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Understanding how an air flow bench works

by kibestar
Tags: bench, flow
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kibestar
#1
Apr14-14, 02:09 PM
P: 10
Hello, I'm new to these forums and they seemed like an excellent way to have my questions answered.

I'll start by saying that I've never had any course that dealt with fluid mechanics (mechanical engineering freshman), so I'm having quite some trouble grasping how readings on an air flow bench are to be correctly made. I'm using wikipedia's top image on its page on air flow benches to try to understand this topic:



Firstly, when the air pump is on, what happens with respect to pressure changes in each plenum? Is the pressure change experienced in every point inside each plenum equally, so a difference in the positioning of the manometers wouldn't cause a difference in readings? And what exact readings of the manometers and calculations would you perform to obtain the air flow volume?

I apologize for any english related mistakes, I'm not a native speaker.

Thanks in advance!
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Seth.T
#2
Apr23-14, 09:50 PM
P: 25
Your question is incomplete, theres no way for me to quantify any exact readings on meters without any given numerals like mass flow rate, known air pressure and temperature.
ares
#3
Apr29-14, 05:15 PM
P: 2
As air flows down a pipe the pressure decreases or there wouldn't be any flow. So fundamentally the pressure in each point of the plenum is not the same. In practice the differences between the pressures in a plenum might be very small compared to the pressure loss across the metering element. It depends. There are standards such as ISO 5167, which deals with metering elements somewhere in the middle of a pipe. It tells you how far you should place the manometers upstream and downstream from the element.

In your diagram the air is dumped straight into the surrounding ambient air so ISO 5167 is not applicable and there might be some other standard that deals with this case. It also seems to depict an orifice plate for which there's a wikipedia article that explains how to get either the mass flow rate or the volume flow rate. I would go with mass flow rate and calculate the volume flow rate as needed because the mass flow rate is typically constant and the volume flow rate rarely is.

edit:
To add to air flowing towards decreasing pressure, that is to say that the static pressure decreases. In an ideal world with frictionless pipes the total or stagnation pressure remains the same and the dynamic pressure or simply air velocity will increase. Your manometer will measure some value in between static and total pressure making practical measurements difficult.

Travis_King
#4
Apr29-14, 06:25 PM
P: 841
Understanding how an air flow bench works

Quote Quote by kibestar View Post
Hello, I'm new to these forums and they seemed like an excellent way to have my questions answered.

I'll start by saying that I've never had any course that dealt with fluid mechanics (mechanical engineering freshman), so I'm having quite some trouble grasping how readings on an air flow bench are to be correctly made. I'm using wikipedia's top image on its page on air flow benches to try to understand this topic:



Firstly, when the air pump is on, what happens with respect to pressure changes in each plenum? Is the pressure change experienced in every point inside each plenum equally, so a difference in the positioning of the manometers wouldn't cause a difference in readings? And what exact readings of the manometers and calculations would you perform to obtain the air flow volume?

I apologize for any english related mistakes, I'm not a native speaker.

Thanks in advance!
I'm not 100% sure about the application, so take my words with a grain of salt (i.e. bear in mind that I may be mistaken) but here's how I see it working:

Each piece of equipment there has known characteristics. The pipe has a set diameter, the test piece has what looks like a valve (a one-way valve, maybe?) which has specific loss characteristics for given differential pressures, same is true for the flow control device. The fan, too, has known flow characteristics for given differential pressures. That is, knowing the pressure upstream of the fan and knowing the pressure downstream, you can determine the flow rate (say, in SCFM) through the fan.

Plenum's are used for precisely the reasons indicated by ares. All pipes with air flow through them experience pressure losses. Plenums (or plena), however, are significantly larger in volume than the pipes connected to them. This is a practical way to ensure that the pressure you are measuring is an accurate representation of the pressure of the fluid (in this case air) at that point as the losses in the plenum are significantly lower than in the attached valves and piping. They are also used because introducing a measuring device into a plenum doesn't disrupt the flow (causing further losses) as much as it would if you had put the device into a pipe.

So, using the inlet pressure and inlet temperature and comparing that to the discharge pressure and discharge temperature, you can use the known characteristics of the pump to determine the airflow.
gmax137
#5
Apr29-14, 06:57 PM
P: 855
Like Travis, I'm not familiar with flow benches but I imagine the plenum dimensions are chosen to ensure a very low flow velocity within; that way the entire plenum is at the same pressure. Or close enough that the variation does not matter.


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