Learning Forum

[Rhandy Delefortrie]

Dear all, 

I have a system with a mass flow inlet through which a nitrogen cooling gas enters a chamber and leaves again through a pressure outlet. I've run simulations in transient, with data sampling for time statistics turned on for 0.04s of sampling time. I want to know if there is a way to determine the average residence time of the cooling gas inside the chamber. 

Kind regards! 

[Rob]

There are several options. Is the flow transient? If you've modelled 0.04s how does that compare to the bulk flow (plug flow) residence time? 

[Rhandy Delefortrie]

Yes, the flow is transient. I'm not sure what you mean with the second question. Once I reached a pseudo-steady state during the simulations, I continued running the simulation with 'Sampling for Time Statistics' option checked in orde to calculate average values, eg minimum facet temperature. I would like to do this as well for the residence time of the cooling gas inside the chamber. 

 

[Rob]

Look up "plug flow residence time", or go ask a Chemical Engineer. Then compare that to the 0.04s you've modelled. 

In the model, you can use pathlines to get an idea of flow residence time, species tracer (like in the experiments) or a scalar method commonly referred to as "mean age of air". The latter isn't covered the documentation, but should be explained in Solutions either on here (I think it's public) or on the Customer Portal (your Prof should have access). 

[Rhandy Delefortrie]

Thank you for the respons. I know about the methods you proposed, however I'm wondering if there is a way to get the same results without having to run additional simulations. 

[Rob]

If you save case & data then switch to steady that opens up the pathlines option with no extra calculation. If you also freeze the flow (look in the Equations in Solver section) you just need to run the scalar calc, that ought to converge in a few hundred iterations and it's only a single equation to solve. 

[Rhandy Delefortrie]

How can I initialize my scalar simulation with the mean velocity and pressure of my transient simulation? 

 

[Rob]

I'd just use the instanteous values. If you want to use the RMS values it's a little more complicated and requires a combination of Patch and UDMs. 

[Rhandy Delefortrie]

Thank you for your time and effort, I managed to get what I needed!