Flucturating depression and flow mesurement - flucturating unstable pressure

[GordonE]

Hi!
Im now beginning to test my newly built pitot flow bench.
It uses 110mm plastic piping adapted(pretty bad adapter) down to 75mm od piping. Then it enters the 2.5" pitot mesuring tube.
After that i use a gate valve to set the depression from the vacuum motors.
The depression is mesuered in the straight cylinder pipe around 20cm down from the cylinder head surface.

Im just beginning to use the flow bench and it seems to mesure my test plates around what they should be. I have machined three calibration plates, 50(21.49mm), 150(37,22mm) and 300(52,65mm)CFM.
The 50cfm plate reads around 52 and the 150cfm reads 154.

My scale isnt steep enough, I have just 200mm of rise on the inclined manometer, so therfore i havnt tested the 300CFM plate yet. The scale bottoms out.

But i have noticed a flucturation of the test pressure and the inclined scale. This appers somewere over 200CFM.
Any suggestions of what is happening? could it be turbulence at the 110-75mm adapter?

It looks like this, its pretty crappy. And I think that it shouldnt affect the depression that is measured upstream of the adaptor!


I am also fittting some sensors to a A/D card to get the signal to the computer. So far I got the depression sensor working!

Thanks Gordon

[slracer]

Hello Gordon, My computer "land line" was off line yesterday due to high winds in the area so I'm a little late responding! First, welcome to the "dark side" from another pitot bench user! We're outnumbered here so it's good to see someone else in the fold! I'm still quite new at this also, but maybe I can help. If I am making any mistakes, will someone please correct me!

My first post described what I was doing during construction of my bench and has a bit about my flow element construction and the flow straighteners I used. It may be helpful:


Bruce thought my own "new bench" writeup was well done, so this might help as well:


Be sure you use the SEARCH function for your questions as many have been answered before. You can get to it by clicking the SEARCH button in the upper right corner of the screen. When using the feature, also be sure that you start at "The Beginning". That is done by using the last pull down menu on the page (which usually reads "This Month" when the page starts up). With all of that said, let's look at your problems.

You haven't said much about the construction of your bench, so some added details and a pic could help. Are you using a settling chamber or plenum under your test piece? This is where the static depression tap is usually located and that has to be in an area that doesn't "feel" any flow (only pressure). Close to one of the inside corners is best. If it is in a turbulent location, it could show pulsing. About the only other thing I can think of here is that it is pulsing, possibily due to your motors surging. You say that it happens around 200CFM, how many motors are you using? Do you have enough power to run all of your motors at the same time? They may be physically turning, but not efficiently.

For the inclined pulsing, yes, that is a crappy adapter! What are you using for a pitot, single point or averaging? Where it is located (how far from the adapter)? Do you have flow straighteners AFTER the adapter? Turbulence could cause the problem and, again, the motor questions apply. Please send more details and we will try to help.

As Bruce likes to say, "Welcome to the addiction!"

Doug

[GordonE]

Hi Doug!
Thanks for your reply.
I am familiar with this forum and I have used the search function a lot. No problem there! I have studied quiet a few threads before i started my build.

I am starting to think that one of my motors is a little worn and is causing this flucturation. I flowed the plates once more yesterday and it started to flucturate a little wile after the motors where turned on.

The static mesurement is placed around 8in down the 110mm pvc pipe. No setteling chamber built yet. Im not sure that it is nessecary. Although I have had plans to bild a flow straightener before the static port.

Im using six used vacuum motors from a lot of different vacuum cleaners. They are run simultanius right now. I have thaught about using some sort of speed control, but I havnt found a suitable solution yet.
I am running two motors on each phase of 240V, and I have 3X16A of power, and 3X32 if its needed. I think that lack of power its not a problem.
First thing is first and I have decided to get the bench working before digging more into the speed control.

I havent had the time to build straighteners after the adapter yet. I have baught 600 soda straws, but I am not redy for therapy yet
I will fix that but I was eager to try it out.

Im using a single point pitot made out of a 1/4" brake line copper tube, and im not going to buy bruces
Its doing its job as it is now. Its around 500mm from the reduction.

The bench is built under a garage bench so its not very attractive to look at. Its mostly pvc pipe and 1m 2.5" pipe.
Ill post pictures later when I come home from work.

I got the flow sensor to work 11PM last night. Its a little bit sensetive because the root function for the flow calculation, but with some averaging it seems to work fine!

Thanks Gordon

[slracer]

Hi Gordon, A worn motor could certainly cause your problem! If you can determine which one it is, shut it down and see if that stops the pulsing. Be sure to close the exhaust off so you don't get reverse flow.

Your 110mm pipe is acting like a (small) settling chamber as, assuming a 100mm ID on it and 65mm ID on the flow element, the flow area is about 2.5 times the flow element. You don't say how long it is, but it is a positive factor. What is the diameter of your bore/head adapter outlet? If it is smaller than the 110mm pipe, a better location for your static depression measurement would be very close to the bottom of that joint (the top of the 110mm pipe). That would keep it out of direct flow.

Your description sounds like you are using only one static pickup. A second pickup located 10-15mm before the pitot is recommended. Remember you are trying to measure the difference between the pitot and static flow at the same place. The flow velocity at the pitot is 2.5 times that in the "settling chamber" (110mm) and you will have losses through that "adapter". I also recommend an averaging pitot. The flow through that adapter is going to be turbulent and the averaging should help smooth it out as it is measuring at 6 places across the tube. You sound like you are handy enough to build your own (although I wouldn't build mine for what Bruce charges for his). Take a look at these sites to see more about location of the holes:


In fact, I read through a lot in the sites before I found the Forum and learned things there also.
While making the new pitot, take a little therapy time and add the soda straws. They should help smooth out the flow sensor readings also.
The pitot location is pretty good. For a 65mm ID, it is almost 8D from the adapter (but the straighteners will reduce this somewhat). Is it straight after the pitot, or does it turn?

I'm looking forward to seeing pictures! My original try at building was not too pretty either (and it wasn't functional either! :p ) The Forum has been a terrific source of info! In fact, I would like to hear more about what you are doing with your flow sensors. Put some of that into the DAQ section. I'm looking forward to seeing the PTS product, but I'm also looking for something to put on some race vehicles I work on, maybe what you are doing will help there.

Doug

PS - What kind of "improvements" do you do to Mercedes?