Fluids

Fluids

Age of Air – What are the units ?

    • Weaerodo
      Subscriber

      Hi 


      I need to calculate the Age of Air


      From the manual I got the udf of diffusivity as follows


      #include "udf.h"


      DEFINE_SOURCE(rt_source,c,t,dS,eqn)


      {


      real source = C_R(c,t);


      dS[eqn]= 0.0;


      return source;


      }


      DEFINE_DIFFUSIVITY(rtd_diff,c,t,i)


      {


      return C_R(c,t)*2.88e-5+C_MU_EFF(c,t)/0.7;


      }


       


      when I have utilised this udf, by creating a scalar and solving it 


       


      I have got the following result.


       


      I am not sure about the results, are the units in seconds??


      because I don't think 7 seconds will be the right answer as the domain is larger.


       


      Apart from this, I have created an injection (massless), and from post-processing, I have calculated the residence time 


      I was getting the residence time as 1061 seconds.


       


      Please suggest me regarding this.


       

    • DrAmine
      Ansys Employee

      Seconds

    • Rob
      Ansys Employee

      What materials are you using?

    • Weaerodo
      Subscriber

      Thanks.


      Do you feel my result is appropriate.?


      Is there a difference between the residence time and age of air?


       

    • Weaerodo
      Subscriber

      The material is air, the density is an incompressible ideal gas.


      Is there anything??


      Are you feeling doubtful about the result?

    • Rob
      Ansys Employee

      How well converged is the scalar? As I don't know the size of the domain I have no idea if it's correct. 

    • DrAmine
      Ansys Employee

      If you do not trust your results you might inject a massless particle and check its residence time. We do not know your case, domain extension and boundaries used here so we cannot comment on unknown things.

    • Weaerodo
      Subscriber

      Thanks for your responses.


       


      I understand I should have specified a few details about the domain.


      The domain is 4*2.5*3 (meters).


       


      Yes, Injection of a massless particle is giving me a value of 1061 seconds, whereas the scalar was giving me 7 seconds.


      Should I go with the injection? (I wonder what was my mistake for the scalar case.!)


       

    • DrAmine
      Ansys Employee

      No no : check the particle tracks perhaps you have a particle stuck somewhere which is not really what you want to have. Do you have inlet /outlet? More details please........ And what value you have at inlet for your UDS...?

    • Weaerodo
      Subscriber

      I have two inlets and two outlets.


       


      The uds is specified a value = 0 at the inlets (Blue colour in the contour), whereas for all the remaining boundaries I have selected the flux=0.


       


      Apart from these things, is their any other important thing to consider while solving for the scalar.?


      Do we need to add source in the cell zone?


       

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