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

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

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.

Steve

2. For a quick answer to some of your questions I suggest you see this document:

Sonic-Nozzle-Brochure.pdf

3. Okay I see, thanks for that.

So the nozzle reduces the flow going through the part? Am i correct in saying that?

So there will be a mass flow rate calculation before the nozzles, then a mass flow rate after the nozzles? So what are the pressure and temperature probes responsible for?

4. First, the mass flow is constant throughout the system. That is the reason that the gas flow velocity is increased in the reduced diameter of the flow nozzle.

The pressure and temperature probes are to allow the determination of mass flow because the density of gases varies with both pressure and temperature; so, while the mass flow does not change, the volume flow in CFM (cubic feet per minute, using our American measurement system) and nozzles flow velocity are dependent upon the pressure and temperature of the gas (in your case, air).

The downstream probes establish the gas conditions that are entering whatever item is being tested by the system. The upstream and downstream probes can also be used to insure the the required 1.4 or greater pressure ratio (for air) needed for a critical (sonic) nozzle flow is present.

5. Thanks for that JAlberts

So why is the gas flow velocity increased? I'm just trying to get my head around what is actually happening in the system.

And when the pressure ratio is mentioned, what is this? The difference between upstream and downstream of the nozzle?

A quick start to finish in this scenario would be very appreciated

6. Yes the pressure ratio is "The difference between upstream and downstream of the nozzle".

As to the velocity increase, think about it this way: if a given amount (mass) of a gas (or liquid) is flowing through a piping system and at some point the inside area of the pipe is reduced to 1/2 of that of previous section of pipe then the velocity of the fluid through the smaller section must be increased to twice that of the larger area pipe. This is no different than what happens when you use a nozzle on the end of a garden hose to shoot a water stream across the yard or rinse your car.

7. Thanks JAlberts.

So eventually the velocity in the nozzle reaches the speed of sound which is then choked or 'capped'. Increasing the pressure prior to this wont change this.

However, when combining nozzles, what does this achieve? Just more mass flow?

So what happens is;

A pressure is placed prior to the nozzle to achieve a mass flow before the nozzle?
When it reaches the plenum after the nozzle, the mass flow is calculated as it leaves the plenum through the component?

am i correct in saying this?

8. I am not sure what you mean by "increasing the pressure prior to this"; but, until the pressure entering the nozzle is a minimum of 1.4 times the pressure after the nozzle, the mass flow in the system is affected by both the pressure before and pressure after the flow nozzle. However, when this pressure ratio reaches and remains above 1.4, then the downstream pressure no longer has any effect on the mass flow through the system. The mass flow is totally controlled by the changes in the upstream pressure. If that pressure is increased then the mass flow will increase and visa-versa.

So now the discussion gets more complicated. What does change below the nozzle is the density of the gas and therefore the volume of the gas; i.e., if the downstream pressure is 1/3 of the upstream pressure then the volume of each lb of gas will be 3 times the volume per lb of the gas before the nozzle. Any temperature change will also have a similar effect on the gas volume, but the mass flow in lbs/mimute through the system does not change.

It critical to understand that the volume flow, not the mass flow, is what determines the velocity of the gas through a nozzle and: therefore, the point at which a sonic flow velocity will be reached in the nozzle.

The reason that the temperature and pressure are measured above the nozzle is to determine (using calculations based upon thermodynamic properties of the gas) the density of the gas entering the nozzle and the sonic velocity through the nozzle and; therefore, the mass flow in lbs/second through the nozzle, which controls the mass flow through the complete test system.

The pressure and temperature measurements downstream of the sonic nozzle are used to determine the volume flow in CFM (or, cubic meters per second, etc) at a given pressure through the downstream piece of equipment being tested; and, this data is used to rate the performance of that equipment for service.

With regard to your question about multiple nozzles, if you are referring to two, or more, sonic nozzles side by side being fed by the same upstream pipe; then the mass flow of the system is multipled by the number of nozzles; for example: two nozzles will provide twice the mass flow rate through the system.

What we are discussing here is complcated and based on gas properties and flow physics, so I understand your difficulties in trying to comprehend all of it. My real understanding of the information I am communicating to you did not come easily for me either; and, was only achieved over some 20 years experience in designing and flow testing pressure relief valves.

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