Air Residence Time

[Bryan Klein]

"Modelling a car park smoke and pollution ventilation system, and want to show a slice at roughly head-height plotting the mean age of air or air residence time.  Is this possible at all?"

https://en.wikipedia.org/wiki/Residence_time

Any suggestions on if/how to do this in FDS?

-Bryan

[Kevin McGrattan]

You can seed the flow with MASSLESS particles and color them with 'PARTICLE AGE'

[dr_jfloyd]

Another option is run the model with the ventilation system for a long enough period of time to reach steady-state flow, then introduce a tracer gas into the supply vents and run for a short period of time. At the end of that short time period you can use the tracer concentration and the length of time to derive the local air exchange rate. 

[Bryan Klein]

Thank you for these suggestions and they answer my question.

They are very much the way I was thinking to go, I was curious if there was some secret quantity or method to extract this directly in a 3D Slice or Statistical output data.

Best Regards,

Bryan

[Michael Abbot]

Team,

I tried a method similar to this, and abandoned it as particle injection absorbed a considerable amount of processing time. The particles need to turn off at some point in time, otherwise as model time increases, the amount of CPU time required appears to increase to process.

I would recommend a minimum velocity criteria within the basement car park space, further, and slices measuring Carbon Monoxide levels throughout. 

Regards,

Michael Abbot

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[fde]

Just brainstorming, 

If we set FIXED_MIX_TIME to a very high number and turn off the combustion while releasing the gas at certain concentration, then measure the whole compartment with a device XB to find AVERAGE value of MIXING TIME quantity, would it work?

[dr_jfloyd]

MIXING TIME would not give you what Bryan is looking for.  MIXING TIME is a measure of how species in the cell are mixing in that cell and not a measure of of transport of species to/from the cell.

[fde]

Thank you.