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Age of air calculation divergence

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Old   September 20, 2021, 12:08
Default Age of air calculation divergence
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Hello, I have ran a few different HVAC simulation models with the inclusion of S2S radiation model, and with air density as "incompressible-ideal-gas".

I am using the following UDF to calculate age of air.

#include "udf.h"
DEFINE_DIFFUSIVITY(mean_age_diff, c, t, i)
return C_R(c,t)*2.88e-05+C_MU_EFF(c,t)/0.7;

DEFINE_SOURCE(mean_age_source, c, t, dS, eqn)
return C_R(c,t);
I run the uds in transient after my models have converged with all other equations turned off. It converges perfectly for one of my models which was run with a constant density instead of "incompressible-ideal-gas". Otherwise it diverges. The age of air (volume average) jumps between really high positive and negative numbers.

Any idea on how to calculate it for the incompressible-ideal-gas cases?
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aoa, fluent, hvac, udf

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