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 Pitot Bench Operation 
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Joined: Sat Jul 10, 2010 8:33 pm
Posts: 8
So I am new to this forum, and i thank you for all the good info. I have a question about the pitot style bench operation. here goes...

So the flow in a pipe has a certai velocity profile across any given cross section. In fully developed, laminar, no pressure gradient, flow it is a parabola. in fully developed turbulent flow, it is almost constant across the section. But in general, this profile may or may not even be axisymetric due to the short pipe lengths and multiple pipe bends in the measurement circuits. Also, this profile could change depending on the flow conditions.

So do people with Pitot benches do a sweep of a cross section for each test condition and then find an average velocity and use that in their mass flow rate calc? do you just assume that the velocity is uniform and use flow numbers for comparison purposes and don't care about the systematic error? how do people handle this?

One way i can think of is to set up a traverse system, where i measure the velocity profile (measure te velocity at multiple points from one wall to the next) for every test point. This would take a really long time to complete a test. The other way i would think to handle this is to use a flow straightener section and long sections of pipe before the pitot probe. I would then spend one good afternoon doing miltiple sweeps at all flow rates achievable and make sure my flow was uniform & axisymmetric. then correlate the average velocity to a measured velocity of the pitot-static probe. But this would add flow losses to the system and reduce the maximum flow rate.

I am trying to decide on which kind of bench to build for small motorcycle engine parts (< 50 cfm). I'm just trying to make sure i understand what is normaly done before i build. Thanks for your input.
-Josh


Thu Dec 30, 2010 5:04 pm
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Joined: Fri Jan 08, 2010 10:36 pm
Posts: 1171
Location: Maryland
Josh,

I think there is a lot to be learned and unitized from a “Flow Element” (APT “Averaging Pito Tube” in a pipe) I/E one of my goals since I work on motorcycles is to build an Flow element and data logger to measure flow against engine rpm in a live environment.

As for flow testing I think the biggest downfall to Pito benches is the ability to reverse flow for testing, all others, math weather stations etc can be overcome.

So to answer your question for all the Flow elements I have seen the Pito pickup is not what you would think it looks like. It is installed perpendicular to the airflow bisecting it in the center of the pipe with multiple impact sensing holes. This design averages the pressure impact across the cross sectional area, I guess if you wanted overkill you could make it in an X ???

Rick.

PS. You should PM Bruce about our new secret project, the PTS CUBE! a small 2 motor bench top ?? orifice bench. I am planning on building the first prototype after my year end work settles down.


Fri Dec 31, 2010 12:08 pm
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