## 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.

• 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

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?