgot to specify this is a transient simulation.

       

    • Rob
      Ansys Employee

      It depends, what is the rate limiting step? Ie does the phase change or reaction limit the overall rate from liquid to decomposed gases? 

    • Robert Hübner
      Subscriber

      Firts of all, thank you for the answers! I will try to simulate this with the DPM + Species model after having reading up on DPM.

      Like I wrote, this is a transient simulation of a real experiment and I have the right geometry. I know the feeding pressure, so I use pressure-inlet and gauge total pressure = feeding pressure. Outlet to atmosphere, so I use pressure-outlet with the default set up.  Do you think, this is correct?

      a picture of the chamber:

    • Rob
      Ansys Employee

      I suspect the rate of evaporation & decomposition could be a factor: liquid to gas is a huge volume change. Then one mole of H2O2 becomes 2 moles each of H2 and O2 

      Outlet pressure may be require a plenum depending on where the pressure drops: 9 bar to 1 bar as a loss is going to result in a fairly rapid acceleration. 

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