Understanding how an air flow bench works

In summary: This forum is dedicated to flow bench design and workings. In summary, the conversation discusses the use of air flow benches to measure air flow volume and the factors that affect pressure changes in each plenum. The forum members suggest joining specific forums and websites for more information and resources on flow bench design and usage.
  • #1
kibestar
16
2
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:

Flow_bench_schematic.GIF


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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  • #2
Your question is incomplete, there's no way for me to quantify any exact readings on meters without any given numerals like mass flow rate, known air pressure and temperature.
 
  • #3
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.
 
Last edited:
  • #4
kibestar said:
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:

Flow_bench_schematic.GIF


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.
 
  • #5
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.
 
  • #6
Hi , join <http://www.flowbenchtech.com all> the information and question/answered you have in regards to " How a flow bench works ", you will find on this fourm.
This forum is dedicated forum to flow bench design and workings.
http://www.flowbenchtech.com/forum/viewtopic.php?f=3&t=626
AND ALSO
Link to the Old Tractorsport Flowbench Forum. It is READ ONLY and is an archive only forum and can not be posted to. BUT holds valuable information on the topic of flow bench and flow testing procedures. http://www.tractorsport.com/forum/
Hope this information helps you
Cheers
 
  • #7

1. What is an air flow bench used for?

An air flow bench is a tool used to measure the flow rate of air through a component, such as an engine cylinder head or intake manifold. It is commonly used in automotive and aerospace industries to optimize the performance of engines.

2. How does an air flow bench work?

An air flow bench works by drawing air through a calibrated orifice while measuring the differential pressure and calculating the flow rate. The air flow is controlled by a variable valve or venturi tube, and the pressure is measured using a manometer or electronic sensor.

3. What factors can affect the accuracy of an air flow bench?

The accuracy of an air flow bench can be affected by several factors, including the temperature, humidity, and pressure of the air being tested, as well as the condition and size of the orifice, and the precision of the measurement equipment.

4. How is an air flow bench calibrated?

An air flow bench is typically calibrated using a known standard, such as a flow meter or a calibrated orifice plate. The flow rate is adjusted until the measured values match the known values, and any discrepancies are corrected through adjustments to the equipment.

5. What are the benefits of using an air flow bench?

Using an air flow bench allows for precise measurement and optimization of air flow, which can lead to improved engine performance and efficiency. It can also help identify potential issues or areas for improvement in an engine's design or components.

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