I am modelling airflow through a building and currently am hoping to analyse the air change efficiency. Is there any way of initially filling the building with one gas and then letting the air carry it out so that I can estimate the time for a certain amount of the original gas to leave the building? If not could someone please suggest another method of analysing the air change efficiency without using the traditional method of just calculating airflow in and airflow out- I am trying to show that certain parts of the building, although they may appear to be well ventilated with the traditional methods, are in fact not.

Hello, I think that the following should be concerned. Firstly, How to define the Air Change Efficiency. What is your defination then. Secondly, When the ventilation began, the incoming will absolutely mixed with the original air.But some part of the building will ventilated well and some part of the building will ventilated poor. That is to say we can estimate the extend that every part of building has been ventilated. thirdly,I think you can initially filling the building with tracers with uniform concertration. then let the fresh air into the building and determine the tracers variation with time to decided the ventilation degree.I think this methods is physically real. Hope this will be helpful to you.

Thanks for the information, but I was wondering if there is any way of actually implementing it in CFX 5.

it sounds like you need to solve a scalar transport equation for the mean age of air to estimate the local "freshness" of the air [seconds] ... (some cfd codes focused on hvac applications include this as a standard post-processing feature) then you can convert that into a ventilation effectiveness using the average age of air one could compute knowing the volume of the space and the total ventilation flowrate.