Age of Air – What are the units ?

- July 20, 2020 at 8:39 am
  
  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.
- July 20, 2020 at 8:40 am
  
  DrAmine
  Ansys Employee
  
  Seconds
- July 20, 2020 at 3:57 pm
  
  Rob
  Forum Moderator
  
  What materials are you using?
- July 20, 2020 at 4:39 pm
  
  Weaerodo
  Subscriber
  
  Thanks.
  
  Do you feel my result is appropriate.?
  
  Is there a difference between the residence time and age of air?
- July 20, 2020 at 4:41 pm
  
  Weaerodo
  Subscriber
  
  The material is air, the density is an incompressible ideal gas.
  
  Is there anything??
  
  Are you feeling doubtful about the result?
- July 21, 2020 at 3:32 pm
  
  Rob
  Forum Moderator
  
  How well converged is the scalar? As I don't know the size of the domain I have no idea if it's correct.
- July 22, 2020 at 11:17 am
  
  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.
- July 22, 2020 at 2:20 pm
  
  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.!)
- July 22, 2020 at 2:35 pm
  
  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...?
- July 22, 2020 at 3:07 pm
  
  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?