Fluids

Fluids

Evaporation-condensation model

    • maliuzair
      Subscriber

      biomass static temperature after initialization and after performing few time stepsHi All, 


      I am simulation biomass drying using thermal phase change evaporation-condensation  model (eulerian multiphase).


      initial conditions:


      Biomass initial volume fraction: 0.6, Temperature: 300 K


      Air: initial temperature 300k... gradually increasing with time


      after initializing, i check contours of static temperature of biomass. It shows 300 k throughout the domain. 


      when i run simulation for 1 time step (or even more), i check the contours. It shows 300 K where biomass is patched and the rest of the domain shows 373 K. Attached pics. 


       


      Then I tried with the energy equation turned off in the beginning, and later I turned on the energy equation. The instant I turn ON the energy equation, the warning comes..


      Temperature limited to 1.000000e+00 in 15337 cells on zone 2 in domain 3


      and the area weighted average of biomass-temperature drops to less than 50 K from 300 K. area weighted average of biomass static temperature


      Can someone please address this issue. the temperature should be 300K throughout the domain. I have checked again and again by turning off the evaporation condensation model. when i turn off the model, there is no such problem.


       


      Thanks

    • DrAmine
      Ansys Employee

      How is the phase change defined? Please attach some Screenshots of phase definition and interactions. Bear in mind thermal phase change ist not proper for multiphase multicomponent cases. Moreover check if you have unphysical backflow 


       

    • maliuzair
      Subscriber

      Dear Sir


      Which model is suitable then for drying, because for Lee model, experimental data is required to adjust the coefficient and there is no calibration required for the thermal phase change model.


       


      Here biomass is a mixture of biomass and liquid water. Drag correlation used  is Gidaspow'sBiomass granular phaseheat transfermass transferMixturesmultiphase model

    • klu
      Ansys Employee

      Hi, 


      May I know how the condensation will be triggered? Is there any wall thermal boundary condition that continues to take heat out of flow? I remember that Thermal Phase Change model cannot deal with condensation occurring on walls. Therefore please make sure all walls can have only insulated conditions.

    • maliuzair
      Subscriber

      Hi KLU,


      I am not sure if I understood properly. I am just a beginner. the walls are adiabatic.


      Besides I have another simple heat transfer model between air, sand and biomass. Both sand and biomass are granular. Initial conditions are 300 K. I let it run for sometime. But it is behaving abnormally. The temperature shouldn't drop below 300 K.


      The boundary conditions are adiabatic walls, velocity inlet for air, and Pressure outlet.Heat Transfer


      Based on this case and the evaporation model case, what do you suggest. Where is the problem? What am I doing wrong. 

    • DrAmine
      Ansys Employee

      Thermal phase change model is not proper for multicomponent flow. You need to drive your bulk phase change by means of relative humdity say partial pressure to saturation pressure. You can even use the Lee model but adjust the saturation temperature to fulfill Antoine equation at RH Equal t0 100%. On the other hand you can make use of the Ddpm Model and get advantage of the more accurate heat and mass transfer for DPM for combustion or droplet or multicomponent particle. 

    • maliuzair
      Subscriber

      Thank you sir. I will have a look in to your suggestions.


      Meanwhile can you also please address the second issue in this post, i.e.  simple heat transfer between air, biomass (granular) and sand (granular). (No mixture)


      Thank you and Best regards

    • DrAmine
      Ansys Employee

      Check if you have any openings conducting cold flow from the outside due to backflow (at your pressure outlet). Backflow quantities are always stagnation quantities. So if your air is flowing back with 300 K then it would enter with temperature slightly smaller then 300 K depending on compressibility and pressure in neighborhood.  Moreover please move to the most actual release: there were some issues which have been fixed and might affect your solution.

    • maliuzair
      Subscriber

      Dear Sir,


      Yes, the backflow temperature is set as 300 K, but the temperature instantaneously drops just above the bed height at the interface of solids and air after 5-10 time steps.


       


    • DrAmine
      Ansys Employee

      Please post the initialization panel here especially temperatures, vof and mass fractions.

    • maliuzair
      Subscriber

      Dear Sir,


      One solid granular phase was patched as 0.4 vof, and the other as 0.2.


      I would like to add that I encountered this problem when I used 2 granular phases.


      There is no problem when I use 1 granular phase.


      Thanks and Best Regards


       


       


       


      initialization panel 1initialization panel 2initialization panel 3

    • DrAmine
      Ansys Employee

      Which heat transfer correlation's have been used? Which release are you using ?

    • maliuzair
      Subscriber

      Sir,


      For gas/solid interactions, GUNN


      For solid/solid interactions, TOMIYAMA


      Version: ANSYS 17.2


       


      Best Regards

    • DrAmine
      Ansys Employee

      So tomiyama is only suitable for interaction with fluids. Is you air compresible? Please use the most current releases as we have enhanced a lot of stuff in the previous versions.

    • maliuzair
      Subscriber

      Sir, the air is in-compressible. 


      As far as tomiyama model is concerned, tomiyama has been used in literature for solids-solids. though i ll try with other suitable model.


      Can there be anything wrong with the 2 granular phases, since it works perfectly fine with 1 granular phase. What can possibly be wrong?


       


      Best regards

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