How is Flow Function Calculated with this setup? (Novice)

[flexywozere]

Hi there,

I am an apprentice at my place of work and my supervisor has put me in the airflow side of the business as an operator.

The airflow rig that we have is a Flow Systems one. I just want to know how exactly it works? So, dumb questions are following.

We measure the Flow Function of the parts. What does this actually represent physically? How is it different to Mass Flow?

I understand that the flow goes through a venturi tube which becomes choked when it hits the speed of sound, which eliminates fluctuations with changes downstream to the venturi tube. However, it's upstream i have the problems with.

The flow then goes through nozzles. What happens here? Do they speed up the velocity of the air? Why? All i know is that the nozzle choice depends on the flow. The air eventually reaches a plenum chamber where the flow comes to rest. There is a pressure tapping in the chamber and air then 'leaks' through the part at the top of chamber.

All i know is that the pressure ratio is set to 1.4. What exactly is the pressure ratio between?

My final question. What is the whole rig measuring actually measuring to calculate the flow function? I haven't been able to find this out.

I realize that these are dumb questions, but once i know exactly how something is working, and more importantly why, I can apply logic to the whole rig.

Thanks in advance.

Steve